effects of pollution research paper

Effects of air pollution on health: A mapping review of systematic reviews and meta-analyses

Affiliations.

• 1 Laboratory of Sport and Exercise Psychology (Lape) - College of Health and Sport Science of the Santa Catarina State University (UDESC), Florianópolis, SC, Brazil.
• 2 Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Cassino, FR, Italy.
• 3 Department of Mechanical Engineering, University of Coimbra, Portugal.
• 4 Laboratory of Sport and Exercise Psychology (Lape) - College of Health and Sport Science of the Santa Catarina State University (UDESC), Florianópolis, SC, Brazil. Electronic address: [email protected].
• PMID: 34116013
• DOI: 10.1016/j.envres.2021.111487

Background: There has been a notable increase in knowledge production on air pollution and human health.

Objective: To analyze the state of the art on the effects of air pollution on human health through a mapping review of existing systematic reviews and meta-analyses (SRs and MAs).

Methods: The systematic mapping review was based on the recommendations for this type of scientific approach in environmental sciences. The search was performed using PubMed, Web of Science, Scopus, Cinahl, and Cochrane Library databases, from their inception through June 2020.

Results: Among 3401 studies screened, 240 SRs and MAs satisfied the inclusion criteria. Five research questions were answered. There has been an overall progressive increase in publications since 2014. The majority of the SRs and MAs were carried out by researchers from institutions in China, the US, the UK, and Italy. Most studies performed a meta-analysis (161). In general, the reviews support the association of air pollution and health outcomes, and analyzed the effects of outdoor air pollution. The most commonly investigated health outcome type was the respiratory (mainly asthma and COPD), followed by cardiovascular outcomes (mainly stroke). Particulate matter (with a diameter of 2.5 μm (PM 2.5 ) and 10 μm (PM 10 ) or less) and nitrogen dioxide (NO 2 ) were the most widely investigated pollutants in the reviews. The general population was the most common sample in the reviews, followed by children, and adults. The majority of the reviews investigated health outcomes of respiratory diseases in children, as well as cardiovascular diseases in all ages. Combining health outcomes and air pollutants, PM 2.5 was included in a higher number of reviews in eight health outcomes, mainly cardiovascular diseases.

Discussion: The majority of SRs and MAs showed that air pollution has harmful effects on health, with a focus on respiratory and cardiovascular outcomes. Future studies should extend the analysis to psychological and social aspects influenced by air pollution.

Keywords: Air pollutants; Cardiovascular diseases; Harmful effects; Nitrogen dioxide; Particulate matter; Respiratory diseases.

Copyright © 2021 Elsevier Inc. All rights reserved.

Publication types

• Meta-Analysis
• Research Support, Non-U.S. Gov't
• Air Pollutants* / analysis
• Air Pollutants* / toxicity
• Air Pollution* / adverse effects
• Air Pollution* / analysis
• Environmental Exposure / adverse effects
• Environmental Exposure / analysis
• Nitrogen Dioxide / analysis
• Particulate Matter / analysis
• Particulate Matter / toxicity
• Systematic Reviews as Topic
• Air Pollutants
• Particulate Matter
• Nitrogen Dioxide

• Open access
• Published: 06 November 2008

Health effects of ambient air pollution – recent research development and contemporary methodological challenges

• Cizao Ren 1 , 2 &
• Shilu Tong 1  

Environmental Health volume  7 , Article number:  56 ( 2008 ) Cite this article

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Exposure to high levels of air pollution can cause a variety of adverse health outcomes. Air quality in developed countries has been generally improved over the last three decades. However, many recent epidemiological studies have consistently shown positive associations between low-level exposure to air pollution and health outcomes. Thus, adverse health effects of air pollution, even at relatively low levels, remain a public concern. This paper aims to provide an overview of recent research development and contemporary methodological challenges in this field and to identify future research directions for air pollution epidemiological studies.

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Introduction

It is well known that exposure to high levels of air pollution can adversely affect human health. A number of air pollution catastrophes occurred in industrial countries between 1950s and 1970s, such as the London smog of 1952 [ 1 ]. Air quality in western countries has significantly improved since the 1970s. However, adverse health effects of exposure to relatively low level of air pollution remain a public concern, motivated largely by a number of recent epidemiological studies that have shown the positive associations between air pollution and health outcomes using sophisticated time-series and other designs [ 2 ].

This review highlights the key findings from major epidemiological study designs (including time-series, case-crossover, panel, cohort, and birth outcome studies) in estimating the associations of exposure to ambient air pollution with health outcomes over the last two decades, and identifies future research opportunities. We do not intend for this to be a formal systematic literature review or meta-analysis, but to discuss issues we feel are vitally important based on the recent literature and our own experience. This paper is divided into two parts: firstly to summarize recent findings from major epidemiological studies, and secondly to discuss key methodological challenges in this field and to identify research opportunities for future air pollution epidemiological studies.

Health effects of ambient air pollution

Time-series studies.

There are a large number of time-series studies on the short-term health effects of air pollution, with the emphasis on mortality and hospital admissions by means of fitting Poisson regression models at a community level or ecological level. This type of time-series design is a major approach to estimating short-term health effects of air pollution in epidemiological studies for the last two decades. Many studies have found associations between daily changes in ambient particulate air pollution and increased cardiorespiratory hospital admissions [ 3 – 6 ], along with cardiorespiratory mortality [ 7 – 9 ] and all cause mortality [ 10 ]. Because numerous air pollution time-series studies show that exposure to air pollution is associated with different kinds of human health outcomes, it is impossible to list results from all studies. Table 1 only selects major time-series studies on short-term health effects of particulate matter (PM) and ozone from different countries around the world published over the last two decades because these two air pollutants are important toxic agents and widely explored by the majority of air pollution epidemiological studies. Early findings have been systematically and thoroughly reviewed by other authors [ 11 , 12 ].

Single-site time-series studies have been criticized because of exposure measurement errors, substantial variation of the air pollution effects and the heterogeneity of the statistical approaches used in different studies [ 13 ]. Recently, several multi-site time-series studies have been conducted in Europe and the United States. Two large collaborative air pollution projects in Europe and U.S. are summarised below.

In Europe, the APHEA (Air Pollution and Health: a European Approach) studies have provided many new insights. Initial studies were based on older data (APHEA-1) [ 14 ] and a new series of studies (APHEA-2) used data of the PM 10 fraction since the late 1990s [ 15 ]. The APHEA-2 mortality studies covered over 43 million people and 29 European cities, which were all studied for more than 5 years in the 1990s. The combined effect estimate showed that all-cause daily mortality increased by 0.6% (95% CI: 0.4%, 0.8%) for each 10 μg/m 3 increase in PM 10 from data involving 21 cities. It was found that there was heterogeneity between cities with different levels of NO 2 . The estimated increase in daily mortality for an increase of 10 μg/m 3 in PM 10 were 0.2% (95% CI: 0.0%, 0.4%), and 0.8% (95% CI: 0.7%, 0.9%) in cities with low and high average NO 2 , respectively [ 16 ]. The APHEA-2 hospital admission study involved 38 million people living in eight European cities. Hospital admissions for asthma and chronic obstructive pulmonary disease (COPD) increased by 1.0% (95% CI: 0.4%, 1.5%) per 10 μg/m 3 PM 10 increment among people older than 65 years [ 15 ].

In the United States, the National Morbidity, Mortality and Air Pollution Studies (NMMAPS) focused on the 20 largest metropolitan areas in the USA, involving 50 million inhabitants, during 1987–94 [ 2 ]. All-cause mortality was increased by 0.5% (95% CI: 0.1%, 0.9%) for each increase of 10 μg/m 3 in PM 10 . The estimated increase in the relative rate of death from cardiovascular and respiratory disease was 0.7% (95% CI: 0.2%, 1.2%). Effects on hospital admissions were studied in ten cities with a combined population of 1 843 000 individuals older than 65 years [ 17 ]. The model used considered simultaneously the effects of PM 10 up to the lag of 5 days and effects of PM 10 on chronic obstructive pulmonary disease admissions to be 2.5% (95% CI: 1.8%, 3.3%) and on cardiovascular disease admissions to be 1.3% (95% CI: 1.0%, 1.5%) for an increase of 10 μg/m 3 in PM 10 . Bell et al. [ 18 ] analysed 95 NMMAPS community data to examine the association between ozone concentration and mortality, showing that a 10-ppb increase in the previous week's ozone was associated with a 0.5% (95% posterior interval (PI), 0.3%, 0.8%) increase in daily mortality and a 0.64% (95% PI, 0.31%, 0.98%) increase in cardiovascular and respiratory mortality. The effect estimates of the exposure over the previous week were larger than those considering only a single day's exposure. Recently, Dominici et al. [ 13 ] examined the short-term association between fine particulate air pollution and hospital admissions and found that exposure to PM 2.5 was associated with different health outcomes. The largest association was observed for heart failure, and a 10 μg/m 3 increase in PM 2.5 was found to be associated with a 1.3% (95% PI: 0.8%, 1.8%) increase in hospital admissions from heart failure on the same day.

Although time-series studies have shown that day-to-day variations in air pollutant concentrations are associated with daily deaths and hospital admissions, it is still unclear how many days, weeks or months of air pollution have brought such events forward. Harvesting or mortality/morbidity displacement means that some cases are occurring only in those to whom it would have happened in a few days anyway [ 19 ]. If so, the increase in cases immediately after exposure would be offset by a deficit in daily deaths a few days later [ 19 , 20 ]. If air pollution has harvesting effects, normal time-series models are unable to estimate the effects due to the issues of collinearity and statistical power. The polynomial distributed lag (PDL) model [ 21 ] and the time-scale model [ 19 ] have been adopted to explore whether air pollution has harvesting or displacement effects on daily deaths or hospital admissions. A few studies suggested potential harvesting effects of ambient air pollution while other studies have shown that there is no evidence for harvesting effects [ 19 , 22 , 23 ]. Although one study shows that potential bias might occur in PDL model [ 24 ], the estimated effects of ambient air pollutants seem to increase when longer lags of air pollution are included [ 19 , 20 ].

Case-crossover studies

Case-crossover study design is an alternative approach to estimating short-term health effects of air pollution in epidemiological studies. In the last two decades, the case-crossover design has been applied in a large number of studies of air pollution and health [ 25 – 28 ]. For example, Neas et al. [ 27 ] used a case-crossover study design to estimate the association between air pollution and mortality in Philadelphia and found a 100 μg/m 3 increment in the 48 hours mean level of TSP was associated with increased all-cause mortality (odds ratio (OR) = 1.06; 95% CI: 1.03, 1.09). A similar association was observed for deaths in individuals over 65 years of age (OR: 1.07; 95% CI: 1.04, 1.11). Levy et al. [ 28 ] estimated the effect of short-term changes in exposure to particulate matter on the rate of sudden cardiac arrest. The cases were obtained from a previously conducted population-based case-control study and were combined with ambient air monitoring data. The results did not show any evidence of a short-term effect of particulate air pollution on the risk of sudden cardiac arrest in people without previously recognised heart disease. Schwartz [ 26 ] conducted a case-crossover study to examine the sensitivity of the association between ozone and mortality when adjusted for temperature and found that 10-ppb increase of maximum hourly ozone was associated with 0.23% (95% CI: 0.01% ~0.44%) increase in daily deaths after adjusting for temperature in 14 US cities. Barnett et al. [ 25 ] examined the association between air pollution and cardio-respiratory hospital admissions in Australia and New Zealand cities. The results show that air pollution arising from common emission sources was significantly associated with cardiovascular health outcomes in the elderly. For example, for a 0.9-ppm increase in CO, there were significant increases in elderly hospital admissions for 2.2% (95% CI: 0.9%, 3.4%) increase of total cardiovascular disease and 2.8% (95% CI: 1.3%, 4.4%) increase of all cardiac disease.

Panel studies

Many air pollution panel studies have been conducted, including several large longitudinal studies of air pollution and health effects such as the Southern California Children's Health Study [ 29 , 30 ], in which children from grades 4, 7, and 10 residing in twelve communities near Los Angeles were followed annually. The results indicated that exposure to ambient particles, NO 2 , and inorganic acid vapour was associated with reduced lung function in children. Another large panel study, the Pollution Effects on Asthmatic Children in Europe (PEACE), was designed to examine the relationship between short-term changes in air pollution and lung function, respiratory symptoms and medication use [ 31 ]. This project was conducted in 14 centres using a common protocol in the winter of 1993–1994. Each PEACE centre involved an urban and a rural panel of symptomatic children and followed at least seventy-five 6–12 year old children [ 31 ]. The pooled estimates of two literature reviews which were separately conducted about the PEACE study and showed that no clear relation could be established for changes in PM 10 , black smoke, SO 2 and NO 2 and changes in respiratory health. The non-significant effects were thought to be possibly due to the short observation period. Ward and Ayres [ 32 ] reviewed 22 panel studies published in the 1990s to estimate the overall effects of ambient particles on children. Results show that the majority of identified panel studies indicated an adverse effect of particulate air pollution. Several recent panel studies also show that particulate air pollution is associated with human health [ 33 – 37 ].

Cohort studies

Compared to time-series and case-crossover studies, there are only a few large cohort studies. About a dozen cohort studies have been conducted in the United States [ 38 – 44 ], Europe [ 45 – 48 ] and Australia [ 49 ]. A cohort study conducted by Dockery et al. [ 39 ] in six U.S. cities shows that there was a statistically significant and robust association between air pollution and mortality. The adjusted mortality rate ratio for the most polluted city was 1.26 (95% CI: 1.08–1.47) compared with the least polluted city. Air pollution was also associated with deaths from lung cancer and cardiopulmonary diseases. Abbey et al. [ 38 ] conducted a cohort study during 1973–1992 to estimate effect of exposure to long-term ambient concentrations of PM 10 and other air pollutants, and show that PM 10 was strongly associated with mortality from respiratory disease for both sexes adjusting for a wide range of potentially confounding factors. The relative risk (RR) for an interquartile range (IQR) difference of PM 10 was 1.18 (95% CI: 1.42, 3.97). Ozone was strongly associated with lung cancer mortality for males for the IQR difference (RR: 4.19; 95% CI: 1.81, 9.69). Sulphur dioxide was also strongly associated with lung cancer mortality for both sexes. Pope et al. [ 44 ] conducted one cohort study in the US to examine the long-term effect of exposure to fine particulate air pollution. They found that fine particulate and sulphur oxide-related pollution were associated with all-cause, lung cancer and cardiopulmonary mortality. A 10 μg/m 3 increase in fine particulate air pollution was associated with an increase of 4%, 6%, and 8% for all-cause, cardiopulmonary, and lung cancer mortality, respectively. Hoek et al. [ 48 ] investigated a random sample of 5000 people and 489 of 4492 (11%) died during 1986–1994 in the Netherlands fining that cardiopulmonary mortality was associated with living near a major road with relative risk of 1.95 (95% CI: 1.09–3.52). A cohort study conducted by Filleul et al. [ 46 ] in France found that urban air pollution to be associated with increased mortality over 25 years in France. Frostad et al. [ 47 ], in a 30-year follow-up cohort study in Norway, found that respiratory symptoms were a significant predictor of mortality from all causes. In Australia, Jalaludin et al.[ 49 ] enrolled a cohort of primary school children with a history of wheeze (n = 148) in an 11-month longitudinal study to examine the association between ambient air pollution and respiratory morbidity. They found that PM 10 and NO 2 , but not ozone, were significantly associated with doctor visits for asthma.

Birth outcome studies

Even though effects of exposure to ambient air pollution on mortality and hospital admissions have been increasingly demonstrated over the past 30 years, exploring its adverse impact on pregnant outcomes has only begun since the last decade [ 50 ]. Because pregnancy is a period of human development particularly susceptible to the influence of many environmental factors due to high cell proliferation, organ develop and the changes of capabilities of fetal metabolism, the relative short-term period provides a unique opportunity to study the adverse effects of ambient toxins on human health [ 51 ]. The majority of birth outcome studies are based on large datasets routinely collected from air pollution monitoring systems and birth registration processes, and therefore, in general, the statistical power is strong [ 52 – 59 ]. Logistic regression models or linear regression models at the individual level are usually adopted to assess the effects of ambient air pollution on adverse birth outcomes adjusting for potential confounders including maternal age, maternal race, parity, fetal gender, season, gestational period, etc. Birth outcomes usually include low birth weight, preterm delivery and other biomarkers such as birth defect and ultrasound measures of head circumference. Personal exposures are often estimated at different terms, including the full gestation, trimesters, month after the pregnancy or before the time of delivery, etc.

Many studies have shown that there are significant associations between exposure to ambient air pollutants and adverse birth outcomes [ 52 – 60 ]. For example, Liu et al. [ 53 ] found that 5-ppb increase of sulfur dioxide was associated with an 11% (95% CI: 1%, 22%) increase of low birth weight (< 2500 grams) during the first month gestation and with a 9% increase of preterm delivery in Vancouver, Canada. A 1.0 ppm increase of carbon monoxide during the last month of pregnancy was associated with an 8% increase of preterm delivery. Parker et al. [ 60 ] selected population within 5 miles of over 40 air pollution monitoring sites across 28 California counties to estimate the adverse effects of air pollution and found that per 10 μg/m 3 PM increase was associated with 13 g (95% CI: 7.6 g, 18.3 g) decrease of birth weight. Similarly, Ritz et al. [ 59 ] conducted a population-nested case-control study to examine associations between air pollution and birth outcomes in Los Angeles and found that air pollution exposure was associated with preterm birth. Hansen et al. [ 58 ] examined the associations of exposure to ambient air pollution during early pregnancy with fetal ultrasonic measurements during mid-pregnancy in Australia. They found that a reduction in fetal abdominal circumference was associated with exposure to O 3 during the days 31–60 of pregnancy (-1.42 mm, 95% CI: -2.74, -0.09), SO2 during the days 61–90 (-1.67 mm, 95% CI: -2.94, -0.40), and PM 10 during the days 90–120 (-0.78 mm, 95% CI: -1.49, -0.08).

Implications of weak health effects

Even though the association of air pollution with health outcomes is weak, it still has strong public health implications. One reason is that air pollution is ubiquitous and affects the whole population in most metropolitan cities. Another reason is that residents are continuously and permanently exposed to air pollution, which may have both short- and long-term effects on health outcomes. Some intervention studies have shown that the reduction in air pollution has resulted in an improvement in population health [ 55 , 61 ]. For example, Hedley et al. [ 61 ] reported that cardiovascular, respiratory and all cause mortality reduced by 2.0% (p < 0.05), 3.9% (p < 0.05) and 2.1% (p < 0.05) respectively in the first 12 months after an introduction of the restrictions on sulphur content of fuel in Hong Kong.

Contemporary methodological challenges

Air pollution epidemiologic research is challenged by the complexity of human exposure to environmental agents and by the difficulty of accurately measuring exposure. Residents are usually ubiquitously exposed to air pollution. In order to detect small effects of air pollution, both high statistical power and sophisticated study design are required. In addition, the characteristics of air pollutants vary and their concentrations change both spatially and temporally. Although everyone is susceptible to high concentration of pollution, its concentrations are not evenly distributed across populations. Due to such complexities, there are still many research questions to be addressed by future air pollution epidemiological studies. The following section discusses these issues.

Shape of exposure response curve

The shape of the exposure and response curve is very important. A key research question to be addressed is whether a threshold exists below which a certain air pollutant has no effect on population health. If such a threshold could be identified, public health benefits would be expected from bringing the pollutant below this level. Both theoretical and empirical works have been done to shed light on this issue [ 62 , 63 ]. In the analysis of NMMAPS data, no threshold evidence was found for the relationship between PM 10 and daily all-cause and cardiorespiratory mortality [ 63 ]. By contrast, a threshold of about 50 μg/m 3 was indicated for non-cardiorespiratory causes of death – viz, below this point, PM 10 had little influence on non-cardiorespiratory mortality. These issues remain to be clarified.

Model uncertainty and bias

The process of model selection includes how to select covariates (eg, meteorological variables and co-pollutants), lag structure for air pollutants and the number of degrees of freedom for smoothing functions to adjust for long-term trend, short fluctuation, seasonality, other covariates and the determination of referent in case-crossover design. Studies have shown that the model choice will impact on estimates of relative risk [ 64 ]. As a result, many authors attempted to estimate the effects using the best single lag or combination of lags for meteorological factors and/or air pollutants and to identify the best degree of freedom for smoothing to adjust for different potential confounders. Some types of data can use several different models. Some authors do not clearly state why they select models and how they conduct data analyses. For example, when we estimate associations between exposure to air pollution and recurrent asthma episodes, based on different assumptions, at least five survival Cox models could be applied to estimate the associations between exposure to air pollution and asthma episodes [ 65 ]. Different assumptions or models may result in different estimates, and sometimes the difference is considerable. The choice of software options may cause this kind of uncertainty as well [ 65 ].

Results presented by the "best" final models are likely to cause publication bias because stronger and positive estimates tend to be published but negative results are usually difficult to be published. Multi-site time series design in which all data are analysed using the same model is one way to solve this problem. However, model uncertainty still exists in a multi-site study to some extent due to the model choice. Some studies have used Bayesian Model Averaging (BMA) to take into account uncertainties in model choice when making an inference [ 64 ]. BMA uses hierarchical models. The predictions and inferences are based on multiple models rather than a single model. Predictions are obtained by forming weighted averages of predictions over the different models where weights depend on the degree to which the data support each model.

Measurement errors of exposure to air pollution and potential confounders usually exist in air pollution epidemiological studies and it is impossible to be solved in most air pollution studies [ 66 ]. Due to spatial and temporal variations, data obtained in air pollution central monitors are not well representative of individual exposures. Some models are used to assess individual exposure to air pollution [ 66 , 67 ], but they could not efficiently adjust for measurement errors. Therefore, potential misclassification bias of exposure is one of the main concerns in air pollution studies.

There are both spatial and temporal variations for exposure and outcomes in air pollution studies [ 68 ]. Both times-series and case-crossover designs at a community level can efficiently adjust for some measured and unmeasured time-invariant characteristics of the subjects (such as gender, age, smoking status and spatial characteristics) via matching, and therefore, the potential confounding from these measured and unmeasured characteristics is minimised [ 69 , 70 ]. The key concern for these designs is how to control for temporal confounding and meteorological variables, such as seasonality, short-term variations and weather conditions (eg, temperature and humidity). In a prospective cohort study design, a major issue is how to identify a cohort with a sufficient variation in cumulative exposures, particularly when data recorded in central monitoring stations are used to measure ambient air pollution levels [ 44 ]. However, in maximizing the geographical variability of exposure the relative risk estimates from cohort studies are likely to be confounded by area-specific characteristics [ 68 ]. Due to collection of relatively detailed individual characters and sufficient adjustment for potential individual social and economic status, such confounding might be efficiently adjusted for.

Birth outcome studies are mainly based on routinely collected data, including exposure, outcome and potential confounders [ 52 – 60 ]. Most studies use pollution data obtained from the different monitors and the closest residential monitoring data are used as exposure proxies [ 58 , 60 , 71 , 72 ]. In general, information related to birth outcomes is well recorded in birth registration systems. However, the data may not include complete and accurate information on other potential confounders, such as maternal social and economic status and life styles. Birth outcome data analyses are usually conducted at an individual level. Therefore, this design is inherently vulnerable to some potential biases, including both temporal and spatial misclassification bias. Ritz and Wilhelm [ 73 ] has discussed the methodological issues of birth outcome designs in detail, and this review would not repeat these issues but rather than focus on potential bias in relation to spatial variation, which was ignored in their review, in the following section.

In birth outcome studies, both exposure and outcome data include temporal and spatial variations to some extent. The majority of birth outcome studies have adjusted for temporal and other confounders which are related to delivery information, including season, maternal age and race, fetal gender, parity, and maternal education attainment [ 52 – 60 , 71 , 72 ]. However, so far, few studies have paid much attention to the potential spatial confounding. Unlike time-series or case-crossover studies, most birth outcome studies lack the ability for automatic adjustment for measured and unmeasured time-invariant spatial variations. Unlike cohort studies, birth outcome study designs also lack the ability to efficiently adjust for personal life styles and social and economic status due to the lack of the detailed information available in routinely collected data. Because both exposure to air pollution and birth outcomes are influenced by some geographic characteristics, such as land use, forest, public infrastructure, and residential social and economic status, etc, the previous birth outcome studies might introduce bias to some extent due to the failure to consider spatial variation. In general, these spatial-related factors are favourable to links between air pollution exposure and birth outcomes. Therefore, we presume that the stronger associations reported in the previous birth outcome studies might partially attribute to this kind of bias. The simple way to adjust for the spatial variation is to add a categorical variable for individual residential areas to fit a fixed effect model or to include the residential areas to fit a mixed model or a random effect model.

Interaction of temperature and air pollution

In many locations, patterns of air pollution are driven by weather. Therefore, concentrations of air pollutants may be associated with temperature. Therefore, it may be possible that temperature and air pollution interact to affect health outcomes. Although effect modification has important public health implications [ 74 ], this issue has so far received limited attention, probably because of methodological complexity and the difficulty in data interpretation. Several studies examined whether or not ambient air pollution and temperature interact to affect human health outcomes, but they produced conflicting results [ 75 – 78 ]). For example, Samet et al. [ 78 ] investigated the sensitivity of the particulate air pollution mortality effect estimate to alternative methods of controlling weather and did not find any evidence that weather conditions modified the associations of particulate air pollution and sulphur dioxide with mortality, regardless of approaches of synoptic weather conditions. Katsouyanni et al. [ 75 ] used a multiple linear regression to investigate the interaction between air pollution and high temperature during a heat wave in Athens in July 1987. They found that while the main effects of air pollution index were not statistically significant, there was statistically significant synergistic effect between high levels of sulphur dioxide and high temperature (P < 0.05). Roberts [ 77 ] found evidence that the effect of particulate air pollution on mortality might depend on temperature but the synergistic effect was sensitive to the number of degrees of freedom used in confounder adjustments. Recently, we found that temperature and particulate matter symmetrically enhanced the effect [ 76 ]. Since then, several multiple-site studies have found evidence that temperature and air pollutants interacted to impact human health but the nature and magnitude of such an interaction varied with geographic area [ 79 – 82 ]. Thus, further research is needed to examine the interactive effects between air pollutants and temperature on mortality and morbidity, especially in different spatial settings.

Many time-series, case-crossover and panel studies have shown that there are consistent short-term effects of air pollution on health outcomes (hospital admissions or deaths). Some cohort studies have also shown long-term health effects of air pollution. In spite of the weak associations of air pollution with human morbidity or mortality, its public health implications are strong because exposure to air pollution is ubiquitous and widespread. However, there are several key methodological challenges in the estimation of the health effects of low-level exposure to air pollution, such as the shape of the exposure response curve, threshold of air pollution, interactive effects of air pollution and weather conditions, and model uncertainty and potential bias. Future research efforts should focus on these important issues.

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Acknowledgements

We thank Prof. Gail Williams, School of Population Health, University of Queensland for the comments on the earlier version of the manuscript. We also thank two reviewers for their insightful and constructive comments.

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CR conceived of the study, participated in its design, and is responsible for the draft of the manuscript. ST participated in the study design and revised the draft of the manuscript.

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Ren, C., Tong, S. Health effects of ambient air pollution – recent research development and contemporary methodological challenges. Environ Health 7 , 56 (2008). https://doi.org/10.1186/1476-069X-7-56

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Research on Health Effects from Air Pollution

Decades of research have shown that air pollutants such as ozone and particulate matter (PM) increase the amount and seriousness of lung and heart disease and other health problems. More investigation is needed to further understand the role poor air quality plays in causing detrimental effects to health and increased disease, especially in vulnerable populations. Children, the elderly, and  people living in areas with high levels of air pollution are especially susceptible.

Results from these investigations are used to support the nation's air quality standards under the Clean Air Act and contribute to improvements in public health.

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Health Effects of Air Pollutants on Vulnerable Populations

Long-term and short-term effects from exposure to air pollutants.

• Multipollutant Exposures and Changes in Environmental Conditions
• Leveraging Big Data for Innovations in Health Science 

Health Effects of Wildfire Smoke

Public health intervention and communications strategies, integrated science assessments for air pollutants.

Research has shown that some people are more susceptible than others to air pollutants. These groups include children, pregnant women, older adults, and individuals with pre-existing heart and lung disease. People in low socioeconomic neighborhoods and communities may be more vulnerable to air pollution because of many factors. Proximity to industrial sources of air pollution, underlying health problems, poor nutrition, stress, and other factors can contribute to increased health impacts in these communities.

There is a need for greater understanding of the factors that may influence whether a population or age group is at increased risk of health effects from air pollution. In addition, advances to analytical approaches used to study the health effects from air pollution will improve exposure estimates for healthy and at-risk groups.

The research by EPA scientists and others inform the required reviews of the primary National Ambient Air Quality Standards (NAAQS), which is done with the development of Integrated Science Assessments (ISAs). These ISAs are mandated by Congress every five years to assess the current state of the science on criteria air pollutants and determine if the standards provide adequate protection to public health. 

Research is focused on addressing four areas:

• Identifying and characterizing whether there are key reproductive factors and critical stages of development that are impacted by air pollution exposures;
• Determining the role of acute and chronic sociodemographic factors in air pollution health disparities;
• Understanding how diet modifies responses to air pollution;
• Evaluating long-term lifestyle and chronic disease effects on air pollution-induced respiratory and cardiovascular responses

A multi-disciplinary team of investigators is coordinating epidemiological, human observational, and basic toxicological research to assess the effects of air pollution in at-risk populations and develop strategies to protect these populations, particularly those with pre-existing disease. The results from these products will improve risk assessments by clarifying the role of modifying factors such as psychosocial stress (e.g. noise) and diet, and determining the impact of individual susceptibility on the relationship between air pollutant exposures and health.

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People can experience exposure to varying concentrations of air pollution. Poor air quality can impact individuals for a short period of time during the day, or more frequently during a given day. Exposure to pollutants can also occur over multiple days, weeks or months due to seasonal air pollution, such as increased ozone during the summer or particulate matter from woodstoves during the winter.

The health impact of air pollution exposure depends on the duration and concentrations, and the health status of the affected populations. Studies are needed to increase knowledge of the exposure duration and the possible cumulative increase in risk.

The research is focused on three main areas: 

• Short-term peak exposures, such as wildfires, traffic-related sources, or other episodic events;
• Intermittent and cumulative exposures;
•  Mechanisms underlying the exposure risks

Researchers are evaluating the health responses of intermittent multiple days versus one-day air pollution exposure in controlled human exposure, animal, and in vitro models and associated cellular and molecular mechanisms. They are employing population-based models and electronic health records to assess the health effects of short-term and long-term exposures and identifying populations at greatest risk of health effects. The work is improving our understanding of the possible cumulative effects of multiple short-term peak exposures and the relationship of these exposures to longer-term exposures and risks.

Multipollutant Exposures and Changes in Environmental Conditions  

EPA research is providing information to understand how individuals may respond to two or more pollutants or mixtures and how environmental conditions may impact air quality.  While risk estimates for exposure to individual criteria air pollutants such as PM and ozone are well established, the acute and cumulative effects of combinations of pollutants is not well understood. In addition, research is needed to determine how changes in the environment affect both pollutant formation and subsequent responsiveness to exposures in healthy and susceptible individuals.

The research is focused on three specific questions: 

• What is the role of temperature and photochemical aging on the health impact of wildfire smoke and air pollution mixtures?
• What is the effect of changing environmental conditions (i.e., temperature and humidity) on responsiveness to air pollution?
• Does prior pollutant exposure modify responsiveness to subsequent exposures?

The integrated, multi-disciplinary research includes:

• Epidemiologic analyses of environmental influences on morbidity and mortality in populations,
• Simulations of changing environmental conditions in multi-pollutant formation in atmospheric chamber studies coupled with clinical and toxicological assessments in healthy and at-risk populations,
• Evaluation of pre-exposure as a modifying effect on subsequent exposures

The results are revealing how changes in environmental conditions affect pollutant formation and subsequent health impact in at-risk populations. The research findings are informing EPA’s Integrated Science  Assessments for criteria air pollutants and assisting with future regulatory decisions on the National Ambient Air Quality Standards (NAAQS).

Leveraging Big Data for Innovations in Health Science  

EPA is at the cutting edge of health science, using electronic health records, novel data systems, tissue-like advanced cellular models, molecular approaches, and animal models to evaluate the health impacts of air pollution.  Researchers are using these powerful new techniques to identify factors that may increase sensitivity and vulnerability to air pollution effects. 

The research is building capacity for future risk assessment and regulatory analyses that go beyond traditional lines of evidence to more clearly define populations and lifestages at increased risk of health effects from air pollution.

To continue to protect public health from poor air quality, researchers must consider new epidemiological, toxicological and clinical approaches to understand the health risks of poor air quality and the biological mechanisms responsible for these risks. At the center of these new research approaches is an explosion of data availability and methodological approaches for handling large clinical and molecular datasets, also known as "big data."

While data of increasing size, depth, and complexity have accelerated research for many industries and scientific fields, big data is sometimes less recognized for the impacts it is having on environmental health studies. Increasingly, researchers are able to examine vulnerable populations with unprecedented precision and detail while also evaluating hundreds of thousands of molecular biomarkers in order to understand biological mechanisms associated with exposure.

• Development of the InTelligence And Machine LEarning (TAME) Toolkit for Introductory Data Science, Chemical-Biological Analyses, Predictive Modeling, and Database Mining for Environmental Health Research (Journal Article)

Larger and more intense wildfires are creating the potential for greater smoke production and chronic exposures in the United States, particularly in the West. Wildfires increase air pollution in surrounding areas and can affect regional air quality.

The health effects of wildfire smoke can range from eye and respiratory tract irritation to more serious disorders, including reduced lung function, exacerbation of asthma and heart failure, and premature death. Children, pregnant women, and the elderly are especially vulnerable to smoke exposure. Emissions from wildfires are known to cause increased visits to hospitals and clinics by those exposed to smoke.

It is important to more fully understand the human health effects associated with short- and long-term exposures to smoke from wildfires as well as prescribed fires, together referred to as wildland fires. EPA is conducting research to advance understanding of the health effects from different types of fires as well as combustion phases. Researchers want to know:

• What is the full extent of health effects from smoke exposure?
• Who is most at risk?
• Are there differences in health effects from different wildfire fuel types or combustion phases (burning versus flaming)?
• What strategies and approaches are most effective in protecting public health?
• What are the environmental, social and economic impacts of wildfire emissions?
• Wildland Fire Research
• Smoke-Ready Toolbox for Wildfires
• Smoke Sense Project and App

Many communities throughout the United States face challenges in providing advice to residents about how best to protect their health when they are exposed to elevated concentrations of air pollutants from motor vehicle and industrial emissions and other sources of combustion, including wildland fire smoke.

Researchers are studying intervention strategies to reduce the health impacts from exposure to air pollution as well as ways to effectively communicate these health risks. To translate the science for use in public health communication and community empowerment, EPA is collaborating with other federal agencies, such as the Centers for Disease Control and Prevention (CDC) and the National Heart, Lung, and Blood Institute (NHLBI), and state and local agencies and tribes. The objectives are to identify ways to lower air pollution exposure or mitigate the biological responses at individual, community or ecosystem levels, and ultimately evaluate whether such interventions have benefits as measured by indicators of health, well-being or economics.

Studies are evaluating the interactions between behavior and social and economic factors to more thoroughly understand how these factors may influence health and well-being outcomes, which can inform effective and consistent health risk messaging. 

• Healthy Heart Toolkit and Research
• Video:  Air Quality Impacts on Public Health

EPA sets National Ambient Air Quality Standards (NAAQS) for six principal criteria air pollutants —nitrogen oxides, sulfur oxides, particulate matter, carbon monoxide, ozone and lead—all of which have been shown to be harmful to public health and the environment.

The Agency’s  Integrated Science Assessments (ISAs)  form the scientific foundation for the review of the NAAQS standards by providing the primary (human health-based) assessments and secondary (welfare-based, e.g. ecology, visibility, materials) assessments. The ISAs are assessments of the state of the science on the criteria pollutants. They are conducted as mandated under the Clean Air Act.

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Impact of Environmental Factors on the Health of Children and Older Adults

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Environmental factors, particularly air pollution, significantly affect human health, leading to conditions such as stroke, heart disease, lung cancer, and respiratory ailments. As climate change progresses, its health implications become increasingly acute through indirect pathways like altered weather patterns, affecting particularly sensitive demographics like children and older adults. Both groups are crucial to the fabric of family, community, and economic spheres, yet they remain highly vulnerable to environmental stressors. This Research Topic will focus on identifying and quantifying the effects of environmental factors, excluding extreme weather events, on these vulnerable populations. A thorough exploration of these impacts will help provide actionable data and inform policy decisions, thus optimizing environmental health management and promoting sustainable practices to support healthy aging and societal resilience. This Research Topic aims to enhance our understanding of how environmental pollutants affect the health of children and older adults, foregoing the direct influence of extreme weather events, thereby contributing to more targeted and effective environmental and health governance strategies. We encourage interdisciplinary and innovative approaches to thoroughly understand how air pollution impacts these specific age groups, thereby strengthening environmental health and resilience initiatives. This Research Topic seeks contributions that explore these areas to offer empirical evidence that can inform and shape policy at all levels and improve health management practices. We welcome submissions from epidemiological research to policy analysis to aid stakeholders, from policymakers to families, in enhancing their environmental health strategies. We invite submissions on a variety of topics, not limited to the ones listed below: ● Impact of air pollution on children: investigating health risks related to air pollution, particularly its effects on development and disease predisposition in children. ● Impact of extreme weather events on children: exploring the specific impacts of extreme climate conditions on children. ● Impact of air pollution on older adults: examining how air pollution exacerbates chronic conditions and increases mortality risks in the older adults. ● Impact of extreme weather events on older adults: studying how extreme weather affects older adults' health. ● Health management strategies: developing targeted prevention and intervention strategies to support the unique needs of children and older adults, informed by empirical evidence.

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• Review Article
• Published: 23 May 2024

Monitoring, trends and impacts of light pollution

• Hector Linares Arroyo   ORCID: orcid.org/0000-0003-0034-3700 1 ,
• Angela Abascal 2 ,
• Tobias Degen 3 , 4 ,
• Martin Aubé 5 , 6 ,
• Brian R. Espey 7 ,
• Geza Gyuk 8 ,
• Franz Hölker   ORCID: orcid.org/0000-0001-5932-266X 3 , 9 ,
• Andreas Jechow   ORCID: orcid.org/0000-0002-7596-6366 3 , 10 ,
• Monika Kuffer 2 ,
• Alejandro Sánchez de Miguel 11 , 12 ,
• Alexandre Simoneau 5 , 6 ,
• Ken Walczak 8 &
• Christopher C. M. Kyba 13 , 14  

Nature Reviews Earth & Environment ( 2024 ) Cite this article

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• Atmospheric chemistry
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• Environmental impact

Light pollution has increased globally, with 80% of the total population now living under light-polluted skies. In this Review, we elucidate the scope and importance of light pollution and discuss techniques to monitor it. In urban areas, light emissions from sources such as street lights lead to a zenith radiance 40 times larger than that of an unpolluted night sky. Non-urban areas account for over 50% of the total night-time light observed by satellites, with contributions from sources such as transportation networks and resource extraction. Artificial light can disturb the migratory and reproductive behaviours of animals even at the low illuminances from diffuse skyglow. Additionally, lighting (indoor and outdoor) accounts for 20% of global electricity consumption and 6% of CO 2 emissions, leading to indirect environmental impacts and a financial cost. However, existing monitoring techniques can only perform a limited number of measurements throughout the night and lack spectral and spatial resolution. Therefore, satellites with improved spectral and spatial resolution are needed to enable time series analysis of light pollution trends throughout the night.

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Acknowledgements

A.A., A.S, C.C.M.K., F.H., H.L.A., M.A., M.K. and T.D. received funding for this work through ESA’s New Earth Observation Mission Ideas (NEOMI) program under contract 4000139244/22/NL. A.S.d.M. has been funded by European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement number 847635 (UNA4CAREER). A.J. was supported by the project BELLVUE “Beleuchtungsplanung: Verfahren und Methoden für eine naturschutzfreundliche Beleuchtungsgestaltung” by the BfN with funds from the BMU (FKZ: 3521 84 1000).

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Effects of air pollution on human health and practical measures for prevention in Iran

Adel ghorani-azam.

Medical Toxicology Research Center, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran

Bamdad Riahi-Zanjani

Mahdi balali-mood.

Air pollution is a major concern of new civilized world, which has a serious toxicological impact on human health and the environment. It has a number of different emission sources, but motor vehicles and industrial processes contribute the major part of air pollution. According to the World Health Organization, six major air pollutants include particle pollution, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Long and short term exposure to air suspended toxicants has a different toxicological impact on human including respiratory and cardiovascular diseases, neuropsychiatric complications, the eyes irritation, skin diseases, and long-term chronic diseases such as cancer. Several reports have revealed the direct association between exposure to the poor air quality and increasing rate of morbidity and mortality mostly due to cardiovascular and respiratory diseases. Air pollution is considered as the major environmental risk factor in the incidence and progression of some diseases such as asthma, lung cancer, ventricular hypertrophy, Alzheimer's and Parkinson's diseases, psychological complications, autism, retinopathy, fetal growth, and low birth weight. In this review article, we aimed to discuss toxicology of major air pollutants, sources of emission, and their impact on human health. We have also proposed practical measures to reduce air pollution in Iran.

INTRODUCTION

Air pollution is a major problem of recent decades, which has a serious toxicological impact on human health and the environment. The sources of pollution vary from small unit of cigarettes and natural sources such as volcanic activities to large volume of emission from motor engines of automobiles and industrial activities.[ 1 , 2 ] Long-term effects of air pollution on the onset of diseases such as respiratory infections and inflammations, cardiovascular dysfunctions, and cancer is widely accepted;[ 3 , 4 , 5 , 6 ] hence, air pollution is linked with millions of death globally each year.[ 7 , 8 , 9 ] A recent study has revealed the association between male infertility and air pollution.[ 10 ]

Air pollution has now emerged in developing countries as a result of industrial activities and also increase the quantity of emission sources such as inappropriate vehicles.[ 11 , 12 , 13 ] About 4.3 million people die from household air pollution and 3.7 million from ambient air pollution, most of whom (3.3 and 2.6 million, respectively) live in Asia.[ 14 ] In Iran, as a developing country, the level of air pollutants has increased gradually since the beginning of industrialization in the 1970s, but it has reached a very harmful level in some megacities such as Tehran, Mashhad, Tabriz, Isfahan, Ahvaz, Arak, and Karaj over the past two decades. Iran is the world's third main polluted country in the world, which results in 16 billion $ annual loss.[ 15 ] In fact, four of the top ten air-polluted cities are in Iran. Ahvaz is the most air polluted city in the world with microdust blowing in from neighboring countries, and particulate levels three times that of Beijing, and nearly 13 times that of London.[ 16 ] Air pollution caused almost 4460 deaths in 2013 only in Tehran although the reality seemed higher and is getting worse every year.[ 17 ] Therefore, it is of great importance to describe the problem, particularly its toxic effects on human health and provide recommendations as a basis for environmental guidelines and standard protocols in the field of air pollution in Iran.

The present article is neither a systematic review nor a descriptive, educational study. It is a problem-based descriptive review in which the authors try to explain a problem which is the major health and ecological problem in developing countries like Iran. In this review, we have tried to summarize the toxicology of air pollutants and related diseases with a possible mechanism of action and appropriate management of the patients. Therefore, it shall be useful for the environmental and health professionals particularly policy makers, emergency physicians, and other clinicians who may be involved in air pollution and related diseases. In this paper, we also discuss sources of air pollution and proposed some feasible solutions which may be beneficial for the environmental legislators and decision makers.

DEFINITIONS

Air pollution is defined as all destructive effects of any sources which contribute to the pollution of the atmosphere and/or deterioration of the ecosystem. Air pollution is caused by both human interventions and/or natural phenomena. It is made up of many kinds of pollutants including materials in solid, liquid, and gas phases.[ 18 ] Air pollutions of indoors will not be specifically considered in this article.

The Pollutant Standard Index (PSI) is a numerical value and indicator of pollutants that is normally used to facilitate risk assessment. It is a numeric value between zero to 500.[ 19 ] PSI is a guideline for reporting air quality which was first introduced by Thom and Ott in 1974.[ 20 ] Hence, it would provide a method of comparing the relative contribution of each pollutant to total risk.[ 21 ] The calculation of PSI is based on the concentration of five major air pollutants including particulate matters (PMs), sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), carbon monoxide (CO), and ozone (O 3 ) in the air.

According to Johnson et al ., “air quality index (AQI) is defined as a measure of the condition of air relative to the requirements of one or more biotic species or to any human need.”[ 22 ] AQI is divided into ranges, in which they are numbered, and each range is marked with color codes. It provides a number from healthy standard level of zero to a very hazardous level of above 300 to indicate the level of health risk associated with air quality. Based on PSI, air quality is classified into six major indices, which is marked by color codes and each color corresponds to a different level of health concerns. Principally, green is defined as a color indicator for healthy air quality; while yellow, orange, red, purple, and maroon colors indicate as moderate, unhealthy for sensitive groups, unhealthy, very unhealthy, and hazardous air quality, respectively. These ranges and codes may differ in the different methods of classifications in different countries.[ 22 ]

TOXICOLOGY OF AIR POLLUTION

Effects of air pollutants on living organism will not only be limited to the human and animal health but also include the whole environment. Different geographical conditions, global climate changes, and the environmental variations affect the human health and the environment including the animal life.

Environmental damages

Ecologically, air pollution can cause serious environmental damages to the groundwater, soil, and air.[ 23 , 24 ] It is also a serious threat to the diversity of life. Studies on the relationship between air pollution and reducing species diversity clearly show the detrimental effects of environmental contaminants on the extinction of animals and plants species.[ 25 ] Air suspended toxicants may also cause reproductive effects in animals.[ 26 , 27 ] Acid rain, temperature inversion, and global climate changes due to the emissions of greenhouse gasses to the atmosphere are other major ecological impacts of air pollution.[ 28 ]

Air pollutants and their toxicities

Every material in the air which could affect human health or have a profound impact on the environment is defined as air pollutants. According to the World Health Organization (WHO), particle pollution, ground-level O 3 , CO, sulfur oxides, nitrogen oxides, and lead (Pb) are the six major air pollutants which harm human health and also the ecosystem. There are many pollutants of suspended materials such as dust, fumes, smokes, mists, gaseous pollutants, hydrocarbons, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), and halogen derivatives in the air which at the high concentrations cause vulnerability to many diseases including different types of cancers.[ 29 , 30 , 31 , 32 ] The most important air pollutants and their toxic effects on different human body organs and related diseases have been briefly described below.

Particle pollutants

Particle pollutants are major parts of air pollutants. In a simple definition, they are a mixture of particles found in the air. Particle pollution which is more known as PM is linked with most of pulmonary and cardiac-associated morbidity and mortality.[ 33 , 34 ] They have varied in size ranging mostly from 2.5 to 10 μm (PM 2.5 to PM 10 ).

The size of particle pollutants is directly associated with the onset and progression of the lungs and heart diseases. Particles of smaller size reach the lower respiratory tract and thus have greater potential for causing the lungs and heart diseases. Moreover, numerous scientific data have demonstrated that fine particle pollutants cause premature death in people with heart and/or lung disease including cardiac dysrhythmias, nonfatal heart attacks, aggravated asthma, and decreased lung functions. Depending on the level of exposure, particulate pollutants may cause mild to severe illnesses. Wheezing, cough, dry mouth, and limitation in activities due to breathing problems are the most prevalent clinical symptoms of respiratory disease resulted from air pollution.[ 35 , 36 , 37 ]

Long-term exposure to current ambient PM concentrations may lead to a marked reduction in life expectancy. The increase of cardiopulmonary and lung cancer mortality are the main reasons for the reduction in life expectancy. Reduced lung functions in children and adults leading to asthmatic bronchitis and chronic obstructive pulmonary disease (COPD) are also serious diseases which induce lower quality of life and reduced life expectancy. Strong evidence on the effect of long-term exposure to PM on cardiovascular and cardiopulmonary mortality come from cohort studies.[ 38 , 39 , 40 ]

Ground-level ozone

O 3 with the chemical formula of O 3 is a colorless gas which is the major constituent of the atmosphere. It is found both at the ground level and in the upper regions of the atmosphere which is called troposphere. Ground-level ozone (GLO) is produced as a result of chemical reaction between oxides of nitrogen and VOCs emitted from natural sources and/or due to human activities. GLO is believed to have a plausible association with increased risk of respiratory diseases, particularly asthma.[ 41 ]

As a powerful oxidant, O 3 accepts electrons from other molecules. There is a high level of polyunsaturated fatty acids in the surface fluid lining of the respiratory tract and cell membranes that underlie the lining fluid. The double bonds available in these fatty acids are unstable. O 3 attacks unpaired electron to form ozonides and progress through an unstable zwitterion or trioxolane (depending on the presence of water). These ultimately recombine or decompose to lipohydroperoxides, aldehydes, and hydrogen peroxide. These pathways are thought to initiate propagation of lipid radicals and auto-oxidation of cell membranes and macromolecules. It also increases the risk of DNA damage in epidermal keratinocytes, which leads to impaired cellular function.[ 42 ]

O 3 induces a variety of toxic effects in humans and experimental animals at concentrations that occur in many urban areas.[ 43 ] These effects include morphologic, functional, immunologic, and biochemical alterations. Because of its low water solubility, a substantial portion of inhaled O 3 penetrates deep into the lungs but its reactivity is scrubbed by the nasopharynx of resting rats and humans in around 17% and 40%, respectively.[ 44 , 45 ] On ecological aspect, O 3 can reduce carbon assimilation in trees leading to deforestation which may affect global food security in long-term exposure.[ 46 , 47 ]

Carbon monoxide

CO is a colorless and odorless gas, which is produced by fossil fuel, particularly when combustion is not appropriate, as in burning coal and wood. The affinity of CO to hemoglobin (as an oxygen carrier in the body) is about 250 times greater than that of oxygen. Depending on CO concentration and length of exposure, mild to severe poisoning may occur. Symptoms of CO poisoning may include headache, dizziness, weakness, nausea, vomiting, and finally loss of consciousness. The symptoms are very similar to those of other illnesses, such as food poisoning or viral infections.

No human health effects have been showed for carboxyhemoglobin (COHb) levels lower than 2%, while levels above 40% may be fatal. Hypoxia, apoptosis, and ischemia are known mechanisms of underlying CO toxicity.[ 48 ] The mechanism of such toxicity is the loss of oxygen due to competitive binding of CO to the hemoglobin heme groups. Cardiovascular changes also may be observed by CO exposures that create COHb in excess of 5%. In the early 1990s, Health Effects Institute performed a series of studies associated with cardiovascular disease to determine the potential for angina pectoris with COHb levels in the range of 2–6%.[ 49 ] The results showed that premature angina can occur under these situations but that the potential for the occurrence of ventricular arrhythmias remains uncertain. Thus, the reduction in ambient CO can reduce the risk of myocardial infarction in predisposed persons.

Sulfur dioxide

SO 2 is a colorless, highly reactive gas, which is considered as an important air pollutant. It is mostly emitted from fossil fuel consumption, natural volcanic activities, and industrial processes. SO 2 is very harmful for plant life, animal, and human health. People with lung disease, children, older people, and those who are more exposed to SO 2 are at higher risk of the skin and lung diseases.

The major health concerns associated with exposure to high concentrations of SO 2 include respiratory irritation and dysfunction, and also aggravation of existing cardiovascular disease. SO 2 is predominantly absorbed in the upper airways. As a sensory irritant, it can cause bronchospasm and mucus secretion in humans. Residents of industrialized regions encountered with SO 2 even at lower concentrations (<1 ppm) in the polluted ambient air might experience a high level of bronchitis.

The penetration of SO 2 into the lungs is greater during mouth breathing compared to nose breathing. An increase in the airflow in deep, rapid breathing enhances penetration of the gas into the deeper lung. Therefore, people who exercise in the polluted air would inhale more SO 2 and are likely to suffer from greater irritation. When SO 2 deposits along the airway, it dissolves into surface lining fluid as sulfite or bisulfite and is easily distributed throughout the body. It seems that the sulfite interacts with sensory receptors in the airways to cause local and centrally mediated bronchoconstriction.

According to the Environmental Protection Agency (EPA) of the USA, the level of annual standard for SO 2 is 0.03 ppm. Due to its solubility in water, SO 2 is responsible for acid rain formation and acidification of soils. SO 2 reduces the amount of oxygen in the water causing the death of marine species including both animals and plants. Exposure to SO 2 can cause damages to the eyes (lacrimation and corneal opacity), mucous membranes, the skin (redness, and blisters), and respiratory tracts. Bronchospasm, pulmonary edema, pneumonitis, and acute airway obstruction are the most common clinical findings associated with exposure to SO 2 .[ 50 ]

Nitrogen oxide

Nitrogen oxides are important ambient air pollutants which may increase the risk of respiratory infections.[ 50 ] They are mainly emitted from motor engines and thus are traffic-related air pollutants. They are deep lung irritants that can induce pulmonary edema if been inhaled at high levels. They are generally less toxic than O 3 , but NO 2 can pose clear toxicological problems. Exposures at 2.0–5.0 ppm have been shown to affect T-lymphocytes, particularly CD8 + cells and natural killer cells that play an important role in host defenses against viruses. Although these levels may be high, epidemiologic studies demonstrate effects of NO 2 on respiratory infection rates in children.

Coughing and wheezing are the most common complication of nitrogen oxides toxicity, but the eyes, nose or throat irritations, headache, dyspnea, chest pain, diaphoresis, fever, bronchospasm, and pulmonary edema may also occur. In another report, it is suggested that the level of nitrogen oxide between 0.2 and 0.6 ppm is harmless for the human population.[ 51 ]

Pb or plumb is a toxic heavy metal that is widely used in different industries.[ 52 ] Pb pollution may result from both indoor and outdoor sources. It is emitted from motor engines, particularly with those using petrol containing Pb tetraethyl. Smelters and battery plants, as well as irrigation water wells and wastewaters, are other emission sources of the Pb into the environment.[ 52 , 53 ] Evaluation of the blood Pb level in traffic police officers shows that environmental pollution may be considered as a source of Pb exposure.[ 54 ] Fetuses and children are highly susceptible to even low doses of Pb.[ 55 ] Pb accumulates in the body in blood, bone, and soft tissue. Because it is not readily excreted, Pb can also affect the kidneys, liver, nervous system, and the other organs.[ 56 ]

Pb absorption by the lungs depends on the particle size and concentration. Around 90% of Pb particles in the ambient air that are inhaled are small enough to be retained. Retained Pb absorption through alveoli is absorbed and induces toxicity. Pb is a powerful neurotoxicant, especially for infants and children as the high-risk groups. Mental retardation, learning disabilities, impairment of memory, hyperactivity, and antisocial behaviors are of adverse effects of Pb in childhood.[ 57 , 58 ] Therefore, it is very important to reduce the Pb level of ambient air.[ 59 ]

Pb exposure is often chronic, without obvious symptoms.[ 60 ] It can affect the different parts of the body including cardiovascular, renal, and reproductive systems, but the main target for Pb toxicity is the nervous system.[ 61 ] Pb disrupts the normal function of intracellular second messenger systems through the inhibition of N-methyl-D-aspartate receptors. Pb may also replace calcium as a second messenger resulting in protein modification through various cellular processes including protein kinase activation or deactivation.

Abdominal pain, anemia, aggression, constipation, headaches, irritability, loss of concentration and memory, reduced sensations, and sleep disorders are the most common symptoms of Pb poisoning. Exposure to Pb is manifested with numerous problems, such as high blood pressure, infertility, digestive and renal dysfunctions, and muscle and joint pain.

Other air pollutants

Other major air pollutants that are classified as carcinogen and mutagen compounds and are thought to be responsible for incidence and progression of cancer in human include VOCs such as benzene, toluene, ethylbenzene, and xylene, PAHs such as acenaphthene, acenaphthylene, anthracene, and benzopyrene, and other organic pollutants such as dioxins, which are unwanted chemical pollutants that almost totally produced by industrial processes and human activity.[ 62 , 63 , 64 ]

In Table 1 , the standard level of some conventional air pollutants is presented in which the values were defined as air quality standards that provide public welfare protection.

Standard level of criteria air pollutants and their sources with health impact based on the United States Environmental Protection Agency

As it can be easily understood, fossil fuel consumption shares the largest part of air contamination. Air pollutants can also be classified into anthropogenic and natural according to their source of emission. From anthropogenic aspect, air contamination occurs from industrial and agricultural activities, transportation, and energy acquisition. While from natural contaminant has different sources of emission such as volcanic activities, forest fire, sea water, and so on.[ 65 , 66 ]

Health hazards

In terms of health hazards, every unusual suspended material in the air, which causes difficulties in normal function of the human organs, is defined as air toxicants. According to available data, the main toxic effects of exposure to air pollutants are mainly on the respiratory, cardiovascular, ophthalmologic, dermatologic, neuropsychiatric, hematologic, immunologic, and reproductive systems. However, the molecular and cell toxicity may also induce a variety of cancers in the long term.[ 67 , 68 ] On the other hand, even small amount of air toxicants is shown to be dangerous for susceptible groups including children and elderly people as well as patients suffering from respiratory and cardiovascular diseases.[ 69 ]

Respiratory disorders

Because most of the pollutants enter the body through the airways, the respiratory system is in the first line of battle in the onset and progression of diseases resulted from air pollutants. Depending on the dose of inhaled pollutants, and deposition in target cells, they cause a different level of damages in the respiratory system. In the upper respiratory tract, the first effect is irritation, especially in trachea which induces voice disturbances. Air pollution is also considered as the major environmental risk factor for some respiratory diseases such as asthma and lung cancer.[ 70 , 71 ] Air pollutants, especially PMs and other respirable chemicals such as dust, O 3 , and benzene cause serious damage to the respiratory tract.[ 72 , 73 , 74 , 75 , 76 , 77 ] Asthma is a respiratory disease which may be developed as a result of exposure to air toxicants.[ 78 ] Some studies have validated associations between both traffic-related and/or industrial air pollution and increasing the risk of COPD.[ 79 , 80 , 81 ] Treatment of respiratory diseases due to air pollution is similar to the other toxic chemical induce respiratory disorders.

Cardiovascular dysfunctions

Many experimental and epidemiologic studies have shown the direct association of air pollutant exposure and cardiac-related illnesses.[ 82 , 83 , 84 , 85 ] Air pollution is also associated with changes in white blood cell counts[ 86 ] which also may affect the cardiovascular functions. On the other hand, a study on animal models suggested the close relationship between hypertension and air pollution exposure.[ 87 ] The traffic-related air pollution, especially exposure to high levels of NO 2 , is associated with right and left ventricular hypertrophy.[ 88 , 89 ] In addition to the antidote therapy that exists only for a few cardiotoxic substances like CO, usual treatment of cardiovascular diseases should be carried out.

Neuropsychiatric complications

The relationship between exposure to air suspended toxic materials and nerve system has always been argued. However, it is now believed that these toxic substances have damaging effects on the nervous system. The toxic effect of air pollutants on nerve system includes neurological complications and psychiatric disorders. Neurological impairment may cause devastating consequences, especially in infants. In contrast, psychiatric disorders will induce aggression and antisocial behaviors. Recent studies have reported the relationship between air pollution and neurobehavioral hyperactivity, criminal activity, and age-inappropriate behaviors.[ 90 , 91 ] Studies have also revealed the association between air pollution and higher risk of neuroinflammation,[ 92 ] Alzheimer's and Parkinson's diseases.[ 93 ] Some studies showed that aggression and anxiety in megacities are in close relationship with the high level of air pollutants.[ 94 , 95 , 96 ]

Other long-term complications

Skin is the body's first line of defense against a foreign pathogen or infectious agent and it is the first organ that may be contaminated by a pollutant. The skin is a target organ for pollution in which the absorption of environmental pollutants from this organ is equivalent to the respiratory uptake.[ 97 ] Research on the skin has provided evidence that traffic-related air pollutants, especially PAHs, VOCs, oxides, and PM affect skin aging and cause pigmented spots on the face.[ 98 , 99 , 100 ]

Theoretically, toxic air pollutants can cause damage to organs when inhaled or absorbed through the skin.[ 101 ] Some of these pollutants are hepatocarcinogen chemicals.[ 102 , 103 ] There are some proven data which highlighted the role of air pollutants, especially traffic-related air pollution on the incidence of autism and its related disorders in fetus and children.[ 104 , 105 , 106 , 107 ] Disrupting endocrine by chemical components of pollutants has been described as a possible mechanistic pathway of autism or other neurological disorders.[ 93 , 108 ] Some studies showed that there are relationships between air pollution exposure and fetal head size in late pregnancy,[ 109 ] fetal growth,[ 110 ] and low birth weight.[ 111 , 112 ]

Many of the diseases that are linked to immune system dysfunction can be affected by several environmental factors such as poor air quality.[ 113 , 114 ] Poor air quality can cause serious complications in the immune system such as an abnormal increase in the serum levels of the immunoglobulin (Ig); IgA, IgM, and the complement component C 3 in humans as well as chronic inflammatory diseases of the respiratory system.[ 115 ] Exposure to these immunotoxicants may also cause immune dysfunction at different stages which can serve as the basis for increased risks of numerous diseases such as neuroinflammation, an altered brain innate immune response.[ 93 , 116 ] Air pollutants modify antigen presentation by up-regulation of costimulatory molecules such as CD80 and CD86 on macrophages.[ 117 ]

The eye is a neglected vulnerable organ to the adverse effects of air suspended contaminants even household air pollution.[ 118 , 119 ] Clinical effects of air pollution on the eyes can vary from asymptomatic eye problems to dry eye syndrome. Chronic exposure to air pollutants increases the risk for retinopathy and adverse ocular outcomes. In addition, there are now evidence suggesting the association between air pollution and irritation of the eyes, dry eye syndrome, and some of the major blinding.[ 118 , 120 ] According to data, the level of air pollution is linked to short-term increases in the number of people visiting the ophthalmological emergency department.[ 121 , 122 ]

Air pollution in Iran

Air pollution in Iran as a developing country has recently caused several health and environmental problem. According to a report, the quality of air in Iran, especially in Tehran metropolis is very unhealthy and most of the pollution indices, specifically indices for CO and PM are above the standard and at sometimes at dangerous level.[ 123 ] Nonstandard motor engines and other traffic-related sources of air pollution are the most important cause of poor air quality. For example, more than 90% of the CO gas as an important air pollutant is generated by motor vehicles in Tehran.[ 124 ] Moreover, reports have shown that more than 80% of air pollution in Iran is attributed to motor vehicles.[ 125 , 126 ] Official reports show that in Tehran, 9.4% of the cars, 22.1% of vans, and 4.7% of taxis are carburetor vehicles. Around 9% of the vehicles in Tehran are responsible for the production of almost 400 tons pollutants annually.[ 127 ] Other reports demonstrated that cars are responsible for 80% of air pollution. Unpublished data show that a motorcycle produces air pollution 60 times more than a standard car. Based on reports, annual average of air toxicants including PM 10, SO 2 , NO 2 and O 3 in Tehran capital city of Iran with around 8.3 million inhabitants were 90.58, 89.16, 85, and 68.82 μg/m 3 , respectively. These values are more than standards defined by EPA and WHO. Therefore, as expected, air pollution is the main casualty of excess 2194 out of total 47284 deaths in a year. According to a recent report, SO 2 , NO 2 , and O 3 , respectively, have caused about additional 1458, 1050, and 819 cases of total mortality in 2011.[ 128 ] According to an official report in 2013, air pollution leaves almost 4,460 deaths annually only in Tehran.[ 129 ]

Reports of the World Bank in 2005 show that mortality due to urban air pollution in Iran has led to about 640 million dollars annual losses which contribute to 0.57% of the gross domestic product.[ 130 ] Another report has also shown that Iran is the world's third main polluted country in the world, which results in 16 billion $ annual loss for the country.[ 16 ] Many regulatory programs including planting projects have been developed to reduce urban air pollution in Iran, but due to lack of enough stewardship and standardization of new technologies including those related to car engines and also nonstandard energy production, no significant output has been obtained so far.[ 17 ]

Practical measures to reduce air pollution in Iran

The industrialization of societies is necessary to develop, but a long-term health problem and ecological impacts of such growth should always be considered prior to imposing a large financial burden on the societies. Therefore, it is suggested to adopt a balance between economic development and air pollution by legislating policies to control all activities resulting in air pollution.[ 131 , 132 , 133 ] There are some temporary but not trustworthy strategies to reduce air pollution in Iran. For example, increasing the price of fuels, planting trees around and inside the city, replacing old cars with modern ones, and increasing road taxes and car insurance may reduce the amount of air pollutants, but in order to keep the constancy or even optimization, these strategies should be continued. The most air-polluted capital cities of Asia are Delhi and Tehran.[ 134 ] Causes of air pollutions including cheap and low quality of vehicle's fuel particularly gas oil, nonstandard motor engines, inappropriate public transport, overuse of fossil fuel, lack of public awareness and transparency, legislation, and cooperation between different departments and green societies are similar in the two cities. Therefore, urgent and concerted actions at national and international levels are required.

Some mega capital cities in the other countries like London and Tokyo have controlled their air pollutions over the years following appropriate legislations and strict controls, whereas moving the capital cities in India and Pakistan in the last century have not solved the problem of air pollution in the long term in these countries. Therefore, moving capital city will not solve the problem of air pollution and only reduces the problem in the short term. Some recommended strategies to reduce the air pollution in Iran are summarized as the followings:

• Standardization of vehicle's fuel as much as possible and also finding a new source of energy for motor engines has attracted great attention. A great part of emission comes from vehicle exhaust, especially those which use diesel and gasoline. Using other clean source of energy such as compressed natural gas (CNG), liquefied natural gas (LNG), and alcohol is of great interests. Hence, exhorting researchers and also companies in the era of interest to find a way for replacing petrol and other fossil fuels with new suitable power generation sources will be beneficial.[ 135 , 136 ] Expanded of more CNG and LNG stations in big cities of Iran and producing more vehicles using CNG and LNG are recommended
• Standardization of motor engines and manufacturing engines with low fuel consumption is another strategy to reduce the level of air pollutants. Surely, motor vehicles will not use fossil fuel and derivatives anymore in the near future. Recently, some car makers in Japan and Western countries have made electrical cars that use electricity from a battery storage for low speeds which is usually enough for driving inside the cities with traffic and/or controlled speeds. Hence, designing new motor engines, companies of interests should look forward.[ 137 ] This might be implemented in Iran in the future
• Improving public transportation systems by using more subways (metro), trams, and electrical bus routes. Reducing the costs for the people who are using such systems is an optimal solution for lowering air pollution.[ 14 ] The people and governments will gain profits from reducing air contaminations in the long term, both economically and in cases of health issues.[ 138 , 139 , 140 ] At the present, there are only limited metro lines in Tehran, Mashhad, and Esfahan. It is recommended to expand the metro lines in these cities. Trams and electrical buses have unfortunately not been established in Iran. They should be implemented soon in big cities of Iran
• Increasing the cost of fuel in Iran can be considered as an effective solution to reduce the proportion of air contaminants. According to the report by Barnett and Knibbs, higher fuel price is associated with lower air pollution level[ 141 ]
• Imposing penalties for polluting industries and implementing low tax policy for clean technologies. Applying more taxes on automobiles in Iran, particularly on those older than 20 years to distinguish between dirty and clean vehicles. The government should establish exhorting plans for car makers and other producers who adhere to environmental standards. Moreover, exhorting plans should be designed for all sources of emissions. These plans may contain discount on taxes or other financial supports for customers and producers
• Since the phenomenon of air pollution was a global issue over the centuries since the industrial revolution, it is proposed to establish an interdisciplinary academic field on air pollution. It is also suggested that more communication and collaboration between specialists in different sciences including toxicology, environmental health, analytical chemistry, mechanics, and applied physics will be performed
• Continuous monitoring of air quality, designing and developing tools to identify the pollutants, finding the origin of the particles, and the use of particulate filter for diesel engines and other nonroad cars are other suggested practical approaches to reduce air pollution
• Extensive media campaign to increase public awareness about air quality, environmental, and public health issues.

Inadequate legislation and also a lack of appropriate policies in Iran result in higher levels of environmental pollution and its impact on the incidence of diseases, which will undeniably impose a heavy financial burden on the community. Increasing risk of diseases due to air contamination has necessitated defining the standard values for air quality and also a normal range for pollutants and daily control of air pollution. Thus, monitoring air quality plays an important role in developing regulatory policies.[ 142 ] These should be implemented in all big cities of Iran.

Regulatory programs should apply high taxes per unit of emission not only as a penalty for air polluters but also should give rise to the cost of pollution for them in order to ensure the efficient reduction of pollutants. Final achievement is to reduce the pollution and not only paying the cost of damages by polluters.[ 143 ]

Advantages and breakpoints of these strategies should be carefully documented. Tax on vehicles is the most controversial issue in controlling and management of air pollution. It would be helpful providing all necessary facilities including subway, and other transportation systems prior to applying penalties. In other word, taxes should be applied, but public transportation systems should be improved, and also its cost should be reasonable enough.

It is important to balance between all suggested strategies, and discuss all aspects of each plan. Each of these controlling policies has an impact on each other and also on the society. Ideally, an optimum solution for the problem of air pollution is that in which no additional problem resulted from controlling policies impose on the society. Therefore, a suitable controlling policy should contain penalties for each unit of air pollutants’ emission. The sources of emission vary from small unit of cigarettes to large volume of emission from motor engines of automobiles and industrial activities.[ 144 ]

CONCLUSIONS AND RECOMMENDATIONS

Air pollutions have major impacts on human health, triggering, and inducing many diseases leading to high morbidities and mortalities, particularly in the developing countries such as Iran. Therefore, air pollutions control is vital and should be on the top of priority list of the governments. The policy makers and legislators in these countries must update all laws and regulations related to air pollutions. Coordination between different departments involving in air pollutions must be leaded by a powerful environmental protection organization. An effective environmental protection organization should have enough budgets for administration, research, development, monitoring, and full control of the environment including air pollution.

Financial support and sponsorship

Conflicts of interest.

The authors have no conflicts of interest.

AUTHORS’ CONTRIBUTIONS

AGA, BRZ, and MBM contributed in study concept, design, and critical revision of the manuscript. AGA and BRZ participated in the drafting of the manuscript. MBM contributed in revision and final approval of the manuscript.

The impact of environmental regulation and economic expectations on crop-livestock integration among hog farmers: a field study from China

• Research Article
• Published: 01 June 2024

Cite this article

• Jing Cao 1 ,
• Jiapeng Xu 1 ,
• Huimin Cao 1 ,
• Fangfang Wang 1 ,
• Zhenyu Yan   ORCID: orcid.org/0000-0002-3154-9763 1 &
• Taimoor Muhammad 1  

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Decoupling of crop-livestock systems increases the risks of pollution, waste of nutrient resources, and biodiversity loss. Crop-livestock integration (CLI) is an effective solution to these problems, and motivating farmers to adopt CLI is the key. Many countries have implemented environmental regulations (ER) aiming to influence farmers’ CLI adoption decisions. Based on a field study of 316 hog farmers from Shaanxi Province of China, this paper applies the triple-hurdle model to empirically examine the impacts of economic expectations (EE) and ER on CLI adoption decisions. It also verifies the income effect of CLI. The results are as follows: 90.5% of farmers are willing to adopt CLI, but the adoption rate is only 40.8% and the average integration degree is only 0.236; CLI not been widely popularized. EE and ER promote farmers’ CLI adoption significantly, while the impact of interaction between EE and ER on CLI adoption differs. IER weakens the positive impact of EE on farmers’ CLI integration degree, which has a “crowding out effect.” GER negatively moderates the impact of EE on farmers’ adoption willingness of CLI. CER strengthens the positive effect of EE on farmers’ adoption behavior and CLI integration degree. CLI increases the farmers’ income. These results contribute to our understanding of the mechanisms of CLI adoption decisions and sustainable policy optimization for green agricultural development.

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When testing for sample selection errors, IMR was not significant. Therefore, only the test results of IMR are explained here, and the overall regression results are not reported (Models 1–8).

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This work was supported by Key Think Tank Research Project of Shaanxi Province on “Social Sciences Helping County Economies Develop in High Quality” (2023ZD0662); Humanities and Social Sciences Project of Fundamental Research Funds for the Central Universities in 2023 (452023307); Soft Science Research Program of Shaanxi Province (2022KRM032; 2023-CX-RKX-103); and Social Science Foundation of Shaanxi Province (2021D058; 2022D022). The authors would like to thank the anonymous referees for their helpful suggestions and corrections on the earlier draft of our paper.

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Jiapeng Xu: conceptualization, supervision, project administration, article modification.

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Cao, J., Xu, J., Cao, H. et al. The impact of environmental regulation and economic expectations on crop-livestock integration among hog farmers: a field study from China. Environ Sci Pollut Res (2024). https://doi.org/10.1007/s11356-024-33616-z

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Received : 03 January 2024

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DOI : https://doi.org/10.1007/s11356-024-33616-z

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