The effects of outdoor air pollution on the respiratory health of Canadian children: A systematic review of epidemiological studies

BACKGROUND:

Outdoor air pollution is a global problem with serious effects on human health, and children are considered to be highly susceptible to the effects of air pollution.

OBJECTIVE:

To conduct a comprehensive and updated systematic review of the literature reporting the effects of outdoor air pollution on the respiratory health of children in Canada.

METHODS:

Searches of four electronic databases between January 2004 and November 2014 were conducted to identify epidemiological studies evaluating the effect of exposure to outdoor air pollutants on respiratory symptoms, lung function measurements and the use of health services due to respiratory conditions in Canadian children. The selection process and quality assessment, using the Newcastle-Ottawa Scale, were conducted independently by two reviewers.

RESULTS:

Twenty-seven studies that were heterogeneous with regard to study design, population, respiratory outcome and air pollution exposure were identified. Overall, the included studies reported adverse effects of outdoor air pollution at concentrations that were below Canadian and United States standards. Heterogeneous effects of air pollutants were reported according to city, sex, socioeconomic status and seasonality. The present review also describes trends in research related to the effect of air pollution on Canadian children over the past 25 years.

CONCLUSION:

The present study reconfirms the adverse effects of outdoor air pollution on the respiratory health of children in Canada. It will help researchers, clinicians and environmental health authorities identify the available evidence of the adverse effect of outdoor air pollution, research gaps and the limitations for further research.     

Outdoor air pollution: ozone health effects

Ozone is a respiratory irritant associated with a spectrum of adverse health events. Ground-level ozone has been shown to cause decreases in lung function and has been associated with other important respiratory health effects. Some reports suggest short-term increases in ozone lead to increased cardiopulmonary mortality. Other studies have found no association between exposure and measured health effects. Outdoor air pollution consists of multiple copollutant exposures complicating definitive assessments about ambient air ozone exposure-effect relations.     

Acute effects of ozone on mortality from the "air pollution and health: a European approach" project

In the Air Pollution and Health: A European Approach (APHEA2) project, the effects of ambient ozone concentrations on mortality were investigated. Data were collected on daily ozone concentrations, the daily number of deaths, confounders, and potential effect modifiers from 23 cities/areas for at least 3 years since 1990. Effect estimates were obtained for each city with city-specific models and were combined using second-stage regression models. No significant effects were observed during the cold half of the year. For the warm season, an increase in the 1-hour ozone concentration by 10 mug/m3 was associated with a 0.33% (95% confidence interval [CI], 0.17-0.52) increase in the total daily number of deaths, 0.45% (95% CI, 0.22-0.69) in the number of cardiovascular deaths, and 1.13% (95% CI, 0.62-1.48) in the number of respiratory deaths. The corresponding figures for the 8-hour ozone were similar. The associations with total mortality were independent of SO2 and particulate matter with aerodynamic diameter less than 10 mum (PM10) but were somewhat confounded by NO2 and CO. Individual city estimates were heterogeneous for total (a higher standardized mortality rate was associated with larger effects) and cardiovascular mortality (larger effects were observed in southern cities). The dose-response curve of ozone effects on total mortality during the summer did not deviate significantly from linearity.     

Outdoor air pollution: particulate matter health effects

Numerous investigations studying multiple populations across a variety of environmental settings have demonstrated a strong association between ambient air particulate matter and cardiopulmonary morbidity and mortality. In most studies, the effect size of ambient air particulate pollution on health outcomes is small. However, the exposed population worldwide is very large. Accordingly, particulate air pollution appears to be an important public health hazard that makes an important contribution to the total burden of disease and death in populations across the world. Much of the evidence linking ambient air particulates with adverse health effects is derived from population-based, observational research with potential unidentified confounding exposures, precluding definitive assessments about causation and providing limited mechanistic insights. A growing body of research suggests particulate-associated adverse health effects result from the induction of proinflammatory responses in the lower respiratory tract. Ambient air particulates may increase lung cancer risk.     

Update on the health effects of outdoor air pollution

The number of studies conducted on the health effects of air pollution has increased exponentially. Important methodological advances include the application of novel observational study designs, in particular the multi-city design, and the development and application of airborne particle concentrators for use in experimental human exposure studies and toxicological studies. Experimental data are validating and providing insight into some surprising observational findings.     

Acute effects of air pollution on respiratory health of 50-70 yr old adults.

The aim of this study was to investigate the association between daily changes in respiratory health and air pollution in 489 adults, aged 50-70 yrs, with and without chronic respiratory symptoms, living in urban and nonurban areas in the Netherlands. Subjects were selected from the general population with a screening questionnaire. During three consecutive winters starting in 1992/1993, peak expiratory flow (PEF) and respiratory symptoms were registered in a daily diary. Daily measurements of particles with a 50% cut-off aerodynamic diameter of 10 microm (PM10), black smoke (BS), sulphate, sulphur dioxide (SO2) and nitrogen dioxide (NO2) were conducted. The difference in PM10, BS and sulphate concentrations between urban and nonurban areas was small, but there was more contrast in the concentrations of SO2 and NO2. In symptomatic subjects from urban areas, PM10, BS, sulphate and SO2 concentrations were associated with the prevalence of large decrements in morning PEF (>20% below the median). BS in particular was also associated with upper respiratory symptoms (URS). The magnitude of the effect estimates was in the order of an 80% increase in PEF decrements and a 20% increase in URS for a 40 microg x m-3 increase of the same day BS concentration. In symptomatic subjects from nonurban areas, no consistent associations between air pollution and health indicators were observed. However, the differences in effect estimates between urban and nonurban symptomatic panels were small and nonsignificant. In nonsymptomatic adults from both areas, no consistent pattern of associations with air pollution was found. In conclusion, air pollution effects were only found in symptomatic adults in the urban areas.     

Human health effects and air pollution control philosophies

Human health protection is one of the major goals of air pollution control, but not the only one. There can be conflict between the goals; an example of this type is discussed. There are various philosophies of how to design air pollution control regulations. The most common — air quality standards — is based on a specific assumption about the shape of the air pollutant dose-response curve. The next most common — emission standards — is indifferent to the data on human health effects. The emission tax and cost-benefit philosophies are not as well known. As most widely discussed, the emission tax philosophy would be indifferent to the health data, and the cost-benefit philosophy would make the greatest demands for detailed, reliable health effects data. The available health effects data are mostly based on epidemiology, and are very sparse and unsatisfying.     

Epidemiology of chronic obstructive pulmonary disease: health effects of air pollution

COPD is one of the leading causes of morbidity and mortality in the industrialized and the developing countries. According to the prediction of the World Health Organization, COPD will become the third leading cause of mortality and the fifth cause of disability in 2020 worldwide. In epidemiology, distinct phenotypic entities converge on the term COPD, so that prevalence and mortality data may be inclusive of chronic bronchitis, emphysema and asthma; moreover, the assessment of prevalence rates may change considerably according to the diagnostic tools used. Thus, a considerable problem is to estimate the real prevalence of COPD in the general population. COPD is determined by the action of a number of various risk factors, among which, the most important is cigarette smoking. However, during the last few decades, evidence from epidemiological studies finding consistent associations between air pollution and various outcomes (respiratory symptoms, reduced lung function, chronic bronchitis and mortality), has suggested that outdoor air pollution is a contributing cause of morbidity and mortality. In conclusion, epidemiological studies suggest that air pollution plays a remarkable role in the exacerbation and in the pathogenesis of chronic respiratory diseases. Thus, respiratory physicians, as well as public health professionals, should advocate for a cleaner environment.     

The effects of air pollution on the respiratory health of children: a cross-sectional study

To investigate the effects of air pollution on the respiratory health of children, a subject of some controversy, a comparative study was undertaken of 2,385 school children who lived in central urban, peripheral urban, and suburban areas. Daily monitoring of sulphur dioxide and total suspended particle concentrations in all areas showed that pollutant concentrations in central and peripheral urban areas were above commonly accepted safety levels for respiratory health, while concentrations in the suburban area were within acceptable limits. A questionnaire administered to each mother assessed environmental exposure to pollutants in the household, the occurrence of respiratory symptoms as well as lung diseases as diagnosed by a physician, and general information. Children were interviewed about smoking habits and any acute respiratory symptoms. Children also performed standard lung function tests. Results showed that children from both urban areas had lessened pulmonary function and a higher prevalence of bronchial secretion with common colds than did those from the suburban area. These differences persisted after corrections for exposure to indoor pollutants, active or passive smoking, socioeconomic status, and sex. Parental cigarette smoking was related to a fall in forced expiratory volume in 1 second and an increased incidence of acute respiratory illnesses and chronic cough in children. Although boys had higher lung volumes and lower air flow, regression analysis showed no significant influence of the interactions "sex-geographic area" and "sex-smoking" on lung function. It was concluded that air pollution has a significant effect on the respiratory health of children.     

Effects of air pollution on respiratory health

Air pollution, which started to become a serious health concern with industrial revolution, has been threatening human health. Epidemiological studies have reported a close relation between air pollution and respiratory morbidity and mortality. Studies investigating the mechanisms underlying respiratory effects of air pollution have reported that pollutants lead to increased respiratory symptoms, decreased respiratory function and induce inflammatory changes in airways. In vitro studies have demonstrated that air pollutants exert their effects by causing cellular injury directly, and indirectly activating intracellular oxidative pathways. Air way epithelial cells, which form the first line of defence against air pollutants, may play an active role in this pathophysiological course.     

Health effects of air pollution. A review of clinical studies.

The database for the acute health effects of common outdoor air pollutants is rapidly increasing but important gaps still exist. Greater technical efforts and innovative studies are required to adequately characterize health effects and understand the underlying mechanisms of toxicity. Controlled human exposures provide relevant data about short-term effects and complement animal and epidemiologic investigations. Except for possibly nitrogen dioxide, the clinical data for ozone, sulfur dioxide, and particulates (H2SO4) at contemporary levels indicate potentially untoward or adverse physiologic or clinical responses in healthy individuals and sensitive groups such as children, adolescents, and asthmatic patients. Exercise, duration, and other exposure factors may potentiate pollutant effects on symptoms, lung function, nonspecific bronchial reactivity, mucociliary clearance, and BAL markers of inflammation. Continued animal, clinical, and epidemiologic research of both short- and long-term health effects is clearly needed to support or limit future regulatory decisions regarding the quality of outdoor air.     

Acute health effects of PM10 pollution on symptomatic and asymptomatic children.

This study assessed the association between daily changes in respiratory health and respirable particulate pollution (PM10) in Utah Valley during the winter of 1990-1991. During the study period, 24-h PM10 concentrations ranged from 7 to 251 micrograms/m3. Participants included symptomatic and asymptomatic samples of fifth- and sixth-grade students. Relatively small but statistically significant (p less than 0.01) negative associations between peak expiratory flow (PEF) and PM10 were observed for both the symptomatic and asymptomatic samples. The association was strongest for the symptomatic children. Large associations between the incidence of respiratory symptoms, especially cough, and PM10 pollution were also observed for both samples. Again the association was strongest for the symptomatic sample. Immediate and delayed PM10 effects were observed. Respiratory symptoms and PEF changes were more closely associated with 5-day moving-average PM10 levels than with concurrent-day levels. These associations were also observed at PM10 levels below the 24-h standard of 150 micrograms/m3. This study indicates that both symptomatic and asymptomatic children may suffer acute health effects of respirable particulate pollution, with symptomatic children suffering the most.