Human Health Risk Assessment for Coarse Particulate Matter: executive summary
Health Canada has completed a detailed risk assessment entitled "Human Health Risk Assessment for Coarse Particulate Matter", a stand-alone piece focusing on the potential health impacts of coarse particulate matter (PM10-2.5). The state of knowledge on the health effects of coarse PM has and continues to advance and approximately 150 new primary scientific publications were evaluated.
The assessment critically evaluated relevant information essential to establishing the weight of evidence for the various health effects associated with exposure to ambient coarse PM, and for establishing whether population health impacts can be expected from current ambient exposures. The health effects of coarse PM have been investigated through a number of different types of studies, with the strongest evidence coming from the epidemiological literature. These studies have primarily evaluated the impact of coarse PM on respiratory and cardiovascular outcomes. The results of work with human volunteers (controlled human exposure studies) and with animals (toxicological studies) in controlled laboratory settings support the epidemiological results and help to better understand the mechanisms of coarse PM impacts on heart and lung health.
The next few paragraphs provide a brief summary of the key health impacts of coarse particles and causality conclusions reached in the assessment.
Acute Effects - Mortality
The health database supporting the relationship between coarse particles and mortality outcomes has grown over the past few years. There are fairly consistent positive associations between short-term exposure to ambient coarse PM and non-accidental, respiratory and cardiovascular mortality in the available epidemiological studies. However, the associations observed in these studies are sometimes not statistically significant, demonstrating only limited strength of association. Generally, the associations between coarse particles and mortality are similar in magnitude, but less precise, than those observed for fine particles, though this lack of precision is not unexpected given the larger exposure measurement error for the coarse fraction. There is also only limited support for coarse PM-related mortality in other lines of evidence; for example, respiratory morbidity is prominent in the toxicological and epidemiological studies, with little indication of effects that may be related to cardiovascular mortality, which comprises the bulk of non-accidental mortality. Uncertainty also remains about the potential for confounding by the fine PM fraction and/or gaseous pollutants. Overall, the epidemiology data are suggestive of a causal relationship between short-term exposure to the coarse PM fraction and mortality.
Acute Effects - Morbidity
Fairly consistent coarse PM-related increases in hospital admissions and emergency room visits (ERVs) for respiratory conditions, most often for asthma in children, have been reported in epidemiological studies. These findings are mostly statistically significant, particularly for hospitalizations, and are supported by even more consistent significant increases in respiratory symptoms and asthma medication use in panel studies of asthmatic children. These findings have been robust across a number of study designs and model specifications, and also across different pollutant mixtures and health care systems. There is also fairly coherent evidence (across hospital admissions, ERVs, panel, controlled human exposure and animal toxicology studies) that asthma exacerbation is a critical effect of coarse PM exposure and, that children and the elderly are susceptible subgroups. Studies have provided experimental support for the epidemiological findings and for emerging plausible mechanisms of action/toxic moieties (e.g. endotoxins). The main proposed mechanisms for PM-induced health effects are inflammation-induced injury and oxidative stress, with the specific component(s) of particles responsible for the stimulation of various biological parameters being presently unknown (although biological matter, metal contaminants, and polycyclic aromatic hydrocarbons such as benzo(a)pyrene have all been implicated). Further, there is some indication of specificity of effect, with coarse PM being associated with effects in the upper respiratory tract and fine PM more with those in the lower respiratory tract, corresponding to their principal respiratory tract regions of deposition. However, important uncertainties remain concerning the possible role of co-pollutants in the observed associations. This is particularly important considering the relatively large measurement error and variation in composition of the coarse fraction. Overall, the epidemiology data and the limited results from controlled human exposure and toxicological studies are suggestive of a causal relationship between short-term exposure to the coarse PM fraction and respiratory effects.
There is little indication of coherence in the limited and sometimes inconsistent cardiovascular findings from available panel and controlled human exposure studies, and uncertainty remains with respect to the possible role of fine PM and gaseous pollutants in the associations observed in many epidemiology studies. However, the risk estimates for cardiovascular hospitalizations and mortality in relation to ambient coarse PM are generally positive, and there were significant and robust coarse PM-related increases in mortality from certain cardiovascular causes in two US multi-centre studies. Overall, the findings in these population-based epidemiology studies are suggestive of a causal relationship between short-term exposure to the coarse PM fraction and cardiovascular effects, though investigations of cardiovascular endpoints in panel studies and in toxicological studies in animals and humans are too limited to shed much light on the weight of evidence.
Chronic Effects - Mortality and Morbidity
In contrast to the large number of studies of short-term variations in air pollutants associated with a range of mortality and morbidity endpoints, there have been relatively few studies that examined the respiratory and cardiovascular effects of long-term exposure to air pollutants. The associations of chronic exposure to ambient coarse particles with mortality and both respiratory and cardiovascular health effects have been examined more recently in prospective cohort studies, and the results do not provide significant insight into the role, if any, played by the coarse PM fraction. In addition, a limited number of studies have investigated the association between chronic exposure to coarse particles and adverse birth outcomes and infant mortality and results have been inconsistent; a small negative effect on birth weight was observed in an American multi-city study, but this association varied widely with the geographical location, the study samples, and the covariates included in the model. There are important uncertainties with respect to the possible role of co-pollutants, the appropriate measure of exposure to PM, and critical periods of exposure during pregnancy. Overall, the epidemiology data are inadequate to infer a causal relationship between chronic exposure to the coarse PM fraction and mortality, respiratory and cardiovascular health effects, as well as with the incidence of developmental outcomes.
The body of evidence on the health effects of coarse PM has grown since the previous assessment but is still limited compared to that available on fine PM. The respiratory system appears to be the critical target for adverse effects following exposure to coarse particles. Overall, the data on the health effects of coarse particles are weaker than for fine particles and subject to larger measurement errors. These particles are also characterized by a more heterogeneous chemical composition. However, based on dosimetric, epidemiological, and toxicological studies performed in industrialized/urban areas, the existence of adverse health effects on the respiratory system resulting from short-term exposure to coarse particles cannot be dismissed.
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