Implications of individual particulate matter component toxicity for population exposure

There is currently much debate regarding which chemical or physical characteristics of particulate matter are responsible for the adverse health effects apparent in epidemiological studies. Toxicological data suggest that the strongest evidence for adverse effects is associated with primary combustion emissions. In contrast, few toxicological studies have observed effects from secondary inorganic particulate components resulting from emissions of ammonia, sulphur dioxide and nitrogen oxides. A study has been performed to examine the contribution of different emission source sectors to the total exposure associated with a variety of individual PM_2.5 components in Europe using the CMAQ air quality model. The results suggest that secondary particulate mass dominates PM_2.5 concentrations, being responsible for 82% of total anthropogenic exposure, with agriculture the dominant contributing sector. However, if the toxic component of PM_2.5 lies in the primary fraction, agriculture contributes less than 3% of primary particulate exposure, with the majority due to emissions from residential sources, transport and industry. Taken together, these results suggest that identification of the particulate matter fraction responsible for toxicity is critical for designing effective emission reduction policies and that the assumption that all PM_2.5 mass is equally toxic may lead to a focus on reducing secondary particulate pre-cursor emissions, which will deliver little in terms of reducing adverse health effects.