Contributions that epidemiological studies can make to the search for a mechanistic basis for the health effects of ultrafine and larger particles

Epidemiology is a rather blunt tool for elucidating biological mechanisms t hat can account for the increased mortality and morbidity associated with p opulation exposures to ambient air particulate matter (PM). However, it has an essential role to play. Recent studies indicate that three readily meas urable ambient air PM concentration indices can be significantly associated with one or more elevations of rates of specific disease or dysfunction ca tegories. These three indices, i.e. ultrafine particle number, fine particl e mass (PM2.5) and thoracic coarse mass (PM10-2.5) differ not only in size range, but also in terms of their sources, deposition patterns, and chemica l reactivities, factors that may account for their different associations w ith human health effects. Further epidemiological studies employing a wider array of air quality and health effects variables should enable us to reso lve some of the outstanding questions related to causal relationships for P M components or, at the minimum, to pose some better questions.