Differential epidemiology of ambient aerosols

There is now a large body of epidemiological evidence associating exposure to ambient particles with short- and long-term effects on health. Most authorities consider that at least some of these associations represent a causal relationship with particles. The size fraction of particles that could potentially harm health is PM10, since only particles less than this size can plausibly reach the small airways and alveoli. Studies of mechanisms and theoretical considerations suggest that the fine (PM2.5) and ultrafine (PM0.1) particles are probably more important than larger particles, because of their relatively greater numbers and deeper penetration of the lung. Because of limited population exposure data, there is little direct epidemiological evidence about the effects of ultrafine particles. Indirect evidence falls into three groups. The first comes from studies that have directly compared the coarse (PM2.5-10) with the fine (PM2.5) fractions: the findings of these few studies have not been consistent. The second comes from studies of chemical species or measures of particles (sulphates, acid aerosol and black smoke) that reside mainly in the fine fraction; many of these have found associations with adverse health effects. The third group are those few studies that have compared the effects of size/number concentrations with size/mass concentrations; the findings of these have either been inconclusive or have suggested that numbers may be more important than mass. Inference about the toxic component of particles will depend on all the evidence, especially from toxicology, as well as epidemiology. At present, epidemiological evidence points towards the fine fraction being important, but an effect of the coarse fraction cannot be excluded. Because of a lack of data, epidemiology has little to say about the relative importance of the ultrafine fraction. This is an urgent research need.