OBJECTIVE Susceptibility of target populations to air pollution is an important issue, because air pollution policies and standards should be based on the susceptibilities of those at particular risk. To evaluate which age group is more susceptible to the adverse health effects of air pollution, we compared the effects of air pollution on mortality among postneonates, those aged 2 to 64 years, and those over 65 years of age. DESIGN Daily counts of total and respiratory death along with daily levels of meteorological variables and air pollutants were analyzed using generalized additive Poisson regression. The relative risks (RR) of mortality for interquartile changes of the levels of particulate matter <10 micro m (PM(10)) were calculated on the same day. RESULTS For postneonates, the RR of total mortality for an interquartile change (42.9 micro g/m(3)) in PM(10) (RR: 1.142; 95% confidence interval [CI]: 1.096-1.190) was greatest among age groups. Next were the elderly over 65 years of age (RR: 1.023; 95% CI: 1.022-1.024). Regarding respiratory mortality, RR for an interquartile change of PM(10) in postneonates (RR: 2.018; 95% CI: 1.784-2.283) was also greater than those in the other groups. CONCLUSIONS These results agree with the hypothesis that infants are most susceptible to PM(10) in terms of mortality, particularly respiratory mortality.