Epidemiological research to identify subpopulations with enhanced susceptibility to air pollution is still at an early stage. From the available studies, there is evidence that both "endogenous" and "exogenous" factors contribute to individual susceptibility. Females and the elderly are at an increased risk of pollution-related diseases. Moreover, some chronic clinical conditions seem to be good candidates for identifying the "frail" populations: chronic obstructive pulmonary disease including asthma, coronary heart diseases, congestive heart failure, and heart rhythm disorders. It seems clear that epidemiological research on susceptibility in the future should investigate the underlying biological and physiological mechanisms, in addition to the environmental and toxicological effects.