Populations at risk: addressing health effects due to complex mixtures with a focus on respiratory effects.

Some individuals in the population may be sensitive or susceptible be to the effects of air pollutants. Such sensitivity may be to specific pollutants or classes of pollutants. However, sensitivity or susceptibility in some individuals can be to all irritants, but the sensitivity is likely to be response specific or organ specific. The U.S. Clean Air Act specifically recognizes that some individuals in the population are sensitive to air pollutants and indicates that such individuals need to be protected by air quality standards. It is usually difficult to determine the cause of sensitivity, though various biological mechanisms have been studied. Biological age may be a factor, with the young being most sensitive and susceptible to being affected. An example is the heightened bronchial lability and responsiveness in the very young that appears to disappear with growth. Susceptibility may be innate (e.g., genetic) and/or induced by events/exposures. Frequently, those with preexisting illnesses are part of the sensitive population because they may often respond, sometimes hyperrespond, to a pollutant exposure that may not affect most people. Asthmatics are excellent examples of individuals who were susceptible to the disease and, once inflicted, are susceptible to the effects of many environmental and nonenvironmental agents. Usually only a fraction of the general population will respond with heightened reactions at lower doses. Such individuals require special evaluation and attention in all exposure-response studies and risk assessments. Thus, the conditions defining populations at risk and the methodologies to discover and study them can be reviewed.