tions in individual susceptibility (i.e., sensitivity) to pollutants (Brain et al. 1988; WHO 1992; NRC 1993; ATS 1996a,b).

The NRC (1993) report concluded that profound differences exist between children and adults. Because infants and children are growing and developing, they are different from adults in composition and in certain metabolic, physiological, and biochemical processes. Before full maturation, damage to a specific organ or organ system might permanently prevent normal physical maturation and increase the incidence of a variety of diseases. That possibility has been demonstrated in studies showing children's sensitivity to the irreversible effects of lead and mercury (Calabrese 1986; Klaassen et al. 1996), enhanced susceptibility to certain radiation-induced cancers (Calabrese 1978), and enhanced risk from a number of carcinogens, e.g., vinyl chloride-induced angiosarcoma (Drew et al. 1983; Calabrese 1986). In addition, certain populations of children might be more sensitive than other children to the effects of chemical agents because of physiological and biochemical factors, such as genetic predisposition, general health status, low socioeconomic status, and possible interactions with certain medications. For certain types of toxicity, children might be more resistant to certain chemical agents, and in such cases, adults might be at greater risk.

Brain et al. (1988) provided a comprehensive analysis of the general principles for variations in human sensitivity to inhaled air pollutants. In that review, Brain et al. (1988) focused primarily on the effects of genetic factors, age and nutrition, gender, smoking, and pre-existing disease states on sensitivity. Non-neoplastic and neoplastic pulmonary diseases were discussed. The analysis was limited, however, to the effects of comparatively low-concentration, long-term exposures to common air pollutants and did not provide substantial guidance relevant to short-term exposures or to the specific compounds—HCl, NO2, and HNO3—examined in this report.

In 1992, the World Health Organization (WHO) reviewed human health effects caused by brief episodes of air pollution and provided some information on the special needs of sensitive populations (WHO 1992). WHO stated that people with pre-existing lung disease or circulation problems usually are more affected by episodes of increased “winter-type” (sulfur dioxide and particulates) pollution than are healthy individuals. On the other hand, for “summer-type” pollution (mostly nitrogen oxides and ozone), WHO could not identify any spe-



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