Atmospheric secondary inorganic particulate matter: the toxicological perspective as a basis for health effects risk assessment.

Epidemiological studies have provided evidence for an association between exposure to ambient particulate matter and increased mortality and morbidity. However, the exact physicochemical nature of the responsible components is not as yet clear. One major constituent of the ambient aerosol is secondary inorganic particles, which are produced within the atmosphere via chemical reactions and are dominated by sulfates and nitrates. This article reviews the biological effects resulting from exposure to these ambient aerosol constituents. It was developed based upon available data from peer reviewed published papers as well as publicly available reports on controlled animal and human clinical exposure studies. The aim was to provide a toxicological basis for addressing the issue of whether ambient concentrations of these secondary aerosols in two venues, namely the United States and the Netherlands, could be causally related to reported human health effects associated with exposure to ambient particulate matter. Evaluation of the toxicological database suggests that these particles have little biological potency in normal humans or animals, or in the limited compromised animal models studied at environmentally relevant levels. There are, however, some critical caveats in this analysis that must be considered. First, it is important to understand the relationship between animal exposure studies and actual human exposures, in terms of both particle size and inhaled dose. Second, it is necessary to consider the physicochemical characteristics of the chemical species within ambient air compared to the characteristics of those used in controlled studies. Third, there is the issue of relevance of the exposure models used in these studies to those populations that may be affected by exposure to ambient particulates. Finally, the potential for interactions between particulates and ambient gases in the total atmospheric mix must be considered in developing conclusions as to exposure concentrations for the former constituents of polluted air that may be hazardous to public health.