of pathogens in biosolids. Advances in microbial risk assessment have occurred since then, but there remains a difference in the maturity of risk-assessment procedures for chemicals and those for pathogens. The question posed is whether this difference is simply an artifact of the different stages of development of these two branches of risk assessment or whether generic differences are attributable to the nature of the agents themselves. In addressing this question, it is useful to consider the four components of the traditional risk-assessment process (hazard identification, dose-response characterization, exposure assessment, and risk characterization) and ask which, if any, of those components has inherent differences in the way pathogens and chemicals are assessed.

Hazard identification is the process of reviewing relevant biological and chemical information on an agent that might pose a health hazard. Although there are obvious differences in the types of information available on chemicals and pathogens, there appears to be little fundamental difference in the process of identifying their hazards. This is supported by a recent NRC report Classifying Drinking Water Contaminants for Regulatory Consideration (NRC 2001), in which no distinction between chemical and biological contaminants is made. In general, however, pathogens usually are grouped into generic classes with less of an agent-specific focus than is common in chemical risk assessment.

The process for characterizing dose-response relationships is not as straightforward for pathogens as it is for chemicals. The process is complicated by the possibility that exposure to a pathogen may engender an immune response that might persist and alter an individual’s subsequent susceptibility to infection or clinical disease. Acquired immunity has no relevant analog for chemical exposures in the risk-assessment context, although there are chemicals for which sustained exposure can result in tolerance for some toxic end points. Also, the converse can be true when an individual becomes sensitized to a chemical and develops serious and persistent hypersensitivity. For infectious agents, however, acquired immunity can be a major modifier of population risk. An exposed population is likely to be an unknown mixture of those with acquired immunity and those without. Moreover, the population can change over time as susceptible individuals become infected and move from one subgroup to the other. Acquired immunity might simply be addressed by developing two dose-response functions in the risk-assessment process, one for the susceptible population without immunity and a second for the population with acquired immunity. The conservative approach would be to conduct an assessment of a totally susceptible population, and although the results could be very conservative, this option would be consistent with EPA’s practice of protecting sensitive subpopulations.

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