the nature and extent of contamination, the existence of potential pathways of human exposure...the size and potential susceptibility of the community within the likely pathways of exposure, the comparison of expected human exposure levels to the short-term and long-term health effects associated with identified hazardous substances and any available recommended exposure or tolerance limits for such hazardous substances, and the comparison of existing morbidity and mortality data on diseases that may be associated with the observed levels of exposure. (ATSDR, 1990a, pp. 2-3, 4)

Although many aspects of EPA's risk assessment process and ATSDR's health assessment process overlap, the distinction between them is based primarily on the intended purpose of each type of assessment. The EPA risk assessment is intended to serve as the quantitative basis for the selection of remedial objectives and strategies for the site; the ATSDR health assessment is intended to provide the community with qualitative information on the public health implications of the site and to identify the need for further action to protect the health of the community or to research the health effects associated with current or past releases from the site.

In response to ATSDR's slow start-up, Congress also set deadlines for many of these mandated activities. Health assessments were to be completed by the end of 1988 for all 951 Superfund sites listed on the NPL before Oct. 17, 1986, and subsequent health assessments were due within one year of proposal for NPL status. Significantly, this meant that ATSDR 's health assessment would normally be completed well before each RI/FS began. Because the RI/FS is the stage at which the most extensive exposure and risk assessment information is produced, ATSDR has divided its health assessments into two stages: the preliminary health assessment (PHA), prepared in the first year after a site is proposed for listing, and the full health assessment, prepared when the RI/FS is complete. The full assessment is used in determining the remediation objectives and the final record of decision. As of Dec. 12, 1990, ATSDR had completed 600 preliminary assessments and 469 full assessments (J. Andrews, ATSDR, personal communication, 1991). With the backlog of unevaluated sites now essentially eliminated, ATSDR can begin to evaluate sites as soon as they are proposed for NPL listing, and it can begin to play a more active role in the development of the RI/FS work plan.

Of the 951 NPL sites evaluated in the first round, ATSDR found that 109 (11.5 percent) constituted a risk to human health because of actual exposures (11 sites) or probable exposures (98 sites) to hazardous chemical agents that could have adverse health consequences.

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