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INITIAL RELEASE DDJC Tracy draft comprehensive RI/feasibility study. The quality assurance and quality control measures followed with regard to chain-or-custody, laboratory procedures, and data reporting are identified in the associated reports. I EVALUATION OF ENVIRONMENTAL CONTAMINATION AND POTENTIAL EXPOSURE PATHWAYS Introduction An exposure pathway is the process by which an individual is exposed to contaminants that originate from a source. To determine whether people were or continue to be exposed to contaminants originating from the DDJC Tracy site, ATSDR evaluated the factors that might lead to human exposure. These factors, or elements, include a source of contamination, an environmental medium in which contaminants may be present, a point of human exposure, a route of human exposure (such as ingestion, inhalation, or skin contact), and a receptor population (e.g., nearby residences). Figure 4 explains the exposure evaluation process in more detail and Appendix A contains a glossary of environmental and health terms used in this document. ATSDR identifies exposure pathways as completed or potential. A completed exposure pathway exists in the past, present, or future if all elements of human exposure link the contaminant source to a receptor population. Potential pathways, however, are defined as situations in which at least one of the elements is missing, but could exist. The potential for adverse health effects from an exposure is also considered in relation to contaminant concentration, exposure variables (e.g., duration and frequency), and the toxicology of the contaminant. ATSDR uses comparison values in selecting contaminants for further evaluation within an exposure pathway. Comparison values are contaminant concentrations in a specific environmental medium to which a person might be exposed that are not expected to produce health effects. Because comparison values do not represent thresholds of toxicity, exposure to concentrations greater than the comparison values does not necessarily produce health effects. Comparison values used in this document include EPA's maximum contaminant levels (MCLs), Environmental Media Evaluation Guides (EMEGs), Reference Dose Media Evaluation Guides (R1vIEGs), and Cancer Risk Evaluation Guides (CREGs). These values are further described in Appendix B. 6