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LLNL Site 300 (USDOE) Final Release 2. noted community health concerns, and 3. the quality and extent of the sampling data we can use to evaluate potential exposure and human health hazard. For inorganic compounds (metals) and radionuclides,background values may also be considered, given that some of these substances occur naturally. For chemicals, the highest environmental concentration detected off site is matched with media-specific comparison values to determine whether further evaluation is warranted. Generally, if a contaminant's concentration exceeds one or more media-specific comparison values, the contaminant is further evaluated,both in this section and in the public heath implications section. A release of a chemical or radioactive material into the environment does not always result in human exposure. For an exposure to occur, a completed exposure pathway must exist. A completed exposure pathway exists when all of the following five elements are present: 1. a source of contamination, 2. an environmental medium through which the contaminant is transported, 3. a point of human exposure, 4. a route of human exposure, and 5. an exposed population. A potential exposure pathway exists when one or more of the elements are missing but available information indicates possible human exposure. An incomplete exposure pathway exists when one or more of the elements are missing and available information indicates that human exposure is unlikely to occur(ATSDR 2001). Figure 4 illustrates the necessary components of an exposure pathway. In addition,for each exposure pathway ATSDR scientists identify whether releases of contaminants and exposures are likely to have occurred in the past, are currently occurring, or could potentially occur in the future. In this report, "on-site contamination"refers to contamination and releases of material within the fenced security area of the site or in areas for which public access is restricted. "Off-site contamination" refers to environmental media(soil, sediment, surface water, groundwater, air, or biota) that are contaminated as a result of chemical or radioactive contaminants leaving the site and are no longer controlled by DOE or LLNL. In this health assessment, on-site sources of contamination are considered only as sources of off-site contamination or for their impact on the community. 12