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slope factors are also given an EPA weight-of-evidence classification. This describes the likelihood that the contaminant causes cancer in humans. Table 5-1 presents the EPA weight-of-evidence classification system for carcinogenicity. Available RfDs and SFs for each contaminant of concern are presented in Table 5-2. The RfDs and SFs values used in this risk assessment were obtained from the following sources: • EPA's Integrated Risk Information System (IRIS) (EPA 1995) on-line database system • EPA's Health Effects Assessment Summary Tables (HEAST) (EPA 1994) EPA has performed toxicity assessments for hundreds of compounds associated with chemical releases from industrial sites. The assessments are based on qualitative and quantitative toxicity information acquired through evaluation of relevant scientific literature. The most directly relevant data come from studies in humans. However, ?,. most of the usable information on the toxic effects of chemicals comes from controlled experiments in animals. The result of toxicity assessments performed by EPA is the r development of chemical-specific toxicity factors for either the inhalation or oral exposure pathway. These toxicity factors are published in IRIS and HEAST. Only toxicity factors that have been verified by EPA science work groups are included in IRIS. HEAST contains provisional or interim toxicity factors that do not appear in IRIS. Table 5-3 lists the chemicals of concern and their carcinogenic and noncarcinogenic effects by ingestion and inhalation exposure routes. 5.2 TOXICITY ASSESSMENT FOR NONCARCINOGENIC EFFECTS Substances that produce noncarcinogenic effects are generally thought to have a 5,. threshold dose below which there are no observable adverse health effects. In developing a toxicity value for noncarcinogenic effects, the approach is to identify this threshold dose of No Observed Adverse Effect Level (NOAEL) through studies with experimental animals or from epidemiological studies. A NOAEL is an experimentally (or epidemiologically) determined highest dose at which there was no statistically or SALDMELLOAPT May 4,1995 5-2 AM"