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A primary objective of this risk assessment was to use conservative parameters so as to not underestimate potential human and groundwater exposures. This risk assessment evaluates exposures to carcinogenic and noncarcinogenic chemicals of concern. The evaluation of noncarcinogenic chemicals includes both subchronic and chronic exposures. The EPA recommends that when evaluating exposure to w noncarcinogenic chemicals, the intake should be calculated by averaging the intake over the duration of exposure (referred to here as averaging time or AT) (EPA 1989b). For carcinogens, intakes were calculated by averaging the total chemical exposures during the exposure period over an average lifetime of 70 years. The use of different averaging times for estimating carcinogenic and noncarcinogenic intakes is based on currently held `r scientific opinion that the mechanism of action of carcinogens and noncarcinogens is different. 4.5.1 Exposure Frequency, Duration, Body Weight Several exposure parameters, such as exposure duration, body weight, and averaging times, have general application in all intake estimations, regardless of pathway. These f general assumptions are detailed below: • Occupational frequency of exposure was estimated to be 5 days/week for 52 weeks for the RME and 5 days/week for 50 weeks (250 days per year) for the average exposure. These rates are conservative because they assume that exposures occur routinely at the site, when in fact, exposures are not routine and may occur less frequently. • Residential frequency of exposure for RME was assumed to have a high- end exposure frequency of 350 days/year (allowing 15 days per year for vacations and holidays, etc.) (USEPA 1991). For average exposures, residents were assumed to have an exposure frequency of 320 days/year (exposure reduced by time away from home). This time away from home assumed that the average individual, compared with the RME individual, spends slightly more than an additional one-half day per week away from home. • The average exposure frequency for showering exposures assumed that residents and occupational employees shower every day that they are on the site (e.g., 350 days/year for RME resident). S:\LDC\YELLO.RPT May 4, 1995 4-15