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`✓ Use of the RfD assumes a level of intake (the RfD) below which it is unlikely that even sensitive individuals will experience adverse health effects over a lifetime exposure. If the average daily intake exceeds the RfD (that is, if the hazard quotient exceeds 1.0), ,., there may be cause for concern for potential noncancer effects. It should be noted, however, that the level of concern does not increase linearly as the RID is approached or exceeded. This is because all RfDs are not equally accurate and are not based on the same severity of toxic effects. Thus, the slopes of the dose-response curve in excess of the RfD can vary widely depending on the substance. Furthermore, it must be emphasized that the hazard quotient does not represent a statistical probability of an effect occurring. The EPA has developed chronic RfDs to evaluate long-term potential exposures (seven :-i years to a lifetime) and subchronic RfDs to evaluate potential exposures occurring over a portion of a lifetime (two weeks to seven years). The child resident scenarios used subchronic RfDs and adult resident and occupational scenarios used chronic RfDs. To assess the overall potential for adverse health effects posed by exposure to multiple chemicals, the hazard quotients for each chemical of concern associated with a given exposure pathway are summed. The resulting sum is referred to by EPA as the hazard `-' index (H.I.). The H.I. approach assumes that multiple subthreshold exposures to several chemicals could result in an adverse health effect. The H.I. is expressed as follows: H.I. = E,/RfD, + E2/RfD2....+ E;/RfD; N where: E; = Exposure level (or intake) for toxicant i RfD; = Reference dose for toxicant i Where a receptor may be subject to chemicals of concern via multiple exposure pathways (e.g., inhalation of volatilies and ingestion of water), the individual hazard indexes from all relevant pathways are summed to obtain the total H.I. for that receptor. For multiple -� chemical exposures, the total H.I. might exceed 1.0 even if no single chemical intake exceeds its RfD. If the total H.I. is less than 1.0, cumulative exposures to the chemicals S:\LDC\YELLO.RPT May 4, 1995 6-2