Risk Assessment for Carcinogens Under California's Proposition 65

Risk assessments for carcinogens are being developed through an accelerated process in California as a part of the state's implementation of Proposition 65, the Safe Drinking Water and Toxic Enforcement Act. Estimates of carcinogenic potency made by the California Department of Health Services (CDHS) are generally similar to estimates made by the U.S. Environmental Protection Agency (EPA). The largest differences are due to EPA's use of the maximum likelihood estimate instead of CDHS use of the upper 95% confidence bounds on potencies derived from human data and to procedures used to correct for studies of short duration or with early mortality. Numerical limits derived from these potency estimates constitute “no significant risk” levels, which govern exemption from Proposition 65's discharge prohibition and warning requirements. Under Proposition 65 regulations, lifetime cancer risks less than 10−5 are not significant and cumulative intake is not considered. Following these regulations, numerical limits for a number of Proposition 65 carcinogens that are applicable to the control of toxic discharges are less stringent than limits under existing federal water pollution control laws. Thus, existing federal limits will become the Proposition 65 levels for discharge. Chemicals currently not covered by federal and state controls will eventually be subject to discharge limitations under Proposition 65. “No significant risk” levels (expressed in terms of daily intake of carcinogens) also trigger warning requirements under Proposition 65 that are more extensive than existing state or federal requirements. A variety of chemical exposures from multiple sources are identified that exceed Proposition 65's “no significant risk” levels.