Arsenic: risk assessment for california drinking water standards

Six California counties contain 15 water systems with arsenic concentrations above the 50 ppb Maximum Contaminant Level (MCL). Arsenic compounds are carcinogenic in humans by oral and inhalation routes. They are also fetotoxic and teratogenic in mice, rats and hamsters and cause a variety of toxic effects in the gastro-intestinal tract, circulatory system, skin, liver, kidney, nervous system and heart. The US EPA has identified an oral human chronic No-Observable-Adverse-Effect Level (NOAEL) of 0.0008 mg/kg-d based on skin and vascular effects. In 1988 EPA estimated the human carcinogenic potency of arsenic in drinking water to be about 2 × 10 −1/μg/kg-d with a 10 −6 lifetime skin cancer risk equivalent to consumption of 21/day at 20 ppt. Recently Smith et al. (1990) estimated the potency of arsenic to be 5.3 × 10 −3μ/g/kg-d based on the same human data. Also Chen et al. (1988) and Chen and Wang (1990) identified the additional tumor sites of liver, lung, bladder, kidney, nasal cavity and prostate. The lifetime risk of developing skin cancer at the 50 ppb MCL level (21/day) is about 8 in 1000. Preliminary analysis of the recent data on other tumor sites indicate comparable risks of: females - lung, 11.0; bladder, 6.7; kidney, 3.4; liver, 0.3; males - lung, 6.1, bladder, 2.2; kidney, 1.4; liver, 0.2. On the basis of a skin cancer potency of 5.3 × 10 −3/μg/kg-d, a Recommended Public Health Level (RPHL) of 2 ppt (0.002 μg/l) is being proposed in accordance with the provisions of the Safe Drinking Water Act of 1989. This value assumes a body weight of 70 kg, a water consumption of 21/day, a relative source contribution of 20%, and a lifetime extra cancer risk of 10 −6.