#62-S ecological study of bladder cancer in counties with high levels of arsenic in drinking water

PURPOSE: Bladder cancer incidence and mortality in US counties were studied to determine the excess risk associated with drinking water with arsenic concentrations above the new USEPA MCL of 10 ppb. METHODS: Rural counties, where the majority of the population obtains drinking water from groundwater with high arsenic concentrations (>10 ppb), were identified from databases compiled by the EPA, state agencies and local water utilities. Age-specific bladder cancer mortality rates for the period 1982 to 1998 were obtained for these 33 ‘exposed’ counties and 105 adjacent ‘unexposed’ rural counties in 11 states. A standardized mortality ratio (SMR) for the aggregate ‘exposed’ population was calculated using reference rates from the aggregate ‘unexposed’ population and compared to SMRs calculated using national and state reference rates. SMRs were also calculated for exposed populations in individual states. Standardized Incidence Ratios (SIRs) were calculated in a similar manner for exposed counties in States included in the National Cancer Instituteʹs Surveillance Epidemiology and End Results cancer registry program. RESULTS: Calculated SMRs and SIRs depended on the choices of exposed and reference populations. Risk of bladder cancer mortality for white males was not elevated for the population drawn from the 33 exposed counties (SMR = 0.93; 95%CI = 0.85, 1.02) or for the exposed population drawn from Utah, irrespective of the reference population. Risks of bladder cancer incidence for exposed populations in New Mexico, however, were slightly elevated compared to unexposed reference populations drawn from adjacent counties within the state (SIR = 1.20; 95%CI = 1.13, 1.26). CONCLUSION: A small but statistically significant excess risk for bladder cancer may be attributed to exposure to arsenic at levels >10 ppb in some populations in New Mexico. Additional analyses to examine potential confounding by ethnicity and smoking prevalence are required. The results demonstrate the potential sensitivity of bladder cancer risks from arsenic exposure on a variety of factors. The study illustrates the potential limitations of ecological studies and risk assessment in US populations exposed to high levels of arsenic in drinking water.