Changing the assessment to meet the changing science: the case of arsenic and drinking water standards in the United States

Purpose As a practical matter, policy decisions and risk assessments are often made before the underlying science can be firmly established. As a result, it is important that these policy decisions be revisited as our understanding of the relevant scientific basis changes over time. Development in our understanding of the relationship between arsenic exposure in drinking water and internal cancers provides an example of why these periodic reevaluations are important. Methods The current U.S. Environmental Protection Agency (EPA) and National Research Council (NRC) estimates of bladder cancer risk associated with exposure to arsenic in drinking water implicitly assumed that the water source differences for the 42 studied villages in southwestern Taiwan were not relevant. Subsequent reanalysis has now used water source as an exposure variable. The resulting risk estimates were compared with the relationship observed between U.S. bladder cancer data and arsenic concentrations in groundwater to determine how well the southwestern Taiwan data predicted the U.S. risk. Results Data from southwestern Taiwan villages with only artesian well sources (median arsenic content = 350–940 μg/L) significantly overpredicted the rate of arsenic-related bladder cancer in the United States. Data from villages with nonartesian water sources (median As content = 0–740 μg/L) revealed no arsenic-related increase in bladder cancer mortality and were consistent with U.S. bladder cancer data. These results suggest that arsenic acts as a carcinogen either at high (>400 μg/L) concentrations or in the presence of a cofactor that is present in the artesian water sources in southwestern Taiwan but not in the United States. These results are consistent with other recent studies of arsenic and bladder cancer in the United States and Argentina. Conclusion Current drinking water standards in the United States are based on the presumption that the relationship between arsenic exposure and cancer risk is solely a matter of the concentration of arsenic. Recent research, however, suggests that this scientific basis needs to be revised, and with it, the policy decisions that have been based on it.