In contrast, one factor discussed above alone would account for a 100-fold variation. It appears that the risk reductions, according to EPA's calculation could easily be lower by much greater than a factor of 100, using NRC's own figures, which EPA does not dispute. Further, significant adverse health effects have not been noted in any study based in the United States with low arsenic concentrations. The NRC report states that the only known plausible mechanisms for arsenic to act as a carcinogen would dictate that arsenic act in a sublinear fashion, i.e., as a threshold chemical. This threshold could be in the neighborhood of 10 or 20 ppb.
Additional research is ongoing to further reduce the uncertainties in the arsenic risk assessment.
This provides the basis for EPA selection of a regulatory option of 10 ppb or above, including the option of 20 ppb. However, that calculation is predicated on an estimate that is very uncertain. It may be equally likely that the number exposed to this risk is less than one hundredth or one thousandth of the EPA estimate.