Using Comparative Exposure Analsis to Validate Low-Dose Human Health Risk Assessment: The Case of Perchlorate

Abstract

Comparative risk assessment is usually performed to inform risk ranking and prioritization exercises. Here it is applied as an innovative tool for testing the scientific validity and reliability of a 2002 USEPA human health risk assessment of perchlorate. Dietary exposure to nitrate is compared with drinking water exposure to perchlorate; both chemicals act on the thyroid gland by iodide uptake inhibition (IUI). The analysis shows that dietary nitrate is predicted to cause orders of magnitude more IUI than perchlorate exposure at environmental concentrations. If the 2002 USEPA risk assessment is scientifically valid and reliable, then a generally accepted decade-old USEPA nitrate risk assessment is fatally flawed, and risk management decisions based on it are severely under-protective. If the nitrate risk assessment is valid and reliable, however, then the 2002 USEPA perchlorate risk assessment is fatally flawed, unreliable and should not be used as the basis for risk management. The origin of this inconsistency is a policy decision to deem IUI a ``key event'' that may lead to changes in thyroid hormones and consequent adverse effects. This implicitly treats IUI as ``adverse.'' Unless large and sustained over a long period, however, IUI is mundane, reversible, and arises at exposure levels orders of magnitude below true adverse effects. In communities where quantitative human health risk assessment is expensive or expertise is lacking, comparative exposure assessment provides a cost-effective means to evaluate the merits of such assessments before taking costly risk management actions.