Acceptable microbial risk: Cost-benefit analysis of a boil-water order for Cryptosporidium

Abstract

Water treatment plants typically issue a boil-water order (BWO) to a potentially affected populace when Cryptosporidium is detected in the finished water. Although BWOs involve costs that may not be justified for very low risks of infection, there is no predetermined risk level or environmental concentration that triggers such an order. In this study, a cost-benefit analysis was used to identify a threshold level of risk for issuing a BWO. A decision tree was constructed, and the threshold level of risk at which the expected benefits of a BWO exceed expected costs was identified. An exponential dose-response model was used to determine the dose of Cryptosporidium oocysts that corresponds to the threshold risk level. Results suggest that a daily risk of nine illnesses out of 10,000 people exposed would justify a BWO. Given typical 3-log removal of oocysts during treatment, this risk level would correspond to a finished water concentration of 0.046 oocysts/L and a raw water concentration of 46 oocysts/L.