Use of risk assessment for development of microbial standards

A single-hit exponential model and a β-distributed 'infectivity probability' model were used to evaluate the potential health risks from drinking water containing low levels of protozoan cysts and viruses, respectively. The models were based on dose response curves developed from human feeding studies and assumed consumption of 2 L/day. The US EPA has suggested that one infection/10,000/year is an acceptable risk for infectious agents acquired through potable water. Based on this risk, acceptable levels of viruses and cysts were determined using the probability models. Thus poliovirus and rotavirus levels should not be more than 0.1 and 0.3 pfu/100 L respectively per day. Giardia levels should be below 0.2 cysts/100 L. To achieve a 1:10,000 annual risk, the geometric average should not exceed 10^-3 organisms/100 L. As monitoring treated water for these levels would be difficult, source waters may be monitored. Geometric averages of 1-100 organisms/100 L require 3-5 logs of treatment reduction to achieve the 1:10,000 risk. The risk models for viruses and protozoa can be used to evaluate the occurrence of pathogens in source water and determine the appropriate treatment needed to obtain specific levels of safety for drinking water.