Microbial risks from wastewater irrigation of salad crops: a screening-level risk assessment.

The potential health risk from viruses, associated with the consumption of lettuce crops spray irrigated with secondary-treated municipal effluent, has been evaluated in the first level investigation of a tiered microbial risk assessment. The study assessed the impact of two factors on the estimated risk of infection: a suitable probability density function for the occurrence of human enteroviruses in irrigation water and appropriate die-off rates for viruses on lettuce crops. A Monte Carlo simulation using a log-normal and a nonparametric, kernel estimated probability density function indicated that slight changes in the upper tail of the probability density function had a relatively low effect on the estimated infection rates. Predicted infection rates were much more sensitive to the decay rate of viruses than occasional high virus numbers. The median and 99th percentile risks of infection from the overall model were 0.10 and 0.51/10000 lettuce consumers, respectively, indicating possible human health concern, and the justification of a more detailed microbial risk assessment.