A probabilistic model for acute bystander exposure and risk assessment for soil fumigants

There is currently significant regulatory interest in estimating exposures and risks to bystanders following applications of soil fumigants. Soil fumigants are generally volatile and some of the fumigant mass will escape the field causing downwind exposures to resident populations. Regulatory agencies in the US are currently considering implementing buffer zones for many of the fumigants. These buffer zones would provide a restricted-entry zone around fumigant applications to mitigate potential exposures. This paper describes a probabilistic dispersion modeling application that has been developed to address this issue. The Probabilistic Exposure and Risk Model for FUMigants (PERFUM) employs the Gaussian dispersion algorithms developed by the US Environmental Protection Agency (US EPA). The model adapts EPA air dispersion algorithms to develop probabilistic estimates of acute exposures to bystanders following fumigant applications. The model considers the potential variability in exposures caused by differences in mass emission rates of the fumigants and the meteorological conditions following the application. The model is generalized to model exposures for fumigants with different exposure averaging times of concern to regulators (1–24 h), varying field sizes (up to 40 acres), and field dimensions. PERFUM also outputs the probabilistic exposures both based on the whole population surrounding the field and for the maximally exposed location only. The model is non-proprietary and publicly available.