Estimating Outside Air Concentrations surrounding Fumigated Grain Mills
Introduction
Sulphuryl fluoride (SF) is an effective fumigant used to control unwanted insects. A new use for SF is in the treatment of grain-milling facilities and warehouses. Sulphuryl fluoride is a gas upon injection, and disperses throughout the structure and into walls and voids where insects may be residing. Gas concentrations of SF must be high enough and sustained long enough to be effective (dosing). Once the dosing period has ended, remaining fumigant within the structure is vented to safe levels before employees can return (aeration). In addition to reducing internal building air concentrations, external SF concentrations must be minimised to limit the potential for bystander inhalation exposure.
Characteristic fumigant procedures entail a 1/2–1 day dosing period (for efficacy) followed by a venting period (∼2–4 h) that is a function of the remaining fumigant concentration at the onset of aeration. A modelling system was constructed that accounts for both fumigation and aeration periods to evaluate off-site SF gas concentrations at specific distances surrounding treated structure(s) such as mills and warehouses. Two US Environmental Protection Agency (EPA) air dispersion models were used in novel ways to mimic fumigation practices for treated buildings. Outside air concentrations of SF at specific vertical and horizontal distances around treated buildings were investigated.
Section snippets
Materials and methods
Mills and warehouses come in an infinite number of styles, dimensions, and volume, room division and construction materials. In addition, meteorological conditions such as wind speed and direction, air stability class, and obstacles surrounding the structure affect airflow patterns and subsequent fumigant transport away from the treated structure. Therefore, one feasible approach to investigate multiple scenarios was the development of a probabilistic numerical system that can encompass a
Results
The ISCST3 model runs on an hourly time step and hourly concentration results are integrated over a specific time interval to address acute, subchronic, or chronic exposure intervals. Fumigation for commercial mill facilities typically takes 1–3 days and is considered to be an acute exposure event from a risk assessment perspective. Assessment of acute inhalation risk for non-occupational populations is normalised to a 24 h time-weighed average exposure period as a standard practice by
Conclusion
A numerical system has been constructed to simulate outside air concentrations arising from fumigation of grain milling facilities. Transparent coupling of two US Environmental Protection Agency air dispersion models [Building Profile Input Program (BPIP) and the Industrial Source Complex dispersion model (ISCST3)] was achieved and a graphical user interface (GUI) was employed for constructing mill and surrounding building geometry (e.g. building footprints, building heights, surrounding
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