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Fumigation is often performed in grain-milling facilities and warehouses to control unwanted insect pests. Fumigants are typically gases or liquids having a high vapour pressure such that the fumigant permeates throughout the facility. Multiple fumigant field studies using commercial mill structures were performed using sulphuryl fluoride (SF). Internal building concentrations, SF leakage rates, and external air concentrations at specific setback distances from the treated structure were quantified. However, field studies are limited to the parametric combinations of management practices, mill geometry, and meteorological conditions that occurred during the experiment. The use of two existing US Environmental Protection Agency air dispersion models [Building Profile Input Program (BPIP) and the Industrial Source Complex dispersion model (ISCST3)], with custom applications of Visual Basic and Monte Carlo algorithms were employed to calculate different exposure scenarios and to extrapolate external SF air concentrations for parameter ranges not used/observed in the field trials. Different inputs included application rate, application date, application timing, meteorological variability, and parameters quantifying building leakage and aeration processes. Results from the numerical system were validated against observations from three US mill field studies located in Kansas, Texas, and Indiana with excellent agreement (average difference from 32·9% to 42·7%), giving credence that the modelling can expand field observations to other diverse situations where mill fumigation may be used. Sensitive input parameters were allowed to vary by Monte Carlo techniques to quantify parametric uncertainty. It was found that the orientation of wind speed and direction surrounding a fumigated mill can influence external air concentrations due to the effect of neighbouring buildings and silos. [Copyright &y& Elsevier]