Elsevier

Biosystems Engineering

Volume 94, Issue 4, August 2006, Pages 557-572
Biosystems Engineering

Estimating Outside Air Concentrations surrounding Fumigated Grain Mills

https://doi.org/10.1016/j.biosystemseng.2006.03.013Get rights and content

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.

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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