Estimating methyl bromide exposure due to offgassing from fumigated commodities.

Methyl bromide (MB) is used to fumigate diverse commodities. During fumigation, the commodity can sorb a substantial mass of MB, which does not chemically react and is termed a residue. During subsequent commodity handling, the residue offgasses and can lead to MB inhalation exposure among processing workers. Although MB has a low 1 ppm 8-hr TLV-TWA as recommended by the American Conference of Governmental Industrial Hygienists (ACGIH(R)) and is considered a potential occupational carcinogen by the National Institute for Occupational Safety and Health, the recent industrial hygiene literature contains no pertinent exposure data. Limited measurements made in 1992 by the California Department of Pesticide Regulation are summarized here, but associated information on exposure determinants is lacking. In this article, mathematical models are used to integrate data on MB residue offgassing with several processing scenarios to estimate potential exposure levels. The main finding is that if a large volume of commodity rapidly offgasses MB and is handled under conditions of low ventilation, the potential exists for MB exposures above the 1 ppm TLV-TWA value. However, the combination of handling a smaller commodity volume and less rapid offgassing may be the more typical scenario. It is recommended that a pilot study be conducted to measure current MB exposure levels, test the validity of the mathematical models, and collect industry-wide data on exposure determinants. By using the latter data as inputs for validated models, public health scientists could estimate the distribution of MB exposure levels across the commodity processing industry.