Environmental Monitoring and

product, as well as manufactured synthetically. With a vapor pressure of approximately 2000 torr at 25 degrees Celsius, methyl bromide is a gas at normal pressure and temperature and a liquid under high pressure or at low temperature. It is colorless (a dye is sometimes added) and odorless except at high concentrations. Methyl bromide is primarily used as a pesticide. The preliminary and final risk characterization by the Department of Pesticide Regulation (DPR) for methyl bromide indicated toxic effects at doses lower than those previously documented (Nelson 1992; Lim 1999). The risk characterization estimated that acute exposure to 0.21 parts per million (ppm, 24-hour time-weighted average) provides a loo-fold margin of exposure (100 times less than the no observed effect level in animal tests). Subsequently, DPR and others conducted off-site and worker air monitoring for methyl bromide field fumigations. Several methyl bromide applications exceeded 0.21 ppm outside the field (Attachment 1). Air concentrations are highly variable, with measured concentrations 30 feet from the field ranging from 0.042- 1.1 ppm, plus 1.7 ppm detected 330 feet from the field for one application. Air concentrations vary with numerous factors such as distance from the field, wind speed, wind direction, application rate, field size and dimensions, and method of application