effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.

AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.

AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death.

Airborne concentrations below the AEGL-1 represent exposure concentrations that could produce mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects. With increasing airborne concentrations above each AEGL, there is a progressive increase in the likelihood of occurrence and the severity of effects described for each corresponding AEGL. Although the AEGL values represent threshold concentrations for the general public, including susceptible subpopulations, such as infants, children, the elderly, persons with asthma, and those with other illnesses, it is recognized that individuals, subject to idiosyncratic responses, could experience the effects described at concentrations below the corresponding AEGL.

SUMMARY

Methyl bromide is a colorless, nonflammable gas, with no taste or odor properties at low concentrations. Methyl bromide is currently used as a fumigant for buildings and soil and as a methylation agent in industry. Methyl bromide is an effective herbicide, rodenticide, nematicide, insecticide, bactericide, and fungicide. In the past, it was used as a refrigerant and fire extinguisher. Worldwide consumption of methyl bromide in 1996 was approximately 68 thousand metric tons. It is available as a liquefied gas in steel cylinders or cans.

Although numerous reports of accidental exposure of humans to methyl bromide that resulted in neurotoxicity or deaths are available in the literature, reliable information on exposure concentrations was not available. Acute, repeat-exposure, subchronic, and chronic studies, primarily with rats and mice, were available. Human case reports and controlled animal studies document that the central nervous system (CNS) is the primary target of methyl bromide. Neurotoxic symptoms can be delayed for several hours. Extremely high concentrations also produce lung edema. The mechanism-of-action of monohalomethanes is not completely understood, but could involve metabolism via the glutathione-S-transferase (GST) pathway to products that alkylate or inactivate cellular proteins. Species with higher cellular concentrations of GST appear to be more sensitive to the effects of methyl bromide than those with lower concentrations. The same is true for humans because of genetic polymorphisms of GST in the human population.



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