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Methyl bromide

Publication Date: 9/93

TRADE OR OTHER NAMES

Trade or common names of methyl bromide containing products include: Brom-o-Gas, Celfume, Embafume, MB, MeBr, Methogas, Profume, Terr-o-Gas, and Zytox.

REGULATORY STATUS

EPA has proposed restrictions on methyl bromide, as well as a schedule of phaseout by the year 2000. Methyl bromide is considered an ozone depleting chemical and thus falls under the Clean Air Act with an ozone depletion potential of 0.2%.

The methyl bromide phaseout is due to action under the Clean Air Act, not FIFRA. Unlike FIFRA, the Clean Air Act does not contain a risk/benefit balancing process that would allow retention of essential or high benefit uses, nor does the listing and phaseout of ozone depleters depend on the availability of alternative products (16).

INTRODUCTION

Methyl bromide is most often used as a gas fumigant against insects, termites, rodents, weeds, nematodes, and soil-borne diseases. It has been used to fumigate agricultural commodities, grain elevators, mills, ships, clothes, furniture and greenhouses.

About 70% of methyl bromide produced in the United States goes into pesticidal formulations. Nonpesticidal uses include degreasing wool and extracting oils from nuts, seeds and flowers. Methyl bromide is also a methylating agent in the chemical industry. In the past, it was used as a refrigerant and as a fire-extinguishing agent in aircraft. The chemical name for this fumigant is bromomethane.

Methyl bromide is a Restricted Use Pesticide (RUP) because of its high acute toxicity to applicators. Restricted Use Pesticides may be purchased and used only by certified applicators.

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Methyl bromide, labeled with a DANGER signal word, is an extremely toxic vapor. In humans, methyl bromide is readily absorbed through the lungs. Most problems occur as a result of inhalation. About 1,000 human poisoning incidents caused by methyl bromide exposure have been documented, with effects ranging from skin and eye irritation to death. Most fatalities and injuries occurred when methyl bromide was used as a fumigant.

Inhalation of 1,600 ppm for 10-20 hours, or 7,900 ppm for 1.5 hours is lethal to humans (8). The lowest inhalation level found to cause toxicity in humans is 35 ppm in air.

Methyl bromide is a dangerous cumulative poison. First symptoms often are due to damage to the nervous system, and may be delayed from 48 hours to as long as several months after exposure. This delay, combined with methyl bromide's lack of odor, means that the victim may not realize that exposure is occurring until much time has passed.

Symptoms of poisoning vary widely. Soon after inhalation of large doses, symptoms may include headache, dizziness, nausea, chest and abdominal pain, and a dry throat. Three to 12 hours after vapor inhalation, symptoms include slurred speech, blurred vision, temporary blindness, mental confusion, and sweating. More severe symptoms may include lung swelling; congestion; hemorrhaging of the brain, heart, and spleen; severe kidney damage; and numbness. Death may occur within 1-30 hours, usually from respiratory failure.

Although skin absorption is not an important route for methyl bromide intoxication, the skin is affected by contact with this chemical. Methyl bromide can cause enormous blisters that are rarely deep enough to destroy the entire skin layer. Small amounts of skin or eye contact brings on shortness of breath and itching. If absorbed through the skin, nausea and vomiting may result. Clothing that can not "breathe" may delay the evaporation of the pesticide from the skin. Continued contact with skin can cause death. Ingestion of methyl bromide may cause hand tremors, and convulsions.

The inhalation LC50 for rats is 3,120 ppm/15 minutes (7), 2,700 ppm/30 minutes, and 1,164 ppm/60 minutes. For guinea pigs, it is 300 ppm/9 hours and it is 2,000 ppm/11 hours in the rabbit (7). The rat LD50 administered in liquid is 20 mg/L (11). The dermal LD50 for rabbits is 15 ppm.

Common animal responses to high doses often include lung irritation, fluid accumulation in the lungs, paralysis, and kidney, liver, and nervous system damage (9).

CHRONIC TOXICITY

Chronic exposures to methyl bromide can cause dizziness, vision and hearing disturbances, depression, confusion, hallucinations, euphoria, personality changes, and irritability. If exposure is severe enough, lung irritation followed by lung swelling and bronchial pneumonia may occur (7).

Reproductive Effects

No reproductive problems involving methyl bromide have been observed in test animals, though few experiments have been performed.

Teratogenic Effects

Inhalation of methyl bromide for 6-7 hours per day during gestation was reported to cause no birth defects on rabbits and rats (7).

Mutagenic Effects

The overall scientific evidence indicates that methyl bromide is a mutagen, but that its potential to cause genetic mutations is relatively low (13). Mutagenic effects were seen in a mouse cell test and two different tests on bacteria (2). In addition, fruit flies showed mutagenic effects as did human white blood cells (6). However, rat liver cells did not display mutagenicity after exposure to methyl bromide (2).

Carcinogenic Effects

Methyl bromide is considered to be a potent cell growth stimulant and is thus a potential promoter of cancerous growth. In one study of industrial workers exposed to various brominated compounds, exposure to methyl bromide was suggested as the possible common factor in two fatal cases of testicular cancer. The mortality rate for this cancer was significantly higher than expected (9).

In another study, methyl bromide induced tumors in the stomach of the rat (2). The chemical was given through the stomach in an oil solution for 90 days. In 13 of 20 rats given high doses, malignant growths developed quickly and in high numbers near the site of application. Some tumor growth was observed in rats given much lower doses. No effects were seen in rats exposed to very low doses of methyl bromide for 90 days (2). Due to a very limited amount of information on the cancer causing effects on humans the EPA has determined that the compound is not classifiable.

Organ Toxicity

Chronic low level exposure causes depression of the central nervous system, injury to the kidneys, and may cause respiratory problems, and irritate the skin and eyes. Methyl bromide and its constituents appear to concentrate in the central nervous system. This chemical can affect muscle control and behavior. Other targets of the fumigant are the heart, nasal cavities, adrenal gland, and the testis (14).

Fate in Humans and Animals

The major route of absorption of methyl bromide vapors is through the lungs. Some of the compound is excreted through the lungs as unchanged methyl bromide, but a significant amount also undergoes metabolic decomposition. The primary breakdown product is the bromide ion, which is detectable in the blood and tissues and is excreted in the urine. Organic bromides also appear in stomach fluids and mucous.

In humans, methyl bromide's half-life in blood is about 12 days (11). As a result, the toxic effects of methyl bromide can be delayed or prolonged. Additionally, once in a cell, this chemical inactivates many enzyme systems, so prolonged small doses can cause severe toxicity.

ECOLOGICAL EFFECTS

Methyl bromide is moderately toxic to aquatic organisms. Acute toxicity for some freshwater fish may occur at concentrations of 11 ppm and around 12 ppm for saltwater fish (7). However, when applied properly, methyl bromide is not expected to enter surface waters via run-off or erosion, and thus poses little risk to aquatic species. It is not highly toxic to most plants.

ENVIRONMENTAL FATE

The amount of bromide ion, the metabolite of methyl bromide, in residues, is proportional to the protein content of the crop. Higher levels of the bromide ion will most likely be found in high-protein plants.

When used as a soil fumigant, only a small amount of methyl bromide is transformed into the bromide ion while much of the gas enters the atmosphere. Transformation of methyl bromide into bromide increases as the amount of organic matter in the soil increases. The rate of degradation in fumigated soil is 6-14% per day at 20 degrees C. More leaching occurs in a sandy versus a loamy soil.

Methyl bromide run-off from fields into surface waters is rare due to the way this chemical is normally applied. If it contacts surface waters, the average half-life for methyl bromide under field conditions has been calculated to be 6.6 hr at 11 degrees C. Another study showed the half-life in water to be 20 days at 25 degrees C in a neutral solution (9). Methyl bromide quickly evaporates at temperatures ordinarily encountered in fumigating.

Some methyl bromide is vaporized during chemical manufacturing and processing operations. It can also enter the environment through auto exhaust (when used as a gas additive) and may play a role in ozone depletion. Bromide compounds have been shown to have destructive effects on the ozone layer.

PHYSICAL PROPERTIES AND GUIDELINES

Methyl bromide is a colorless gas or volatile liquid which is usually odorless, but has a sweet, chloroform-like odor at high concentrations.

Since the chemical is a gas at ordinary temperatures, it is compressed and applied as a liquid. Methyl bromide is not spontaneously combustible, but it poses a hazard when exposed to sparks or flame. Although stable under normal temperatures and pressures, when heated to decomposition, methyl bromide emits highly toxic and corrosive fumes of bromides.

This compound presents a severe explosion hazard when combined with aluminum, magnesium, or zinc. Delayed explosions may occur in the presence of dimethyl sulfoxide. A vigorous reaction occurs when it is mixed with strong oxidizers or ethylene oxide (5).

Exposure Guidelines:

NOEL: 16 ppm on rats, rabbits, guinea pigs, and monkeys (7).
TOL: odorless at low concentrations; no odor until at a very high concentration (above TLV) so odor is a poor warning signal.
PEL: 20 ppm in the air (4).
TLV: 5 ppm skin
TWA: 5 ppm skin
RfD: 1.4 x 10 to the minus 3 mg/kg/day

Physical Properties:

CAS #: 74-74-83-9
Solubility in water: Methyl bromide is considered very slightly soluble in water. 1.34 g/100g (25 degrees C); 1.75 g/100g (20 degrees C, 748 mm Hg) (12)
Solubility in solvents: Easily miscible with ethanol, ether, carbon disulfide, chloroform; freely soluble in benzene, carbon tetrachloride.
Melting point: 93.6 degrees C (12)
Boiling point: 3.56 degrees C (at 760 torr) (12)
Vapor pressure: 1250 mg Hg (at 20 degrees C) (5); 1824 mm at 25 degrees C (3)
Kow: 1.69 (1971 med); 109 (1979 mth)

BASIC MANUFACTURER

Great Lakes Chemical Corporation
P.O. Box 2200
West Lafayette, Indiana 47906
Emergency 501-862-5141

Review by Basic Manufacturer:

Comments solicited: October, 1992
Comments received:

REFERENCES

  1. American Conference of Governmental Industrial Hygienists. Threshold Limit Values for Chemical Substances in the Work Environment. 1984-85.
  2. Danse, L.H., F.L. Van Velsen, and C.A. Vander Heijden. Toxic. 1984. Methylbromide: Carcinogenic Effects in the Rat Forestomach. Toxicology and Applied Pharmacology. 72: 262-271.
  3. Gosselin, R.E., R.P. Smith, H.C. Hodge, J.E. Braddock. 1984. Clinical Toxicology of Commercial Products. Fifth Edition. Williams & Wilkins.
  4. National Institute for Occupational Safety and Health. 1978. Occupational Health Guidelines for Methyl Bromide.
  5. Occupational Health Services, Inc. Material Safety Data Sheet on Methyl Bromide. 19 February 1987.
  6. Lewis, R.J. and R.L. Tatken, eds. 1979. Registry of Toxic Effects of Chemical Substances. Tracor Jitco, Inc.
  7. National Library of Medicine. Hazardous Substances Databank. Methyl Bromide. May 10, 1992.
  8. U.S. Environmental Protection Agency. Chemical Fact Sheet for Methyl Bromide. Fact Sheet No. 98. 22 August 1986.
  9. U.S. Environmental Protection Agency. Chemical Hazard Information Profile. "Methyl Bromide." 20 February 1986.
  10. Vettorazzi, G. 1979. Bromomethane. International Regulatory Aspects for Pesticide Chemicals. Vol. I. CRC Press, Inc: Boca Raton, FL.
  11. Wagner, S.L. 1983. Fumigants. Clinical Toxicology of Agricultural Chemicals. Noyes Data Corp.
  12. Windholz, M., et al. eds. 1983. The Merck Index, 10th ed. Merck & Co., Inc.
  13. Gehring, P.J., R.J. Nolan, P.G. Watanabe, and A.M. Schumann. 1991. Solvents, Fumigants, and Related Compounds. In Handbook of Pesticide Toxocology, Volume 2, Classes of Pesticides. Wayland J. Hayes Jr. and Edward R. Lawes, Jr. editors. Academic Press, Inc., NY.
  14. Mitsumori, K., K. Maita, T. Kosaka, T. Miyaoka and Y. Shirasu. 1990. Two-Year oral chronic toxicity and carcenogenicity study in rats of diets fumigated with methyl bromide. Food and Chemical Toxicology. Vol 28 number 2. pp. 109-119.
  15. World Health Organization. 1986. IARC Monogrpahs of the Evaluation of the Carcenogenic risk of Chemicals to Humans: Some Halogenated Hydrocarbons and Pesticide Exposures. International Agency for Research on Cancer, Volume 41.
  16. Federal Register. 58 FR 15030. March 18, 1993. U.S. Government Printing Office. Washington, DC.