Methyl bromide
(endorsed 2011)
Guideline
Based on human health concerns, methyl bromide in drinking water should not exceed 0.001 mg/L.
Related chemicals
Methyl bromide (CAS 74-83-9) belongs to the fumigant class of chemicals. Other pesticides in this class include chloropicrin, dimethyl disulfide, ethylene dibromide and sulfuryl fluoride (Tomlin 2006).
Human risk statement
With good water quality management practices, the exposure of the general population is expected to be well below levels that may cause health concerns.
If present in drinking water as a result of a spillage or through misuse, methyl bromide would not be a health concern unless the concentration exceeded 0.001 mg/L. Excursions above this level would need to occur over a reasonably significant period to be a health concern, as the health-based guideline is based on medium-term effects.
With good water quality management practices, pesticides should not be detected in source waters used for drinking water supplies. Persistent detection of pesticides may indicate inappropriate use or accidental spillage, and investigation is required in line with established procedures in the risk management plan for the particular water source.
General description
Uses: Methyl bromide is a broad-spectrum fumigant effective against a variety of pests including insects, nematodes, rodents, bacteria, viruses, fungi, mites and weeds. It is used in grain silos, buildings, ships’ holds and cargos, and in soil.
There are registered products that contain methyl bromide in Australia. The products are intended for professional use only, with restrictions on availability to authorised or licensed persons. Methyl bromide is available as liquefied gas (under pressure) to be applied either by hand equipment or machinery. Methyl bromide is currently in the phase-out stage due to environmental concerns, with very restricted uses allowed only under Critical Use Exemptions (CUEs) and for quarantine uses. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to methyl bromide is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Use of methyl bromide on soil may potentially lead to contamination of source waters through absorption into soil and subsequent entry into groundwater.
A major route of exposure to methyl-bromide is likely to be inhalation from gaseous sources such as leaking and venting from fumigation chambers, and gas escape from treated products.
Typical values in Australian drinking water
No reports of methyl bromide in Australian drinking waters have been identified.
Treatment of drinking water
No reports of the treatment of methyl bromide in drinking water were found. Methyl bromide is a volatile halogenated methane and would be expected to respond to most treatment processes in a similar manner to other halogenated methanes such as dichloromethane and chloroform.
Measurement
Methyl bromide can be measured in drinking waters using the same techniques as those routinely applied to dihalomethanes and trihalomethanes. Most commonly these substances are analysed by liquid-liquid extraction followed by gas chromatography with electron caption detection. The typical limit of quantitation for this approach is less than 1 µg/L.
History of the health values
The current acceptable daily intake (ADI) for methyl bromide is 0.0004 mg per kg body weight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.4 mg/kg bw/day from a short-term (90-day) dietary rat study. The NOEL is based on injury to the forestomach. The ADI incorporates a safety factor of 1000 and was established in 2001.
A health value has not been previously established by NHMRC.
Health considerations
Metabolism: Methyl bromide is readily and extensively absorbed from the gastrointestinal tract in rats. It is widely distributed to tissues, extensively metabolised to bromide, excreted in urine and bile, and exhaled as . Bromide accumulates in tissues in rats followed by slow elimination. The half-life of bromide has been reported to be 12 days in humans.
Acute effects: Methyl bromide has moderate acute oral toxicity and high dermal toxicity in rats. There are no quantitative data available on skin sensitisation. However, dermal exposure may result in redness, dermatitis, itching, swelling and blistering.
Short-term effects: Short-term dietary studies in rats reported focal hyperaemia and dose-related hyperplasia of the forestomach at doses of 2 mg/kg bw/day and above. The NOEL for these effects was 0.4 mg/kg bw/day and this is the basis for the current ADI. Inflammation, fibrosis and hyperkeratosis were reported at doses of 25 mg/kg bw/day and above. Methyl bromide is much more toxic via inhalation than via the oral route.
Long-term effects: A long-term dietary study in rats reported decreased bodyweight gain at a dose of 16.75 mg/kg bw/day. Relative brain and kidney weights were increased in males at higher doses.
Carcinogenicity: From oral administration studies in rats, there is some evidence of induction of forestomach squamous cell carcinoma. From inhalational studies in mice and rats, there is no evidence of carcinogenicity for methyl bromide.
Genotoxicity: Methyl bromide produced positive results in some in vitro assays in bacteria and cultured mammalian cells, as well as in mouse and rat bone marrow cells in vivo and in Drosophila. It was negative in the mouse dominant lethal assay. Overall, it has some genotoxic potential; however, there is no evidence that this leads to cancer formation.
Reproductive and developmental effects: Two-generation reproduction studies in rats and rabbits did not produce any evidence of reproductive effects. A developmental toxicity study in rabbits by inhalation exposure produced developmental effects at maternotoxic dose levels only. No oral exposure study was available.
Poisons Schedule: Methyl bromide is included in Schedule 7 of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010), with an Appendix J (Conditions for availability and use of Schedule 7 poisons) rider restricting its availability to authorised or licensed persons. Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline
The health-based guideline of 0.001 mg/L for methyl bromide was determined as follows:
where:
0.4 mg/kg bw/day is the NOEL based on a short-term (90-day) dietary study in rats.
70 kg is taken as the average weight of an adult.
0.1 is a proportionality factor based on the assumption that 10% of the ADI will arise from the consumption of drinking water.
2 L/day is the estimated maximum amount of water consumed by an adult.
1000 is the safety factor applied to the NOEL derived from animal studies. This safety factor incorporates a factor of 10 for interspecies extrapolation, 10 for intraspecies variation, and an additional 10 because the NOEL is based on a short-term study.
References
NOTE: The toxicological information used in developing this fact sheet is from reports and data held by the Department of Health, Office of Chemical Safety.
DoHA (2010) The Poisons Standard; Schedule 1-Standard for the Uniform Scheduling of Medicines and Poisons, Department of Health and Ageing, Commonwealth of Australia, Canberra.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
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