BROMOFORM

CAS Number: 75-25-2

Synonyms: Methyl tribromide; Tribromomethane; Methenyl tribromide

Molecular Formula: CHBr₃

TLV–TWA, 0.5 ppm (5.2 mg/m³)

A3 — Confirmed Animal Carcinogen with Unknown Relevance to Humans

TLV^® Recommendation

A TLV–TWA of 0.5 ppm (5.2 mg/m³) is recommended for occupational exposure to bromoform to minimize the potential for possible liver and kidney damage reported in rodent studies (Bowman et al., 1978; Chu et al., 1980; Condie et al., 1983; Kroll et al., 1994a, b; Aida et al., 1992), as well as for upper respiratory tract irritation and lacrimation reported in humans (von Oettingen, 1955). The dose of bromoform from exposure to the recommended TLV–TWA value is well below the lowest-observed-adverse-effect level (LOAEL) for liver effects in orally-exposed rats (56 mg/kg) (Aida et al., 1992). An A3, Confirmed Animal Carcinogen with Unknown Relevance to Humans, notation is designated based on increased incidences of adenomatous polyps and adenocarcinomas of the large intestine of rats treated with bromoform (NTP, 1988).

Sufficient data were not available to recommend a Skin or sensitization (SEN) notation or a TLV–STEL. Although bromoform is absorbed through the skin (Xu et al., 2002), the low systemic toxicity and lethality of bromoform (NLM, 2007; Chu et al., 1982a) suggests that dermal exposure is unlikely to lead to toxic effects. The reader is expected to be familiar with the section on Excursion Limits in the Introduction to the Chemical Substance TLVs of the current edition of the Documentation of the TLVs^® and BEIs^® for the guidance and control of excursions above the TLV–TWA, even when the eight-hour TWA is within the recommended limit.

TLV^® Basis

Liver damage; upper respiratory tract irritation; eye irritation.

Chemical and Physical Properties

Bromoform is a colorless, non-flammable, heavy liquid, similar to chloroform in odor and taste. Chemical and physical properties include (NLM, 2007):

Molecular weight: 252.73
Specific gravity: 2.89 at 20°C
Freezing point: 8°C
Boiling point: 149.1°C
Vapor pressure: 5 torr at 20°C
Vapor density: 8.7
Percent in saturated air: 0.7 at 25°C
Solubility: 3.1 g/l water at 25°C; miscible with organic solvents
Reactivity: liquid bromoform will attack some forms of plastics, rubber, and coatings
Conversion factors at 25°C and 760 torr: 1 ppm = 10.35 mg/m³; 1 mg/m³ = 0.097 ppm

Major Uses

Bromoform has been used as a chemical intermediate; in the synthesis of pharmaceuticals; and as a solvent for waxes, greases, and oils. It has also been used to make high-density liquids for geologic analysis and in the shipbuilding, aircraft, and aerospace industries (NLM, 2007).

Animal Studies

Acute

Dogs exposed to 7000 ppm bromoform or more became deeply anesthetized after eight minutes and died after one hour (Irish, 1963). The lowest lethal four-hour inhalation concentration of bromoform was reported to be 4500 mg/m³ for rats (Izmerov et al., 1982). Oral LD₅₀ values for bromoform were greater than 1 g/kg in rats and mice (NLM, 2007; Chu et al., 1982a). Liver and kidney damage were observed in rats and mice (Bowman et al., 1978; Chu et al., 1980; Condie et al., 1983). The subcutaneous LD₅₀ was 1820 mg/kg in mice (Kutob and Plaa, 1962). Narcosis and hepatotoxic effects were observed, and bromoform was rated as 38 times less hepato-toxic than tetrabromomethane (carbon tetrabromide). A single intraperitoneal dose of 758 mg/kg produced kidney dysfunction in rats (Kroll et al., 1994a, b). Bromoform was a less potent renal toxicant than the other trihalomethanes investigated (dichlorobromomethane, chloroform, and dibromochloromethane). Unlike other halomethanes, the hepatotoxicity of bromoform was not potentiated by prior treatment with chlordecone (Agarwal and Mehendale, 1983).

Undiluted bromoform was moderately irritating to rabbit eyes, and healing was complete in one to two days. Repeated skin contact caused moderate irritation to rabbit skin (Torkelson and Rowe, 1981).

Subchronic

Groups of 10 male rats were given 0, 5, 50, or 500 ppm bromoform in their drinking water for 28 days. Growth rate and food intake were unaffected and a slight increase in kidney weight was observed in the 500 ppm group. No histopathological changes were seen in the tissues examined (Chu et al., 1982a). Groups of Wistar rats (seven males and seven females) were given a diet containing micro-capsules of bromoform at 0.07, 0.2, and 0.6% of the diet for one month. Suppression of body weight gain was seen in the high-dose males. Vacuolization and swelling of the liver were noted in all groups given bromoform. Renal lesions were not observed. The LOAEL for bromoform was 56.4 mg/kg (Aida et al., 1992).

Groups of 20 male and female rats were given 0, 5, 50, 500, or 2500 ppm bromoform in their drinking water for 90 days (equivalent to approx 0, 0.3, 3, 30, and 150 mg/m³). Ten rats from each group were killed at 90 days and the remaining animals were given tap water for an additional 90 days. Mild histological changes in livers and thyroids were observed at 90 days in all treatment groups but not in the 90-day recovery groups (Chu et al., 1982b). The reversibility of these effects and the types of changes seen suggest that they are adaptive changes rather than adverse effects. Statistical analysis of the data was not presented. Male ICR mice given various concentrations of bromoform in drinking water for up to 90 days did not exhibit significant behavioral toxicity (Balster and Borzelleca, 1982).

Chronic/Carcinogenicity

Groups of 50 F-344/N rats of each sex and 50 female B₆C₃F₁ mice were administered 0, 100, or 200 mg/kg bromoform by corn oil gavage five days per week for 103 weeks. Male B₆C₃F₁ mice were administered 0, 50, or 100 mg/kg bromoform on the same schedule. Reduced survival of male rats in the high dose group (200 mg/kg) was noted. There was some evidence of carcinogenicity of bromoform in male rats and clear evidence in female rats based on increased incidences of adenomatous polyps and adenocarcinomas of the large intestine. There was no evidence for carcinogenic activity in mice (NTP, 1988). Administration of bromoform in drinking water (1.1 g/l) to male F344/N rats and B₆C₃F₁ mice for 13 weeks induced aberrant crypt foci in the colons of rats but not mice (DeAngelo et al., 2002). Aberrant crypt foci are thought to be preneoplastic lesions involved in colon cancer. Feeding rats a folate-deficient diet for 26 weeks increased the incidence of aberrant crypt foci due to bromoform (0.5 g/l in drinking water) (Geter et al., 2005). Repeated intraperitoneal administration of bromoform produced pulmonary adenomas in strain A mice (Theiss et al., 1977).

Genotoxicity

Genotoxicity testing with bromoform has given mixed results. Bromoform has been reported to be positive, inconclusive, and negative in the Salmonella mutagenicity assay (Torkelson and Rowe, 1981; Haworth et al., 1983; Zeiger, 1990; Roldan-Arjona and Pueyo, 1993; Le Curieux et al., 1995; DiMarini et al., 1997; Landi et al., 1999; Kargalioglu et al., 2002). Bromoform was consistently mutagenic in Salmonella strain RSJ100, which contains the rat glutathione S-transferase theta-1 gene, suggesting that this form of glutathione S-transferase may be involved in bromoform bioactivation (DiMarini et al., 1997; Landi et al., 1999). However, there was no difference in the weak induction of DNA damage by bromoform in human whole blood cultures from individuals expressing glutathione S-transferase theta versus individuals lacking glutathione S-transferase theta as assessed by the Comet assay (Landi et al., 1999). Bromoform induced DNA damage in E. coli PO37 with or without a metabolic activation system and was clastogenic in the newt micronucleus assay (Le Curieux et al., 1995). Bromoform was positive in Drosophila sex-linked recessive lethal assays but negative in Drosophila reciprocal translocation assays (Woodruff et al., 1985).

Bromoform was positive in the mouse lymphoma cell mutation assay (Myhr et al., 1990) and produced sister-chromatid exchanges in Chinese hamster ovary cells (Galloway et al., 1985; Anderson et al., 1990), rat erythroblastic leukemia cells (Fujie et al., 1993), and human lymphocytes (Morimoto and Koizumi, 1983; Banerji and Fernandes, 1996). Administration of up to 253 mg/kg bromoform to rats intraperitoneally or orally produced chromosomal aberrations in bone marrow (Fujie et al., 1990); conflicting results were obtained with mice (Morimoto and Koizumi, 1983; Stocker et al., 1997). Oral gavage of male F-344 rats with up to 380 mg/kg bromoform for seven days did not produce DNA strand breaks (Potter et al., 1996).

Reproductive/Developmental Toxicity

No significant alterations in the number of resorption sites, fetuses per litter, fetal body weights, fetal malformations, or visceral anomalies were observed in the offspring of rats administered up to 200 mg/kg/day bromoform in corn oil by oral gavage on gestational days 6–15 (Ruddick et al., 1983).

Absorption, Distribution, Metabolism, and Excretion

Bromoform is metabolized by cytochromes P450 in vitro (Wolf et al., 1977; Ahmed et al., 1977) and in vivo (Anders et al., 1978) to ultimately form carbon monoxide. Free radicals were detected during incubation of bromoform with rat hepatocytes (Tomasi et al., 1985).

Detailed studies of the biotransformation of bromoform have not been reported. Oral administration of ¹⁴C-bromoform to Sprague-Dawley rats (100 mg/kg) and B₆C₃F₁ mice (150 mg/kg) resulted in 60–90% absorption, wide organ distribution, and excretion primarily by exhalation of the parent compound or CO₂ (Mink et al., 1986). Forty percent of the bromoform dose was converted to CO₂ in mice while only 4% was converted in rats. A nonlinear relationship between oral dose (63 and 126 mg/kg) and area-under-the-curve (AUC) in rat blood suggested saturation of metabolic processes in this dose range (da Silva et al., 1999). Co-administration of bromoform and other trihalomethanes and related water disinfection by-products increased the AUC of bromoform, indicating inhibition of bromoform metabolism by these compounds (da Silva et al., 1999, 2000; St.-Pierre et al., 2003).

Human Studies

Exposure to bromoform vapor was reported to cause irritation of the respiratory tract, pharynx, and larynx, as well as lacrimation and salivation. Accidental ingestion of the liquid has produced central nervous system depression with coma and loss of reflexes. Smaller doses have produced listlessness, headache, and vertigo (von Oettingen, 1955). Quantitative dosimetry information was not available.

Bromoform is absorbed through human breast skin in vitro with a dermal absorption coefficient of 0.21 cm/hour at 25°C (Xu et al., 2002).

TLV^® Chronology

1965: proposed: TLV–TWA, 5 ppm; Skin
1966: proposed: TLV–TWA, 0.5 ppm; Skin
1967–1996: TLV–TWA, 0.5 ppm; Skin
1995: proposed: A3, Confirmed Animal Carcinogen with Unknown Relevance to Humans
1996–2008: TLV–TWA, 0.5 ppm; Skin; A3
2008: proposed: TLV–TWA, 0.5 ppm; A3
2009: Adopted: TLV–TWA, 0.5 ppm; A3 Literature search current through mid-2008

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