Guidance

Chloroform: toxicological overview

Updated 6 August 2024

Main points

Kinetics and metabolism

Chloroform is readily absorbed from the lungs, gastrointestinal tract and skin.

Chloroform undergoes metabolism via an oxidative or a reductive pathway.

Chloroform is mainly excreted via the lungs unchanged or as the main metabolite carbon dioxide.

Health effects of acute exposure

Harmful, irritant, and possibly carcinogenic after prolonged exposure.

Acute inhalation of chloroform can cause systemic effects such as excitement, nausea, vomiting followed by dizziness, ataxia, and drowsiness. Convulsions, coma, and death may occur following substantial exposures.

Delayed effects of chloroform exposure include renal and hepatic damage (up to 48 hours post exposure).

Local effects are observed following inhalation (irritation of the nose and throat), ingestion (burning sensation of the mouth and throat), ocular (stinging) and dermal exposure (irritation and redness).

Health effects of chronic exposure

Chronic inhalation or ingestion of chloroform may cause hepatic damage.

The International Agency for Research on Cancer (IARC) classified chloroform as a category 2B carcinogen, therefore, possibly carcinogenic to humans.

Kinetics and metabolism

Absorption

Chloroform is readily absorbed from the lungs, gastrointestinal tract and skin (1, 2). Approximately 60 to 80% of inhaled chloroform is absorbed (1).

Distribution

Following absorption, chloroform is distributed throughout the body. Human and animal inhalation and oral exposure studies have recorded high chloroform concentrations in adipose tissue, brain, liver, kidneys, adrenals and blood (1, 2).

Metabolism

Chloroform is metabolised in humans and animals by oxidative or reductive cytochrome P450 dependent pathways. It has been suggested that chloroform metabolism mainly occurs via the oxidative pathway (3, 4). In the presence of oxygen (oxidative pathway) chloroform undergoes oxidative dechlorination to form trichloromethanol, which spontaneously dehydrochlorinates to form phosgene. Subsequent hydrolysis of phosgene forms hydrochloric acid and the main metabolite carbon dioxide (3, 4), Phosgene may also react with cellular macromolecules (such as enzymes, proteins or the polar heads of phospholipids), leading to the formation of covalent adducts. The adducts can interfere with molecular function which may result in loss of cellular function and cell death (3).

In the absence of oxygen (reductive pathway), the main metabolite is dichloromethyl free radical, which is extremely reactive and forms covalent adducts with microsomal enzymes and fatty acid tails of phospholipids. This may result in the loss of microsomal enzyme activity and can also lead to lipid peroxidation (3). Almost all tissues are capable of metabolising chloroform, the highest levels of metabolism occur in the liver, kidney and nasal mucosa (3).

Excretion

Chloroform is excreted via the lungs unchanged or in the form of the metabolite carbon dioxide, with small amounts of either detectable in the urine and faeces (2).

Sources and route of human exposure

Exposure to chloroform may occur via (1, 2, 4):

• inhalation (contaminated air)
• ingestion (contaminated food, beverages and water)
• possibly through dermal contact (showering, cleaning and swimming)

Chloroform may be released into the environment from its use, production and transport. It is also indirectly formed as a result of the reaction of chlorine with organic compounds (1). Processes known to contribute to the indirect formation and emission of chloroform include paper bleaching with chlorine and chlorination of municipal water, swimming pools and wastewater (1, 2). Under certain conditions some bacteria can dehalogenate carbon tetrachloride to release chloroform (2).

The majority of chloroform that enters the environment will eventually enter the atmosphere, due to its volatility (1, 2). The degradation of chloroform involves a reaction with hydroxyl radicals; the half-life for degradation is reported to be approximately 100 to 180 days (1).

Chloroform is a by-product of water chlorination and is therefore present in drinking water. The drinking water quality guideline for chloroform is 0.2 mg L^-1 (5). It has also been detected in sea, waste, and ground waters (1).

Atmospheric chloroform levels in remote, urban, and source-dominated areas have been reported to range from 0.1 to 0.25, 0.9 to 9.9 and 4.1 to 110 µg m^-3, respectively. The indoor air concentration of chloroform can be up to ten-fold higher than outdoor air concentrations. The use of chlorinated water in homes is thought to significantly contribute to levels of chloroform in indoor air (1). Concentrations of chloroform may exceed 1000 µg m^-3 in a shower cubical, as a result of volatilisation from hot water (2).

Exposure to chloroform may also occur in the workplace. Individuals that work at or near facilities that manufacture or use chloroform (for example, drinking water-plants, waste water-treatment plants and pulp and paper plants) may be exposed to considerably higher levels of chloroform than the general population (2).

Health effects of acute or single exposure

Human data

General toxicity

The main target organs of chloroform-induced toxicity are the central nervous system and the liver. The main symptoms of acute chloroform poisoning depend upon the concentration of chloroform absorbed, rather than the route of exposure (6). Older clinical reports involving patients exposed to chloroform as a method of anaesthesia, have reported that exposure to 40,000 ppm chloroform (195,600 mg m^-3) for several minutes may be lethal (2, 7). Dizziness and vertigo were observed in humans exposed to 920 ppm chloroform (4,498 mg m^-3) for 3 minutes (2). Chloroform can cause symptoms of illness at 2,490 mg m^-3 and discomfort at levels below 249 mg m^-3 (1).

Chloroform also causes progressive central nervous system depression. Initial symptoms include (6, 8):

• excitement
• nausea
• vomiting

followed by:

• ataxia
• dizziness
• drowsiness
• convulsions
• coma

In severe cases paralysis of the medullary respiratory centre may lead to respiratory failure and sudden death (6).

Early death following exposure to high levels of chloroform is often due to cardiac arrhythmias. Chloroform may also cause hypotension (1, 2, 6).

In the past, chloroform was extensively used to induce and maintain medical anaesthesia. Its use as an anaesthetic was abandoned because it caused hepatic damage and deaths due to respiratory and cardiac arrhythmias and failure (1). Chloroform levels of 3,000 to 30,000 ppm (14,670 to 146,700 mg m^-3) were used to induce anaesthesia (2).

Inhalation

Inhalation of chloroform can cause severe acute toxicity, as described in the general toxicity section. Inhalation of concentrated chloroform vapour causes irritation of exposed mucous membranes, including the nose and throat (7). Shortness of breath may also occur (6).

Other effects reported following the use of chloroform as an anaesthetic include:

• hypothermia
• depression of gastrointestinal tract motility
• respiratory acidosis
• hyperglycaemia
• ketoacidosis
• constriction of the spleen
• an increase in leukocyte count

Ingestion

Ingestion of chloroform can cause severe acute toxicity, as described in the general toxicity section. Local effects following ingestion of chloroform include gastrointestinal irritation with abdominal pain, nausea, vomiting and diarrhoea (6).

There are considerable inter-individual differences in susceptibility to chloroform following acute ingestion. Serious illness has been reported following ingestion of 7.5g chloroform. Fatal doses have been reported to be as low as 14.8g, whereas other individuals have survived a dose of 270g chloroform (1, 2). The mean lethal dose for adults is estimated to be approximately 45g (1).

Dermal or ocular exposure

Chloroform may be absorbed across the skin and prolonged exposure may result in systemic toxicity, as described in the inhalation section. Skin exposure causes irritation and redness at the site of contact, especially sensitive areas such as the eyelids and neck (6). Prolonged contact may result in burns and dermatitis (7).

Liquid chloroform splashed in the eye causes immediate burning pain, tearing and reddening of the conjunctiva. The corneal epithelium is usually injured but regeneration is prompt and as a rule the eye returns to normal within 1 to 3 days (9).

Delayed effects following acute exposure

Individuals who survive an acute exposure to chloroform may develop hepatic dysfunction several days later. Symptoms include prostration, nausea, vomiting, jaundice, coma and in some cases death (1). Necrosis of the liver may occur, resulting in increased concentrations of serum bilirubin and transaminases (6).

Renal damage is less common than hepatic damage, but it may occur following acute exposure to chloroform. Renal tubular necrosis and renal dysfunction (anuria, proteinuria, and uraemia) have been reported in individuals who were exposed to chloroform as an anaesthetic (4).

Animal data and in-vitro data

The acute toxic effects of chloroform in animals are similar to those observed in humans. The main target organs are the liver, kidneys, and the central nervous system.

Inhalation

Exposure to chloroform at concentrations in the range of 10 to 100 g m^-3 resulted in anaesthesia in mice, rabbits, guinea pigs and cats. Cardiac effects including decreased diastolic pressure, reduction of stroke volume and decreased cardiac output were reported in rabbits exposed to 224 mg m^-3 chloroform for one minute. Liver toxicity was observed in mice and rats exposed to 490 and 1,410 mg m^-3 chloroform, respectively. Kidney effects including necrosis of the proximal and distal tubules were reported in male mice exposed to 3,400 to 5,400 mg m^-3 chloroform for 1 to 3 hours (1).

Ingestion

Oral administration of chloroform can result in anaesthesia. The (effective dose 50) ED50 in mice for acute neurological effects (ataxia, incoordination and anaesthesia) was 484 mg kg^-1 bw. Hepatic effects (centrilobular fatty infiltration) have been observed in mice at doses as low as 35 mg kg^-1 bw and chloroform administered at 250 mg kg^-1 bw by gavage has caused hepatic necrosis in mice (1, 4). In rats, piloerection, sedation, flaccid muscle tone, ataxia, prostration, reduced food intake and kidney and liver effects following exposure to ≥ 546 mg kg^-1 bw [1].

The acute oral (lethal dose 50) LD50 for chloroform in mice ranges from 36 to 1,366 mg kg^-1 bw and for rats it ranges from 450 to 2,000 mg kg^-1 bw.

Dermal or ocular exposure

Moderate necrosis, hyperaemia and eschar formation were observed when chloroform (1,000 or 4,000 mg kg^-1 bw) was applied under a patch to the skin of rabbits for 24 hours. Systemic effects including weight loss and degenerative changes in the tubules of the kidneys were also reported (1, 7).

Eye contact with chloroform liquid has resulted in corneal injury and conjunctivitis in rabbits (1, 7).

Health effects following chronic or repeated exposure

Human data

Hepatic and central nervous system toxicity are the main effects following long term exposure to chloroform.

Inhalation

In an occupational study, workers exposed to 14 to 400 ppm (68 to 1,956 mg m^-3) developed toxic hepatitis and other effects, including jaundice, nausea and vomiting without fever (2, 7).

In another study, 68 workers were occupationally exposed to chloroform concentrations of 10 to 1,000 mg m^-3 for 1 to 4 years. A higher frequency of hepatitis was found in the group of workers compared with city inhabitants. Seventeen of the workers had hepatomegaly, 3 of which developed hepatitis (1).

Chronic occupational exposure to 375 to 1,330 mg m^-3 chloroform, with a peak concentration of 5,680 mg m^-3, for 8 exposed workers over periods of 3 to 10 years was reported to cause:

• lassitude
• thirst
• gastrointestinal distress
• frequent and scalding urination
• lack of concentration
• depression
• irritability

Nine workers who were exposed to lower concentrations of chloroform (110 to 350 mg m^-3 for 10 to 24 months) also experienced the same effects, but to a lesser degree (1).

A case study of an individual who abused chloroform for approximately 12 years reported delusions, hallucinations, psychotic episodes, and convulsions. Withdrawal symptoms, including dysarthria and ataxia have been reported following the abrupt discontinuation of chloroform use (2).

Ingestion

There is limited data available regarding the health effects of chronic exposure to chloroform in humans.

Hepatitis and nephrosis were observed in a man who ingested cough mixture containing 1.6 to 2.6 g of chloroform daily for 10 years (6). No renal or hepatic effects were observed in humans who used ingested 0.34 to 0.96 mg kg^-1 bw day^-1 chloroform in mouthwash for up to 5 years (2).

Genotoxicity

There are currently no data available regarding the genotoxic effects of chloroform in humans.

Carcinogenicity

Chlorinated drinking water typically contains chloroform, other trihalomethanes and a wide variety of other disinfection by-products. There has been considerable epidemiological research into the question of associations between chlorinated drinking-water and various diseases, and this research continues. The results have raised concern that chlorination byproducts in drinking-water may increase the risk of certain cancers. However, no conclusions can be drawn specifically about chloroform from these studies.

The expert advisory Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment (COC) considered that the evidence was inconclusive but advised that efforts to minimise exposure to chlorination by-products remain appropriate, providing that they do not compromise the efficiency of disinfection of (10).

The IARC has concluded that there is inadequate evidence in humans for the carcinogenicity of chloroform but sufficient evidence in experimental animals for the carcinogenicity of chloroform. It is classified as possibly carcinogenic to humans (Group 2B) (11).

Reproductive and developmental toxicity

There are no data available regarding the reproductive and developmental effects of chloroform per se.

There has been considerable epidemiological research into the question of associations between chlorinated drinking-water and various diseases, and this research continues. The results have raised concern that chlorination by-products in drinking-water may increase the risk of certain adverse reproductive outcomes. However, no conclusions can be drawn specifically about chloroform from these studies.

The expert advisory Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) considered that the evidence was inconclusive but advised that efforts to minimise exposure to chlorination by-products remain appropriate, providing that they do not compromise the efficiency of disinfection of drinking (12).

Animal data

Inhalation

The liver, kidneys and nasal cavity are primary targets for chloroform-induced toxicity following chronic exposure (1, 2, 4). In a medium-term study, BDF1 mice were exposed to chloroform 0, 59, 123, 245, 490, or 980 mg m^-3 6 hours day^-1, 5 days week^-1, for 13 weeks. All the females survived but reduced growth and deaths occurred in males in all chloroform groups. At exposure levels of 59 mg m^-3 and above, male mice showed kidney (necrosis of the proximal tubules) and nasal cavity effects (bone thickening and degeneration of the olfactory epithelium). Nasal cavity toxicity was observed in female mice at all exposure levels. Liver toxicity was not observed in either sex at up to 245 mg m^-3 chloroform, abnormal cells were seen in females at 490 mg m^-3 and swelling and necrosis at 980 mg m^-3 (4).

Ingestion

Several studies have reported nephrotoxic and hepatotoxic effects in laboratory animals chronically exposed to chloroform (1, 2, 4). In one study it was reported that chloroform administered by gavage in corn oil was significantly more hepatotoxic compared with chloroform administered in aqueous emulsion. Male and female B6C3F1 mice were administered 0, 130 and 270 mg kg^-1 chloroform for 90 days. Liver body weight ratios were higher in all dose groups when chloroform was administered in corn oil. Disruption of hepatic architecture including cirrhosis was observed in the group administered the high dose of chloroform in corn oil. No pathological changes were observed in any of the animals administered chloroform in aqueous emulsion (1).

Hepatic damage was observed in beagle dogs administered 15 mg kg^-1 chloroform in toothpaste over a period of 7.5 years (1).

Genotoxicity

The genotoxic potential of chloroform has been tested in several in-vitro and in-vivo assays. The majority of results for the Ames Salmonella assay and Escherichia coli test system were negative, both with and without metabolic activation. Tests for unscheduled DNA synthesis have produced negative results in human and animal cells. Chloroform did not induce chromosome aberrations in human lymphocytes in culture. Sister chromatid exchange assays using human lymphocytes gave mixed results. Chloroform also induced sister chromatid exchange in mouse bone marrow cells in vivo and chromosome aberrations in rat bone marrow in vivo (4). Three out of four bone marrow micronuclei in-vivo studies in mice produced negative results, the fourth study gave a weekly positive result (1, 4). Overall, the evidence suggests that chloroform does not have significant genotoxic potential.

Carcinogenicity

The carcinogenicity of chloroform has been investigated in rodents. Chloroform produced renal adenomas and carcinomas in male mice exposed via inhalation or ingestion and in male rats following oral exposure. An increase in incidence of hepatocellular adenomas and carcinomas was observed in mice of either sex, exposed to chloroform by ingestion. Thyroid tumours were noted in female rats orally administered chloroform. The IARC has concluded that there is sufficient evidence in experimental animals for the carcinogenicity of chloroform (11, 13).

There is experimental evidence to suggest that kidney and liver tumours in rodents are secondary to the cytotoxic effects of metabolites (such as phosgene and hydrogen chloride) and persistent associated reparative cell proliferation (1, 4). At the high dose levels used in the carcinogenicity bioassay in which tumours were produced, chloroform induced cytotoxicity and regenerative cell proliferation in the target organs for cancer consistent with the mode of action for tumourgenesis in the liver and kidney involving cytotoxicity (11).

Reproductive and developmental toxicity

Developmental studies involving oral exposure in rats have reported effects on the fetus (reduced fetal body weight) but only at dose levels that were maternally toxic. No teratogenic effects were observed (4).

Few data were found regarding the effects of chloroform on reproduction.

In a continuous breeding reproductive toxicity study in CD-1 mice no effects were reported on fertility or reproduction in the F1 generation. The mice had been exposed in utero and during lactation (as a result of maternal treatment) and then by gavage at 41 mg kg^-1 bw day^-1, through to young adulthood. The dose level did not produce any effect on fertility but did induce hepatotoxicity in the parent animals (4).

A three-generation reproductive toxicity study has been reported using ICR mice. Chloroform was administered at 0.1, 1 or 5 mg ml^-1 in drinking water. The only statistically significant effects noted were reduced fertility, litter size, gestation index and viability index, at the highest dose level. Some evidence of hepatotoxicity was seen at all dose levels and mortality was seen at the highest dose (1).

References 

1.International Programme on Chemical Safety (IPCS) ‘Chloroform’ Environmental Health Criteria 163: 1994

2. Agency for Toxic Substances and Disease Registry (ATSDR) ‘Toxicological Profile for Chloroform’ 1997

3. US Environmental Protection Agency (US EPA) ‘Toxicological Review of Chloroform’ 2001

4. International Programme on Chemical Safety (IPCS) ‘Chloroform’ Concise International Chemical Assessment: Document 58: 2004

5. World Health Organisation (WHO) ‘Guidelines for Drinking-Water Quality’ Third edition: volume 1: Recommendations: 2004

6. IPCS ‘Chloroform’ Poisons Information Monograph (PIM) 121

7. Bingham B, Cohrssen B, and Powell C (eds) ‘Patty’s Toxicology’ fifth edition: volume 5: 2001

8. National Poisons Information Service (NPIS) ‘Chloroform’ TOXBASE® 2004

9. Grant WM and, and S Schuman J ‘Toxicology of the Eye’ fourth edition: volume 1: 1993

10. COC ‘Chlorinated drinking water and cancer’ COC/99/S2 1999

11. IARC ‘Some Chemicals that Cause Tumours of the Kidney or Urinary Bladder in Rodents and Some Other Substances’ volume 73 in IARC Monographs on the Evaluation of Carcinogenic Risks to Humans 1999

12. COT) ‘COT Statement on chlorinated drinking water and reproductive outcomes’ COT/04/8 2004

13. IARC ‘Some Halogenated Hydrocarbons’ volume 20 in IARC Monographs on the Evaluation of Carcinogenic Risks to Humans 1979

Further information

The information contained in this document from the UKHSA Radiation, Chemicals and Environment Directorate is correct at the time of its publication.

Email chemcompendium@ukhsa.gov.uk if you have any questions about this guidance or enquiries@ukhsa.gov.uk if you have any other questions.