Hazardous Chemical Information System (HCIS)

Methylene chloride

Adopted Exposure Standard - TWA:               50 ppm, 174 mg/m³

                                                                        Carcinogen Category 3

                                                                        Skin absorption

Skin absorption notice: Absorption through the skin may be a significant source of exposure.

Carcinogen category notice: Category 3. Substances suspected of having carcinogenic potential are those substances which have possible carcinogenic effects on humans but in respect of which the available information is not adequate for making a satisfactory assessment. There is some evidence from appropriate animal or epidemiological studies, but this is insufficient to place the substance in Category 2.

NOHSC revised the exposure standard for methylene chloride after May 1995.  

No standard should be applied without reference to Guidance on the interpretation of Workplace exposure standards for airborne contaminants.

Documentation notice: National Occupational Health and Safety Commission documentation available for these values.

 

1.          SUBSTANCE IDENTIFICATION

 

CAS No:                                                      75-09-02

Synonym(s):                                               Dichloromethane

                                                                   methane chloride

                                                                   methylene bichloride

Formula:                                                    CH₂Cl₂

Molecular weight:                                    84.93

 

 

2.          CHEMICAL AND PHYSICAL PROPERTIES

 

Methylene chloride is a volatile, colourless and non-flammable liquid with a sweetish ether-like odour.   Commercial grades usually have a purity of 99 per cent and normally contain stabilisers such as phenol, hydroquinone, alcohols and amines⁽¹⁾.

 

The vapour may form explosive mixtures with an atmosphere having a high oxygen content⁽²⁾.   Methylene chloride can be decomposed by contact with hot surfaces and open flame, emitting highly toxic fumes of phosgene and chlorine.   Methylene chloride reacts violently with active metals and strong bases⁽²⁾.

 

Its chemical and physical properties include⁽¹⁾:

 

Freezing point:                                       -96.7^oC

Boiling point:                                          39.8^oC

Specific gravity:                                     1.320 at 20^oC

Vapour pressure:                                    46.5 kPa at 20^oC⁽³⁾

Vapour density:                                      2.93

Solubility:                                              Soluble in alcohol and ether,

                                                          slightly soluble in water

Odour threshold:                                    230 ppm⁽⁴⁾

Partition coefficient log P_ow :                                  1.25⁽⁵⁾

 

 

3.          OCCURRENCE, PRODUCTION AND USE

 

Methylene chloride has considerable solvent capacity, stability and volatility. Because of these properties it is used in a variety of industrial or process applications, for example, a component of paint and varnish strippers, and of adhesive formulations; vapour pressure depressant in aerosols; extractant in the food and pharmaceutical industries; extractant of fats and paraffins; vapour degreasing agent for metal cleaning; process solvent in cellulose ester production, fibre and film foaming, and polycarbonate production; as a refrigerant or heat transfer medium; and blowing agent in foam manufacture and in the manufacture of plastics⁽¹⁾.   Methylene chloride is used in Australia as a blowing agent in foam production as an alternative to CFCs.

 

 

4.          MEASUREMENT TECHNIQUES AND ANALYTICAL METHODS

 

Several methods have been described for the analysis of methylene chloride in air.   NIOSH Method 1005⁽⁶⁾ is suitable for sampling and analysing airborne levels of methylene chloride.   The technique involves sampling onto coconut shell charcoal, extraction with carbon disulfide and analysis using gas chromatography coupled to a flame ionisation detector.   The estimated limit of detection is 0.01 mg per sample and the working range is nine to 3000 ppm for a one-litre air sample.

 

 

5.          TOXICOLOGY

 

5.1       Toxicokinetics

 

Methylene chloride vapour is readily absorbed via the lungs and gastrointestinal tract, with its uptake being directly proportional to the magnitude of exposure.   Dermal absorption also occurs.

 

 

5.1.1    Metabolism

 

Two enzyme systems for methylene chloride metabolism have been identified, that is: the microsomal cytochrome P-450 dependent mixed function oxidase system which produces carbon monoxide (CO), carbon dioxide (CO₂) and chloride ions as end products; and the cytosolic glutathione-S-transferase system which metabolises methylene chloride to formaldehyde and formic acid and CO₂⁽⁷⁾.

 

The cytochrome P-450 system has a high affinity but low capacity for methylene chloride and is saturated at exposure concentrations of 500 ppm in all species studied⁽⁸⁾, whereas the glutathione-S-transferase system has a low affinity, but a high capacity for methylene chloride⁽⁸⁾.   In vitro and in vivo studies have shown that at low exposure levels, methylene chloride is metabolised predominantly by the cytochrome P-450 pathway.   Increasing the dose above the saturation level of 500 ppm does not lead to extra metabolism by this pathway.   The glutathione pathway is known to be highly active in mice at high exposure levels⁽⁸⁾.   However, metabolism by this route appears to be relatively minor in rats in vivo and in rat, hamster or human tissue fractions in vitro at any dose⁽⁸⁾.

 

In blood, carbon monoxide combines with the haemoglobin to produce carboxyhaemoglobin (COHb).   The formation of COHb is concentration-dependent and saturable.

 

`Stewart et al., exposed volunteers to atmospheric concentrations of methylene chloride as high as 500 ppm for 7.5 hours and found a direct correlation between peak blood COHb saturations and the magnitude of the exposure up to 250 ppm but not at 500 ppm.   These findings indicate that an eight-hour exposure to atmospheric concentrations of methylene chloride exceeding 250 ppm may lead to saturation of the enzymes responsible for metabolism of methylene chloride to carbon monoxide.’ [cited in Ref. 9.]

 

Pankow et al.⁽¹⁰⁾, found that rats pretreated with aromatic hydrocarbons, such as benzene, xylenes and toluene before exposure to methylene chloride had enhanced COHb formation, whereas simultaneous exposure to methylene chloride and the aromatic hydrocarbons inhibited the formation of COHb.   The authors suggested that the stimulation or inhibition of COHb formation by the aromatic hydrocarbons is due to the induction of cytochrome P-450 isoenzyme P-450 IIE1 or competition between methylene chloride and the aromatic solvents for this isoenzyme.   These findings are consistent with an earlier study by the same authors where isoniazid, ethanol or acetone and concurrent exposure to methylene chloride enhanced the formation of COHb.

 

 

5.1.2    Animals

 

5.1.2.1   Absorption

 

Tsuruta et al.⁽¹¹⁾ studied the percutaneous absorption of eight chlorinated solvents, including methylene chloride, through mouse abdominal skin.   Of the solvents tested those which had the highest solubility in water showed the greatest percutaneous absorption rate.   Methylene chloride was absorbed more readily than other solvents tested.   The percutaneous absorption rate determined for methylene chloride in this study is 1291 +/- 72.1 nmole/min/cm² of skin⁽¹¹⁾.

 

In rats, methylene chloride readily passes through the placenta⁽¹²⁾. Results show exposure of pregnant rats to methylene chloride is accompanied by detectable amounts of both methylene chloride and CO in the foetus.

 

 

5.1.2.2   Distribution and elimination

 

McKenna et al.⁽¹³⁾ investigated the pharmacokinetics and metabolism of inhaled methylene chloride in Sprague-Dawley rats following exposure to 50, 500 or 1500 ppm ¹⁴ C-methylene chloride for six hours.   The plasma levels of methylene chloride reached a steady-state after two hours of inhalation exposure (50 ppm: 0.05 m g/ml; 500 ppm: 2.38 m g/ml; and 1500 ppm: 8.94 m g/ml).   Following cessation of exposure plasma methylene chloride levels decreased rapidly in an exponential manner.   Blood COHb levels reached a steady state after one hour of exposure (50 ppm: three per cent COHb; 500 and 1500 ppm: 10-13 per cent COHb).   When exposure was terminated, blood COHb levels remained elevated until approximately 90 minutes post exposure, whereafter they decreased in a log-linear manner.   The body burdens in rats following exposure to 50, 500 and 1500 ppm are 5.53, 48.41, and 109.41 mg equivalent ¹⁴C-methylene chloride per kg⁽¹³⁾.

 

Di Vincenzo et al.⁽¹⁴⁾ investigated the fate and distribution of radiolabelled methylene chloride.   Following intraperitoneal administration of [¹⁴C] methylene chloride, the primary route of elimination of radioactivity was in the breath.   More than 75 per cent of the dose was exhaled within the first two hours.   At 24 hours approximately 90 per cent of the dose was eliminated unchanged in the breath.   The metabolites consisted of: CO (2%); CO₂ (3%); and an uncharacterised metabolite (1.5%).   Organs found to contain the highest radioactivity after 24 hours were the liver, kidneys and adrenal glands.   Low levels of radioactivity were detected in the brain.   There was little accumulation of radioactivity in fatty tissues.   Similar findings were observed by Mc Kenna et al.⁽¹³⁾

 

5.1.3    Humans

 

5.1.3.1   Absorption

 

Stewart et al.⁽¹⁵⁾ studied the absorption of methylene chloride through human skin.   Volunteers immersed their thumbs in liquid methylene chloride for 30 minutes.   Methylene chloride penetrated the skin several times more rapidly than other solvents tested.   The subjects had a mean peak breath concentration of 3.1 ppm methylene chloride after the exposure period⁽¹⁵⁾.

 

Engstrom et al.⁽¹⁶⁾ found that in volunteers exposed for one hour to 750 ppm of methylene chloride, the absolute uptake was proportional to the body weight and the amount of body fat present.

 

Male and female volunteers were exposed for 7.5 hours to atmospheric concentrations of 50 to 200 ppm methylene chloride⁽⁹⁾.   During the first hour of exposure pulmonary uptake of methylene chloride was rapid and ranged from 69 to 75 per cent of the inhaled vapour.   After 7.5 hours, blood and breath concentrations of methylene chloride were found to be directly proportional to the magnitude of exposure.   The blood methylene chloride concentration increased gradually and by the end of exposure appeared to reach a plateau.

 

The combination of exercise and exposure to methylene chloride can result in increased pulmonary absorption of methylene chloride compared to sedentary individuals^(17,18).

 

Methylene chloride has been found in the breast milk of exposed lactating women [cited in Ref 19].

 

 

5.1.3.2   Distribution

 

In subjects exposed by inhalation for one hour to 750 ppm of methylene chloride, the mean concentration of methylene chloride in subcutaneous fat was 10.2, 8.4 and 1.7 mg/kg, one, four and 22 hours following exposure, respectively.   Obese volunteers had a greater calculated amount of methylene chloride in their fat tissues when compared to slim volunteers⁽¹⁶⁾.

 

In volunteers exposed for 7.5 hours to methylene chloride at concentrations ranging from 50 to 200 ppm, blood COHb levels increased with increasing concentrations of methylene chloride⁽⁹⁾.   Exposure to methylene chloride at 50 and 200 ppm for 7.5 hours resulted in blood COHb levels of 1.9 per cent and 6.8 per cent, respectively.   However, blood COHb levels did not reach a plateau after 7.5 hours.   Similarly, breath and blood concentrations of CO increased in direct proportion to the exposure concentration but did not reach a plateau by the end of exposure⁽⁹⁾.

 

Volunteers exposed to methylene chloride vapour at concentrations ranging from 100 to 200 ppm for 7.5 hours/day for five consecutive days, showed no evidence of systemic accumulation of methylene chloride⁽⁹⁾.

 

From these studies Di Vincenzo et al.⁽⁹⁾ concluded that in sedentary non-smokers an eight hour exposure to 100 ppm of methylene chloride vapour will produce a blood COHb saturation of about three per cent, which is less than the increase in blood COHb saturation’s produced by an exposure to CO alone at 35 ppm for eight hours⁽⁹⁾ ! .

 

The combination of physical exercise and exposure to methylene chloride resulted in increases in the conversion of methylene chloride to CO and blood COHb values⁽¹⁷⁾.

 

 

5.1.3.3   Elimination

 

In sedentary individuals inhaling ambient air, the half-life of COHb in blood is five to six hours.   In comparison, workers exposed to 180 - 200 ppm of methylene chloride for eight hours, the half-life of COHb is 13 hours⁽²¹⁾. This prolonged excretion of CO results from the storage of methylene chloride in body tissues, especially fatty tissue⁽¹⁶⁾, with conversion to CO continuing well after exposure has ceased.   In effect, the storage of methylene chloride in body tissues, produces a continuous exposure to CO from endogenous sources⁽²¹⁾.

 

Following inhalation exposure to 100 ppm for two hours at rest, the half-life of methylene chloride in three body compartments was as follows: blood: five-10 minutes; aqueous tissues: 50-60 minutes; and fatty tissue: approximately 400 minutes⁽²²⁾.

 

In the study by Di Vincenzo et al.⁽⁹⁾ on sedentary individuals, post-exposure elimination of methylene chloride in exhaled breath was less than five per cent indicating that the breath is a minor route of elimination at these concentrations.   The concentration of CO in expired air was directly proportional to the magnitude of the exposure both during and after the exposure.   The post exposure elimination of CO in breath and COHb from blood was more gradual than that of methylene chloride and reached pre-exposure levels after 24 hours.

 

The net pulmonary excretion of CO increased considerably with exercise⁽¹⁷⁾, as did the conversion of methylene chloride to CO.   Breath elimination times for methylene chloride were prolonged.

 

 

5.1.3.4   Smoking studies

 

Di Vincenzo et al.⁽¹⁷⁾ also studied the effect of smoking on methylene chloride metabolism in humans.   Volunteers were exposed for 7.5 hours to 100 ppm of methylene chloride vapour.   Cigarettes were smoked before, during and after exposure.   Compared to non smokers, higher concentrations of methylene chloride were measured in the breath of smokers during exposure.   Also, blood concentrations of methylene chloride for smokers were elevated compared to those for non-smokers both during and after exposure.   The combined effect of smoking and exposure to methylene chloride vapour produced an additive increase in blood COHb values⁽¹⁷⁾.

 

 

5.2       Health Effects in Humans

 

The acute effects associated with exposure to methylene chloride are central nervous system (CNS) depression and elevated COHb levels.   The oxygen carrying capacity of the blood is reduced as COHb levels increase.   Blood COHb levels in excess of 50 per cent can lead to coma and death # .

 

In volunteers, direct skin contact with liquid methylene chloride for 30 minutes caused, within 10 minutes, an intense burning sensation, numbness and pain.   After 30 minutes exposure, paraesthesia, mild erythema and a slight degree of white scale was reported⁽²³⁾.   Numbness and skin irritation from direct skin exposure to methylene chloride vapour have been reported⁽²⁴⁾.   The irritant effects of methylene chloride can be accentuated if the chemical is confined by clothing, gloves or if contact is prolonged⁽³⁾.

 

Hughes et al.⁽²⁵⁾ reported a fatal poisoning following ingestion of approximately 300 mls methylene chloride as ‘nitromors’.   The victim was found deeply unconscious and died 25 days later.   Autopsy revealed acute tubular necrosis, extensive upper oesophageal, gastric and duodenal ulceration with perforation and peri-pancreatic abscess formation with multiple areas of fat necrosis.

 

Two fatalities have been reported from occupational exposure to paint strippers containing 65-85 per cent methylene chloride and approximately 10 per cent methanol⁽²⁶⁾.   One victim was found dead and slumped forward in an immersion tank.   An autopsy revealed first and second degree burns, an enlarged liver with mild fatty change and congestion of the viscera.   The blood concentration of methanol and methylene chloride were 2.4 and 0.4 g/litre, respectively.   Another victim at a different workplace was found unconscious and also slumped forward in an immersion tank.   The victim had first and second degree burns and died seven days later.   Autopsy revealed necrotising aspiration pneumonitis and brain swelling.   The author states that ‘the pyschoactive effects of the solvent vapours are believed to have contributed directly to the deaths’.   Symptoms reported by workers at these two workplaces following exposure to solvent vapours during furniture stripping operations include irritation, dizziness, nausea and hallucinations to loss of consciousness⁽²⁶⁾.

 

In another case report⁽²⁷⁾, two men were found dead in a well where drums of solvent and mixed waste were being buried.   Up to 168,000 ppm methylene chloride was measured in the area.   Autopsy of both victims revealed similar findings to the above reports — extensive brain and lung oedema and congestion, microhaemorrhagic changes of the stomach and congestion in other organs.   Blood collected at autopsy contained COHb levels of 30 per cent⁽²⁷⁾.

 

A further 37 cases of poisoning or death following inhalation or dermal exposure to methylene chloride have also been reported.   In most instances the incidents occurred in confined spaces, with similar health effect findings as above^{(28, 29)}.

 

The above case reports of high level exposure to methylene chloride demonstrate the narcotic effect of methylene chloride and the dangers of its use in confined spaces without adequate ventilation.

 

Severe pulmonary injury and death have been reported due to phosgene poisoning caused by the use of methylene chloride near a heat source⁽³⁰⁾.

 

Cherry et al.⁽³¹⁾ investigated a group of 29 workers to determine if there was evidence of neuropsychological damage following exposure over several years to a mixture of methylene chloride and methanol (9:1), at atmospheric concentrations of 75 to 100 ppm.   Following clinical examination, ECG measurements, measurements of motor conduction velocities or neurobehavioural tests, Cherry et al.⁽³¹⁾ found no evidence of methylene chloride-related long-term damage.

 

Acute behavioural effects were investigated in 56 workers exposed to a mixture of methylene chloride and methanol (9:1), at atmospheric concentrations ranging from 28 to 173 ppm.   Statistically significant changes in sleepiness, physical tiredness and mental tiredness were observed in morning shift workers compared with controls⁽³²⁾.

 

Gamberale et al.⁽³³⁾ investigated psychological functions in subjects exposed for two hours to 250, 500, 750 or 1000 ppm.   No statistically significant impairment in reaction time, short-term memory and numerical ability was detected.

 

Volunteers exposed to methylene chloride vapour ranging from 500 to 1000 ppm for either one or two hours experienced light headedness, central nervous system depression and visual-evoked response alterations⁽¹⁵⁾.   During the 24-hour period following exposure haematological and clinical chemistry, blood parameters were normal while blood COHb levels remained elevated⁽¹⁵⁾.

 

In subjects exposed for four hours to 300, 500 or 800 ppm methylene chloride, a significant effect on a ‘Critical Flicker Fusion’ test was noted at 300 ppm.   At 500 and 800 ppm significantly lower results were noted in an alertness test.   The author of this study maintains ‘that the decreases in alertness are probably due to an interaction between hypoxia, induced by carbon monoxide and the narcotic effects of methylene chloride’ [cited in Ref. 19].

 

A cohort study of retired airline mechanics carried out to determine whether long-term exposure to low levels of methylene chloride (mean TWA range: 82-236 ppm) produced lasting effects on the CNS, Lash et al.⁽³⁴⁾ found no statistically significant differences in a battery of physiological and psychological tests.   However, there were subtle differences in attention and memory when compared with controls⁽³⁴⁾.

 

In a Finnish case-control study⁽³⁵⁾ of women working in the pharmaceutical industry, the frequency of spontaneous abortion was higher among those exposed to solvents including methylene chloride.   Women exposed at least once a week to methylene chloride during the first trimester had a higher odds ratio (2.8) of spontaneous abortion compared with women exposed less than once a week (odds ratio 2.0).   No information was given on exposure levels.   However, the authors have indicated that prior to improved work practices, the concentrations of methylene chloride were occasionally high, as it was common to allow the solvent to evaporate into the air of the working environment.

 

No allergic effects for methylene chloride have been described in the data reviewed.

 

 

5.2.1    Epidemiology

 

A series of mortality studies of workers chronically exposed to methylene chloride have been conducted at the Eastman-Kodak company, Rochester, New York.

 

In the first study of the series reported in 1978, the objective was to determine whether methylene-chloride exposed workers exhibit an increased risk for specific causes of mortality that could be attributed to the work environment.   The cause of death of 334 male workers with continuous low level exposures (30 to 125 ppm) for 30 years was investigated.   The number of deaths due to malignant neoplasms was not significantly different from employed referents (71 observed versus 73.4 expected), and ischaemic heart disease deaths were only 95 per cent of the expected number⁽³⁶⁾.   In the same study, a group of 252 male workers who had a minimum of 20 years work exposure were also investigated.   In this group, no increased risk for malignancy, circulatory disease or total mortality was identified when compared with the New York State population or employed referents⁽³⁶⁾.

 

A follow-up study⁽³⁷⁾ of a cohort of workers (n=1013) exposed during 1964 to 1976 and observed to 1984, in the same company as above, showed that compared with both the New York State general population and employed referents, there were no statistically significant increases for specific hypothesised causes such as lung and liver cancer and ischaemic heart diseases.   However, there was an excess number of deaths due to pancreatic cancer (eight versus 3.1 expected)⁽³⁷⁾.   The mean exposure was 26 ppm TWA for 23 years.   A further mortality update on this cohort to include additional data from 1985 to 1988 showed lower than expected death rates for cancer and ischaemic heart disease when compared with the New York State general population and employed referents.   No pancreatic cancer deaths occurred since the 1984 follow up⁽³⁸⁾.

 

Ott et al.⁽³⁹⁾ and Lanes et al.^(40,41), have reported the mortality of a cohort of workers exposed to methylene chloride at a cellulose fibre production plant in South Carolina in the United States, from 1954 to 1990.   The main focus of the study by Ott et al.⁽³⁹⁾ was mortality from cardiovascular disease, in particular ischaemic heart disease.   Mortality was studied in a cohort of 1271 workers (male and female).   Each person in the cohort was employed for at least three months between 1 January 1954 and 1 January 1977.   Methylene chloride was the main solvent used in the areas under study and concentrations ranged from below detectable limits to 1700 ppm.   Methanol and acetone were also used in these areas but their concentrations were reported to be lower than that of methylene chloride.   No significant increase in overall mortality or deaths due to ischaemic heart disease or malignant neoplasms was observed compared with the general population.   In 1990, Lanes et al.⁽⁴⁰⁾ published an update for this cohort which included data up to September 1986.   Significant excess mortality was found for cancer of the biliary passage and a non-significant excess for cancer of the buccal cavity, pharynx and for melanoma.   No excess of cancer of the pancreas nor of ischaemic heart disease was observed.   In 1993, Lanes et al.⁽⁴¹⁾ reported a further mortality update of the same cohort which included data up to December 1990⁽⁴¹⁾.   No new deaths were observed from cancer of the liver or biliary tract.   No excess mortality was observed for cancer of the pancreas or for ischaemic heart disease⁽⁴¹⁾.

 

 

5.3       Toxicity in Animals

 

5.3.1    Acute

 

5.3.1.1   Oral

 

The oral LD₅₀ in rats ranges from 2388 to 4368 mg/kg⁽⁴²⁾.   The oral LD₅₀ in mice is 1987 mg/kg⁽⁴²⁾.

 

 

5.3.1.2   Inhalation

 

The six hour LC₅₀ values in rodents are⁽⁴²⁾: rats, 16,100 to 18,150 ppm; mice 14,100 ppm; and guinea pigs, 11,500 ppm.

 

 

5.3.1.3   Eye

 

Following instillation into rabbit eyes, methylene chloride was severely irritating and caused transitory thickening of the cornea and increased intraocular tension⁽⁴²⁾.

 

 

5.3.1.4   Skin

Methylene chloride on occlusive application to rabbit skin caused severe erythema and oedema with necrosis and acanthosis⁽⁴²⁾.

 

 

5.3.1.5   Sensitisation

 

There are no reports of sensitisation occurring.

 

 

5.3.2    Short term repeated dose

 

The primary effects of exposure to high concentrations of methylene chloride are hepatotoxicity and neurotoxicity.

 

Female ICR mice exposed continuously by inhalation to 5000 ppm of methylene chloride for seven days, showed histological fatty changes in the liver and swelling of the rough endoplasmic reticulum.   Increased levels of liver triglycerides and decreased activity, food and water intake and change in appearance were also noted.   Liver fatty change was found to be partially reversible⁽⁴³⁾.

 

In a study by Heppel et al.⁽⁴⁴⁾, dogs, rats, rabbits and guinea pigs were exposed to a methylene chloride concentration of 17 mg/L (approx. 5000 ppm) seven hours/day, five days/week for 15 weeks.   The exposure had no effect on dogs, rats or rabbits, but in guinea pigs there was an adverse effect on growth rate and centrilobular degeneration of the liver.   In the same study, monkeys, dogs, rats, rabbits and guinea pigs were exposed to 10,000 ppm for seven hours/day, five days/week for eight weeks.   Fatty degeneration of the liver occurred in dogs and guinea pigs.   Several of the rats and rabbits died from acute pulmonary congestion and oedema during the experiment.

 

In a further study by Heppel et al.⁽⁴⁵⁾ exposure to 5,000 ppm by inhalation for one hour caused a great diminution in the running activity of rats on a running wheel.   ‘Similar results were described in mice after a three hour exposure to 1000 ppm and after a 24 hour exposure to 5000 ppm’ [cited in Ref. 42].

 

Rosengren et al.⁽⁴⁶⁾ found changes in levels of certain brain proteins in the gerbil brain following continuous exposure to 210 or 350 ppm methylene chloride for seven weeks.   There was a high mortality rate in this study.   Neurotoxic effects were not found to correlate with the endogenous formation of carbon monoxide⁽⁴⁶⁾.

 

Mattson et al.⁽⁴⁷⁾ conducted a neurotoxicologic evaluation of F344 rats exposed to 0, 50, 200 or 2000 ppm methylene chloride for six hours/day, five days/week for 13 weeks.   Mattson et al., concluded that there was `no persistent discernible effects on either function or structure of the nervous system’⁽⁴⁷⁾.

 

 

5.3.3    Teratogenicity and embryotoxicity

 

Nitschke et al.⁽⁴⁸⁾ evaluated reproductive parameters in F344 rats following exposure by inhalation to methylene chloride for two successive generations.   Male and female rats were exposed to 0, 100, 500 or 1500 ppm methylene chloride for six hours/day, five days/week for 14 weeks.   No significant changes were observed in the gross pathology or histopathology of tissues in adults or weanlings of each generation.

 

In a study by Hardin et al.⁽⁴⁹⁾ female Long-Evans hooded rats were exposed by inhalation to methylene chloride to determine whether exposure before and during gestation is more detrimental to reproductive outcome than exposure either before or during gestation alone.   Rats were exposed to 4500 ppm methylene chloride or filtered air six hours/day, seven days/week, before and during, or only during days one to 17 of gestation.   In animals exposed to methylene chloride only during gestation, a low degree of foetotoxicity and maternal toxicity was shown by reduced foetal bodyweight and increased maternal liver weight, respectively.   Exposure prior to and during gestation did not increase the incidence of maternal toxicity or embryotoxicity.   This study was considered to be more realistic of workplace exposure as exposure to methylene chloride occurred before and during gestation.   Both exposure patterns resulted in the same low degree of maternal and embryotoxicity⁽⁴⁹⁾.

 

Using the same experimental conditions as Hardin et al⁽⁴⁹⁾, Bornschien et al.⁽⁵⁰⁾ found that methylene chloride exposure to maternal rats at 4500 ppm resulted in changes in general activity of pups of both sexes from the age of 10 days, and persisting in male pups to 150 days.

 

Sprague Dawley rats and Swiss Webster mice were exposed by inhalation to 1250 ppm of methylene chloride for seven hours/day from days six to 15 of gestation⁽⁵¹⁾.   Elevated COHb concentrations were found in all treated mice and rats.   The effect of methylene chloride on embryonal and foetal development was related to variations in the development of the sternum.   Among the litters of rats, the incidence of delayed ossification of sternebrae was significantly greater than in controls.   Among litters of mice, a significant number of litters contained pups with a single extra center of ossification in the sternum⁽⁵¹⁾.

 

 

5.3.4    Chronic toxicity/carcinogenicity

 

In a two year inhalation study, Sprague-Dawley rats (male and female) and female Golden Syrian hamsters were exposed six hours/day, five days/week to 0, 500, 1500 or 3500 ppm methylene chloride⁽⁵²⁾.   Female rats in the high dose group had reduced survival.   In male and female rats, exposure-related fatty changes in liver were observed.   In male and female rats there was an increase in red cell indices and COHb.   An increase in the total number of benign mammary neoplasms was noted in male and female rats, but more pronounced in females.   There was no indication of an increased number or incidence of malignant mammary tumours in either male or female rats exposed to methylene chloride.   The authors stated that there was a high background incidence of mammary tumours for this strain of rat⁽⁵²⁾.   In male rats of the high dose group there was a statistically significant increased incidence of sarcomas in the salivary gland region.   These tumours appeared to be of mesenchymal cell origin with different cell types.   This was an unexpected finding as this type of tumour response has not been observed in previous studies with methylene chloride.   The authors state that these findings could be due to the combination of a viral infection which affects the salivary gland and exposure to high concentrations of methylene chloride⁽⁵²⁾.

 

In hamsters there was no significant increases in the incidence of tumours.   In female hamsters decreased mortality was observed.   In both male and female hamsters there were increases in haemoglobin, haematocrit, COHb and red cell indices⁽⁵²⁾.

 

In a two generation inhalation reproductive study, Sprague-Dawley rats (male and female) were exposed to methylene chloride concentrations of 0, 50, 200 or 500 ppm, for six hours/day, five days/week for 20 months (male) or two years (female).   Female rats in the 500 ppm dose group had a slightly increased incidence of both multinucleated hepatocytes and benign mammary tumours per tumour-bearing female rat.   No effects on mammary glands were reported in male rats.   No salivary gland tumours were observed in male rats⁽⁵³⁾.

 

Results of the National Toxicity Program (NTP) two year inhalation studies (six hours/day, five days/week) conducted in rodents have been reported by IARC⁽⁵⁴⁾.   Male and female F344 rats were exposed to methylene chloride (technical grade) at concentrations of 0, 1000, 2000, or 4000 ppm for 6 hours/day, 5 days/week.   Increased incidences of benign mammary gland tumours were observed in females.   In male rats there was a positive trend in the incidence of benign tumours in the mammary gland area.   At the two highest dose levels there was an increased incidence of mononuclear cell leukaemia in female rats.   In male and female B6C3F1 mice exposed to 0, 2000, or 4000 ppm of methylene chloride for the same duration, a significantly increased incidence of alveolar/bronchialor adenomas, and hepatocellular adenomas and carcinomas was noted⁽⁵⁴⁾.   Mennear et al.⁽⁵³⁾ report that ‘the NTP interprets these inhalation studies of methylene chloride in male and female F-344 rats and B6C3F1 mice to show evidence of carcinogenicity on the basis of increased incidences of neoplasms of the mammary glands in male and female rats and increased incidences of lung and liver neoplasms in male and female mice.’

 

Methylene chloride was administered at 0, 5, 50, 125 or 250 mg/kg/day for 104 weeks in the drinking water of female and male Fischer 344 rats⁽⁵⁶⁾. Dose related increases in mean haematocrit, haemoglobin and red cell count were observed in both sexes at the three highest doses.   Decreases in alkaline phosphatase, creatinine, blood urea nitrogen, total protein and cholesterol values were noted in both sexes in the high dose group.   Significant decreases in bodyweight gain at the middle and high dose group was observed in both sexes.   Treatment-related hepatic changes were observed including increased incidence of foci/areas of cellular alteration and fatty change in both sexes at all but the lowest dose level.   Female rats had higher incidences of neoplastic nodules at all doses and of hepatocellular carcinomas in the middle and high dose groups.   The authors state that the incidence of hepatocellular carcinomas is significant compared with matched controls, but not to historical controls⁽⁵⁶⁾.

 

Male and female B6C3F1 mice received in their drinking water 0, 60, 125, 185 or 250 mg/kg/day methylene chloride for two years⁽⁵⁷⁾.   Mice in the 250 mg/kg dose group had a slight increase in the incidence of fatty livers.   In male mice, an elevated but not significant incidence of hepatocellular adenomas and carcinomas, alone or combined, was observed compared with historical controls.

 

 

5.4       Genotoxicity

 

5.4.1    In vitro

 

Methylene chloride is mutagenic in Salmonella typhimurium when assayed both with and without metabolic activation⁽⁵⁸⁾. The interpretation of studies in Salmonella are complicated by the fact that methylene chloride is metabolised by Salmonella to CO and CO₂⁽⁵⁹⁾. It is considered likely that the mutagenic activity observed in Salmonella follows the conversion of methylene chloride to unstable active intermediates by bacterial enzymes⁽⁵⁹⁾.

 

Negative results were observed in the point mutation assay at the HGPRT locus in Chinese hamster epithelial cells (V79) and unscheduled DNA synthesis assays in human fibroblasts⁽⁶⁰⁾. However, a weak positive effect in the sister chromatid exchange assay was observed in Chinese hamster epithelial cells (V79)⁽⁶⁰⁾.

 

Chromosome aberrations were observed after exposure to methylene chloride in Chinese hamster ovary cells⁽⁶¹⁾, human lymphocytes and L5178Y mouse lymphoma cells with or without metabolic activation⁽⁶²⁾. However, there was no increase in sister chromatid exchanges in these cells⁽⁶²⁾.

 

 

5.4.2    In vivo

 

No mutagenic activity was revealed in in vivo studies for chromosomal aberrations, micronuclei and unscheduled DNA synthesis^(63,64).

 

DNA from either liver, lung or salivary gland did not bind covalently with methylene chloride or its metabolites⁽⁵⁹⁾.

 

 

6.          CONCLUSION

 

Occupational exposure to methylene chloride is primarily via inhalation and by skin contact.   In humans, methylene chloride is readily absorbed via the lungs and to some extent through the skin. The uptake of methylene chloride through the lungs is directly proportional to exposure. Its absorption increases with increased physical activity and body fat percentage.

 

The metabolism of methylene chloride is via two metabolic pathways: the cytochrome P450-dependent transformation to carbon monoxide and the glutathione-S-transferase-dependent metabolism to formaldehyde and formic acid. The first metabolic pathway results in elevated levels of COHb and increased levels of CO in expired air. This pathway is rate limited by enzyme saturation so that at high doses, the blood level of COHb becomes constant and independent of dose. Blood COHb is elevated for longer periods for methylene chloride exposure than for CO exposure alone. This is due to the continued metabolism of methylene chloride retained in body tissues following exposure.

 

The combined effect of smoking and exposure to methylene chloride will produce an additive increase in blood COHb values.

 

In humans, the major health effects of acute exposure to methylene chloride are central nervous system depression and elevated blood COHb levels. A number of deaths have been reported from high acute exposures to the chemical. Data from controlled human studies indicate that for sedentary, non-smoking individuals exposure to methylene chloride vapour to 100 ppm for eight hours will produce a blood COHb of about three per cent, which is less than the increase in blood COHb levels produced by an exposure to CO alone at 35 ppm for eight hours.

 

From a series of epidemiological studies conducted at two plants, with the mean exposure of 26 ppm at one plant and median exposure from 140 to 475 ppm at the other, there was no increase in lung and liver cancers, ischaemic heart disease or mortality compared with that of the general population. There was an equivocal increase in biliary cancer in one study and pancreatic cancer in the other.

 

In short-term repeated dose studies in animals, exposure to methylene chloride concentrations greater than 1000 ppm indicate that the liver and CNS are the primary target organs. At concentrations greater than 10,000 ppm, deaths from pulmonary congestion occurred. In reproductive studies there was no evidence of teratogenicity.

 

Chronic inhalation studies in rats, hamsters and mice at high dose levels have revealed: in rats, an increase in benign mammary tumours, and in male rats an increased incidence of sarcomas in the salivary gland region; in mice, an increase in the incidence of lung and liver tumours; and in hamsters no significant increase in the incidence of tumours.

 

Methylene chloride is mutagenic in the Ames assay but the interpretation of these results are complicated by the fact that the bacteria used in these assays metabolise methylene chloride. Mutagenic activity was observed in some mammalian cell cultures. No mutagenic activity was observed in vivo.

 

 

7.          OVERSEAS EXPOSURE STANDARDS

 

ACGIH:                                        50 ppm TLV-TWA, Carcinogen A3.

Germany:                                    100 ppm MAK; 500 ppm Peak Limitation

(30 mins, two times per shift). Carcinogen IIIB.

United Kingdom HSE:            100 ppm MEL; 250 ppm STEL (10 minutes).

Sweden                                       35 ppm TWA; 70 ppm STEL. Carcinogen
Skin notation.

 

 

8.          RECOMMENDATION

 

The Exposure Standards Working Group are of the view that, assuming no concurrent exposure to exogenous carbon monoxide occurs, the current time-weighted average exposure standard of 50 ppm for methylene chloride should ensure that workers do not exceed a blood carboxyhaemoglobin level of five per cent.

 

A time weighted average exposure standard of 50 ppm should also protect against possible carcinogenic effects which have been demonstrated in laboratory animals. The Working Group are also of the view that the current Carcinogen Category 3 classification should remain until further evidence becomes available regarding the carcinogenic potential of methylene chloride. The reader is encouraged to review the section on ‘Carcinogenic Effects’ in the National Commission’s Approved Criteria for Classifying Hazardous Substances [NOHSC:1008(1994)] for guidance on the classification of carcinogens.

 

Based on the available information, the Working Group recommends that a skin notation be assigned for this substance. Skin absorption has been reported to occur in both human and animal studies and absorption through the skin may be a significant route of exposure. In addition, significant irritation and burns may occur following skin contact with methylene chloride. The reader is encouraged to review the chapter on ‘Effects on the skin’ in the Guidance Note on the Interpretation of Exposure Standards for Atmospheric Contaminants in the Occupational Environment [NOHSC:3008(1995)] for a more detailed discussion of skin notation.

 

 

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