Effects on laboratory mammals and in vitro test systems. Systemic toxicity. The acute and short-term toxicity of methanol varies greatly between different species, toxicity being highest in species with a relatively poor ability to metabolize formate. In such cases of poor metabolism of formate, fatal methanol poisoning occurs as a result of metabolic acidosis and neuronal toxicity, whereas, in animals that readily metabolize formate, consequences of CNS depression (coma, respiratory failure, etc.) are usually the cause of death. Sensitive primate species (humans and monkeys) develop increased blood formate concentrations following methanol exposure, while resistant rodents, rabbits and dogs do not. Humans and non-human primates are uniquely sensitive to the toxic effects of methanol. Overall methanol has a low acute toxicity to non-primate animals. The LD50 values and minimal lethal doses after oral exposure range from 7000 to 13 000 mg/kg in the rat, mouse, rabbit and dog and from 2000 to 7000 mg/kg for the monkey. Rats exposed to levels of methanol up to 6500 mg/m3 (5000 ppm) for 6 h/day, 5 days/week for 4 weeks, exhibited no exposure-related effects except for increased discharges around the nose and eyes. These were considered reflective of upper respiratory irritation. Rats exposed to methanol vapour levels up to 13 000 mg/m3 (10 000 ppm) for 6 h/day, 5 days/week for 6 weeks, failed to demonstrate pulmonary toxicity. In the rabbit, methanol is a moderately irritant to the eye. It was not skin-sensitizing in a modified maximization test. Toxic effects found in methanol-exposed primates include metabolic acidosis and ocular toxicity, effects that are not normally found in folate-sufficient rodents. The differences in toxicity are due to differences in the rate of metabolism of the methanol metabolise formate. For instance, the clearance of formate from the blood of exposed primates is at least 50% slower than for rodents. Monkeys receiving methanol doses higher than 3000 mg/kg by gavage demonstrated ataxia, weakness and lethargy within a few hours of exposure. These signs tended to disappear within 24 h and were followed by transient coma in some of the animals. In monkeys exposed to methanol for 6 h/day for 5 days a week, 20 repeated exposures to 6500 mg/m3 (5000 ppm) methanol failed to elicit ocular effects. Genotoxicity and carcinogenicity. Methanol has given negative results for gene mutation in bacteria and yeast assays, but it did induce chromosomal malsegregation in Aspergillus. It did not induce sister chromatic exchanges in Chinese hamster cells in vitro but caused significant increases in mutation frequencies in L5178Y mouse lymphoma cells. Methanol inhalation did not induce chromosomal damage in mice. There is some evidence that oral or intraperitoneal administration increased the incidence of chromosomal damage in mice. There is no evidence from animal studies to suggest that methanol is a carcinogen, although the lack of an appropriate animal model is recognized. Reproductive toxicity, embryotoxicity and teratogenicity. Conflicting results have been reported on the effects of inhalation of methanol for up to six weeks on gonadotropin and testosterone concentrations. The inhalation of methanol by pregnant rodents throughout the period of embryogenesis induces a wide range of concentrationdependent teratogenic and embryolethal effects. Treatment-related malformations, predominantly extra or rudimentary cervical ribs and urinary or cardiovascular defects, were found in fetuses of rats exposed 7 h/day for 7-15 days of gestation to 26 000 mg/m3 (20 000 ppm) methanol. Slight maternal toxicity was found at this exposure level, and no adverse effects to the mother or offspring were found in animals exposed to 6500 mg/m3 (5000 ppm), which was interpreted as the no-observed-adverse-effect level (NOAEL) for this test system. Increased incidences of exencephaly and cleft palate were found in the offspring of CD-1 mice exposed 7 h/day, on days 6-15 of gestation, to methanol levels of 6500 mg/m3 (5000 ppm) or more. There was increased embryo/fetal death at 9825 mg/m3 (7500 ppm) or more and an increasing incidence of full-litter resorptions. Reduced fetal weight was observed at 13 000 and 19 500 mg/m3 (10 000 or 15 000 ppm). The NOAEL for developmental toxicity was 1300 mg/m3 (1000 ppm) methanol. There was no evidence of maternal toxicity at methanol exposure levels below 9000 mg/m3 (7000 ppm). When litters of pregnant CD-1 mice were given 4 g methanol/kg by gavage, the incidences of adverse effects on resorption, external defects including cleft palate, and fetal weight were similar to those found in the 13 000 mg/m3 (10 000 ppm) inhalation exposure group, presumably due to the greater rate of respiration of the mouse. The mouse is more sensitive than the rat to developmental toxicity resulting from inhaled methanol. Transient neurological signs and reduced body weights were found in CD-1 dams exposed to 19 500 mg/m3 (15 000 ppm) for 6 h/day throughout organogenesis (gestational days 6-15). Fetal malformations found at 13 000 and 19 500 mg/m3 (10 000 and 15 000 ppm) included neural and ocular defects, cleft palate, hydronephrosis and limb anomalies. Effects on humans. Humans (and non-human primates) are uniquely sensitive to methanol poisoning and the toxic effects in these species is characterized by formic acidaemia, metabolic acidosis, ocular toxicity, nervous system depression, blindness, coma and death. Nearly all of the available information on methanol toxicity in humans relates to the consequences of acute rather than chronic exposures. A vast majority of poisonings involving methanol have occurred from drinking adulterated beverages and from methanol-containing products. Although ingestion dominates as the most frequent route of poisoning, inhalation of high concentrations of methanol vapour and percutaneous absorption of methanolic liquids are as effective as the oral route in producing acute toxic effects. The most noted health consequence of longer-term exposure to lower levels of methanol is a broad range of ocular effects. The toxic properties of methanol are based on factors that govern both the conversion of methanol to formic acid and the subsequent metabolism of formate to carbon dioxide in the folate pathway. The toxicity is manifest if formate generation continues at a rate that exceeds its rate of metabolism. The lethal dose of methanol for humans is not known for certain. The minimum lethal dose of methanol in the absence of medical treatment is between 0.3 and 1 g/kg. The minimum dose causing permanent visual defects is unknown. The severity of the metabolic acidosis is variable and may not correlate well with the amount of methanol ingested. The wide interindividual variability of the toxic dose is a prominent feature in acute methanol poisoning. Two important determinants of human susceptibility to methanol toxicity appear to be (1) concurrent ingestion of ethanol, which slows the entrance of methanol into the metabolic pathway, and (2) hepatic folate status, which governs the rate of formate detoxification. The symptoms and signs of methanol poisoning, which may not appear until after an asymptomatic period of about 12 to 24 h, include visual disturbances, nausea, abdominal and muscle pain, dizziness, weakness and disturbances of consciousness ranging from coma to clonic seizures. Visual disturbances generally develop between 12 and 48 h after methanol ingestion and range from mild photophobia and misty or blurred vision to markedly reduced visual acuity and complete blindness. In extreme cases death results. The principal clinical feature is severe metabolic acidosis of the anion-gap type. The acidosis is largely attributed to the formic acid produced when methanol is metabolized. The normal blood concentration of methanol from endogenous sources is less than 0.5 mg/litre (0.02 mmol/litre), but dietary sources may increase blood methanol levels. Generally, CNS effects appear above blood methanol levels of 200 mg/litre (6 mmol/litre); ocular symptoms appear above 500 mg/litre (16 mmol/litre), and fatalities have occurred in untreated patients with initial methanol levels in the range of 1500-2000 mg/litre (47-62 mmol/litre). Acute inhalation of methanol vapour concentrations below 260 mg/m3 or ingestion of up to 20 mg methanol/kg by healthy or moderately folate-deficient humans should not result in formate accumulation above endogenous levels. Visual disturbances of several types (blurring, constriction of the visible field, changes in colour perception, and temporary or permanent blindness) have been reported in workers who experienced methanol air levels of about 1500 mg/m3 (1200 ppm) or more. A widely used occupational exposure limit for methanol is 260 mg/m3 (200 ppm), which is designed to protect workers from any of the effects of methanol-induced formic acid metabolic acidosis and ocular and nervous system toxicity. No other adverse effects of methanol have been reported in humans except minor skin and eye irritation at exposures well above 260 mg/m3 (200 ppm). Effects on organisms in the environment. LC50 values in aquatic organisms range from 1300 to 15 900 mg/litre for invertebrates (48-h and 96-h exposures), and 13 000 to 29 000 mg/litre for fish (96-h exposure). Methanol is of low toxicity to aquatic organisms, and effects due to environmental exposure to methanol are unlikely to be observed, except in the case of a spill.