Methanol exposure in the general population primarily occurs through oral intake of meat or vegetables. Fruit and vegetable juice (average 140 mg/L, range 12–640 mg/L), fermented beverages (1500 mg/L or more), and beverages such as soft drinks also provide a considerable amount of methanol. In addition, local traditional liquor, Korean wine, whiskey, and vodka contain a considerable amount of methanol [10]. Smoking a cigarette provides methanol during the vapor phase (approximately 180 μg methanol). Approximately 10% of aspartame, an artificial sweetener, is converted (when hydrolyzed) into methanol in the human body [1].

Background monitoring of a Japanese population revealed that the concentration of methanol in the urine is less than 2 mg/L in most cases and less than 5 mg/L at the 95th percentile [14]. The arithmetic mean value of methanol in urine has been reported as 1.9 mg/L, and the geometric mean value has been reported as 1.7 mg/L [14]. The following values for urinary methanol have been reported in Brazil: 2.26 ± 1.26 mg/L (the arithmetic mean and standard deviation), 2.10 mg/L (the geometric mean), with a range from 0.50–4.78 mg/L [15]. In South Korea, in terms of the exposure concentration and biological monitoring of the general population, few reports are available, unfortunately. It is possible that not more than 2 mg/L of methanol is found in urine, in alignment with the Japanese and Brazilian reports.

Linear regression analysis of urinary methanol levels at the end of a work shift revealed a methanol concentration of 5500 mg/L of in ambient air [14]. In another study, there was a correlation between blood and urinary methanol concentrations of 500 mg/L and 1100 mg/L (650 and 1430 mg/m³), respectively, from methanol vapor [16]. Table 1 shows the methanol concentration of workroom air and urinary concentrations. The urinary methanol level in male employees was 21.8 ± 20.0 mg/L on average (19.2 mg/L, median and 0.6–57.3 mg/L, range) in exposed groups at the end of a work shift and 1.1 ± 0.9 mg/L on average (0.6–2.9 mg/L, range) in a non-exposed group. It has been estimated that a median urinary methanol concentration of 19.2 mg/L corresponds to a breathing concentration geometric mean of 93 mg/L in the workers’ atmosphere. In reports of exposure to methanol concentrations of 200 mg/L for 8 h, approximate levels of 40 mg/L [17], 42 mg/L (Kawai et al.), and 26 mg/L (arithmetic mean, Yasugi et al.) have been reported for urinary methanol concentrations correlated with workroom air concentrations [18].

As shown in Table 1, methanol exposure doses from ambient air and the response to urinary methanol concentrations could allow for the following regression equation. The regression equation showed 0.21117 for a and 4.69191 for b for Y = aX + b. Pearson’s correlation coefficient was 0.792. The equation showed a linearity between the concentration of methanol in the atmosphere (X) and the urinary methanol concentration (Y). The equation confirmed a dose-response relationship between the inhalation level (X) and urinary concentration of methanol (Y) [19].

The lethal dose of methanol in human beings has not yet been clearly reported (Table 2). If medically untreated, the lethal dose was approximately 1000 mg/kg (oral intake) if there was no simultaneous ingestion of ethanol. Serious intoxication appeared when the blood formate level was 500 mg/L or more for 10 h of exposure. In an IPCS report, the human lethal dose through oral ingestion is approximately 300–1000 mg/kg [1]. In the OHS report, the lethal dose was reported as 60 ~ 240 ml in a case where a human drank pure methanol. The lowest lethal dose in the report was 10 mg/kg for women, in some cases [20]. In case reports, acute methanol poisoning (60 ~ 600 ml of pure methanol) caused acute metabolic acidosis in 28 young men in Papua New Guinea in 1977. These young men had serious visual impairment and acute pancreatitis 8–36 h after drinking methanol. Four of them died within 72 h after admission, but 16 of the 24 people did not experience symptoms, six were visually impaired, two people demonstrated speech defects and visual impairment [1]. According to the autopsy reports, 6 people died with an 840–5430 mg/L blood methanol concentration and a formic acid concentration of 640 ~ 1100 mg/L [21]. Therefore, using 300 mg/kg, which has been reported as the minimum lethal dose, and assuming the average Korean weight of 65 kg, then 19,500 mg can be calculated as the lethal dose. If a lethal dose of 1000 mg/kg is used, as previously reported, a lethal dose of 65,000 mg can be calculated. Therefore, the lethal dose ranges from 19,500–65,000 mg in humans. In grams (g), the final volume can be estimated as 19.5–65 g per person for a lethal dose. Mass is multiplied by 0.79 for the specific gravity of methanol (calculated as 15.8–189.6 g), and 47.4–474 g/per person is the lethal dose range (based on reports of 20–240 ml from OHS MSDS and 60–600 ml from IPCS), therefore, a small amount of methanol taken orally might be a lethal dose. Therefore, the median lethal dose is estimated as a representative value (56.2 g/per person) based on data using the lethal dose of pure methanol.

In case reports of respiratory exposure through vapor, women died at levels of 4000–13,000 mg/L after 12 h of methanol exposure with limited ventilation in a closed space [22].

Intoxication by exposure level initially manifests as temporary sickness and drowsiness similar to ethanol poisoning. In most cases, methanol intoxication has a latent period of 6 to 30 h. Then, metabolites from methanol can cause vomiting, dizziness, abdominal pain, diarrhea, difficulty breathing, acidosis (Kussmaul breathing), blurred vision, hyperemia of the optic nerve head, blindness, optic disc congestion, manic excitement, manic disorder, delirium, and brain edema [23, 24].

Permanent blindness has been reported to be induced by oral ingestion at levels of 4 ~ 10 ml or 15 ml. However, even after ingesting 500 to 600 ml of methanol, one patient recovered; individual differences appear to be related to whether there is simultaneous exposure to ethanol [25]. With oral exposure, abdominal discomfort and abdominal colic frequently occur, and nausea, vomiting, and diarrhea appear in approximately 50% of cases. Acute pancreatitis, accompanied by amylase viremia and high amylase urine disease, could occur following abdominal pain [23, 26]. Headache and breathing changes appeared at doses of 429 mg/kg dose (from drinking) [20]. Eyes were affected when the subjects drunk 3429 mg/kg of methanol. Changes in circulation, vomiting and dyspnea appeared when subjects ingested 6422 mg/kg of methanol [20, 27, 28]. If these data were converted to 65 kg body weight, the lethal dose range would be 27.8–417 g/person. In these oral intake cases, the exposure was not to pure methanol but rather ethanol. Reportedly, there is not a great difference between the exposure levels that trigger a lethal dose and clinical symptoms (Table 3).

In patients who were treated in the intensive care unit for serious methanol poisoning, one study reported using hemodialysis treatment in cases of >200 mg/L of blood methanol concentrations from deliberate drinking; the study also reported metabolic acidosis and vision abnormalities [29]. Permanent blindness appeared when the formate blood concentration was 3220 mg/L for 20 h or more from the inhibition of cytochrome oxidase (cytochrome oxidase) in the mitochondria and oxidative phosphorylation process [22]. In a case report of a 4-year-old girl who was hospitalized with nausea, vomiting and abdominal pain in the emergency room, her blood methanol concentrations was 790 mg/L [30]. In a 1978 study of Swedish alcoholism, a cleaning liquid containing up to 80% methanol was ingested. Since 1978, the cleaning liquid methanol has reportedly limit the amount of methanol to less than 5%. The misuse of cleaning solution for alcohol intoxication has been frequently reported, and the methanol concentration in the blood ranged from 1000 to 2000 mg/ from 1 to 2 weeks of exposure [31]. Women 26 years of age undergoing pregnancy care at gestational week 38 consumed 250–500 ml of methanol. Five hours after ingestion, they demonstrated light acidosis, 2300 mg/L of serum methanol, and 336 mg/L of formic acid concentration. Frenia and Schauben [32] have reported toluene (43.8%), methanol (22.3%), methylene chloride (20.5%) and propane (12.5%) in the vapor of a liquid carburetor cleaner. Blood methanol levels from inhalation exposure ranged from 504 to 1286 mg/L, and blood formic acid levels of 120, 193, and 480 μg/ml resulted in altered vision according to eye examinations [32].

Amblyopia from vapor inhalation in occupational exposure to methanol (visually impaired) and approximately 100 deaths were reported until 1912 [1, 33]. Most patients had been exposed to methanol vapor (300 mg/L) [34, 35]. Wood heel manufacturing industry workers were exposed to a concentration of methanol vapor ranging from 160 to 780 mg/L. However, the workers did not show any health symptoms [22]. Workers repeatedly exposed to 200–375 mg/L methanol vapor report recurrent headaches [27]. Chronic intoxication occurs when individuals are exposed to a concentration of 800 ~ 3000 mg/L in ambient air through inhalation from occupational exposure [16]. In the case of chronic intoxication through inhalation, it is possible to experience the first signs of visual impairment. For instance, light blurred vision, diminished visual field and permanent blindness could occur. In repeated or long-term respiration exposure, clinical symptoms can occur that are similar to those reported in acute toxicity cases from oral intake. The signs and symptoms from repeated exposure at levels of 365–3080 mg/L were headache, dizziness, vomiting, severe upper abdominal pain, low back pain, breathing difficulties, cloudy view and diarrhea [22]. In a patient who inhaled 3000 mg/L of methanol, bradycardia, visual changes and headache were reported [20]. Workers exposed to methanol concentrations of 1200 ~ 8000 mg/L for 4 years demonstrated diminished visual field and liver expansion [20]. The 23 workers exposed to a poor working environment (power failure prevention work during wartime) at a concentration of approximately 8300 mg/L showed symptoms of temporary blindness (Table 4) [22].

In a recent methanol addiction accident in Korea, close attention should be directed toward the exposure level of methanol and the initial exposure concentration range. Symptoms of optic neuritis were supposed to appear at an initial concentration of 228.5 mg/L; however, a methanol exposure concentration range of 228.5 to 417.7 mg/L is a somewhat narrow range. Blindness was supposed to occur at an initial exposure concentration of 1103 mg/L, ranging from 1103 to 2220 mg/L, according to the Korean Confederation of Trade Unions, 2016. These initial exposure concentrations for optic neuritis and blindness were the lowest exposure concentrations reported in recent studies of methanol exposure [36].

In 2016, the methanol exposure limit for workers in ACGIH were shown to be 200 mg/L for TWA and 250 mg/L for STEL. These exposure limit levels were thought to be high because health symptoms can appear at 160 mg/L, recurrent headaches at 200 mg/L and optic neuritis, unconsciousness and vomiting at 228.5 mg/L for the initial exposure concentration (Table 4) [36].

Toxicity through the skin occurs in animals and humans. When the skin comes in contact with methanol liquid, there is a possibility of skin irritation and skin absorption [1, 22]. Metabolic acidosis can result in adverse effects in the eyes and central nervous system. Chronic and repeated continuous contact with methanol can cause erythema, degreasing of the skin, squama, eczema, and dermatitis. For chronic absorption, metabolic acidosis can demonstrate the same symptoms described for acute ingestion [22]. Repeated prolonged contact can cause conjunctivitis, which accumulates a high concentration of methanol when exposed daily to methanol vapor [1].