Methanol, often referred to as wood alcohol, is a light, volatile, colorless, and flammable liquid. Although it shares a similar appearance and odor to common drinking alcohol (ethanol), methanol is highly toxic to humans. Its acute toxicity is not due to the compound itself, but rather the poisonous byproducts created when the body processes it. This metabolic process generates compounds that can rapidly lead to severe illness, permanent disability, and death, even from small ingestions.
What is Methanol
Methanol is the simplest alcohol. Historically, it gained the name “wood alcohol” because it was first produced through the destructive distillation of wood. Today, the majority of methanol is manufactured industrially by the hydrogenation of carbon monoxide. This process makes it a foundational substance for producing a vast array of other chemicals, including formaldehyde and acetic acid.
Methanol is widely used in industrial and consumer products due to its effectiveness as a solvent and fuel. It serves as a solvent in paints, varnishes, and inks, and is a component in various types of antifreeze, including windshield washer fluid. It is also deliberately added to ethanol to “denature” it, rendering it unfit for human consumption, creating methylated spirits. Methanol is also a clean-burning fuel used in applications such as racing cars and camping stoves.
The Mechanism of Methanol Poisoning
While methanol itself acts as a central nervous system depressant, the life-threatening danger comes from its metabolism. Once ingested, methanol is quickly absorbed and enters the bloodstream. The liver is the primary site for its metabolism, where a two-step process converts it into highly toxic compounds.
The enzyme alcohol dehydrogenase (ADH) first converts methanol into formaldehyde. Formaldehyde is then rapidly converted into formic acid, which exists as the formate ion in the body. This formate ion is the primary toxic agent responsible for severe health complications. Because the human body is poor at metabolizing formate into harmless carbon dioxide and water, the poison dangerously accumulates.
The accumulation of formate leads to profound metabolic acidosis and cellular toxicity. Metabolic acidosis occurs because formic acid is a strong acid that dramatically lowers the blood’s pH. Cellular toxicity results from formate inhibiting cytochrome c oxidase, a protein essential for cellular energy production. This inhibition leads to widespread tissue damage.
The Immediate and Long-Term Dangers
The initial symptoms of methanol poisoning can be misleading because they mimic simple alcohol intoxication, including headache, dizziness, and decreased consciousness. This delay in severe symptoms is known as the latent period, which can last from 12 to 24 hours after exposure, or longer if ethanol was consumed simultaneously.
Once the toxic metabolites accumulate, the patient develops severe metabolic acidosis accompanied by nausea, vomiting, and abdominal pain. The most specific long-term consequence is ocular toxicity, ranging from blurred vision to complete and permanent blindness. Ingestion of as little as 4 to 10 milliliters can cause permanent visual damage because the formate directly damages the optic nerve and the retina.
Severe intoxication can also cause permanent central nervous system damage, often manifesting as lesions in the basal ganglia. Individuals who survive severe poisoning may suffer from long-term neurological deficits, including movement disorders that resemble Parkinsonism. The minimal lethal dose for an adult is highly variable, but estimates range from 30 to 240 milliliters.
Sources of Exposure
Methanol is not typically found in commercially produced alcoholic beverages because it is not generated in toxic amounts during fermentation or distillation. However, poisoning frequently occurs when methanol is illicitly added to alcoholic drinks as a cheap substitute for ethanol. This contamination is a significant concern in the production of counterfeit or unregulated spirits, particularly in the developing world.
Accidental ingestion is a common source of exposure, especially among children or individuals who mistake it for potable alcohol. Products like windshield washer fluid, some types of antifreeze, and industrial solvents often contain high concentrations of methanol. Inhalation of concentrated vapors or extensive skin exposure can also lead to systemic toxicity, as methanol is absorbed through the skin.
Emergency Treatment and Antidotes
Prompt medical intervention is necessary to prevent the formation of toxic formate. Supportive care, including managing the airway and correcting electrolyte imbalances, is the first step. The key to treatment is administering an antidote that blocks the action of the alcohol dehydrogenase enzyme.
The preferred antidote is fomepizole, a medication that competitively inhibits alcohol dehydrogenase. It binds to the enzyme more effectively than methanol, stopping the toxic conversion process. If fomepizole is unavailable, medical-grade ethanol can be used as an alternative antidote. Ethanol also binds to alcohol dehydrogenase with a higher affinity than methanol, effectively slowing the creation of formate.
To combat severe metabolic acidosis, patients are often given sodium bicarbonate infusions. This alkaline therapy helps maintain the toxic formic acid in its less harmful dissociated form. Hemodialysis is frequently required for severe cases, as it is highly effective at removing both methanol and accumulated formate from the blood. Additionally, folic acid is administered to enhance the body’s natural metabolism of formate, helping to clear the poison more quickly.