Methanol, sometimes called wood alcohol, is the simplest of the class of chemicals chemists call alcohols. Ethanol, the spirit many enjoy in beer, wine, and cocktails, is closely related. Both can be made naturally when yeast ferment the natural chemicals in grains and fruits. And like all chemicals, both can be toxic when you are exposed to too much. But, when you consume methanol, the way your body metabolizes it makes it much more toxic than ethanol. Methanol poisonings were common during the Prohibition Era of the 1920’s and 30’s because methanol was intentionally added to industrially produced ethanol. This was to prevent bootleggers from using it for alcoholic beverages. Ethanol treated to prevent it from being consumed is called “denatured” alcohol (so never drink denatured alcohol!). Because of concerns over possible toxicity, ethanol denatured with methanol is not allowed for use in things we apply to our skin, like cosmetics. Methanol in hand sanitizers used to combat COVID-19 has recently been in the news with the US FDA warning the public and recalling a number of ethanol based hand sanitizers contaminated with methanol.
Why is Methanol Toxic, and How is it Different From Ethanol Toxicity?
The answer to this question lies in the differences in what your body does to these two chemicals, often referred to as metabolism. In the case of ethanol your liver first metabolizes it to something called acetaldehyde. Acetaldehyde is rapidly metabolized to something called acetate, a far less toxic molecule that is readily eliminated from the body. For the average person there is no significant build-up of metabolic products. We hasten to note however that you can certainly poison yourself with ethanol if you drink too much too fast and overwhelm your body’s ability to get rid of it. Moderation in drinking is wise!
Methanol on the other hand is converted first into formaldehyde and then into formic acid. High levels of formic acid cause a range of different effects including something called acidosis, where the acidity of your blood gets too high and a number of organs (like the kidney) begin to malfunction. Formic acid is also a primary cause for damage to the nervous system (what toxicologists call neurotoxicity). Damage to the optic nerve and subsequent permanent blindness is a hallmark for non-lethal methanol toxicity. Methanol is a great example of how your body can actually make a chemical more toxic.
How Much Methanol is Toxic?
The lethal human dose of pure methanol is estimated to be about 2.5 ounces. This is the median lethal dose, meaning about 50% of people that consume this much may die. Consuming about half an ounce of pure methanol could cause blindness. By comparison, the lethal human dose of ethanol is estimated to be about 6 ounces for an average sized person. Since alcoholic drinks are usually 45% ethanol or less, 6 ounces of pure ethanol is equal to about 14 drinks (assuming a drink with a 1 oz shot of a typical liquor). If a typical bottle of liquor was all methanol instead of ethanol it would only take about 1 drink to cause permanent blindness. Please note that these are estimates for comparative purposes.
Methanol is much more toxic than its close cousin ethanol and is a great example of how differences in the way our bodies handle different chemicals has an influence on both the nature and the extent of toxic effects. But, as always, the dose makes the poison and just because something may contain methanol (e.g. many natural foods) does not mean ingesting it, or being exposed to it through air or skin, will cause harm.