It depends on which glycol you mean. The word “glycol” covers a family of chemical compounds, and their toxicity ranges from deadly to generally safe. Ethylene glycol, the main ingredient in antifreeze, is highly toxic and can be fatal in small amounts. Propylene glycol, used in food and medicine, is recognized as safe by the FDA. Diethylene glycol, an industrial solvent, is dangerously toxic and has caused mass poisoning events around the world. Knowing which glycol you’re dealing with is the most important distinction.
Ethylene Glycol: The Dangerous One
Ethylene glycol is the glycol most people encounter, and it’s the one that causes the most harm. It’s the primary ingredient in automotive antifreeze, typically making up about 95% of the product. You’ll also find it in some brake fluids, windshield-washer fluids, de-icers, paints, detergents, and certain cosmetics.
The estimated minimum lethal dose for a 70-kilogram adult is roughly 100 milliliters, which is less than half a cup. That small volume makes it one of the more dangerous substances commonly stored in garages and homes. In 2015, U.S. poison control centers documented 6,895 ethylene glycol exposures and 22 deaths.
What makes ethylene glycol especially dangerous is that it doesn’t taste bad. It has a sweet flavor, which is part of why accidental ingestion happens, particularly in children and pets. Since 2012, antifreeze manufacturers in the United States have added a bittering agent to discourage drinking it, but this doesn’t make the product safe.
Why Ethylene Glycol Becomes Toxic
Ethylene glycol itself isn’t the real problem. Your liver converts it into a series of breakdown products, and those are what cause the damage. The liver processes ethylene glycol using the same enzyme it uses for alcohol. The first dangerous byproduct is glycolic acid, which floods the bloodstream with acid and throws off the body’s pH balance. Further breakdown produces oxalic acid, which is directly toxic to the kidneys. Oxalic acid forms crystals that physically damage kidney tissue.
This is also why the medical treatment works the way it does. Doctors administer an antidote that blocks the liver enzyme responsible for breaking down ethylene glycol. By stopping the conversion process, the ethylene glycol stays in its less harmful form and can be filtered out of the body before it turns into something destructive. In severe cases, dialysis may be needed to clear the toxins directly from the blood.
Three Stages of Ethylene Glycol Poisoning
Ethylene glycol poisoning typically unfolds in three phases, though not everyone experiences all of them, and the timeline can vary.
The first stage begins within 30 minutes to 12 hours after ingestion. It looks a lot like alcohol intoxication: slurred speech, drowsiness, loss of coordination, disorientation, and restlessness. Because it mimics drunkenness, people sometimes don’t realize a poisoning has occurred.
The second stage, roughly 12 to 24 hours after ingestion, affects the heart and lungs. Blood pressure can swing high or low, heart rhythm disturbances can develop, and fluid may build up in the lungs. Breathing becomes rapid as the body tries to compensate for rising acid levels in the blood. This stage can be fatal.
The third stage hits the kidneys, typically 24 to 72 hours after exposure. Kidney damage shows up as a sharp drop in urine output and flank pain. Without treatment, this can progress to complete kidney failure.
Propylene Glycol: Generally Safe
Propylene glycol is a completely different story. The FDA classifies it as “Generally Recognized as Safe” (GRAS) for use in food, and it appears in a wide range of everyday products. You’ll find it in ice cream, salad dressings, flavoring extracts, soft drinks, medications, and even injectable drugs. It’s also the base liquid in many fog machines and some vaping products.
The FDA sets maximum levels for propylene glycol in different food categories: up to 5% in alcoholic beverages, up to 24% in confections and frostings, up to 2.5% in frozen dairy products, and up to 2% in most other foods. At these concentrations, it poses no meaningful risk to healthy adults. Your body metabolizes propylene glycol efficiently and clears it without producing the dangerous byproducts that ethylene glycol creates.
That said, propylene glycol isn’t completely inert at very high doses or with prolonged exposure, particularly in people with kidney problems who can’t clear it as quickly. In normal dietary and consumer product amounts, though, it’s considered safe.
Diethylene Glycol: A Hidden Danger
Diethylene glycol is an industrial chemical used in solvents and antifreeze. It’s toxic to both the kidneys and liver, and it has been responsible for some of the worst pharmaceutical contamination disasters in modern history.
The most well-known incidents involve diethylene glycol being substituted for pharmaceutical-grade glycerin in liquid medications, often cough syrups or acetaminophen syrups intended for children. An outbreak in Haiti from 1995 to 1996 caused acute kidney failure in 86 children, most of them under age 5. Earlier outbreaks in the United States, Nigeria, Bangladesh, and India followed the same pattern: a contaminated medication, followed by kidney failure and death.
In the Haiti outbreak, victims typically developed a nonspecific fever first, followed within two weeks by complete kidney failure, liver inflammation, pancreatic damage, and neurological decline progressing to coma. Kidney biopsies showed acute damage to the tubules consistent with toxic exposure.
Risks to Pets
Ethylene glycol is one of the most common causes of poisoning in dogs and cats. The sweet taste attracts animals, and the lethal dose is disturbingly small. Cats are especially vulnerable: as little as 1.4 milliliters per kilogram of body weight can be fatal. For a 4-kilogram cat (about 9 pounds), that’s roughly one teaspoon of undiluted antifreeze. Dogs need a somewhat larger dose, around 4.4 to 6.6 milliliters per kilogram, but spills and puddles in a garage can easily provide that amount.
The bittering agents added to antifreeze since 2012 help but don’t guarantee safety. Some animals will still drink it. If you store antifreeze or coolant, keep containers sealed and clean up any spills immediately.
How to Tell Which Glycol You’re Dealing With
Product labels are your best tool. Automotive antifreeze and coolant almost always contain ethylene glycol unless specifically labeled as “pet-safe” or “non-toxic,” in which case it’s usually propylene glycol-based. Food, pharmaceutical, and personal care products that list glycol as an ingredient are nearly always using propylene glycol.
The confusion between these chemicals is more than academic. Ethylene glycol is cheaper than propylene glycol, which is exactly why it gets used in industrial applications and, in contamination cases, why it sometimes ends up where it shouldn’t be. If a product is designed for engines, plumbing, or industrial use, treat it as potentially toxic unless the label confirms otherwise.