Antifreeze is a chemical additive, most commonly ethylene glycol or propylene glycol, mixed with water to create a fluid that resists both freezing and boiling. In vehicles, this mixture circulates through the engine to regulate temperature, preventing the cooling system from cracking in winter or overheating in summer. The same basic chemistry shows up in HVAC systems, industrial machinery, and even airplane deicing fluid.
How Antifreeze Works
Pure water freezes at 32°F (0°C) and boils at 212°F (100°C). Neither extreme is safe for an engine. Antifreeze works by disrupting the way water molecules organize themselves. When water freezes, its molecules lock into a rigid hexagonal structure (ice). Dissolving a substance like glycol into the water interferes with that structure, meaning the liquid needs to lose even more energy before it can solidify. The result: a much lower freezing point.
The same principle raises the boiling point. A typical 50/50 mix of antifreeze and water won’t freeze until around -34°F (-37°C) and won’t boil until roughly 265°F (129°C). That wide operating range is what keeps your engine safe in both January and July. The stronger the concentration of glycol, the more dramatic the shift, though going much beyond 70% antifreeze actually reduces cooling efficiency because you lose the heat-absorbing properties of water itself.
Ethylene Glycol vs. Propylene Glycol
Most automotive antifreeze uses ethylene glycol. It transfers heat efficiently, has low viscosity, and depresses the freezing point more effectively per volume than the alternative. The tradeoff is toxicity. In humans, a lethal dose of ethylene glycol is estimated at roughly 1,400 to 1,600 mg per kilogram of body weight. For an average adult, that translates to as little as 150 mL (about half a cup) in some reported fatalities, though the range extends higher. Cats and dogs are even more vulnerable, and ethylene glycol has a slightly sweet taste that can attract pets.
Propylene glycol is the safer option. Its toxicity is low enough that it’s also used in cosmetics, pharmaceuticals, and even some food products. The downside is performance: it’s more viscous and less efficient at transferring heat, so you need a higher concentration to match the same freeze protection. That makes it more expensive per application. Propylene glycol antifreeze is common in systems where a leak could contact drinking water or food, like brewery chillers or residential heating loops.
Both glycols are readily biodegradable under normal environmental conditions, and research published in the journal Geothermics found that neither produces toxic or persistent breakdown products in soil or groundwater. The environmental concern with used antifreeze comes not from the glycol itself but from what it picks up during use: heavy metals like lead, cadmium, and chromium that leach from engine components over time.
Coolant Types and Color Coding
Antifreeze isn’t just glycol and water. It contains corrosion inhibitor additives that protect the various metals inside your engine and radiator. The type of inhibitor package is what separates the three main coolant technologies, and it’s why antifreeze comes in different colors.
- IAT (Inorganic Additive Technology): The classic green coolant. It uses silicates, phosphates, and borates to protect metal surfaces. IAT was standard for decades but depletes relatively quickly, typically needing replacement every two to three years or 30,000 miles. You’ll mainly find it in older vehicles today.
- OAT (Organic Acid Technology): Uses organic acids instead of silicates for longer-lasting protection, often rated for five years or 150,000 miles. GM’s orange Dex-Cool is the most recognizable OAT coolant. Some Volkswagen, Ford, Chrysler, and Fiat models also use OAT formulas.
- HOAT (Hybrid Organic Acid Technology): Combines organic acids with a small amount of silicate or phosphate for broader corrosion coverage. HOAT coolants come in pink, blue, or purple depending on the subtype. Toyota, Honda, Hyundai, and other Asian manufacturers commonly use a phosphate-based HOAT (often pink). European brands like BMW, Mercedes-Benz, and Porsche typically use a silicate-based version that’s blue or purple.
Color alone isn’t a reliable guide because there’s no universal standard. Your owner’s manual specifies the correct type. Mixing incompatible coolant chemistries can cause the additives to react with each other, forming gel or sludge that clogs passages and accelerates corrosion rather than preventing it.
What Antifreeze Protects Against
Beyond freeze and boil protection, the additive package in antifreeze serves a second critical job: preventing corrosion inside the cooling system. Engines contain aluminum, cast iron, copper, brass, and solder, all in contact with hot liquid. Without inhibitors, the coolant itself would slowly eat through these metals, producing rust and scale that clog narrow radiator tubes and reduce heat transfer.
Fresh coolant is slightly alkaline, which helps keep metal surfaces passivated (coated with a thin protective layer). Over time, the inhibitors get used up and the pH drops. Once coolant falls below a pH of 7.0, it has turned acidic and begins actively corroding the system. You can spot aging coolant visually: healthy fluid is brightly colored, while depleted coolant turns brownish or rust-colored.
Testing Your Coolant
A basic antifreeze tester, the kind sold at auto parts stores for a few dollars, is a floating-ball hydrometer. You draw a sample of coolant into the tube, and the number of balls that float tells you roughly how much freeze protection the mixture provides. These aren’t precision instruments, but they reliably tell you whether your protection level is adequate. One important detail: test with warm coolant, since the reading shifts with temperature.
A refractometer gives a more precise measurement. You place a drop of coolant on the lens and read the glycol concentration directly off a scale. Shops that do cooling system services typically use refractometers. For pH testing, inexpensive test strips designed for coolant will tell you whether the additive package is still doing its job or whether the fluid has gone acidic.
Disposal and Safety
Used antifreeze can contain enough heavy metals to qualify as regulated hazardous waste. The EPA prohibits dumping it on the ground, into storm drains, ditches, dry wells, or septic systems. Fines for illegal disposal can reach $25,000. Many sewage treatment agencies also forbid or discourage pouring it into sanitary sewers, since treatment plants aren’t designed to handle glycol-contaminated water.
The correct disposal route is recycling. Many auto parts stores and municipal hazardous waste facilities accept used coolant for free. Commercial recycling processes filter out the metals and contaminants, then restore the additive package so the glycol can be reused. If you’re doing a coolant change at home, collect the old fluid in a clean, sealed container and drop it off at one of these locations. Keep the container out of reach of children and pets in the meantime, especially if the coolant is ethylene glycol-based.