Ethylene Glycol

Ethylene glycol (EG) is the base fluid in the vast majority of automotive antifreeze and coolant products. At 50/50 dilution with water, EG solutions provide freeze protection to approximately −34°F and boil-over protection to approximately +265°F. The compound works by lowering the freezing point of water through colligative depression and raising its boiling point.

The primary health concern with EG is acute oral toxicity. EG has a sweet taste — a characteristic shared with diethylene glycol — that makes ingestion by pets and young children a documented real-world hazard. The compound is metabolized to glycolic acid and then oxalic acid in the liver; oxalic acid precipitates calcium oxalate crystals in the kidneys, causing acute kidney failure. The GHS classification is H302 (Cat 4 acute oral toxicity, harmful if swallowed) for most formulations; some industrial-grade products carry H300 (fatal if swallowed, Cat 1/2). Retail consumer coolant products are classified H302.

Ethylene glycol is listed on the California Proposition 65 list. Spent coolant containing EG must be collected and recycled at auto parts store collection programs (AutoZone, O'Reilly, Advance Auto Parts). Draining to the ground, storm sewer, or sanitary sewer is prohibited in most jurisdictions.