Polyethylene glycol (PEG) is sometimes present in antifreeze formulations, but it is not the main active ingredient, which often causes confusion. Antifreeze is a functional fluid designed to prevent the coolant mixture in an engine from freezing in cold conditions and boiling over when the engine is hot. The similar-sounding names of Polyethylene Glycol, Ethylene Glycol (EG), and Propylene Glycol (PG) lead many people to assume they share the same purpose and toxicity profile. The distinction between these compounds is important because they have vastly different safety implications, particularly regarding ingestion.
Ethylene and Propylene Glycol Bases
The base of nearly all modern antifreeze is composed of one of two primary chemicals: ethylene glycol (EG) or propylene glycol (PG). Both compounds are alcohols that function by disrupting the formation of water crystals, which depresses the freezing point of the mixture. They also raise the boiling point of the coolant, enhancing the engine’s ability to operate at higher temperatures.
Ethylene glycol is the traditional foundation for automotive antifreeze due to its superior efficiency in lowering the freezing point and transferring heat. A 50/50 mixture of EG and water provides better freeze protection than a similar PG mixture. However, EG is highly toxic if ingested, which led to the development of alternative formulations.
Propylene glycol was introduced as a less hazardous option to address safety concerns associated with EG, especially the risk of accidental ingestion. While PG is less toxic, it is also slightly less effective at depressing the freezing point and tends to have a higher viscosity at low temperatures. PG-based coolants are often marketed as “safer” alternatives, making them a common choice where accidental exposure is a risk.
Polyethylene Glycol: An Additive, Not the Base
Polyethylene glycol (PEG) is chemically distinct from the simple monomer glycols that form the base of antifreeze. Unlike EG and PG, which are small molecules known as diols, PEG is a polymer—a long chain made up of repeating ethylene glycol units. This polymerization results in a structurally different compound with unique properties that make it a poor primary freezing point depressant.
When PEG appears on an ingredient list, it is included in a minor capacity as a functional additive, not the main antifreeze agent. PEG functions as a surfactant, a lubricant, or a carrier for other additives like corrosion inhibitors. Its presence helps keep the internal engine components clean and protected from wear.
The molecular size of PEG fundamentally changes its role in the solution. EG and PG work by dissolving uniformly and lowering the chemical activity of the water. However, PEG’s larger, chain-like structure is better suited for surface-active roles, making it suitable for secondary functions rather than primary thermal regulation.
Safety Profile and Toxicity Differences
The significant difference in the safety profile among the three glycols is the main reason the confusion over their names is important. Ethylene glycol is highly toxic because of how the body metabolizes it. Enzymes in the liver convert EG into toxic byproducts, including glycolic acid and, ultimately, oxalic acid.
The accumulation of glycolic acid causes severe metabolic acidosis. Oxalic acid binds with calcium to form sharp calcium oxalate crystals. These crystals deposit in the kidneys, leading to severe and potentially fatal acute kidney failure and permanent organ damage. The orally lethal dose of pure ethylene glycol for an adult is estimated to be around 1.4 milliliters per kilogram of body weight, making ingestion a medical emergency.
Propylene glycol, in contrast, is designated as “Generally Recognized as Safe” (GRAS) by the U.S. Food and Drug Administration for use in food, cosmetics, and pharmaceuticals. If ingested in large quantities, PG is primarily metabolized into lactic acid, which the body can process without causing the catastrophic organ damage seen with EG. The overall toxicity is remarkably low compared to ethylene glycol.
Polyethylene glycol is considered minimally toxic and is widely used in medicine, such as in prescription and over-the-counter laxatives and as a pharmaceutical carrier. Due to its large molecular size, PEG is poorly absorbed by the gastrointestinal tract and is largely excreted unchanged. This means it does not break down into the dangerous metabolites that make ethylene glycol so hazardous. Its inclusion as a minor additive in antifreeze does not contribute significantly to the overall toxicity.