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A man pouring antifreeze into his vehicle.

Antifreeze is a water-based liquid coolant used in gasoline and diesel engines. Compounds are added to the water to reduce the freezing point of the mixture below the lowest temperature that the engine is likely to be exposed to, and to inhibit corrosion in cooling systems which often contain a range of electrochemically incompatible metals (aluminum, cast iron, copper, lead solder, etc.). The term 'colligative agent' is to be preferred as, in warm climates, the benefit of these compounds is to increase the boiling point of the coolant, which should then be more properly referred to as 'anti-boil', and as anti-freeze decreases and increases both properties, respectively, 'colligative agent' more accurately describes the liquid.

Until the late 1930s, methanol was the most widely used antifreeze. While effective in preventing the coolant from freezing, its low boiling point and low specific heat capacity led to considerably less cooling than water alone. Also, the concentration of methanol would tend to be reduced over time due to its greater tendency to evaporate than the water with which it was mixed.

Ethylene glycol solutions became available in 1937 and were marketed as "permanent antifreeze", since their higher boiling points provided advantages for summertime use as well as during cold weather. They are still used today. Ethylene glycol antifreezes are poisonous and should be kept away from any person or animal (children and especially dogs) that might be tempted by its taste. They form calcium oxalate crystals in the kidneys and can cause acute renal failure and death. All spills should be cleaned, or else an area in which it may be present should be kept inaccessible to those who might ingest it.

Should ingestion of antifreeze occur, ethanol (alcoholic beverages) can be administered until proper treatment can be started in order to slow the conversion of methanol to formaldehyde and formic acid which are the substances responsible for methanol's toxicity.

For this reason bittering agent (denatonium benzoate — trade name Bitrex) is usually added to engine coolant to make it taste unpleasant. In the United States, there is legislation before Congress (H.R.2567/S.1110) that would make the use of a bittering agent mandatory.

Propylene glycol, on the other hand, is considerably less toxic and may be labelled as "non-toxic antifreeze". It is used as antifreeze where ethylene glycol would be inappropriate, such as in food-processing systems or in pipes in homes, as well as numerous other settings. It is also used in food, medicines, and cosmetics, often as a binding agent. Propylene glycol is "generally recognized as safe" by the FDA for use in food. However, it should not be thought that propylene glycol based antifreeze is safe for consumption. In the event of accidental exposure emergency medical services should be contacted.

Most commercial antifreeze formulations include corrosion inhibiting compounds, and a colored dye (commonly a green, red or blue fluorescent) to aid in identification. A 1:1 dilution with water is usually used, resulting in a freezing point of approximately −40°F (−40°C). In warmer areas weaker dilutions are used.

Glycol antifreeze solutions should generally be replaced with fresh mixture every two years. Many modern cars now come filled with organic acid technology (OAT) antifreeze, which has an extended service life of five years. OAT solutions are not compatible with glycol and, if changing from one type to the other, the cooling system must be thoroughly flushed with clean water. Typically OAT antifreeze contains a red or pink dye to differentiate it from glycol (blue or green).

If ingested, the antidote for antifreeze is usually ethanol or fomepizole.

Further reading