[h=3]Ethylene glycol[/h]

Ethylene glycol
Ethylene glycol solutions became available in 1926 and were marketed as "permanent antifreeze" since the higher boiling points provided advantages for summertime use as well as during cold weather. They are used today for a variety of applications, including
automobiles, but gradually being replaced by
propylene glycol due to its lower toxicity.
Propylene glycol
Propylene glycol, on the other hand, is considerably less toxic than ethylene glycol and may be labeled as "non-toxic antifreeze". It is used as antifreeze where ethylene glycol would be inappropriate, such as in food-processing systems or in water pipes in homes where incidental ingestion may be possible. As confirmation of its relative non-toxicity, the FDA allows propylene glycol to be added to a large number of processed foods, including ice cream, frozen custard, and baked goods.
Propylene glycol oxidizes when exposed to air and heat, forming lactic acid.[8][9] If not properly inhibited, this fluid can be very corrosive,[citation needed] so pH buffering agents are often added to propylene glycol, to prevent acidic corrosion of metal components.
Besides cooling system corrosion, biological fouling also occurs. Once bacterial slime starts to grow, the corrosion rate of the system increases. Maintenance of systems using glycol solution includes regular monitoring of freeze protection, pH, specific gravity, inhibitor level, color, and biological contamination. Propylene glycol should be replaced when it turns a reddish color.