Propylene glycol

Propylene glycol, known also by the systematic name propane-1,2-diol, is an organic compound (a diol alcohol), usually a tasteless, slightly sweet, odorless, and colorless clear viscous liquid that is hygroscopic and miscible with water, acetone, and chloroform.

Chirality
Propylene glycol contains an asymmetrical carbon atom, so it exists in two stereoisomers. The commercial product is a racemic mixture. Pure optical isomers can be obtained by hydration of optically pure propylene oxide.

Production
Industrially propylene glycol is produced by propylene oxide hydration. Different manufacturers use non-catalytic high-temperature process at 200-220 °C or catalytic route which proceeds at 150-180 °C in presence of ion exchange resin or small amounts of sulfuric acid or alkali. Final products contain 20% 1,2-propanediol, 1.5% of dipropylene glycol and small amount of other polypropylene glycol. Propylene glycol can also be converted from glycerol, a biodiesel byproduct.

Applications
Propylene glycol is used:
 * As a moisturizer in medicines, cosmetics, food, toothpaste, mouth wash, and tobacco products
 * As a medical and sexual lubricant (A.K.A. "personal lubricant")
 * As an emulsification agent in angostura and orange bitters
 * As a solvent for food colors and flavorings
 * As a humectant food additive, labeled as E number E1520
 * As a carrier in fragrance oils
 * As a less-toxic antifreeze
 * As a solvent used in mixing photographic chemicals, such as film developers
 * In smoke machines to make artificial smoke for use in firefighters' training and theatrical productions
 * In electronic cigarettes to make the produced vapor better resemble cigarette smoke
 * In hand sanitizers, antibacterial lotions, and saline solutions
 * In cryonics
 * As a working fluid in hydraulic presses
 * To regulate humidity in a cigar humidor
 * As the killing and preserving agent in pitfall traps, usually used to capture ground beetles
 * To treat livestock ketosis
 * As the main ingredient in deodorant sticks.

Propylene glycol has similar properties as ethylene glycol (MEG). The industrial norm is to replace ethylene glycol by propylene glycol.

Safety
Cases of propylene glycol poisoning are related to either inappropriate intravenous use or accidental ingestion by children. The oral toxicity of propylene glycol is very low. In one study, rats were provided with feed containing as much as 5% PG over a period of 104 weeks and they showed no apparent ill effects. Because of its low chronic oral toxicity, propylene glycol is generally recognized as safe (GRAS) for use as a direct food additive.

Serious toxicity will occur only at extremely high intakes over a relatively short period of time that result in plasma concentrations of over 4 g/L. Such levels of ingestion would not be possible when consuming reasonable amounts of a food product or dietary supplements containing at most 1 g/kg propylene glycol.

The U.S. Food and Drug Administration (FDA) has determined propylene glycol to be "generally recognized as safe" for use in food, cosmetics, and medicines. Like ethylene glycol, propylene glycol affects the body's chemistry by increasing the amount of acid. Propylene glycol is metabolized into lactic acid, which occurs naturally as muscles are exercised, while ethylene glycol is metabolized into oxalic acid, which is toxic.

However, propylene glycol is not approved for use in cat food. The U.S. Food and Drug Administration has determined that propylene glycol in or on cat food has not been shown by adequate scientific data to be safe for use. Use of propylene glycol in or on cat food causes the feed to be adulterated and in violation of the Federal Food, Drug, and Cosmetic Act. 21CFR589.1001

Prolonged contact with propylene glycol is essentially non-irritating to the skin. Undiluted propylene glycol is minimally irritating to the eye, and can produce slight transient conjunctivitis (the eye recovers after the exposure is removed). Exposure to mists may cause eye irritation, as well as upper respiratory tract irritation. Inhalation of the propylene glycol vapors appears to present no significant hazard in ordinary applications. However, limited human experience indicates that inhalation of propylene glycol mists could be irritating to some individuals. Therefore inhalation exposure to mists of these materials should be avoided. Some research has suggested that propylene glycol not be used in applications where inhalation exposure or human eye contact with the spray mists of these materials is likely, such as fogs for theatrical productions or antifreeze solutions for emergency eye wash stations.

Propylene glycol does not cause sensitization and it shows no evidence of being a carcinogen or of being genotoxic.

Post menopausal women who require the use of an estrogen cream may notice that brand name creams made with propylene glycol often create extreme, uncomfortable burning along the vulva and perianal area. In these cases, patients can request that a local compounding pharmacy make a "propylene glycol free" cream which is much more tolerable.

Research has suggested that individuals who cannot tolerate propylene glycol probably experience a special form of irritation, but that they only rarely develop allergic contact dermatitis. Other investigators believe that the incidence of allergic contact dermatitis to propylene glycol may be greater than 2% in patients with eczema.

Patients with vulvodynia and interstitial cystitis may be especially sensitive to propylene glycol. Women struggling with yeast infections may also notice that some OTC creams can cause intense burning.