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.
Industrially propylene glycol is produced by propylene oxide . Different manufacturers use either non-catalytic high-temperature process at 200 C (392 F) to 220 C (428 F), or a catalytic method, which proceeds at 150 C (302 F) to 180 C (356 F) in the presence of ion exchange resin or a small amount of sulfuric acid or alkali.
Final products contain 20% 1,2-propanediol, 1.5% of dipropylene glycol and small amounts of other polypropylene glycols. Propylene glycol can also be converted from glycerol, a biodiesel byproduct.
Propylene glycol is used:
As a solvent in many pharmaceuticals, including oral, injectable and topical formulations. Notably, diazepam, which is insoluble in water, uses propylene glycol as its solvent in its clinical, injectable form.
As a humectant food additive, labeled as E number E1520
As an emulsification agent in Angostura and orange bitters
As a moisturizer in medicines, cosmetics, food, toothpaste, mouth wash, and tobacco products
As a carrier in fragrance oils
As an ingredient in massage oils
In hand sanitizers, antibacterial lotions, and saline solutions
In smoke machines to make artificial smoke for use in firefighters’ training and theatrical productions
In electronic cigarettes, it is used to deliver vaporized nicotine
As a solvent for food colors and flavorings
As an ingredient, along with wax and gelatin, in the production of paintballs
As a moisture stabilizer (humectant) for snus (Swedish style snuff).
As a cooling agent for beer and wine glycol jacketed fermentation tanks
As a non-toxic antifreeze for winterizing drinking water systems, and in applications where the used antifreeze eventuallyl be drained into the soil, water, or a septic system.
As a less-toxic antifreeze in solar water heating systems
As a solvent used in mixing photographic chemicals, such as film developers
As a working fluid in hydraulic presses
As a coolant in liquid cooling systems
To regulate humidity in a cigar humidor
As the killing and preserving agent in pitfall traps, usually used to capture ground beetles
As an additive to pipe tobacco to prevent dehydration.
To treat livestock ketosis
As the main ingredient in deodorant sticks.
To de-ice aircraft.
As an ingredient in UV or blacklight tattoo ink
Propylene glycol has properties similar to those of ethylene glycol (monoethylene glycol, or MEG). (Note: Infrequently propylene glycol may also use the acronym MEG, but as an abbreviation of methyl ethyl glycol- the industry standard acronym for propylene glycol is PG or MPG (monopropylene glycol). The industrial norm is to replace ethylene glycol with propylene glycol when safer properties are desired.
Propylene glycol is a component in newer automotive antifreezes and de-icers used at airports. Like ethylene glycol, the freezing point of propylene glycol is depressed when mixed with water due to disruption of hydrogen bonding. Unlike ethylene glycol, propylene glycol is much lower in toxicity. Both are readily biodegradable.
Freezing Points of Propylene Glycol – Water Mixtures
Percent Propylene Glycol (wt. %)
Freezing Point (F)
Freezing Point (C)
Propylene glycol is metabolized in the human body into pyruvic acid, which is a normal part of the glucose metabolism process and is readily converted to energy.
The oral toxicity of propylene glycol is very low, and large quantities are required to cause perceptible health damage in humans. Serious toxicity will occur only at plasma concentrations over 4g/L, which requires extremely high intake over a relatively short period of time. It would be nearly impossible to reach toxic levels by consuming foods or supplements, which contain at most 1g/kg of PG. Cases of propylene glycol poisoning are related to either inappropriate intravenous use or accidental ingestion of large quantities by children.
The potential for long-term toxicity is also 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 was classified by the U. S. Food and Drug Administration as “generally recognized as safe” (GRAS) for use as a direct food additive.
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.
There is limited evidence that intravenous injection of propylene glycol can cause adverse responses in a small number of people. A Clinical Journal of Medicine article describes two cases of adult men experiencing psychosis from use of injected phenytoin that contained PG as a solvent. Their symptoms resolved when they were switched to a phenytoin formulation without propylene glycol.
Veterinary data indicates that propylene glycol is toxic to 50% of dogs at doses of 9mL/kg, although the figure is higher for most laboratory animals (LD50 at levels of 20mL/kg).
However, propylene glycol may be toxic to cats in ways not seen in other animals. The U.S. Food and Drug Administration has determined that its presence in or on cat food has not been shown by adequate scientific data to be safe for use. Any such use is considered an adulteration of the cat food and a violation of the Federal Food, Drug, and Cosmetic Act.
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. 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.
Additionally, some electronic cigarette users who inhale propylene glycol vapor may experience dryness of the throat. As an alternative, some suppliers will put Vegetable Glycerin in the “e-liquid” for those who are allergic (or have bad reactions) to propylene glycol.
Possible air germicide
Studies conducted in 1942 by Dr. Oswald Hope Robertson of University of Chicago’s Billings Hospital showed vaporized propylene glycol inhalation in laboratory mice may prevent pneumonia, influenza, and other respiratory diseases. Additional studies in monkeys and other animals were undertaken to determine longterm effects, especially the potential for accumulation in the lungs. After a few months of treatment, no ill effects were discovered.
Propylene glycol is known to exert high levels of biological oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen aquatic organisms need to survive. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose ethylene glycol.
The oxygen depletion potential of airport deicing operation discharges is many times greater than that of raw sewage. For example, before application, Type I propylene glycol-based deicing fluid is generally diluted to a mixture containing approximately 50% propylene glycol. Pure propylene glycol has a five-day biological oxygen demand (BOD5) concentration of approximately 1,000,000 mg/L. A typical diluted propylene-based deicing fluid could therefore have a BOD5 concentration of approximately 500,000 mg/L. In comparison, raw sewage typically has a BOD5 concentration of approximately 200 mg/L. The amount of fluid used to deice a single jet depends on the nature of the precipitation event and the size of the aircraft but can range from a couple hundred to several thousand gallons. Therefore, deicing a single jet can generate a BOD5 load greater than that of one million gallons of raw sewage. A large hub airport often has several hundred flights each day.
Sufficient DO levels in surface waters are critical for the survival of fish, macroinvertebrates, and other aquatic organisms. If oxygen concentrations drop below a minimum level, organisms emigrate, if able and possible, to areas with higher oxygen levels or eventually die. This effect can drastically reduce the amount of useable aquatic habitat. Reductions in DO levels can reduce or eliminate bottom-feeder populations, create conditions that favor a change in a community species profile, or alter critical food-web interactions.
^ Merck Index, 11th Edition, 7868.
^ “1,2-Propanediol”. ChemIndustry.ru. http://chemindustry.ru/1,2-Propanediol.php. Retrieved 2007-12-28.
^ Chauvel, Alain; Lefebvre, Gilles. Petrochemical Processes (Volume 1: Synthesis-Gas Derivatives and Major Hydrocarbons). Editions Technip. pp. 26. ISBN 978-2-7108-0562-5.
^ 1,2-propanediol: chemical product info at CHEMINDUSTRY.RU
^ “Janusz Szajewski, MD , Warsaw Poison Control Centre (August, 1991). “Propylene glycol (PIM 443)”. IPCS INChem. http://www.inchem.org/documents/pims/chemical/pim443.htm. Retrieved July 2, 2009.
^ “Antifreeze & Coolant, Gallon”. West Marine. http://www.westmarine.com/1/1/16186-antifreeze-coolant-gallon-from-seafit-chemicals.html.
^ Flanagan RJ;Braithwaite RA;Brown SS;Widdop B;de Wolff FA;. The International Programme on Chemical Safety: Basic Analytical Toxicology. WHO, 1995.
^ National Library of Medicine;.Propylene glycol is used in antifreezes Human Toxicity Excerpts: CAS Registry Number: 57-55-6 (1,2-Propylene Glycol). Selected toxicity information from HSDB. 2005.
^ Gaunt, IF, Carpanini, FMB, Grasso, P and Lansdown, ABG, Long-term toxicity of propylene glycol in rats, Food and Cosmetics Toxicology, Apr. 1972, 10(2), pages 151 – 162.
^ A Guide to Glycols (http://www.dow.com/PublishedLiterature/dh_0047/0901b803800479d9.pdf#page=36), page 36.
^ 1,2-Dihydroxypropane SIDS Initial Assessment Profile (http://www.chem.unep.ch/irptc/sids/OECDSIDS/57-55-6.pdf), UNEP Publications, SIAM 11, U.S.A, January 23-26, 2001, page 21.
^ Title 21, U.S. Code of Federal Regulations. 1999.
^ Peterson, Michael; Talcott, Patricia A. (2006). Small animal toxicology. St. Louis: Saunders Elsevier. pp. 997. ISBN 0-7216-0639-3.
^ See 21 CFR 589.1001
^ American Medical Association, Council on Drugs (1994). AMA Drug Evaluations Annual 1994 (Chicago, Illinois: American Medical Association): 1224.
^ Elizabeth Vliet MD, Screaming To Be Heard: Hormonal Connections That Women Suspect and Doctors Ignore”. M. Evans and Company, Inc. New York 1995
^ “Air Germicide”. TIME. Time, Inc.. 1942-11-16. http://www.time.com/time/magazine/article/0,9171,932876,00.html. Retrieved 2009-05-23.
^ Environmental Impact and Benefit Assessment for Proposed Effluent Limitation Guidelines and Standards for the Airport Deicing Category, July 2009 EPA PDF
Propylene glycol website
WebBook page for C3H8O2
ATSDR – Case Studies in Environmental Medicine: Ethylene Glycol and Propylene Glycol Toxicity U.S. Department of Health and Human Services (public domain)
Propylene Glycol – chemical product info: properties, production, applications.
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