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Propylene glycol
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Propylene glycol
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Propylene glycol[1]
Propylene glycol
Propylene glycol
ball-and-stick model
ball-and-stick model
Space-filling model
Space-filling model
Names
Preferred IUPAC name
Propane-1,2-diol
Other names
  • Propylene glycol
  • α-Propylene glycol
  • 1,2-Propanediol
  • 1,2-Dihydroxypropane
  • Methyl ethyl glycol
  • Methylethylene glycol
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.000.307 Edit this at Wikidata
EC Number
  • 200-338-0
E number E1520 (additional chemicals)
KEGG
RTECS number
  • TY6300000
UNII
  • InChI=1S/C3H8O2/c1-3(5)2-4/h3-5H,2H2,1H3 checkY
    Key: DNIAPMSPPWPWGF-UHFFFAOYSA-N checkY
  • CC(O)CO
Properties
C3H8O2
Molar mass 76.095 g·mol−1
Appearance colourless liquid
Odor odorless
Density 1.036 g/cm3
Melting point −59 °C (−74 °F; 214 K)
Boiling point 188.2 °C (370.8 °F; 461.3 K)
Miscible
Solubility in ethanol Miscible
Solubility in diethyl ether Miscible
Solubility in acetone Miscible
Solubility in chloroform Miscible
log P −1.34[2]
Vapor pressure 10.66 Pa (20 °C)
Thermal conductivity 0.34 W/m·K (50% H2O @ 90 °C (194 °F))
Viscosity 0.042 Pa·s
Thermochemistry
189.9 J/(mol·K) [3]
Pharmacology
QA16QA01 (WHO)
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
1
0
Related compounds
Related glycols
 Ethylene glycol, 1,3-propanediol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Propylene glycol (IUPAC name: propane-1,2-diol) is a viscous, colorless liquid. It is almost odorless and has a faintly sweet taste. Its chemical formula is CH3CH(OH)CH2OH. As it contains two alcohol groups, it is classified as a diol. An aliphatic diol may also be called a glycol. It is miscible with a broad range of solvents, including water, acetone, and chloroform. In general, glycols[6] are non-irritating and have very low volatility.[7]

For certain uses as a food additive, propylene glycol is considered as GRAS by the US Food and Drug Administration, and is approved for food manufacturing.[8] In the European Union, it has E-number E1520 for food applications. For cosmetics and pharmacology, the number is E490. Propylene glycol is also present in propylene glycol alginate, which is known as E405.

Propylene glycol is approved and used as a vehicle for topical, oral, and some intravenous pharmaceutical preparations in the US and Europe.

Structure

[edit]

The compound is sometimes called (alpha) α-propylene glycol to distinguish it from the isomer propane-1,3-diol, known as (beta) β-propylene glycol. Propylene glycol is chiral. Commercial processes typically use the racemate. The S-isomer is produced by biotechnological routes.

Production

[edit]

Industrial

[edit]

Industrially, propylene glycol is mainly produced from propylene oxide (for food-grade use). According to a 2018 source, 2.16 M tonnes are produced annually.[7] 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.[9]

Final products contain 20% propylene glycol, 1.5% of dipropylene glycol, and small amounts of other polypropylene glycols.[10] Further purification produces finished industrial grade or USP/JP/EP/BP grade propylene glycol that is typically 99.5% or greater. Use of USP (US Pharmacopoeia) propylene glycol can reduce the risk of Abbreviated New Drug Application (ANDA) rejection.[11]

Propylene glycol can also be obtained from glycerol, a byproduct from the production of biodiesel.[7] This starting material is usually reserved for industrial use because of the noticeable odor and taste that accompanies the final product.

Laboratory

[edit]

(S)-Propanediol is synthesized via fermentation methods. Lactic acid and lactaldehyde are common intermediates. Dihydroxyacetone phosphate, one of the two products of breakdown (glycolysis) of fructose 1,6-bisphosphate, is a precursor to methylglyoxal. This conversion is the basis of a potential biotechnological route to the commodity chemical 1,2-propanediol. Three-carbon deoxysugars are also precursor to the 1,2-diol.[7]

A small-scale, nonbiological route from D-mannitol is illustrated in the following scheme:[12]

Applications

[edit]

Polymers

[edit]

Forty-five percent of propylene glycol produced is used as a chemical feedstock for the production of unsaturated polyester resins. In this regard, propylene glycol reacts with a mixture of unsaturated maleic anhydride and isophthalic acid to give a copolymer. This partially unsaturated polymer undergoes further crosslinking to yield thermoset plastics. Related to this application, propylene glycol reacts with propylene oxide to give oligomers and polymers that are used to produce polyurethanes.[7] Propylene glycol is used in water-based acrylic architectural paints to extend dry time which it accomplishes by preventing the surface from drying due to its slower evaporation rate compared to water.

Food and drug

[edit]

In regulated amounts, propylene glycol is designated as safe for food manufacturing as an anticaking agent, emulsifier, flavor agent, humectant, texturizer, stabilizer, solvent, antioxidant, antimicrobial agent, and thickener.[8][13][14]

As regulated by the US FDA for substances deemed as GRAS, propylene glycol is "not subject to premarket review and approval by FDA because it is generally recognized, by qualified experts, to be safe under the intended conditions of use."[13] The scientific panel evaluating propylene glycol for food manufacturing defined its conclusion as: "There is no evidence in the available information on [propylene glycol] that demonstrates, or suggests reasonable grounds to suspect, a hazard to the public when they are used at levels that are now current or might reasonably be expected in the future."[13]

The FDA law defined maximum limits for the use of propylene glycol in various food categories under good manufacturing practices:[8]

  • 2.0% for general food categories
  • 2.5% for frozen dairy products
  • 5% for alcoholic beverages
  • 5% for nuts and nut products
  • 24% for confections and frostings
  • 97% for seasonings and flavorings

The European Food Safety Authority authorizes propylene glycol for use in food manufacturing, establishing a safe daily intake of 25 mg per kg of body weight.[15] Specifically for ice cream or ice milk products, Health Canada permits use of propylene glycol mono fatty acid esters as an emulsifier and stabilizer at a maximum level of use of 0.35% of the ice cream made from the ingredients mix.[16]

Propylene glycol is used in a variety of other edible items, such as baked goods, desserts, prepared meals, flavoring mixes, candy, popcorn, whipped dairy products, and soda.[17] It is also used in beer to stabilize the foam.[18]

Vaporizers used for delivery of pharmaceuticals or personal-care products often include propylene glycol among the ingredients.[7] In alcohol-based hand sanitizers, it is used as a humectant to prevent the skin from drying.[19] Propylene glycol is used as a solvent in many pharmaceuticals, including oral, injectable, and topical formulations. Many pharmaceutical drugs which are insoluble in water utilize propylene glycol as a solvent and carrier; benzodiazepine tablets are one example.[20] Propylene glycol is also used as a solvent and carrier for many pharmaceutical capsule preparations. Additionally, certain formulations of artificial tears use propylene glycol as an ingredient.[21]

Propylene glycol is commonly used to de-ice aircraft.

Antifreeze

[edit]

The freezing point of water is depressed when mixed with propylene glycol. It is used as aircraft de-icing and anti-icing fluid. A 50% water-diluted and heated solution is used for removal of icing accretions from the fuselages of commercial aircraft on the ground (de-icing), and 100% undiluted cold solution is used only on wings and tail surfaces of an aircraft in order to prevent ice accretion from forming during a specific period of time before takeoff (anti-icing). Normally, such time-frame is limited to 15–90 minutes, depending on the severity of snowfall and outside air temperature.[7][22] Water-propylene glycol mixtures dyed pink to indicate the mixture is relatively nontoxic are sold under the name of RV or marine antifreeze. Propylene glycol is frequently used as a substitute for ethylene glycol in low toxicity, environmentally friendly automotive antifreeze. It is also used to winterize the plumbing systems in vacant structures.[23] The eutectic composition/temperature is 60:40 propylene glycol:water/−60 °C.[24][25] The −50 °F/−45 °C commercial product is, however, water rich; a typical formulation is 40:60.[26]

Electronic cigarettes liquid

[edit]

Propylene glycol, vegetable glycerin,[27] or a mixture of both, are the main ingredients in e-liquid used in electronic cigarettes. They are aerosolized to resemble smoke and serve as carriers for substances such as nicotine and flavorants.[28]

Miscellaneous applications

[edit]
  • As a solvent for many substances, both natural and synthetic.[29]
  • As a humectant (E1520).
  • As a freezing point depressant for slurry ice.
  • In veterinary medicine as an oral treatment for hyperketonaemia in ruminants.[30]
  • In the cosmetics industry, where propylene glycol is very commonly used as a carrier or base for various types of makeup.[31]
  • For trapping and preserving insects (including as a DNA preservative).[32]
  • For the creation of theatrical smoke and fog in special effects for film and live entertainment. So-called 'smoke machines' or 'hazers' vaporize a mixture of propylene glycol and water to create the illusion of smoke. While many of these machines use a propylene glycol-based fluid, some use oil. Those which use propylene glycol do so in a process that is identical to how electronic cigarettes work; utilizing a heating element to produce a dense vapor. The vapor produced by these machines has the aesthetic look and appeal of smoke, but without exposing performers and stage crew to the harms and odors associated with actual smoke.[33][34]
  • As an additive in polymerase chain reaction (PCR) to reduce the melting temperature of nucleic acids for targeting of GC rich sequences.[citation needed]
  • As a surfactant,[citation needed] it is used to prevent water from beading up on objects. It is used in photography for this purpose to reduce the risk of water spots, or deposits of minerals from water used to process film or paper.
  • As an ingredient in couplants for ultrasonic testing and medical ultrasound.[35]

Safety in humans

[edit]

When used in average quantities, propylene glycol has no measurable effect on development and/or reproduction on animals and probably does not adversely affect human development or reproduction without active use.[36] The safety of electronic cigarettes—which utilize propylene glycol-based preparations of nicotine or THC and other cannabinoids—is the subject of much controversy.[37][38][39] Vitamin E acetate has also been identified in this controversy.[40]

Oral administration

[edit]

The acute oral toxicity of propylene glycol is very low, and large quantities are required to cause perceptible health effects in humans;[13] in fact, the toxicity of propylene glycol is one third that of ethanol.[41] Propylene glycol is metabolized in the human body into pyruvic acid (a normal part of the glucose-metabolism process, readily converted to energy), acetic acid (handled by ethanol-metabolism), lactic acid (a normal acid generally abundant during digestion),[42] and propionaldehyde (a potentially hazardous substance).[43][44][45] According to the Dow Chemical Company, the LD50 (dose that kills 50% of the test population) for rats is 20 g/kg (oral/rat).[46][47]

Toxicity generally occurs at plasma concentrations over 4 g/L, which requires extremely high intake over a relatively short period of time, or when used as a vehicle for drugs or vitamins given intravenously or orally in large bolus doses.[48] It would be nearly impossible to reach toxic levels by consuming foods or supplements, which contain at most 1 g/kg of PG, except for alcoholic beverages in the US which are allowed 5 percent = 50 g/kg.[8][13] Cases of propylene glycol poisoning are usually related to either inappropriate intravenous administration or accidental ingestion of large quantities by children.[49]

The potential for long-term oral toxicity is also low.[50] In a National Toxicology Program continuous breeding study, no effects on fertility were observed in male or female mice that received propylene glycol in drinking water at doses up to 10100 mg/kg bw/day. No effects on fertility were seen in either the first or second generation of treated mice.[36] In a 2-year study, 12 rats were provided with feed containing as much as 5% propylene glycol, and showed no apparent ill effects.[51]

Skin and eye contact

[edit]
Propylene glycol is often used in electronic cigarettes.

Propylene glycol may be non-irritating to the skin, see section Allergic reaction below for details on allergic reactions.[52] Undiluted propylene glycol is minimally irritating to the eye, producing slight transient conjunctivitis; the eye recovers after the exposure is removed.

A 2018 human volunteer study found that 10 male and female subjects undergoing 4 hours exposures to concentrations of up to 442 mg/m3 and 30 minutes exposures to concentrations of up to 871 mg/m3 in combination with moderate exercise did not show pulmonary function deficits, or signs of ocular irritation, with only slight symptoms of respiratory irritation reported.[53]

Propylene glycol has not caused sensitization or carcinogenicity in laboratory animal studies, nor has it demonstrated genotoxic potential.[54][55]

Inhalation

[edit]

Inhalation of propylene glycol vapors appears to present no significant hazard in ordinary applications.[56] Due to the lack of chronic inhalation data, it is recommended that propylene glycol not be used in inhalation applications such as theatrical productions, or antifreeze solutions for emergency eye wash stations.[57] Recently, propylene glycol (commonly alongside glycerol) has been included as a carrier for nicotine and other additives in e-cigarette liquids, the use of which presents a novel form of exposure. The potential hazards of chronic inhalation of propylene glycol or the latter substance as a whole are as-yet unknown.[58]

According to a 2010 study, the concentrations of PGEs (counted as the sum of propylene glycol and glycol ethers) in indoor air, particularly bedroom air, has been linked to increased risk of developing numerous respiratory and immune disorders in children, including asthma, hay fever, eczema, and allergies, with increased risk ranging from 50% to 180%. This concentration has been linked to use of water-based paints and water-based system cleansers. However, the study authors write that glycol ethers and not propylene glycol are the likely culprit.[59][60][61]

Intravenous administration

[edit]

Studies with intravenously administered propylene glycol have resulted in LD50 values in rats and rabbits of 7 mL/kg BW.[62] Ruddick (1972) also summarized intramuscular LD50 data for rat as 13–20 mL/kg BW, and 6 mL/kg BW for the rabbit. Adverse effects to intravenous administration of drugs that use propylene glycol as an excipient have been seen in a number of people, particularly with large bolus dosages. Responses may include CNS depression, "hypotension, bradycardia, QRS and T abnormalities on the ECG, arrhythmia, cardiac arrhythmias, seizures, agitation, serum hyperosmolality, lactic acidosis, and haemolysis".[63] A high percentage (12–42%) of directly-injected propylene glycol is eliminated or secreted in urine unaltered depending on dosage, with the remainder appearing in its glucuronide-form. The speed of renal filtration decreases as dosage increases,[64] which may be due to propylene glycol's mild anesthetic / CNS-depressant properties as an alcohol.[65] In one case, intravenous administration of propylene glycol-suspended nitroglycerin to an elderly man may have induced coma and acidosis.[66] However, no confirmed lethality from propylene glycol was reported.

Animals

[edit]

Propylene glycol is an approved food additive for dog and sugar glider food under the category of animal feed and is generally recognized as safe for dogs,[67] with an LD50 of 9 mL/kg. The LD50 is higher for most laboratory animals (20 mL/kg).[68] However, it is prohibited for use in food for cats due to links to Heinz body formation and a reduced lifespan of red blood cells.[69] Heinz body formation from MPG has not been observed in dogs, cattle, or humans.

Allergic reaction

[edit]

Estimates on the prevalence of propylene glycol allergy range from 0.8% (10% propylene glycol in aqueous solution) to 3.5% (30% propylene glycol in aqueous solution).[70][71][72] The North American Contact Dermatitis Group (NACDG) data from 1996 to 2006 showed that the most common site for propylene glycol contact dermatitis was the face (25.9%), followed by a generalized or scattered pattern (23.7%).[70] Investigators believe that the incidence of allergic contact dermatitis to propylene glycol may be greater than 2% in patients with eczema or fungal infections, which are very common in countries with lesser sun exposure and lower-than-normal vitamin D balances. Therefore, propylene glycol allergy is more common in those countries.[73][improper synthesis?]

Because of its potential for allergic reactions and frequent use across a variety of topical and systemic products, propylene glycol was named the American Contact Dermatitis Society's Allergen of the Year for 2018.[74][75] Recent publication from The Mayo Clinic reported 0.85% incidence of positive patch tests to propylene glycol (100/11,738 patients) with an overall irritant rate of 0.35% (41/11,738 patients) during a 20-year period of 1997–2016.[76] 87% of the reactions were classified as weak and 9% as strong. The positive reaction rates were 0%, 0.26%, and 1.86% for 5%, 10%, and 20% propylene glycol respectively, increasing with each concentration increase. The irritant reaction rates were 0.95%, 0.24%, and 0.5% for 5%, 10%, and 20% propylene glycol, respectively. Propylene glycol skin sensitization occurred in patients sensitive to a number of other concomitant positive allergens, most common of which were: Myroxylon pereirae resin, benzalkonium chloride, carba mix, potassium dichromate, neomycin sulfate; for positive propylene glycol reactions, the overall median of 5 and mean of 5.6 concomitant positive allergens was reported.

Environmental impacts

[edit]

Propylene glycol occurs naturally, probably as the result of anaerobic catabolism of sugars in the human gut. It is degraded by vitamin B12-dependent enzymes, which convert it to propionaldehyde.[77]

Propylene glycol is expected to degrade rapidly in water from biological processes, but is not expected to be significantly influenced by hydrolysis, oxidation, volatilization, bioconcentration, or adsorption to sediment.[54] Propylene glycol is readily biodegradable under aerobic conditions in freshwater, in seawater and in soil. Therefore, propylene glycol is considered as not persistent in the environment.

Propylene glycol exhibits a low degree of toxicity toward aquatic organisms. Several guideline studies available for freshwater fish with the lowest observed lethal concentration of 96-h LC50 value of 40,613 mg/L in a study with Oncorhynchus mykiss. Similarly, the lethal concentration determined in marine fish is a 96-h LC50 of >10,000 mg/L in Scophthalmus maximus.

Although propylene glycol has low toxicity, it exerts high levels of biochemical oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen needed by aquatic organisms for survival. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose propylene glycol.[78]: 2–23 

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Propylene glycol

View on Grokipedia
from Grokipedia
Propylene glycol, chemically known as 1,2-propanediol, is a synthetic with the molecular formula C₃H₈O₂ and a molecular weight of 76.09 g/mol. It appears as a clear, colorless, viscous, and nearly odorless liquid that is hygroscopic and miscible with water, acetone, and ethanol. With a of 187.6–188.2°C, a of -59 to -60°C, and a of 1.04 g/cm³ at 20°C, it exhibits low volatility and is combustible with a of 99°C. This compound is widely utilized as a , , and stabilizer across multiple industries due to its ability to absorb water and dissolve a variety of substances. In the sector, it functions as an emulsifier, flavor carrier, and moisture retainer, and is approved for use in products like baked goods, beverages, and confections. In pharmaceuticals, it serves as a for oral, topical, and intravenous medications, such as formulations, and helps maintain moisture in drug products. The industry employs it in lotions, creams, and deodorants for its and properties, while industrial applications include solutions, de-icing fluids, resins, and fluids. Additionally, it is used to generate artificial smoke or fog in theatrical productions, training, and simulations. Propylene glycol is produced industrially through the hydrolysis of propylene oxide derived from petroleum. It demonstrates low acute toxicity, with an oral LD50 of about 20 g/kg in animal studies and no evidence of carcinogenicity or reproductive hazards in standard tests. However, high doses can lead to metabolic acidosis, hemolysis, or central nervous system depression, particularly in vulnerable populations like infants or those with kidney impairment. Common exposure occurs via ingestion in foods and drugs, dermal contact in cosmetics, or inhalation in occupational settings, but it metabolizes rapidly in the body within 48 hours into lactic acid and other benign compounds. The U.S. Food and Drug Administration (FDA) classifies it as "generally recognized as safe" (GRAS) for food use under 21 CFR 582.1666, while the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and European Food Safety Authority (EFSA) set an acceptable daily intake (ADI) of 0–25 mg/kg body weight, though recent evaluations propose potentially higher thresholds based on updated toxicological data. Environmentally, it biodegrades quickly in air (24–50 hours), water, and soil (days to weeks) and is found at low levels near hazardous waste sites.

Overview

Definition and nomenclature

Propylene glycol is an organic compound known chemically as 1,2-propanediol, with the molecular formula C₃H₈O₂ and a molecular weight of 76.09 g/mol. It is a synthetic diol, characterized as a clear, colorless, viscous liquid that is hygroscopic and miscible with water. Common synonyms for this compound include propylene glycol (PG), monopropylene glycol (MPG), and 1,2-dihydroxypropane. According to IUPAC nomenclature, it is designated as propane-1,2-diol, reflecting the positions of the hydroxyl groups on the propane chain. The molecule features a chiral center at the carbon bearing one hydroxyl group, resulting in (R)- and (S)-enantiomers; however, industrial synthesis typically produces a racemic mixture in a 1:1 ratio. Unlike (HOCH₂CH₂OH), propylene glycol possesses an additional (HOCH₂CH(OH)CH₃), which alters its to produce less toxic byproducts such as , rendering it significantly safer for human and environmental exposure. This structural difference contributes to its classification as (GRAS) by regulatory bodies for use in , pharmaceuticals, and , in contrast to the of .

History

Propylene glycol, chemically known as propane-1,2-diol, was first synthesized in 1859 by French chemist Charles-Adolphe Wurtz through the of propylene glycol diacetate. This laboratory preparation marked the initial identification of the compound, though it remained a curiosity in for decades without immediate practical applications. Commercial production of propylene glycol began in the early 1930s, pioneered by Carbide and Carbon Chemicals Corporation (a predecessor to ) using the chlorohydrin process to generate , followed by hydration to yield the glycol. This development coincided with the expansion of the and enabled large-scale availability for industrial uses, initially as a and . During , demand increased as propylene glycol served as a less toxic alternative to in some formulations for , and as a substitute for scarce in pharmaceuticals. By the late , propylene glycol's safety profile supported broader adoption; the U.S. Food and Drug Administration (FDA) included it in its inaugural list of substances (GRAS) for use in 1958, affirming its role as a and solvent. Throughout the , its applications evolved from primarily industrial solvents in the 1930s–1940s to widespread incorporation in pharmaceuticals, , and additives by the 1970s, driven by its non-toxicity and versatility. Production methods evolved in the late from the chlorohydrin process to more efficient routes. Since the 2000s, bio-based synthesis from , a byproduct, has grown, representing about 10% of global production as of 2025. As of 2025, propylene glycol faces increased regulatory and scientific scrutiny due to its prevalent use in e-cigarette liquids, where heating transforms it into potentially harmful compounds like and , raising concerns about respiratory and developmental health risks. Studies have linked vaping aerosols containing propylene glycol to mitochondrial damage and altered fetal skull development in animal models, prompting calls for further assessments.

Properties

Molecular structure

Propylene glycol, chemically known as 1,2-propanediol, possesses the molecular formula \ceC3H8O2\ce{C3H8O2} and the \ceCH3CH(OH)CH2OH\ce{CH3CH(OH)CH2OH}. This arrangement consists of a linear three-carbon backbone, with a primary hydroxyl group (\ce−CH2OH\ce{-CH2OH}) attached to the first carbon, a secondary hydroxyl group (\ce−OH\ce{-OH}) on the second carbon, and a methyl group (\ce−CH3\ce{-CH3}) attached to the third carbon. The hydroxyl groups enable intermolecular hydrogen bonding, which is responsible for the compound's relatively high viscosity compared to similar hydrocarbons. The of propylene glycol features tetrahedral coordination around each carbon atom, with typical C-C-O bond angles approaching 109.5°, facilitating the spatial arrangement that supports effective hydrogen bonding between molecules. Propylene glycol exhibits due to a chiral center at the second carbon atom (C2), which bears four different substituents: the , the , a , and a hydroxyl group. This results in two enantiomers: (R)-1,2-propanediol and ()-1,2-propanediol. However, the commercial product is typically a , containing equal proportions of both enantiomers produced during industrial synthesis. In comparison to related diols, propylene glycol shares structural similarities with (\ceHOCH2CH2OH\ce{HOCH2CH2OH}), which lacks the methyl substituent and has two groups, and with (\ceHOCH2CH(OH)CH2OH\ce{HOCH2CH(OH)CH2OH}), which features three hydroxyl groups including two primary and one secondary. These differences influence their respective hydrogen bonding capacities and applications.

Physical properties

Propylene glycol appears as a clear, colorless, at , nearly odorless with a faintly taste. Key physical constants of propylene glycol under standard conditions include the following:
PropertyValueConditions/Source
188.2 °C760 mmHg; NTP, 1992
Melting point-59 °CLide, 2007
Density1.036 g/cm³20 °C; Lide, 2007
1.43220 °C; Lide, 2007
Propylene glycol is miscible with water, acetone, and chloroform, and soluble in ether, ethanol, benzene, and many essential oils, though immiscible with fixed oils. Its hygroscopic nature causes it to absorb moisture from the air, which can influence storage and handling practices. Propylene glycol exhibits good thermal stability under normal conditions but begins to decompose above 200 °C, oxidizing to form products such as , , , and acetic acid.

Chemical properties

Propylene glycol, with its two hydroxyl groups—a primary and a secondary alcohol—exhibits reactivity typical of vicinal diols. It undergoes esterification reactions with carboxylic acids or their derivatives to form mono- or diesters, such as propylene glycol monoacetate, and etherification to produce ethers like under acidic or basic conditions. Additionally, it can be oxidized by strong agents like or , leading to products including , , and aldehydes such as , though it does not readily form lactones under mild conditions. The compound demonstrates good under normal storage conditions in cool, closed containers, showing resistance to in aqueous solutions and compatibility with sterilization by autoclaving when mixed with , , or glycerin. However, it oxidizes upon prolonged exposure to air at elevated temperatures above 280°C, and it is flammable with a closed-cup of 99°C, indicating non-flammability under ambient conditions but potential when heated. Its secondary hydroxyl group has a pKa of 14.8 at 25°C, reflecting weak acidity comparable to other secondary alcohols and limiting its role in acid-base reactions. Propylene glycol serves as a in condensation polymerization, reacting with dicarboxylic acids like adipic or to form polyesters, including unsaturated polyester resins used in composites. For analytical identification, (IR) spectroscopy reveals a characteristic broad O-H stretching peak around 3400 cm⁻¹, confirming the presence of its alcohol functionalities, alongside C-H stretches near 2900 cm⁻¹ and C-O bands at 1000–1200 cm⁻¹.

Production

Industrial production

The primary industrial production method for propylene glycol involves the acid-catalyzed hydration of , which is manufactured from using either the chlorohydrin process or the process. In this process, undergoes ring-opening with excess water in the presence of as the catalyst, producing 1,2-propanediol (propylene glycol) with yields exceeding 90%. The reaction can be represented as: \ce(CH3−CH−CH2)O+H2O−>[H2SO4]CH3−CH(OH)−CH2OH\ce{(CH3-CH-CH2)O + H2O ->[H2SO4] CH3-CH(OH)-CH2OH} Global production capacity for propylene glycol reached approximately 4.1 million metric tons per year as of 2024, with leading manufacturers such as Dow Chemical operating multiple facilities worldwide. An emerging alternative since the is the bio-based route, which converts derived from corn-based via catalytic hydrogenolysis to yield propylene glycol. Side reactions in the primary hydration process produce minimal byproducts, mainly , which can be separated and valorized.

Laboratory synthesis

In laboratory settings, propylene glycol is synthesized using small-scale methods that emphasize high purity, controlled reaction conditions, and ease of operation, often contrasting with cost-optimized by prioritizing analytical-grade product isolation. A classic laboratory method involves the of 1,2-dichloropropane with a base in . This direct proceeds in weak alkaline conditions, such as with , to yield propylene glycol alongside byproducts like inorganic salts. Similarly, propylene bromohydrin (1-bromo-2-propanol) can be hydrolyzed under basic conditions to form the intermediate, which is subsequently opened with to afford propylene glycol, a route adapted from early protocols for chiral variants. Modern laboratory routes focus on reduction reactions for greater selectivity and compatibility with renewable feedstocks. One such approach is the reduction of lact (CH₃CH(OH)CHO), an intermediate derived from . using a reticulated vitreous carbon loaded with 5% Ru/C catalyst at 70 °C and converts lactaldehyde to , with yields increasing linearly with applied current (10–100 mA) and selectivity favoring the at higher currents over 95% in optimized setups. Another contemporary method is the of hydroxyacetone (also known as acetol, CH₃COCH₂OH) to propylene glycol. The reaction, CH₃COCH₂OH + H₂ → CH₃CH(OH)CH₂OH, employs earth-abundant catalysts such as Ni/C nanoparticles under mild electrochemical conditions at sufficient negative potentials (−1.5 V vs. Ag/AgCl), achieving 80% conversion of hydroxyacetone with 89% selectivity to propylene glycol and minimal over-reduction to propanol. Following synthesis, propylene glycol is purified by to remove water and unreacted intermediates, typically at reduced pressure (e.g., 10–20 mmHg) to lower the and prevent decomposition, yielding colorless, high-purity product suitable for analytical use. These laboratory methods generally provide 70–85% overall yields under conditions, depending on efficiency and substrate purity.

Applications

Polymers and materials

Propylene glycol serves as a vital building block in the synthesis of various polymers and materials, particularly due to its structure that facilitates esterification and formation reactions. In the polymers sector, it is predominantly employed in the production of polyols and resins, contributing to the flexibility, durability, and processability of end products. Globally, the construction industry, which utilizes propylene glycol extensively for resins and coatings in polymers, accounts for over 38% of the market demand. In polyurethane production, propylene glycol is polymerized to form polypropylene glycol (PPG), a polyether polyol that reacts with diisocyanates to create polyurethane foams and coatings. This reaction yields flexible, resilient materials commonly used in insulation, furniture cushioning, and protective coatings, where PPG's low viscosity and hydrophobicity enhance the final product's performance. For instance, PPG-based polyurethanes provide superior elasticity and weather resistance in automotive and building applications. As a key component in unsaturated polyester resins (UPRs), propylene glycol acts as the primary glycol, condensing with unsaturated acids like and saturated acids such as to form resins for fiberglass-reinforced composites. These UPRs are cross-linked with styrene to produce strong, lightweight materials ideal for boat hulls, automotive parts, and corrosion-resistant panels, with propylene glycol imparting and flexibility to the cured structure. Its use in UPRs is preferred over for applications requiring impact resistance and reduced brittleness. Propylene glycol derivatives function as effective plasticizers in (PVC) and formulations, enhancing flexibility without compromising strength. In PVC, compounds like poly(1,2-propylene glycol adipate) are incorporated to reduce rigidity, making the material suitable for flexible tubing, films, and wire insulation, while offering an alternative to traditional . Similarly, in , propylene glycol serves as a and , improving processability in inks and packaging films by lowering the temperature and preventing cracking. In resins for paints and varnishes, propylene glycol is integrated as a during esterification with fatty acids and , yielding oil-modified resins that provide , , and to coatings. These alkyds, often used in architectural and industrial paints, benefit from propylene glycol's ability to balance drying time and film hardness, ensuring even application and long-term weatherability.

Food and pharmaceuticals

Propylene glycol serves multiple roles in the , primarily as a to retain moisture in products like baked goods and confections, preventing them from drying out. It also functions as a and carrier for flavorings and colors, helping distribute aromas evenly and preserve them longer; it often serves as the base in products with natural flavors, sometimes mixed with water, enhancing their dispersion in beverages and other formulations, and as an to extend in items such as cakes and soft drinks. The U.S. (FDA) has affirmed its status as (GRAS) for these direct food uses under good manufacturing practices, as outlined in 21 CFR 184.1666. In the , it is approved as the food additive E1520 for similar purposes, authorized in most categories, with carry-over limits such as 1,000 mg/kg in fine bakery wares and 1,000 mg/L in flavoured drinks, as per Regulation (EC) No 1333/2008. In pharmaceuticals, propylene glycol acts as a for both oral and injectable medications, particularly for water-insoluble drugs like , where it comprises a significant portion of the formulation to ensure stability and bioavailability. It is also employed as an in topical creams and ointments, aiding in and maintaining product consistency. The FDA recognizes its safety in these applications when used within established limits, with oral exposure considered low-risk based on metabolic studies. Within cosmetics, propylene glycol functions as a and in lotions and creams, drawing to the skin to improve hydration and texture. It additionally serves as a adjuster and thickener in formulations, contributing to a smooth, stable paste that resists drying. The Cosmetic Ingredient Review (CIR) Expert Panel has deemed it safe for these uses at concentrations up to 50% in leave-on products, based on and data. Regulatory dosage limits ensure safe consumption; for instance, the FDA permits up to 2.5% in frozen dairy desserts and confections under GRAS guidelines, while the sets an of 25 mg/kg body weight. In semi-moist dog foods, it is GRAS without a specified upper limit beyond good manufacturing practices, though prohibited in cat foods due to concerns.

Antifreeze and coolants

Propylene glycol serves as a key component in and formulations, typically mixed with at concentrations of 30-50% by volume for use in automotive engines and (HVAC) systems. These mixtures exploit propylene glycol's ability to depress the freezing point of ; for instance, a 50% solution achieves a freezing point of approximately -34°C (-29°F), while higher concentrations around 60% can provide protection down to -50°C, preventing ice formation in cold climates. Compared to , the traditional base, propylene glycol offers notable advantages, including significantly lower toxicity—making it safer for applications where accidental exposure or leakage is a concern—and greater biodegradability under both aerobic and anaerobic conditions without producing persistent byproducts. These properties have driven its adoption in environmentally sensitive uses, though it requires larger volumes and provides slightly inferior efficiency. Ethylene glycol is generally cheaper but more toxic, while propylene glycol is preferred for HVAC applications due to its lower toxicity. Approximate bulk industrial prices as of 2024 are roughly $4-9 per gallon for ethylene glycol and $9-18 per gallon for propylene glycol, with inhibited HVAC-grade versions often higher due to additives and certification. Specific prices for 2026 are not available, as they are future market-driven values subject to fluctuations in supply, demand, energy costs, and raw material prices; consult specialized chemical market reports for projections. Beyond automotive and HVAC applications, propylene glycol features in solar heating fluids, often at a 50:50 ratio with to balance freeze protection and while minimizing risks in closed-loop systems. It is also integral to airport de-icing operations, where (FAA)-approved mixtures, such as Type I fluids based on propylene glycol, are heated and applied to surfaces for safe removal of and . In terms of market utilization, approximately 25% of global propylene glycol production is directed toward automotive and coolants, reflecting its growing role in safer solutions. Commercial formulations often incorporate additives like silicate-based corrosion inhibitors to protect metal components such as radiators and heat exchangers from degradation in mixed solutions.

E-liquids and other consumer uses

Propylene glycol (PG) serves as a primary base ingredient in e-liquids for electronic delivery systems (ENDS), typically comprising 50-70% of the formulation alongside glycerin (VG). This ratio facilitates the delivery of and acts as a for flavoring agents, while contributing to formation through and subsequent condensation with VG and flavors. In common commercial e-liquids, PG and VG together account for 80-95% of the total volume, enabling the production of inhalable mist that carries active components. In the , PG functions as a in cigarettes, helping to retain moisture in the filler and prevent drying during manufacturing and storage. Added in small quantities, it maintains the structural integrity and burn characteristics of the product, with levels typically ranging from 1-5% by weight in processed . Beyond vaping and , PG finds use in various household consumer products. As a , it is incorporated into inks to adjust and improve flow properties, ensuring even application and . In pet foods, particularly semi-moist varieties, PG acts as a and , extending by binding water and inhibiting microbial growth at concentrations up to 12%. Propylene glycol is also used in wet wipes primarily as a humectant, solvent, and mild preservative to retain moisture, dissolve ingredients, and prevent spoilage of the wipes themselves, supporting cleaning and moisturizing but lacking sufficient antimicrobial strength at typical concentrations to disinfect surfaces or hands by killing viruses or most bacteria. Additionally, PG is a key component in fluids, where it generates artificial smoke effects by vaporizing into fine droplets, often mixed with water and glycerin for theatrical or atmospheric applications. Post-2020, regulatory actions such as the U.S. FDA's enforcement policy banning unauthorized flavored cartridge-based e-cigarettes have influenced e-liquid formulations, prompting shifts toward or flavors and potentially altering PG usage as a flavor carrier, though direct limits on PG levels remain absent. While PG's role in inhalation-based products like e-liquids and fog fluids raises concerns about respiratory effects from aerosolized exposure, its overall profile in these uses is supported by regulatory approvals for food-grade variants.

Health and safety

Oral and dietary exposure

Propylene glycol is rapidly absorbed from the following oral ingestion, with maximum plasma concentrations achieved within 1 hour in humans. It is primarily metabolized in the liver by to lactaldehyde, which is further oxidized to (lactate), and subsequently incorporated into gluconeogenic pathways or excreted. The elimination in adults with normal liver and kidney function typically ranges from 2 to 4 hours. Acute oral toxicity of propylene glycol is low, with an LD50 exceeding 20 g/kg body weight in rats, often accompanied by symptoms such as and prior to death at lethal doses. In chronic studies, the no-observed-adverse-effect level (NOAEL) has been established at 2.5 g/kg/day in rats over 2 years, with no significant histopathological changes observed. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has set an acceptable daily intake (ADI) for propylene glycol at 0–25 mg/kg body weight based on human and animal data, reaffirmed in subsequent evaluations. A 2024 review suggests the ADI may be conservative and proposes higher safe intake levels (up to 400–1000 mg/kg bw-day in adults) based on human data, though official limits remain unchanged as of 2025. Human studies indicate that oral exposure at levels typical in foods and pharmaceuticals poses minimal risk, though higher doses can lead to mild osmotic laxative effects due to water retention in the intestines. Rare cases of hyperosmolarity have been reported in infants exposed to elevated oral doses via medications, potentially contributing to metabolic disturbances. The U.S. Food and Drug Administration (FDA) classifies propylene glycol as generally recognized as safe (GRAS) for use as a direct food additive at levels not exceeding current good manufacturing practices, including in oral pharmaceutical products such as mouthwashes. In sensitive individuals, oral exposure may occasionally trigger allergic reactions, though this is uncommon.

Dermal and ocular contact

Propylene glycol exhibits low dermal absorption through intact , with penetration limited primarily to the outermost layers of the (up to approximately 10 μm depth after application of a 10% ), and negligible amounts reaching the . studies using human abdominal have shown relative absorption rates around 10-20% for propylene glycol and its simple derivatives under steady-state conditions, though actual systemic remains minimal due to its high solubility and the 's . It is commonly incorporated into topical formulations at concentrations of 10-30% as a and , enhancing the delivery of other active ingredients without significant self-absorption concerns in cosmetic or pharmaceutical products. Regarding skin irritation, propylene glycol acts as a mild irritant at concentrations exceeding 10%, potentially causing faint or in sensitive individuals upon prolonged exposure to undiluted forms. Patch testing data indicate a low rate of 2-4%, with positive reactions occurring in approximately 0.85-2.8% of tested patients over extended periods, often distinguishing between true allergic responses and irritant effects. These reactions contribute to occasional , though they are infrequent in the general population. For ocular contact, propylene glycol in eye drops may cause temporary stinging or mild discomfort upon instillation, but it does not result in lasting damage. Rabbit studies demonstrate only slight-to-moderate conjunctival hyperemia that resolves within 2-3 days, with no evidence of corneal opacity, ulceration, or permanent injury even at higher concentrations. In occupational settings, propylene glycol is considered safe for dermal exposure up to 100% concentration when using protective gloves (such as nitrile or neoprene), though adequate ventilation is recommended to minimize any potential vapor inhalation or prolonged contact. The Cosmetic Ingredient Review (CIR) Expert Panel has deemed propylene glycol safe for use in cosmetics as currently formulated, at concentrations up to 99% in diluted products and 73% in leave-on applications, provided formulations are nonirritating overall.

Inhalation exposure

Propylene glycol has a low vapor pressure of approximately 0.07 mm Hg at 20°C, resulting in minimal risk of airborne exposure under normal conditions, though aerosols generated in applications such as theatrical fog or electronic cigarette vaping can increase inhalation potential. Acute inhalation exposure to propylene glycol vapors or aerosols at high concentrations, such as above 50 ppm, may cause irritation of the respiratory tract, including the nose, throat, and lungs, as observed in animal studies and human volunteers. The International Agency for Research on Cancer (IARC) classifies propylene glycol as Group 3, not classifiable as to its carcinogenicity to humans, based on inadequate evidence from animal and human studies. Chronic exposure, particularly from vaping where propylene glycol serves as a primary in e-liquids, has been associated with symptoms such as and dry in user reports and clinical studies from the , though no evidence of long-term damage has been established in humans. As of 2025, recent studies indicate PG in vaping aerosols can cause , disrupt in human airway , and generate toxic carbonyls like , though long-term human effects remain unestablished. The American Industrial Hygiene Association recommends a workplace environmental exposure limit (WEEL) of 10 mg/m³ (approximately 3.2 ppm) averaged over an 8-hour workday to prevent . In e-liquids, propylene glycol can thermally decompose at high temperatures during vaping, producing and other carbonyl compounds, which contribute to potential respiratory depending on device settings and usage patterns.

Intravenous use

Propylene glycol functions as a co-solvent in intravenous formulations of several medications, particularly benzodiazepines that are poorly soluble in water, such as and . In injections, it constitutes approximately 40% of the solution, while injections contain about 80% propylene glycol. These concentrations allow effective delivery of the active s for conditions like , alcohol withdrawal, and in critical care settings. Despite its utility, propylene glycol's metabolism—primarily via to lactaldehyde and then —can overload pathways at high doses, leading to . Toxicity manifests as intravascular , acute renal dysfunction including tubular , and hyperosmolarity, particularly when cumulative exposure exceeds safe thresholds. Such effects are more pronounced in patients with impaired renal or hepatic function, where clearance is reduced. Regulatory guidelines from the establish maximum acceptable daily intakes for intravenous propylene glycol to minimize risks: 500 mg/kg body weight for adults and children over 5 years, 50 mg/kg for children aged 1 month to 5 years, and 1 mg/kg for preterm and term neonates under 1 month. Monitoring is essential, especially in neonates and vulnerable populations, including serial assessments of serum osmolality, , lactate levels, and renal function to detect early signs of accumulation or . Rare case reports from the 1990s highlight severe outcomes from excessive intravenous exposure, including neurological complications like seizures in high-dose infusions. For instance, a report described propylene glycol from infusion exceeding 479 g over 24 hours, contributing to refractory seizures and hemodynamic instability in a postoperative . Similarly, a 2000 case involved a receiving over 3,000 mg of (delivering substantial propylene glycol) in 24 hours, resulting in profound (pH 7.16, 31), hyperosmolality (600 mOsm/kg), elevated (2.2 mg/dL), and requiring emergent for recovery. To avoid these risks, clinicians often opt for propylene glycol-free alternatives, such as injections, which use aqueous solutions without this , providing comparable effects in scenarios like intensive care or .

Effects on animals

Propylene glycol exhibits low in various . In , oral LD50 values range from 20 to 30 g/kg body weight, indicating minimal risk from single exposures. Similar low is observed in birds, with acute oral LD50 values exceeding 20 g/kg, classifying it as practically non-toxic under standard guidelines. For aquatic like , the 96-hour LC50 exceeds 10,000 mg/L, further demonstrating negligible in environments. Chronic exposure studies in rats reveal no reproductive or developmental toxicity at doses up to 2 g/kg/day, with no observed adverse effects on , , or offspring viability. Propylene glycol is metabolized in animals via pathways similar to those in humans, primarily through to and subsequent , leading to rapid elimination without significant accumulation. In pets, propylene glycol is generally safe in at concentrations up to 5%, with no adverse effects reported in long-term feeding studies at this level. Propylene glycol is prohibited in by the U.S. FDA due to hematologic issues like formation in cats, as it causes the feed to be adulterated. Rare cases of intoxication occur from of propylene glycol-based , presenting with and ; these are treatable supportively, often with as a competitive inhibitor if needed early. For , propylene glycol shows minimal potential due to its low (log Kow = -1.07), ensuring it does not concentrate in food chains. Toxicity testing under guidelines, including acute oral, dermal, and inhalation studies in and birds, consistently demonstrates low hazard classifications for propylene glycol across animal models. These findings parallel its safety profile in human exposure scenarios.

Allergic reactions

Allergic reactions to propylene glycol primarily manifest as (ACD), a type IV delayed response mediated by T-cell activation upon re-exposure to the in sensitized individuals. This mechanism involves the haptenation of propylene glycol with skin proteins, triggering an immune cascade that leads to inflammation typically 48-72 hours after contact. Cross-reactivity with related compounds, such as —the chemical precursor to propylene glycol—has been noted in some cases, potentially due to structural similarities, though direct evidence remains limited and often involves co-sensitization rather than true . The prevalence of propylene glycol among patch-tested populations, often those suspected of , ranges from 0.8% to 3.5%, with higher rates observed in studies using higher test concentrations like 30%. This variability reflects differences in testing protocols and patient cohorts, but it underscores propylene glycol as a common yet underrecognized in dermatological practice. In occupational settings, such as or healthcare, exposure can elevate risk, with 2020s studies highlighting cases of ACD from hidden sources like sanitizers or adhesives during the era. Symptoms of propylene glycol allergy typically include eczematous reactions such as redness, itching, and scaling at the site of contact, resembling classic ACD. Urticaria () may occur as a less common immediate response, while systemic contact dermatitis—triggered by oral or widespread topical exposure—can present with generalized eczema, , or gastrointestinal upset. is exceedingly rare and usually linked to high-dose parenteral administration rather than routine contact. Diagnosis relies on epicutaneous patch testing, the gold standard for confirming , performed with propylene glycol at a 10% aqueous concentration to balance sensitivity and minimize irritant reactions. Readings are taken at 48 and 96 hours, with positive results indicating if clinically relevant; lower concentrations (5%) may miss weak sensitizations, while higher ones (20-30%) risk false positives from irritation. The EPIC test specifically refers to this standardized patch application method, aiding differentiation from irritant . Management centers on strict avoidance of propylene glycol in personal care products, medications, and occupational exposures, with hypoallergenic alternatives like glycerin recommended as a safer humectant substitute due to its lower sensitization potential. Patient education on reading labels is crucial, as propylene glycol appears in diverse formulations; in occupational dermatitis cases, protective barriers and substitution have proven effective in recent cohort studies.

Environmental impacts

Persistence and degradation

Propylene glycol is readily biodegradable under aerobic conditions, achieving greater than 70% degradation within 28 days in standardized tests such as 301F, where aerobic microbes primarily convert it to and . This rapid microbial breakdown occurs in various environmental settings, including simulations, confirming its classification as inherently biodegradable. In , the of propylene glycol due to ranges from 1 to 4 days under aerobic conditions and 3 to 5 days under anaerobic conditions, while in , it is equal to or slightly shorter, typically 1 to 5 days. These short half-lives indicate minimal persistence in these compartments, with degradation driven by and microorganisms. In air, propylene glycol has an estimated of 24–50 hours due to gas-phase reaction with photochemically produced hydroxyl radicals. Abiotic degradation of propylene glycol is negligible in environmental compartments; it remains stable to across 5–9 and undergoes slow photolysis in aqueous solutions, with no significant transformation via oxidation in surface waters. Due to its miscibility with water and low soil organic carbon-water partition coefficient (Koc ≈ 1), propylene glycol exhibits high mobility in , with minimal adsorption to soil particles and a strong potential to leach into . Propylene glycol has been detected in municipal and industrial wastewater at concentrations typically below 1 mg/L, though higher levels up to 19,000 mg/L can occur in airport storm runoff from de-icer applications.

Ecological effects

Propylene glycol exhibits low to aquatic organisms, with values exceeding 5,000 mg/L for such as and for daphnia species like . Chronic exposure studies show no adverse effects on reproduction or growth in aquatic invertebrates at concentrations below 1,000 mg/L, with NOEC values as high as 13,020 mg/L for Ceriodaphnia dubia. Similarly, fish species demonstrate high tolerance, with LC50 values often surpassing 50,000 mg/L for fathead minnows. In terrestrial environments, lower concentrations of propylene glycol may serve as a carbon source enhancing bacterial productivity in . Spills or runoff, such as from operations, can attract including birds and mammals to contaminated areas, increasing risks of secondary exposure and disruption. Bioaccumulation of propylene glycol in organisms is negligible, with bioconcentration factors (BCF) below 10, indicating minimal potential for buildup in food chains. Fish kills directly attributable to propylene glycol are rare, but 2010s studies on glycol runoff have documented impacts on invertebrates, including reduced abundance of benthic macro due to localized and alteration. Indirect ecological effects primarily stem from high-concentration spills, where propylene glycol's elevated leads to dissolved oxygen depletion in receiving waters, stressing and invertebrate communities. For instance, deicing discharges have been linked to oxygen sags in near , exacerbating vulnerability in low-flow conditions.

Regulatory status

Propylene glycol is listed on the Toxic Substances Control Act (TSCA) inventory as an active . In the , it is registered under the REACH regulation as propane-1,2-diol, with no harmonized for hazards. Under the Globally Harmonized System (GHS), propylene glycol is generally not classified as hazardous for most uses, though safety data sheets note potential mild irritancy from mists or vapors. For environmental discharges, particularly from airport deicing, the U.S. Environmental Protection Agency (EPA) has established effluent limitation guidelines under 40 CFR Part 449 (finalized in 2010), requiring airports to monitor and treat propylene glycol-based deicing fluids to limit (BOD) and (TOC) in effluents, aiming to reduce pollutant discharges by millions of pounds annually. The (WHO) has established an acceptable daily intake (ADI) for propylene glycol of 25 mg/kg body weight, based on evaluations by the Joint FAO/WHO Expert Committee on Food Additives (JECFA). It is approved under as food additive INS 1520 (also known as E1520 in the ), permitting its use as a carrier, , , and in various foods without specified maximum levels in many categories. In the United States, the (FDA) classifies propylene glycol as (GRAS) for use as a direct food additive, with specifications outlined in 21 CFR 184.1666. Recent regulatory updates include ongoing monitoring of propylene glycol in vaping products; in the , the Tobacco Products Directive (TPD) regulates e-cigarette liquids containing propylene glycol as a base, limiting concentrations to 20 mg/mL and requiring ingredient notifications, while U.S. FDA oversight emphasizes premarket authorization for such products without specific prohibitions on propylene glycol itself. For labeling, U.S. products must declare propylene glycol by name in the ingredients list if added directly, while in the it appears as E1520; industrial applications require Safety Data Sheets (SDS) detailing handling and low-hazard status under OSHA and equivalent standards.

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