A glycol is any organic compound that carries two hydroxyl (OH) groups, each attached to a different carbon atom in the molecule. 1Kirk-Othmer Encyclopedia of Chemical Technology. Glycols That two-OH structure gives glycols an unusual ability to mix with both water and oily substances, which is why they turn up in everything from car radiators to cake frosting. But the word “glycol” covers a surprisingly diverse family of chemicals, and the differences between them matter a great deal for safety, performance, and the products you encounter every day.
Ethylene Glycol
Ethylene glycol (EG) is the simplest and most widely produced glycol. Its two carbon atoms each carry one hydroxyl group, giving it a syrupy consistency, a faintly sweet taste, and an exceptional ability to lower the freezing point of water. That last property is why EG dominates the automotive antifreeze market and is a staple in aircraft deicing fluid. It is also the starting material for polyester fibers and PET plastic bottles, so even if you never pop a car hood, you interact with EG-derived products constantly.
Industrially, EG is made from ethylene oxide. Two main routes exist. Direct hydration mixes ethylene oxide with a large excess of water, while an indirect route first converts ethylene oxide to ethylene carbonate and then hydrolyzes it. The indirect path uses far less energy and produces roughly a third of the greenhouse gas emissions of the direct method, making it the preferred choice in newer facilities.2Nature. A comparative study of mono ethylene glycol economic production via different techniques
Where EG earns its bad reputation is toxicity. The liquid itself is relatively mild, but once the body metabolizes it, it breaks down into a chain of increasingly harmful acids: glycoaldehyde, glycolic acid, glyoxylic acid, and finally oxalic acid.3PubMed Central. Acute oxalate nephropathy caused by ethylene glycol poisoning Oxalic acid is the real culprit. It combines with calcium in the blood to form tiny calcium oxalate monohydrate (COM) crystals, and those crystals accumulate inside kidney cells. Research on isolated kidney mitochondria has shown that COM crystals directly damage these energy-producing structures, impairing their ability to generate energy and triggering a cascade that kills the cells.4Toxicological Sciences. Calcium Oxalate Monohydrate, a Metabolite of Ethylene Glycol, Is Toxic for Rat Renal Mitochondrial Function The crystals also increase reactive oxygen species and alter cell membranes, compounding the damage.5PubMed. Are calcium oxalate crystals involved in the mechanism of acute renal failure in ethylene glycol poisoning? Those metabolites can also depress the central nervous system and cause severe metabolic acidosis. In short, EG poisoning can shut down the kidneys, sometimes permanently, and it remains one of the most common causes of toxic alcohol ingestion in emergency rooms.
Propylene Glycol
Swap one hydrogen for an extra methyl group on ethylene glycol’s backbone and you get propylene glycol (PG), a three-carbon glycol with a dramatically different safety profile. PG is classified as “generally recognized as safe” for use in food in the United States, and it appears on ingredient lists for ice cream, salad dressings, soft drinks, and baked goods, where it works as a humectant, solvent, and texture stabilizer. Some estimates of daily dietary exposure approach or exceed the acceptable daily intake of 25 mg per kilogram of body weight, which has prompted researchers to argue that the current safety margin may actually be more conservative than necessary because the way PG is processed in the human body is well understood.6Food and Chemical Toxicology. Considerations for deriving a safe intake of propylene glycol
PG also shows up in pharmaceuticals as a solvent for drugs that do not dissolve well in water, in theatrical fog machines, and in veterinary medicine. At normal consumer exposure levels it is considered benign, but it is not risk-free at every dose. In children receiving high intravenous doses of PG-containing medications, reported adverse effects include central nervous system depression, abnormally high blood osmolarity, seizures, and lactic acidosis.7PubMed Central. Propylene glycol toxicity in children The distinction that matters is route and dose: eating a small amount in salad dressing is a world apart from receiving concentrated PG intravenously in a hospital.
How Propylene Glycol Helps Lotions and Drugs Penetrate Skin
If you read the label on a moisturizer, serum, or medicated cream, you will often find propylene glycol listed. It is not just a filler. PG acts as a penetration enhancer, helping active ingredients cross the outermost layer of skin. Molecular simulations of model skin-lipid membranes show that PG does not simply dissolve into the fatty interior of skin cells. Instead, it parks itself at the interface between the water-loving head groups and the fatty tails of the lipids, nudging them slightly apart and introducing a bit of disorder in the lipid tails. That loosening of the skin’s barrier structure makes it easier for other molecules to pass through.8PubMed Central. Mechanisms of the Drug Penetration Enhancer Propylene Glycol Interacting with Skin Lipid Membranes
PG is not the only glycol used in cosmetics. Hexylene glycol and 1,3-butylene glycol serve similar roles as solvents and humectants. In laboratory tests of antimicrobial activity, hexylene glycol outperformed the other two: a 10% hexylene glycol solution killed a panel of common bacteria and fungi within 20 hours, while propylene glycol and butylene glycol needed concentrations around 30% to achieve comparable results.9Acta Dermato-Venereologica. Antibacterial and antifungal properties of propylene glycol, hexylene glycol, and 1,3-butylene glycol in vitro That antimicrobial edge has made hexylene glycol attractive as a preservative booster in cosmetic formulations.
Diethylene Glycol and the Birth of Modern Drug Regulation
Diethylene glycol (DEG) is essentially two ethylene glycol molecules linked by an oxygen atom. It is cheap, mixes well with water, and dissolves many drugs, which is exactly why it has been involved in repeated poisoning disasters over the past century. After ingestion, DEG is metabolized into compounds that cause severe metabolic acidosis, acute kidney injury, neurological damage, and death.10Egyptian Journal of Forensic Sciences. Diethylene glycol poisoning: a narrative review of mechanism of toxicity, detection methods, regulatory failures and recurrent mass poisonings Research points to a metabolite called HEAA as the main driver of both the kidney and nervous system toxicity.11PubMed. Diethylene glycol poisoning
The most historically consequential DEG disaster happened in the United States in 1937. A pharmaceutical company used diethylene glycol as the solvent in a liquid sulfanilamide preparation, an early antibiotic, without conducting any toxicity testing. One hundred and five people died.12PubMed. Elixirs, diluents, and the passage of the 1938 Federal Food, Drug and Cosmetic Act At the time, no law required a drugmaker to prove a product was safe before selling it. The public outcry from the “Elixir Sulfanilamide” catastrophe led directly to the 1938 Federal Food, Drug, and Cosmetic Act, which for the first time required proof of safety before a new drug could be released to market and fundamentally reshaped the role of the Food and Drug Administration.13BMJ. When I use a word . . . Medicines regulation—diethylene glycol
Tragically, DEG poisonings have not stopped. Most documented cases since 1937 have been mass outbreaks in which DEG was substituted for a legitimate pharmaceutical solvent, often in countries with weaker supply-chain oversight.11PubMed. Diethylene glycol poisoning DEG has also been detected in health products sold over the counter and imported from various countries, underscoring the importance of quality controls throughout the pharmaceutical supply chain.14PubMed Central. Diethylene glycol in health products sold over-the-counter and imported from Asian countries
Polyethylene Glycol in Medicine and Industry
Chain many ethylene glycol units together and you get polyethylene glycol (PEG), a family of polymers that ranges from waxy solids to thick liquids depending on chain length. PEGs are among the most widely used polymers in medicine. Short-chain PEG 3350 is the active ingredient in popular over-the-counter laxatives. Longer-chain PEGs are used in “PEGylation,” a technique that attaches PEG chains to protein drugs to extend their life in the bloodstream. PEG also serves as a building block for drug-delivery vehicles: pH-sensitive micelles made with PEG shells can carry anticancer agents and release them preferentially in the acidic environment around tumors. Research on these micelles has found that lower-molecular-weight PEG tends to produce more compact carriers that are more readily taken up by cancer cells, while higher-molecular-weight PEG promotes faster drug release in acidic conditions.15Journal of Applied Polymer Science. Influence of polyethylene glycol molecular weight on the anticancer drug delivery of pH‐sensitive polymeric micelle Outside medicine, PEGs appear in toothpaste, cosmetics, industrial lubricants, and as plasticizers in ceramics.
Glycol Ethers Are a Different Chemical Family
A common point of confusion is the term “glycol ether,” which is not just another name for a glycol. Glycol ethers are made by reacting a glycol with an alcohol or an epoxide, producing a hybrid molecule that has one ether linkage and one or more hydroxyl groups. They are powerful solvents used in paints, cleaning products, and semiconductor manufacturing. The safety distinction between ethylene glycol-based ethers and propylene glycol-based ethers is stark. Propylene glycol ethers do not cause the blood cell destruction, reproductive harm, or organ damage that some lower-molecular-weight ethylene glycol ethers are known for, even at much higher exposure levels.16Patty’s Toxicology. Glycol Ethers: Ethers of Propylene, Butylene Glycols, and Other Glycol Derivatives Because of that difference, many industrial formulations have shifted toward propylene glycol-based ethers over the past few decades. If you see “glycol ether” on a safety data sheet, the specific parent glycol makes all the difference.
Antifreeze Performance Differences
Both ethylene glycol and propylene glycol lower water’s freezing point, and both are sold as antifreeze. PG-based antifreeze is marketed as “non-toxic” or “pet-safe,” which matters because EG’s sweet taste has long made it a poisoning hazard for animals and small children. But there is a trade-off in cooling performance. Laboratory testing of heavy-duty truck radiators showed that a 50/50 propylene glycol and water mixture starts losing cooling performance at higher flow rates than a 50/50 ethylene glycol and water mixture. Specifically, the PG blend’s performance dropped off when flow fell below about 5 liters per second, while the EG blend held steady down to about 3.8 liters per second. Pure water performed best, staying efficient at flows as low as 2.5 liters per second.17SAE Technical Paper Series. Heat Transfer Performance of Propylene Glycol Versus Ethylene Glycol Coolant Solutions in Laboratory Testing In practical terms, PG antifreeze works fine in passenger cars but can struggle in heavy-duty or high-heat applications where EG remains the default choice.
Propylene Glycol Vapor and E-Cigarettes
Propylene glycol is one of the two main carrier liquids in electronic cigarettes, along with vegetable glycerin. While PG is safe to eat in small amounts, the lungs are a different story. Airway cells exposed to PG at concentrations found in e-cigarette vapor showed reduced ability to grow, increased cell death, and signs of DNA damage, with worse effects at higher concentrations. Cells from people with chronic obstructive pulmonary disease were more vulnerable than healthy cells.18PubMed Central. Propylene glycol, a component of electronic cigarette liquid, damages epithelial cells in human small airways
A separate line of research found that PG aerosol exposure thickened airway mucus in both sheep and human airway cells grown in the lab. PG slowed the beating of cilia, the tiny hair-like structures that sweep mucus out of the airways. The mechanism appears to involve PG being converted to methylglyoxal inside airway cells, a reactive compound that disrupts the ion channels responsible for keeping airway surfaces properly hydrated.19PubMed Central. E-cigarette aerosols of propylene glycol impair BK channel activity and parameters of mucociliary function The upshot: a substance that is genuinely safe in food can still cause harm when inhaled repeatedly as a heated aerosol. Route of exposure matters as much as the chemical itself.
Bio-Based Production
Conventional propylene glycol comes from petroleum-derived propylene oxide, but greener routes are gaining ground. PG can be made from plant-derived feedstocks through several paths: direct fermentation of sugars, or through intermediates like lactic acid, glycerol, or sorbitol.20Biochemical Engineering Journal. Biobased propylene glycol production in a sugarcane biorefinery through lactic acid, glycerol, or sorbitol One particularly well-studied route converts glycerol, a byproduct of biodiesel production, into propylene glycol through catalytic hydrogenolysis.21PubMed Central. Insights on production mechanism and industrial applications of renewable propylene glycol Since biodiesel plants already produce large amounts of surplus glycerol, turning that waste stream into a valuable chemical is an appealing economic proposition.
On the environmental side, glycol-based deicing fluids used at airports have drawn attention because they wash off runways into waterways. The glycol itself biodegrades, but the process consumes dissolved oxygen. Anaerobic treatment systems can handle glycol-rich wastewater effectively, achieving pollutant removal rates above 90% under the right conditions while generating methane that can be captured for energy.22Water Research. Anaerobic biodegradation of aircraft deicing fluid in UASB reactors That dual benefit, cleaning the water and producing biogas, makes biological treatment an increasingly common approach at airports with heavy winter operations.
Detecting Glycol Contamination
Given the recurring tragedy of DEG showing up in medicines, there has been a push to develop cheap, fast tests that can be deployed in the field rather than requiring a well-equipped laboratory. Recent work has demonstrated that ethylene glycol can be distinguished from propylene glycol, glycerol, and diethylene glycol using simple enzyme-based assays and equipment as basic as a standard plate reader. Even cheaper, off-the-shelf saliva and breast-milk alcohol test strips costing about a dollar each could detect EG down to roughly 0.5 to 2% concentration in under two minutes. Disposable breathalyzers, also costing about a dollar, could identify both DEG and EG from other alcohols in just ten seconds.23Nature. Rapid screening of ethylene glycol and diethylene glycol in raw materials and medicinal syrups using low-cost field deployable assays Tools like these could make a real difference in pharmaceutical supply chains in lower-resource settings, where the cost and complexity of laboratory testing have historically left gaps that counterfeit or contaminated products slip through.