Propylene Glycol: The Everywhere Humectant Behind the Allergy Question

What propylene glycol is

Propylene glycol — PG for short, systematically propane-1,2-diol — is a small molecule with two hydroxyl groups on a three-carbon chain, which is what makes it a diol and what gives it its character. It is a colourless, slightly oily, sweet-tasting, almost odourless liquid that dissolves easily in water, and the European cosmetics database COSMILE describes it in exactly those terms, noting that alcohols like it are, next to water, the most important solvents for cosmetic preparations. It does not occur in nature; the FDA's food regulation describes it as manufactured by treating propylene with chlorinated water and then with sodium carbonate solution, or by heating glycerol with sodium hydroxide. That industrial origin is sometimes held against it in marketing — "synthetic" on a label is rarely a compliment — but it is also why the ingredient is so consistent: every batch is the same small molecule, with no plant harvest, no season, and no botanical variability to manage.

The name causes more confusion than the chemistry. Propylene glycol is not ethylene glycol, the toxic automotive antifreeze, and it is not polyethylene glycol (PEG), the family of larger polymers whose manufacturing can leave traces of 1,4-dioxane. Ethylene glycol has two carbons; propylene glycol has three, with an extra methyl group, and that one-carbon difference is the difference between a poison and a food additive. It is also not the same as its own esters: propylene glycol dicaprylate, propylene glycol laurate and their relatives are different ingredients, assessed separately by the Cosmetic Ingredient Review panel, and a product free of propylene glycol may still contain one of its esters. And it is not interchangeable with its glycol relatives — butylene glycol, pentylene glycol, hexylene glycol, caprylyl glycol — which share the diol structure and some of the same jobs, but have their own allergy profiles, as the 2026 Antwerp study showed. Getting the name right matters here more than usual, because "glycol-free" on a label can mean several different things, and only the full ingredient list tells you which one you are buying.

Tip: If a product claims to be "propylene glycol free" but lists pentylene glycol or caprylyl glycol near the top of its ingredient list, it has swapped one glycol for a relative, not removed glycols. That swap helps some people and not others — the Antwerp data on pentylene glycol allergy is the reason to check, not assume.

Why it is used everywhere

Propylene glycol earns its place in a formula by doing several unglamorous jobs at once, and that versatility is why it is so hard to avoid. As a humectant it holds and retains moisture, keeping a cream from drying out in the jar and helping the outer skin layer stay hydrated. As a solvent and solubiliser it dissolves ingredients that would not dissolve in water alone — fragrance components, preservatives, and drug actives — and keeps them evenly distributed through the product. As a viscosity-controlling agent it adjusts how thick or thin a formula feels. And as a vehicle in medicines it carries the active drug to the skin: a steroid cream is, in part, a delivery system, and propylene glycol is one of the commonest delivery vehicles in dermatology, which is why topical corticosteroids are among the commonest sources of propylene glycol allergy in the patch-test literature. COSMILE lists its cosmetic functions as fragrance-functional, humectant, skin-conditioning, solvent and viscosity-controlling, and its occurrences as skin and hair products, tooth, mouth and lip care, sunscreens and deodorants — a list that covers most of a bathroom shelf.

Its second virtue is penetration enhancement, and it is a virtue with a sting. Propylene glycol helps other ingredients pass through the outer skin layer, which is exactly what a formulator wants when the active ingredient needs to reach living skin, and exactly what a sensitive user does not want when the other ingredients include fragrance allergens or preservatives. The advocacy database EWG Skin Deep (labelled here as an advocacy source, not a regulator) lists "enhanced skin absorption" among its concerns for propylene glycol, and the Cosmetic Ingredient Review's own summaries record the same property. This is not a hidden flaw so much as a trade-off stated plainly: a vehicle that delivers drugs well also delivers everything else in the formula a little better. For a person with no sensitivities that is irrelevant. For a person whose dermatitis is driven by a fragrance or preservative in the same cream, the vehicle may be part of why the reaction is worse than the culprit ingredient alone would explain.

Its third job is outside cosmetics entirely, and it explains the antifreeze confusion. Because it lowers the freezing point of water and has markedly lower acute toxicity than ethylene glycol, propylene glycol is the glycol chosen for systems that may contact food, drinking water or people — food-plant cooling, HVAC loops, RV and marine winterising — and for de-icing and heat-transfer fluids generally. The same low acute toxicity is why the FDA could affirm it for food at all. None of that makes a bottle of industrial antifreeze safe to drink: commercial antifreeze formulations contain corrosion inhibitors and other additives, and "lower toxicity than ethylene glycol" is not the same as "non-toxic". But it does explain why the word "antifreeze" on a propylene glycol safety sheet is not, by itself, an argument against its cosmetic use — the molecule is the same, the formulation and the dose are not.

Where you will find it

• Moisturisers, lotions and creams — as humectant and solvent, often high in the ingredient list; the commonest cosmetic source in patch-test series
• Cleansers, toners and facial products — as solvent and viscosity agent, including products marketed for sensitive skin, where its presence surprises buyers
• Deodorants and antiperspirants — as solvent and humectant in sticks and roll-ons, a source that matters because the underarm is occluded skin
• Sunscreens and hair products — as solvent and conditioning agent in products applied over large skin areas
• Toothpastes, mouthwashes and lip care — where it can contribute to lip and mouth dermatitis that is easily mistaken for a flavour allergy
• Topical medicines — steroid creams and ointments, antifungal creams, and other dermatological preparations, where it is the vehicle carrying the drug; topical corticosteroids were the second-commonest source in the North American patch-test series
• Foods, as additive E1520 — as humectant, solvent and carrier for flavours and colours, in confections, frostings, seasonings and flavourings at the FDA's category caps, and in lower amounts across other foods
• Wet wipes, sanitisers and household products — as solvent and humectant, and in some e-liquids as the carrier liquid for nicotine and flavourings

Related: Alcohol Denat & Drying Alcohols: The Toner Ingredient That Splits Skincare — the other alcohol family on this site, and the one propylene glycol is most often confused with: one dries by evaporation, the other holds moisture, and the label distinction matters.

The concern: an irritant, an allergen, or both

The concern with propylene glycol is not that it is acutely poisonous — the safety reviewers agree it is not — but that a minority of people react to it on their skin, and that the reaction is unusually difficult to pin down. The difficulty has three parts, and keeping them separate is what makes the evidence readable.

First, it irritates as well as sensitises, and the two look alike on a patch test. Propylene glycol is a weak irritant: applied under occlusion (under a patch, under a deodorant stick, under a bandage) it can redden and sting skin without any immune learning involved. It is also, in some people, a true sensitiser: the immune system learns to recognise it and reacts on re-exposure. A patch test that puts a concentrated solution under occlusion for two days can produce a red square for either reason, and dermatologists have argued for decades about how many propylene glycol positives are allergy and how many are irritation misread as allergy. The Mayo Clinic series illustrates the ambiguity in numbers: among 11,738 patients patch tested between 1997 and 2016, 100 (0.85%) tested positive and 41 (0.35%) had reactions judged irritant, with positive rates rising with concentration — 0% at 5%, 0.26% at 10%, 1.86% at 20% — and irritant rates moving in the opposite direction in places. A test that changes its answer with its own concentration is a test that needs an experienced reader, and a home "test" — dabbing a product on your wrist — is not a substitute for one.

Second, the prevalence numbers depend entirely on who was tested and at what concentration. The largest series is the North American Contact Dermatitis Group's retrospective analysis of 23,359 patients patch tested between 1996 and 2006: 810 (3.5%) had allergic patch-test reactions to 30% propylene glycol, of which 12.8% were of definite clinical relevance (a reaction to a personal product containing it) and 88.3% were considered currently relevant in the broader sense, with personal care products the commonest source (53.8%), followed by topical corticosteroids (18.3%) and other topical medicaments (10.1%), and the face the commonest site among patients positive only to propylene glycol (25.9%). Those are patch-test-clinic patients — people referred because they already had dermatitis — not the general public, so 3.5% is not the rate in shoppers. The Australian low figures come from lower test concentrations: the Mayo Clinic's 0.85% at 5–20% and a Skin Health Institute series in Victoria finding 0.31% positive at 5% in petrolatum and 10% in water, with 42.9% of those relevant, point the same way. The newest data, from Antwerp University Hospital, tested 1,979 patients between 2021 and 2024 and found 55 (2.79%) allergic to propylene glycol, with detection rising steeply with concentration — 1.06% at 5% in petrolatum, 0% at 10% in water, 0.66% at 20% in water, 2.19% at 30% in water, and 7.82% at 100% in selectively tested patients — and concluded that 30% in water is more suitable for screening than lower concentrations, with 100% useful in selected cases. Read together, the series do not contradict each other: they describe a dose-response curve, and any article quoting a single percentage without its test concentration is hiding the most important part of the number.

Third, in a small subset, the reaction is not only where the product touches. Systemic contact dermatitis — dermatitis flaring after ingestion or injection of a substance to which the skin is already sensitised — is documented for propylene glycol in case reports, including a woman whose dermatitis cleared when topical propylene glycol products were stopped but flared again after eating foods containing it, with flares at the old patch-test site among the clues. That pattern is uncommon, and it should not be self-diagnosed: most dietary propylene glycol in a sensitised person causes no flare, and most flaring dermatitis has other causes. But it is the reason dermatologists ask about food and medicines, not just cosmetics, when a propylene glycol allergy is confirmed, and the reason a "propylene glycol free" cosmetic routine that still includes propylene-glycol-containing medicines or foods can appear to fail. The same logic applies in reverse to medicines: the European Medicines Agency's excipient guidance requires a skin-irritation warning for topical medicines containing propylene glycol at any concentration, and leaflet warnings for oral and parenteral medicines above dose thresholds (400 mg/kg in adults, 200 mg/kg in children, for the alcohol-like-symptoms warning), with stricter limits for children and neonates, whose livers and kidneys cannot clear it as adults can — the basis for a 2025 restriction on a carbamazepine oral suspension in neonates. Those are medicine rules, not cosmetic rules, and they should not be quoted as cosmetic limits; but they are the clearest statement anywhere that propylene glycol's safety is dose- and age-dependent, not absolute.

Related: Methylisothiazolinone (MI): The Preservative That Became 'Allergen of the Year' — the preservative whose allergy epidemic earned it the same Allergen of the Year title propylene glycol received in 2018, and a useful contrast: MI's problem was allergy at tiny doses, while propylene glycol's is irritation and allergy at ordinary ones, tangled together.

What regulators and reviewers say

The US Food and Drug Administration's position is the broadest permission in this story, and it is a food permission, not a cosmetic one. Under 21 CFR 184.1666, propylene glycol (C3H8O2, CAS 57-55-6) is affirmed as generally recognised as safe for direct food use, at levels consistent with current good manufacturing practice, with maximum levels as served of 5% for alcoholic beverages, 24% for confections and frostings, 2.5% for frozen dairy products, 97% for seasonings and flavourings, 5% for nuts and nut products, and 2.0% for other food categories. The 97% figure for seasonings and flavourings looks alarming until it is understood: flavourings are concentrates used in drops, and the "as served" level in the finished food is far lower. A GRAS affirmation is also not a cosmetic approval — the FDA does not pre-approve cosmetic ingredients the way it approves food additives — so quoting the food rule as proof that a moisturiser is safe is a category error, even though the underlying toxicology (low acute toxicity, no carcinogenicity signal in the Cosmetic Ingredient Review's summary) points the same way.

The Cosmetic Ingredient Review panel, the US industry's safety-review body, assessed propylene glycol first in 1994, concluding it was safe in cosmetics at concentrations up to 50%, and reopened the assessment in 2010 to acknowledge use at higher concentrations, concluding that propylene glycol, tripropylene glycol and the polypropylene glycols are safe as used in cosmetics when formulated to be non-irritating. That qualifier — "when formulated to be non-irritating" — is doing real work: it is the panel's way of saying the ingredient is safe at the concentrations and in the formulations actually used, provided the formulation does not itself irritate, which is a formulation judgement, not a blanket percentage. The European position, as presented by the industry database COSMILE, is quieter: it lists propylene glycol's functions and occurrences without a percentage restriction note of the kind attached to capped preservatives, and notes that a suspected contact allergy to it can be routinely tested by epicutaneous (patch) testing at a dermatologist. The EU has not banned it, capped it for cosmetics, or required a warning on cosmetic labels — and that absence is itself informative: the European system, which caps or bans ingredients far more readily than the US system, has left propylene glycol alone as a cosmetic ingredient while restricting its cousins and its uses in medicines.

The advocacy database EWG Skin Deep (labelled here as an advocacy source, not a regulator) takes the hazard side of the same evidence: its ingredient page says propylene glycol has been associated with irritant and allergic contact dermatitis and with contact urticaria, with sensitisation effects reported at concentrations as low as 2%, and rates its concerns as low for cancer and developmental toxicity, low-to-moderate for allergies and immunotoxicity, and high for irritation of skin, eyes or lungs, with data availability rated fair — while also noting that it is not currently on EWG's Restricted or Unacceptable lists. As with every EWG profile on this site, the distinction matters: EWG scores hazard (what a substance can do), while the FDA and CIR assess risk (what it does at a real dose in a real product). A "high" irritation rating and a "safe when formulated to be non-irritating" conclusion are not contradictory; they are the same observation — this ingredient irritates — expressed once as a property and once as a conditional permission.

The debate

The position this site takes follows the test concentration rather than either slogan: propylene glycol at ordinary cosmetic concentrations, on intact skin, in a person with no history of dermatitis, is a minor concern; propylene glycol in a daily leave-on product on the face or hands of a person whose dermatitis has not responded to treatment, or in the steroid cream prescribed for that dermatitis, is a genuine reason to ask for patch testing. The ingredient is not secretly sensitising ordinary users at scale, and it is not harmless because it is in food. It is a useful, mildly irritating vehicle whose risk concentrates exactly where its use concentrates — on the face, on the hands, under occlusion, and in people already seeing a dermatologist — and the sensible response is to match the product to the person: ordinary products for most, a properly read patch test and a genuinely propylene-glycol-free routine (medicines included) for the minority whose skin says otherwise.

Who should avoid it, or ask first

• People with dermatitis that has not responded to a steroid cream — the classic propylene glycol trap: the vehicle in the treatment may be maintaining the condition the drug is meant to treat; ask for patch testing including propylene glycol before concluding the steroid has failed
• People with facial or hand dermatitis of unclear cause — the face and hands were the commonest sites in both the North American and Antwerp series, and cosmetics were the commonest source; a face or hand pattern plus heavy cosmetic use is the profile to test
• People with a confirmed propylene glycol allergy — avoidance needs to cover topical medicines and foods as well as cosmetics, since systemic flares from ingestion are documented in a subset; ask your dermatologist what level of avoidance your case warrants, rather than assuming total avoidance or none
• People with eczema or a damaged skin barrier using leave-on products daily — damaged skin absorbs more and reacts more readily, and leave-on products give the longest contact; rinse-off products are a lower, though not zero, exposure
• Parents and carers of neonates and young infants, for medicines — the European Medicines Agency's thresholds are medicine rules, but they are the reason to check the excipients of any medicine given to a newborn, and to raise propylene glycol content with the prescriber rather than assuming a liquid medicine is inert
• People who react to one glycol and are offered another as a swap — pentylene glycol and caprylyl glycol carry their own allergy rates in the Antwerp data, so a swap needs the same sceptical label check, and ideally a patch test, rather than a marketing claim

How to spot propylene glycol on a label

• Propylene Glycol — the standard INCI name on cosmetics; usually listed in the middle of a long ingredient list, which roughly (not reliably) signals a moderate concentration
• 1,2-Propanediol / Propane-1,2-diol — the systematic chemical names, used on some technical and non-cosmetic labels
• E1520 — the food-additive number, used on food labels in the EU and Australia instead of the name
• Its usual companions — in moisturisers, glycerin, water, fragrance and a preservative such as phenoxyethanol; in steroid creams, the active drug itself plus other vehicle components; in flavourings, it may be the carrier and therefore the bulk of a "flavouring" ingredient that is not itemised further
• What it is not — ethylene glycol (toxic antifreeze, a different molecule), polyethylene glycol / PEG compounds (larger polymers with their own contamination story), propylene glycol esters such as propylene glycol dicaprylate (different ingredients, separately assessed), and butylene, pentylene, hexylene or caprylyl glycol (relatives, not the same ingredient)

Related: Phenoxyethanol: The Everyday Preservative Behind the Baby-Product Debate — the preservative most often found alongside propylene glycol in the same moisturiser, with its own, very different argument about babies, France and the SCCS.

Alternatives: the same job, without this glycol

The honest alternative depends on which job propylene glycol was doing in the product, because it does several. As a humectant, the classic replacement is glycerin (glycerol), which COSMILE lists alongside propylene glycol as the other great cosmetic humectant and solvent: it draws moisture similarly, it has a longer and less contested allergy record, and it is the base of many products marketed as propylene-glycol-free. As a solvent and vehicle in medicines, the replacement is not a consumer choice at all — it is a different formulation of the same drug, which a prescriber or pharmacist must supply, since the drug's absorption depends on its vehicle. As a carrier in flavourings and foods, there is usually no consumer-visible alternative: the flavouring is bought as a concentrate, and the choice is between products, not ingredients. And as a glycol swap, butylene glycol, pentylene glycol and caprylyl glycol are the usual candidates — with the Antwerp caveat attached: pentylene glycol allergy was found in 6.74% of the patients tested for it in that study, and caprylyl glycol reactions were also recorded, so a glycol-for-glycol swap trades one allergy profile for another and should be treated as an experiment to be patch-tested, not a solution to be assumed. The safest framing for a confirmed allergy is not a single substitute ingredient but a routine: products whose full ingredient lists have been checked, medicines whose excipients have been checked with the prescriber, and a dermatologist's clearance for the products used daily on the face and hands. Everything else is marketing.

Checked against 21 CFR 184.1666, the Cosmetic Ingredient Review amended assessment, Warshaw et al. 2009, the Mayo Clinic patch-test series, Kanokrungsee et al. 2026, and the EWG Skin Deep database (an advocacy source) · Last updated 2026-11-04 · How we assess ingredients

Read more

Methylisothiazolinone (MI): The Preservative That Became 'Allergen of the Year'Phenoxyethanol: The Everyday Preservative Behind the Baby-Product DebateAlcohol Denat & Drying Alcohols: The Toner Ingredient That Splits Skincare