Propylene glycol: humectant, solvent and occasional irritant

Propylene glycol sits near the top of the ingredient list on an enormous number of products: moisturizers, deodorants, shampoos, hand sanitizers, and a large share of prescription creams. It’s not a preservative and it’s not a fragrance, so most people never give it a thought. Then a dermatologist says “you’re reacting to propylene glycol” and suddenly it’s everywhere. Here’s what it does, when it causes trouble, and why the amount matters more than the name.

What propylene glycol is, and what it does

Propylene glycol is a small, synthetic alcohol, a clear, nearly odourless liquid that mixes with both water and oil. That dual nature is what makes it useful. DermNet lists its jobs as humectant, solvent, emulsifier, penetration enhancer and emollient. In plain terms:

• A humectant attracts water and holds it, which keeps a cream from drying out in the jar and helps hydrate the skin surface.
• A solvent dissolves ingredients that won’t dissolve in water alone, so the formula stays clear and even.
• An emulsifier helps oil and water stay mixed instead of separating.
• A penetration enhancer helps other ingredients, including medicines, get through the outer layer of skin.

It’s also a food additive and a pharmaceutical ingredient, and it’s the base of most e-cigarette liquids.

Why it’s in so many products

Because it does four things for the price of one, formulators reach for it constantly. DermNet cites a 2016 analysis of the American Contact Dermatitis Society’s product database in which about 38% of products contained propylene glycol. The US Cosmetic Ingredient Review (CIR), the independent panel that reviews cosmetic ingredient safety, noted that by 2009 it was reported in 9,747 cosmetic formulations, up sharply from the previous decade.

The concentration varies wildly. CIR’s records show a range from 0.0008% to 99%. The 99% figure is in bath products that get diluted in the tub; the highest leave-on use is in deodorants and antiperspirants, where it reaches 73%. In a typical moisturizer it’s a small fraction of that.

That range is the key to understanding propylene glycol. The same ingredient that is unremarkable at 2% in a lotion can sting and redden the skin at 70% in a deodorant stick.

What the safety reviews say

On toxicity, the verdict is calm. CIR’s 2012 assessment describes propylene glycol as “generally nontoxic” and non-carcinogenic. Its original 1994 review concluded it was safe in cosmetics up to 50%; when use climbed above that, the panel reopened the file, reviewed a repeat-insult patch test in which 99 volunteers used an antiperspirant with 86% propylene glycol without becoming sensitized, and in 2012 amended its conclusion to “safe as used in cosmetic formulations when formulated to be nonirritating”.

Read that last clause twice. The panel found “an absence of dermal sensitization at use concentrations, although concerns about irritation remained”. In other words, propylene glycol rarely causes true allergy, but it can irritate, and the manufacturer is expected to formulate around that.

Irritation versus allergy: why the two get confused

This distinction is the heart of the propylene glycol story, so it’s worth being precise.

Irritant contact dermatitis is direct damage: the substance itself disturbs the skin barrier, and anyone gets it if the dose is high enough. Allergic contact dermatitis is an immune reaction: your body has learned to recognize the substance, and even small amounts trigger an itchy rash a day or two later. Only the second is a true allergy.

Propylene glycol does both. A 2018 systematic review in the journal Dermatitis puts it neatly: it “acts as both a weak sensitizer and an irritant, confounding the results of positive patch tests”. A patch test applies the substance under a patch on your back for two days; propylene glycol at the 30% concentration used in North America can produce a faint red patch from irritation alone that’s hard to tell from a weak allergic reaction. The right test concentration has been debated for years and, as DermNet notes, “many reactions are weak” and easily over- or under-read.

That confusion is why the American Contact Dermatitis Society named propylene glycol its Allergen of the Year for 2018. The title isn’t a ranking of danger; it’s given to allergens that are under-recognized or whose exposure patterns have changed, and the society wanted dermatologists to look harder at this one.

How common is a real reaction?

Among people referred for patch testing because of a rash, DermNet gives a positive rate of up to 3.5%, the figure the North American Contact Dermatitis Group has reported. A Vancouver contact dermatitis clinic reported a lower 1.57%, 13 of 828 patients. Remember that these are people who already have a skin problem, so the rate in the general population is lower still.

When a reaction is real, the face is the most common site, followed by a widespread pattern, according to DermNet. Rarely, someone who is allergic through the skin reacts to propylene glycol they’ve eaten, a phenomenon called systemic contact dermatitis. The British Society for Cutaneous Allergy notes that eating it is unlikely to cause serious complications, but it can flare eczema in sensitized people.

The hiding place that matters most: prescription creams

Here’s the twist that makes propylene glycol different from most “free-from” ingredients. It is a favourite solvent in topical corticosteroids, the anti-inflammatory creams and ointments prescribed for eczema and other rashes. A 2011 review by dermatologists in Vancouver found propylene glycol in 64% of the steroid products they examined, and DermNet describes it as the most frequently reported allergen in topical corticosteroids.

Think about what that means for someone with eczema. They’re prescribed a cream for a rash, the cream contains an ingredient they’ve become sensitized to, and the rash doesn’t improve or gets worse. It looks like the treatment is failing. This is one of the main reasons dermatologists patch test people whose eczema won’t settle, and it’s why, if you have a confirmed propylene glycol allergy, your pharmacist should check the non-medicinal ingredients of every prescription cream, not just the drug.

What to do with this

If you’ve never had a problem with propylene glycol, there’s no reason to start avoiding it. Regulators and safety panels consider it safe as used, and true allergy is uncommon.

If a deodorant or a high-concentration product stings or reddens your skin, that’s most likely irritation, not allergy. Switching to a product where propylene glycol is lower on the list, or absent, usually solves it.

If patch testing has confirmed an allergy, avoidance is the treatment, and it takes work. The British Society for Cutaneous Allergy’s advice is blunt: always check the ingredient list on the product, the package or the insert, because “these lists can never be complete and ingredients change”. Look at cosmetics, prescription creams, ear and nose drops, wound dressings, e-cigarette liquid and food labels. Your pharmacist can help find propylene-glycol-free versions of topical medicines.

And if a rash isn’t better after two weeks of avoiding the products you suspect, see your doctor or pharmacist rather than continuing to guess.

Propylene glycol is a workhorse that earns its place in most formulas. It’s neither toxic nor a strong allergen. Its one real fault is that it can irritate at the high concentrations some products use, and that irritation is easy to mistake for allergy. Knowing the difference, and knowing where it hides, is most of what you need.