Polyhydroxy acids (PHAs) are chemical exfoliants that smooth, hydrate, and strengthen skin while causing significantly less irritation than traditional acids like glycolic or lactic acid. They work on the skin’s surface to dissolve dead cells and improve texture, but their larger molecular size means they absorb more slowly and gently, making them one of the few chemical exfoliants suitable for sensitive, reactive, or rosacea-prone skin.
How PHAs Exfoliate Differently
PHAs belong to the same family as alpha hydroxy acids (AHAs), but their molecules are substantially larger. Where glycolic acid (the smallest AHA) penetrates quickly and deeply into the outer skin layer, PHAs take their time. That slower absorption rate is the key to their gentleness: you get the same surface-level exfoliation, dissolving the bonds between dead skin cells so they shed more evenly, without the stinging, redness, or peeling that glycolic acid can trigger.
Each PHA molecule carries two or more hydroxyl groups, which are water-attracting units. Traditional AHAs have just one. Those extra hydroxyl groups do double duty: they help break down dead skin while simultaneously pulling moisture into the skin’s surface. This is why PHAs hydrate as they exfoliate, something glycolic acid doesn’t do on its own.
The Three PHAs You’ll See in Products
Most skincare formulations use one of three PHAs, each with slightly different strengths:
- Gluconolactone is the most common PHA in over-the-counter products. It exfoliates, attracts moisture, and has demonstrated the ability to protect against UV-induced damage in lab studies. In one clinical comparison, glycolic acid caused a significant increase in sunburn cells after UV exposure, while gluconolactone did not.
- Lactobionic acid is technically classified as a bionic acid (a subtype of PHA). It functions as a strong humectant, promotes cell turnover, and acts as an antioxidant by binding to iron molecules that would otherwise generate skin-damaging free radicals. It also inhibits enzymes called matrix metalloproteinases that break down collagen when skin is exposed to UV light, giving it a role in preventing visible sun damage over time.
- Maltobionic acid shares many of lactobionic acid’s properties, including antioxidant activity and the ability to form a gel-like moisture layer on the skin’s surface. This gel matrix can have a soothing, protective effect on irritated or inflamed skin.
Moisture and Barrier Benefits
PHAs are genuinely hygroscopic, meaning they pull water from the environment and hold it against your skin. Lactobionic acid and maltobionic acid are especially effective at this. When their water-rich solutions dry down, they form a thin gel matrix on the skin’s surface that locks in hydration and acts as a physical buffer.
This goes beyond simple moisturizing. PHA-treated skin shows less transepidermal water loss, which is the technical term for moisture escaping through your skin into the air, compared to skin treated with glycolic or lactic acid. PHAs also strengthen the stratum corneum (the outermost barrier layer), making skin more resistant to irritants. In one study, skin treated with PHAs held up better against sodium lauryl sulfate, a common detergent used to test barrier resilience, than AHA-treated skin did.
Why PHAs Suit Sensitive and Reactive Skin
If you’ve tried glycolic acid or even lactic acid and experienced burning, tightness, or flaking, PHAs are worth considering. Clinical comparisons between PHA and AHA regimens have found consistently better tolerance with PHAs. In a 12-week trial, subjects using AHAs reported significantly more stinging, burning, and overall sensitivity at both the 6-week and 12-week marks compared to those on a PHA regimen.
PHAs have also been tested specifically in people with rosacea and atopic dermatitis (eczema), two conditions where most chemical exfoliants are off the table. In rosacea patients, a gluconolactone-based regimen reduced facial dryness, burning, stinging, itching, and skin tightness. Researchers noted visible improvement in background redness and attributed it to the barrier-strengthening effects of gluconolactone. PHAs are also commonly recommended after cosmetic procedures like chemical peels or laser treatments, when skin is temporarily more vulnerable.
PHAs and Sun Sensitivity
One well-known drawback of AHAs is that they increase your skin’s sensitivity to UV radiation. Glycolic acid thins the outermost layer of dead cells, leaving fresh skin more exposed to sunburn. PHAs don’t appear to carry the same risk. In a study published in Dermatologic Surgery, gluconolactone provided up to 50% protection against UV-induced damage in lab models and did not cause a significant increase in sunburn cells in human skin after UV exposure. That’s a meaningful difference if you’re using an exfoliant daily, especially in warmer months.
This doesn’t mean you can skip sunscreen. PHAs aren’t a substitute for UV protection. But the fact that they don’t actively increase your sun vulnerability is a practical advantage over glycolic and lactic acid.
How PHAs Compare to AHAs and BHAs
All three acid families exfoliate, but they work at different depths and suit different skin types. AHAs like glycolic and lactic acid are water-soluble and work on the skin’s surface, dissolving dead cells to brighten and smooth. BHAs (primarily salicylic acid) are oil-soluble, so they penetrate into pores and clear out sebum and debris. PHAs work at the surface like AHAs but absorb more slowly and deliver stronger hydration.
In practical terms: if your main concerns are clogged pores and excess oil, salicylic acid is the better tool. If you want brightening, anti-aging exfoliation, and your skin can handle it, glycolic acid is effective. But if you want gentle exfoliation with built-in hydration and you have sensitive, dry, or reactive skin, PHAs cover that ground without the tradeoffs. People with dry skin can benefit from either AHAs or PHAs, but PHAs will generally cause less flaking and tightness because of their humectant properties.
How to Use PHAs
PHAs appear in toners, serums, cleansers, and moisturizers. Because they’re gentler than AHAs, most people can use them daily without building up tolerance first. You’ll typically find them listed as gluconolactone or lactobionic acid in the ingredient list. Start with one PHA product in your routine rather than layering multiple exfoliants, and apply it to clean skin before heavier creams or oils so the acid can make contact with the skin’s surface.
If you’re already using retinoids or other active ingredients, PHAs are generally easier to combine than AHAs because they cause less irritation and don’t thin the skin barrier in the same way. That said, introducing any new active one at a time lets you gauge how your skin responds before adding more.