AHA (alpha hydroxy acid) and PHA (polyhydroxy acid) are both chemical exfoliants that dissolve dead skin cells to reveal smoother, brighter skin underneath. They belong to the same family of hydroxy acids but differ in molecular size, which changes how deeply they penetrate, how much irritation they cause, and which skin types they work best for. AHAs are the classic, well-studied option for visible results, while PHAs are a gentler next-generation alternative designed for people whose skin can’t tolerate traditional acids.
How AHAs Work
Alpha hydroxy acids are small, water-soluble molecules that loosen the “glue” holding dead skin cells together on the surface of your skin. More specifically, their acidic nature lowers the skin’s pH, which interferes with the bonds (called desmosomes) between cells in the outermost layer. This triggers those dead cells to shed faster than they would on their own, a process called desquamation.
The most common AHAs in skincare are glycolic acid, derived from sugarcane, and lactic acid. Glycolic acid is the smallest and simplest of the group, which means it penetrates skin quickly and delivers the most noticeable exfoliation. Lactic acid is slightly larger and tends to be milder. Other AHAs you might see on ingredient lists include mandelic acid, citric acid, and tartaric acid, though glycolic and lactic dominate the market.
Beyond surface exfoliation, AHAs promote increased expression of collagen and hyaluronic acid in deeper skin layers, which improves plumpness and hydration over time. They also reduce calcium levels in skin cells in a way that encourages new cell growth while slowing premature differentiation, which is part of why they make skin look younger with consistent use.
What AHAs Do for Your Skin
The results from AHA use are well documented. In one clinical study, applying 8% glycolic acid or lactic acid for 22 weeks led to noticeable improvement in the appearance and smoothness of sun-damaged skin for roughly 70 to 76% of participants. A separate trial found that 12% lactic acid used for three months increased both epidermal and dermal firmness and thickness, with visible improvement in lines, wrinkles, and skin smoothness. Even at lower concentrations, 5% glycolic acid cream improved skin texture and discoloration after three months.
In a 21-day study using an AHA-based skincare system, wrinkle depth decreased by about 16% after just one week and by 32.5% after three weeks. By day 21, over 73% of participants reported visible reduction in fine lines, and nearly 56% noticed improvement in skin pigmentation. These are faster results than many people expect from a topical product.
How PHAs Differ
Polyhydroxy acids work through the same basic mechanism as AHAs, breaking down bonds between dead skin cells to encourage exfoliation. The key difference is size. Gluconolactone, the most common PHA, has a molecular weight of 178, compared to just 76 for glycolic acid. That larger molecule penetrates skin more gradually, which is why PHAs provide similar exfoliating and anti-aging effects without the stinging, burning, or redness that AHAs often cause.
The two PHAs you’ll encounter most often are gluconolactone and lactobionic acid. Both have multiple hydroxyl groups in their structure (hence “polyhydroxy”), and those extra groups give PHAs a bonus feature: they attract and hold water against the skin surface, acting as humectants. This means PHAs hydrate while they exfoliate, whereas AHAs can sometimes leave skin feeling tight or dry, especially at higher concentrations.
PHAs and Sensitive Skin
PHAs were specifically developed to solve the irritation problem that limits AHA use for many people. Research confirms that PHAs provide similar effects to AHAs but do not cause the sensory irritation responses, like stinging and burning, that make traditional acids difficult for reactive skin. If you’ve tried glycolic acid and found it too harsh, a PHA product is the logical next step.
PHAs are also well tolerated alongside other potentially irritating treatments. Clinical studies have combined PHA products with retinoids for adult acne treatment and found good tolerability. PHAs paired with pro-vitamin A showed significant anti-aging benefits like skin smoothing and plumping. They’ve even been used alongside procedures like laser treatments and microdermabrasion to enhance results without compounding irritation.
Sun Sensitivity: A Major Difference
One of the most important practical differences between these two acids is how they interact with UV exposure. AHAs are known to increase your skin’s sensitivity to the sun, which is why sunscreen is non-negotiable when using them. The U.S. FDA has specifically flagged this concern in its guidance on AHA-containing products.
PHAs tell a very different story. Gluconolactone provided up to 50% protection against UV radiation in laboratory skin models and did not significantly increase sunburn cells in human skin. So rather than making your skin more vulnerable to sun damage, PHAs may actually offer a mild protective effect. This doesn’t replace sunscreen, but it does make PHAs a more forgiving choice if you’re often outdoors or inconsistent with sun protection.
Concentration Guidelines
For AHAs, the U.S. FDA recommends that over-the-counter cosmetic products contain no more than 10% AHA with a pH of 3.5 or higher. The European Union sets stricter limits: up to 4% glycolic acid at a pH of at least 3.8, or up to 2.5% lactic acid at a pH of at least 5. Products above 10% are generally considered professional-grade, and concentrations above 30% are treated as medical-level peels.
If you’re new to AHAs, starting at 5% or below is a safe approach. At these low concentrations, regulatory bodies have found no evidence of harm even for more vulnerable populations. You can increase concentration gradually as your skin builds tolerance. PHA concentrations aren’t as tightly regulated because the irritation risk is so much lower, but most consumer products contain PHAs in roughly the same 4 to 15% range.
Using AHAs or PHAs With Other Products
A common concern is whether acids can be combined with retinol or vitamin C. The short answer: yes. There is no research showing that AHA or BHA exfoliants deactivate or reduce the effectiveness of retinol when used in the same routine. In fact, combining AHAs with retinol has been shown to improve results for hyperpigmentation and fine lines beyond what either ingredient achieves alone. This works in part because retinol is dissolved in oil, making it a waterless ingredient with no pH to conflict with acidic products.
Vitamin C and retinol also pair well together, providing complementary antioxidant defense against free radicals. And niacinamide, another popular active, can be layered with retinol without issue. Many dermatologists recommend this combination because niacinamide helps keep skin calm while retinol targets wrinkles. PHAs are even easier to layer than AHAs since they’re less likely to tip your skin into irritation when combined with other actives.
Choosing Between AHA and PHA
If your primary goal is maximum exfoliation, faster cell turnover, or treating noticeable sun damage and hyperpigmentation, AHAs deliver stronger, faster results. Glycolic acid in particular penetrates deeply and has decades of clinical evidence behind it. The tradeoff is a higher likelihood of irritation, dryness, and increased sun sensitivity.
PHAs are the better fit if you have sensitive or reactive skin, if you’re already using other strong actives like retinoids, or if you want gentle daily exfoliation with built-in hydration. They won’t produce results as quickly as a high-concentration glycolic acid, but they achieve similar outcomes over time with far less risk of irritation. Many people also use both: a PHA product for daily maintenance and an AHA treatment once or twice a week for deeper exfoliation.