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Alpha hydroxy acids are the oldest actives in modern cosmetic dermatology and the most consistently misused. Glycolic acid in particular has a reputation as the strong one, which is accurate, and a reputation as the best one, which is not. The choice between glycolic, lactic, and mandelic acid is a genuine trade-off between penetration speed and tolerability, and the regulatory limits on concentration and pH exist for reasons worth understanding before you buy a 30% peel off the internet.
What AHAs Actually Are
Alpha hydroxy acids are water-soluble organic acids with a hydroxyl group attached to the carbon adjacent to the acid group. The family includes glycolic acid (from sugar cane), lactic acid (from milk fermentation, though cosmetic grades are synthesised), mandelic acid (from bitter almonds), malic acid (apples), and tartaric acid (grapes).
Their mechanism is well characterised: they weaken the ionic bonds holding corneocytes — dead surface skin cells — together, which accelerates the natural shedding process at the top of the stratum corneum. The immediate visible result is smoother texture and better light reflection. Over longer use, the literature also describes increased dermal glycosaminoglycan and collagen content, which is the basis for the anti-ageing claims attached to them.
A useful review in Cosmetics (2023) covering topical AHA formulations, mechanisms, and efficacy notes that while the effect on texture and collagen synthesis is real, meaningful uncertainty remains about which concentration, pH, and vehicle combination is optimal — which is a fair summary of why product results vary so much.
Why Molecule Size Decides Everything
Glycolic acid has the smallest molecular weight of any AHA at roughly 76 daltons. That single fact drives almost everything else about it. Smaller molecules pass through the stratum corneum faster and reach deeper, which makes glycolic the most potent AHA per unit concentration — and, inseparably, the most irritating.
Lactic acid is meaningfully larger at about 90 daltons. It penetrates more slowly, which makes it gentler, and it has an additional property glycolic doesn't: lactic acid is a component of skin's natural moisturising factor, so it is genuinely humectant as well as exfoliating. For dry or sensitive skin, lactic acid is frequently the better choice and is under-recommended relative to glycolic.
Mandelic acid is larger still, around 152 daltons, and penetrates slowest of the three. That makes it the most tolerable option for reactive skin and for deeper skin tones, where aggressive exfoliation carries a higher risk of post-inflammatory hyperpigmentation. It is not a weaker ingredient so much as a slower one.
The practical framing: pick the acid for your skin's tolerance, not for its reputation. A lactic acid product you use three times a week will do more than a glycolic product that drives you to stop after four days.
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View on Amazon →Concentration and pH: The Real Rules
This is where most consumer confusion lives, and there are actual published limits. For consumer products, the assessment position is that glycolic and lactic acid are safe at concentrations of 10% or less, at a final formulation pH of 3.5 or higher, provided the product is formulated to avoid increasing sun sensitivity or carries directions for daily sun protection. For salon and professional use, the limits open up to 30% at pH 3.0 or higher, in products designed for brief application followed by thorough rinsing, applied by trained practitioners.
That distinction matters. A 30% glycolic peel is a professional-use concentration with a rinse-off protocol behind it, not a leave-on product for home use, and the strength-equals-results instinct is exactly backwards here.
pH is as important as percentage and almost never disclosed. AHAs exfoliate in their free acid form, and the proportion of free acid rises as pH falls. A 10% glycolic product at pH 3.8 is a meaningfully milder product than a 10% glycolic product at pH 3.2, despite identical labelling. When a brand publishes its pH, that is a genuine signal of formulation transparency.
One more variable worth knowing: partially neutralised formulas — where some of the acid is buffered into salt form — trade a little potency for a lot of tolerability, and are often what makes a daily-use AHA toner workable.
AHA vs BHA: Which Problem Are You Solving?
The distinction is solubility, and it maps cleanly onto two different problems. AHAs are water-soluble and work on the skin's surface, which makes them the right tool for texture, dullness, uneven tone, and fine surface lines. Salicylic acid, the BHA covered in our dedicated guide, is oil-soluble and can travel into the sebum-filled pore, which makes it the right tool for blackheads and congestion.
If your complaint is rough texture and a dull, flat look, an AHA is the answer. If it's blackheads on the nose and clogged pores, a BHA is. If it's both, you don't need a combination product on day one — you need to introduce one, establish tolerance, and then consider alternating them on different nights.
Polyhydroxy acids (PHAs) such as gluconolactone deserve a mention as the gentlest tier. Their molecules are larger again, penetration is slower still, and they carry additional humectant properties. For rosacea-prone or genuinely intolerant skin, a PHA is often the only exfoliant that works out.
The Sun Sensitivity Question
AHAs increase UV sensitivity, and unlike most claims in this space, this has been directly measured rather than assumed. In FDA-supported work, volunteers using an AHA product for four weeks showed roughly an 18% increase in sensitivity to UV-induced redness. The effect was also shown to be reversible: a week after stopping, there was no significant difference in UV sensitivity compared with untreated skin.
That is a real effect but a manageable one, and the practical conclusion is straightforward — daily sunscreen is not optional alongside AHA use. It is worth noting separately that in animal studies, glycolic acid did not affect photocarcinogenesis, so the concern here is sunburn risk and pigmentation rebound rather than a demonstrated cancer risk.
Applying AHAs at night is sensible, not because they are deactivated by daylight, but because it puts the window of peak sensitivity during the hours you aren't exposed and gives skin an uninterrupted period to recover.
How to Introduce It Without Wrecking Your Barrier
Start at twice a week, at night, on dry skin, and hold there for two to three weeks before increasing. The most common self-inflicted injury in skincare is a new exfoliant used daily from day one, which produces a barrier that is stripped, shiny, stinging, and — confusingly — often looks good for about five days before it doesn't.
Don't layer an AHA with retinoids in the same session while you're establishing tolerance. Alternate nights instead. The same goes for stacking an AHA and a BHA, or an AHA and a high-strength vitamin C, none of which is dangerous but all of which make it impossible to attribute a reaction to a cause.
Some tingling on application is normal. Stinging that persists, burning, or visible redness that outlasts the application by more than a few minutes is not — that is your signal to reduce frequency or drop to a gentler acid. And if your skin becomes tight, unusually reflective, and reactive to products that never bothered it before, that is over-exfoliation, and the fix is to stop entirely for two weeks and rebuild with a bland moisturiser rather than to add a repair serum on top.
Frequently Asked Questions
Is glycolic acid better than lactic acid?+
Not better — stronger. Glycolic acid's smaller molecule (about 76 daltons vs 90 for lactic) penetrates faster and deeper, which means more potency and more irritation. Lactic acid is gentler and additionally humectant because it's part of skin's natural moisturising factor. For dry or sensitive skin, lactic is often the smarter choice.
What percentage of glycolic acid should I use?+
For leave-on home products, the safety assessment position is 10% or less at pH 3.5 or higher. Higher concentrations — up to 30% at pH 3.0 — are professional-use peels intended for brief application and thorough rinsing. Starting at 5-7% is sensible for almost everyone; going stronger rarely improves results and reliably increases irritation.
Do I really need sunscreen with AHAs?+
Yes. This is one of the few sun-sensitivity claims that has been directly measured: four weeks of AHA use increased UV-induced redness sensitivity by roughly 18%. The effect reverses about a week after stopping, but while you're using an AHA, daily sunscreen is genuinely necessary rather than a generic recommendation.
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Glowstice Editorial
The Glowstice editorial team consists of skincare researchers, cosmetic chemists, and science writers dedicated to translating peer-reviewed dermatology into practical guidance for curious consumers.
