AHA or BHA: glycolic, lactic and salicylic acid compared
Alpha hydroxy acids are water soluble and act on the surface layers, while salicylic acid is lipid soluble, with a log octanol water partition coefficient of 2.25, so it follows sebum into the pore. Strength is set by pH as much as by percentage, because an alpha hydroxy acid drives cell turnover mainly in its un-ionised form, and that turnover peaks near pH 3. A European committee put leave-on glycolic acid at up to 4 percent at pH 3.8 or above and lactic acid at 2.5 percent at pH 5 or above, while EU law caps salicylic acid at 2.0 percent in most leave-on products.
What separates an alpha acid from a beta acid
An alpha hydroxy acid is a carboxylic acid carrying a hydroxyl group on the alpha carbon. Glycolic acid is alpha hydroxy acetic acid, lactic acid is alpha hydroxy propionic acid, and citric acid, 2-hydroxyoctanoic acid and 2-hydroxydecanoic acid sit in the same family. Salicylic acid is sold as a beta hydroxy acid, but both its hydroxyl and its carboxyl group sit on a benzene ring, which makes it a phenolic aromatic acid, and a review of the chemistry states that it does not act physiologically as one.
Size and solubility decide where a molecule stops. Glycolic acid is the smallest of the group and crosses the stratum corneum most readily; lactic acid weighs 90 g per mol, mandelic acid 152 and citric acid 192, and larger molecules cross more slowly, so mandelic acid is gentler. All are small polar molecules that weaken the bonds holding dead cells together, so the top layers shed. Salicylic acid, at 138.12 g per mol with a log partition coefficient of 2.25, leaves water for oil and collects in the sebum filled follicle.
Hydroxy acids have been used in dermatology from 2 percent to 70 percent, for acne, keratoses, psoriasis and sun damaged skin. At 4 to 10 percent they are everyday cosmetic ingredients; above 20 percent they are peels. Salicylic acid runs from 0.5 to 10 percent for acne and 20 to 30 percent for a superficial peel, left on 3 to 5 minutes and repeated 3 to 6 times at 2 to 4 week intervals. Those last figures describe a clinic, and the guide to peel depths explains why.
| Acid | Class and solubility | Where it acts | Leave-on figure used in Europe | Effect on UV sensitivity |
|---|---|---|---|---|
| Glycolic acid | Alpha hydroxy, water soluble, smallest of the group | Upper stratum corneum, whole face | 4 percent at pH 3.8 or above, committee advice | Raised it at 10 percent; back to normal 1 week after stopping |
| Lactic acid | Alpha hydroxy, water soluble, 90 g per mol | Upper stratum corneum, also holds water | 2.5 percent at pH 5 or above, committee advice | A 1.4 percent blend at pH 4.2 changed nothing over 6 months |
| Mandelic acid | Alpha hydroxy, water soluble, 152 g per mol | Shallower and slower than glycolic acid | No separate figure | Not measured separately |
| Citric acid | Alpha hydroxy, 192 g per mol, often a pH adjuster | Surface, usually a minor ingredient | No separate figure | Not measured separately |
| Salicylic acid | Phenolic aromatic acid, lipid soluble, 138.12 g per mol | Inside the sebum filled follicle | 2.0 percent in most leave-on products, 0.5 percent in body lotion, eye and lip products and roll-on deodorant | 2 percent did not change it in a 14 person study |
| Polyhydroxy acids such as lactobionic acid | Larger molecules with several hydroxyl groups | Surface, more slowly, with less irritation | No separate figure | Not measured separately |
Why the pH matters more than the percentage
An alpha hydroxy acid drives cell turnover mainly while it is un-ionised, that is, while it still holds its proton. Raise the pH and more of it sits as a salt that does little. Turnover in the stratum corneum is highest around pH 3, and the useful band runs from roughly pH 2.8 to 4.8.
The same arithmetic governs irritation, and it outweighs the percentage. Data gathered for the European position paper showed irritation falling sharply as pH rose from 3 to 7, and cumulative testing found 13 percent glycolic acid at pH 4.4 milder than 8 percent at pH 3.25. The rest of the formula counts too: one 8 percent product at pH 3.6 was rated milder than several 4 percent products at pH 3.7.
Salicylic acid behaves differently again. Its pKa is 2.9, so at the pH of an ordinary cleanser much of it is already ionised, and its oil solubility rather than its acidity carries it where it is wanted. That is the working split: an alpha hydroxy acid is a surface treatment that needs an acidic formula, a beta hydroxy acid is a pore treatment that is less fussy about pH. The Skin Type Finder sorts out which suits you.
What a leave-on product is allowed to contain
The first position came from industry review in the United States. In 1997 the Cosmetic Ingredient Review panel judged glycolic and lactic acid safe in consumer cosmetics up to 10 percent at pH 3.5, provided the product avoids increasing sun sensitivity or carries directions for daily sun protection. For salon products it accepted up to 30 percent at pH 3.0, for brief use, rinsed off and applied by a trained person.
Europe went lower. A position paper adopted on 28 June 2000 applied the precautionary principle and suggested glycolic acid at up to 4 percent with a pH of 3.8 or above, and lactic acid at up to 2.5 percent with a pH of 5 or above, with warnings to keep the product out of the eyes and to protect the skin from UV during use. The committee looked again in 2004 and kept those figures. That is committee advice rather than a figure in the annexes of the Cosmetics Regulation, so stronger products are still on sale.
Salicylic acid is different: it is written into the Regulation itself. It is allowed at 3.0 percent in rinse-off hair products, 2.0 percent in most other products and 0.5 percent in body lotion, eye shadow, mascara, eyeliner, lipstick and roll-on deodorant, and not at all in preparations for children under 3 years of age. The Scientific Committee on Consumer Safety reviewed those numbers in an opinion adopted on 6 and 7 June 2023 and kept them, calculating with an assumed dermal absorption of 60 percent.
Shelves are wider than the advice: the product survey behind the 2000 position paper found consumer products with 1 to 26 percent glycolic acid and salon preparations with 3 to 67 percent.
What the acids do to sun sensitivity
In 16 volunteers with phototypes 1 to 3, 2 mg per square centimetre of a product declared as 10 percent glycolic acid at pH 3.5 went on the mid back daily for 4 weeks. The dose of simulated sunlight needed to redden the treated site fell to 11.8, against 14.4 on the vehicle site and 14.7 on untreated skin, a drop of about 18 percent. Sunburn cells rose to 2.97 per high power field against 1.53 and 1.10.
The effect does not last. Nothing was applied in a fifth week, and by the end of it the reddening dose read 13.4 against 14.3 and sunburn cells 1.30 against 1.26, with no statistical difference. Weaker products do less: 4 percent glycolic acid at pH 3.8 twice a day for 3 months lowered the reddening dose by 13 percent, although 3 subjects fell by 48 to 50 percent, and a 1.4 percent blend at pH 4.2 twice a day for 6 months changed nothing measurable.
Salicylic acid did not do this. In a separate study of 14 subjects, 10 percent glycolic acid raised sensitivity to simulated solar radiation while 2 percent salicylic acid did not. The DNA evidence is thin: a 12 subject study found more pyrimidine dimers on glycolic acid treated skin, 42.9 against 39.7 for vehicle, but the difference was not significant, and the statistician calculated that at least 108 subjects would be needed. Every one of these trials recruited phototypes 1 to 3.
Skin that needs a different answer
Inflamed skin is the first exclusion. Even weak acids are felt: when 100 people applied a 0.5 percent mixture of lactic and glycolic acid to the face once a day for 6 weeks, 26 percent reported itching or stinging. If you have eczema or rosacea, treat the flare first and keep acids away from it. Absorption through diseased skin is higher, and toxic effects from salicylic acid have followed application to large areas of it. The Barrier Check is a way to judge that.
Pregnancy changes the salicylic acid answer. Salicylic acid peels are not recommended during pregnancy, because the molecule is close kin to acetylsalicylic acid. Salicylate toxicity starts above a blood level of 35 mg per decilitre, and a case has been reported after 20 percent salicylic acid was applied to half the body surface. EU law separately bars salicylic acid from products for children under 3 years. If you are pregnant or breastfeeding, ask your midwife or doctor first.
With a deeper skin tone the risk moves from redness to pigment. In a salicylic acid peel series of 35 Korean patients, 8.8 percent had redness lasting more than 2 days and 32.3 percent had dryness. In a separate series of 25 people with darker skin, 16 percent had mild side effects: 1 case of temporary crusting and pale patches that cleared in 7 days, and 3 of dryness or darkening that settled in 7 to 14 days, with no scarring and no lasting pigment change. Start low, use one acid at a time, and see how often to exfoliate. Wear sunscreen every morning while an alpha hydroxy acid is in use and for the week after, and see a doctor if you get burning, blistering or a spreading rash.
Choosing between the two
- Oily skin, blackheads and closed comedones point to salicylic acid, because at a log partition coefficient of 2.25 it follows sebum into the follicle.
- Dry or easily stung skin points to lactic acid at 2.5 percent or below with a pH of 5 or above.
- For texture and dark marks across the whole face, 4 percent glycolic acid at pH 3.8 or above is the European guide figure.
- Read the pH as well as the percentage: 13 percent at pH 4.4 was milder than 8 percent at pH 3.25 in cumulative testing.
- Use one acid at a time, patch test on the inner forearm for 3 to 5 days, and wear sunscreen every morning while an alpha hydroxy acid is in use and for 1 week afterwards.
- Skip acids while eczema or rosacea is active, and skip salicylic acid peels in pregnancy.
Frequently asked
Is salicylic acid really a beta hydroxy acid?
Chemically it is a phenolic aromatic acid, because both the hydroxyl and the carboxyl group sit on a benzene ring. A review of hydroxy acid chemistry states that it does not act physiologically as a beta hydroxy acid, although the label has stuck to the shelf.
Can I use an AHA and a BHA in the same routine?
They do a similar job by different routes, and stacking them mainly buys irritation. In one panel of 100 people using a 0.5 percent mixture of lactic and glycolic acid once a day for 6 weeks, 26 percent reported itching or stinging, so start with one acid and add nothing for at least 4 weeks.
Do I need sunscreen with a BHA as well?
The measured sun sensitivity problem belongs to the alpha hydroxy acids: in a study of 14 subjects, 10 percent glycolic acid raised sensitivity to simulated sunlight while 2 percent salicylic acid did not. Daily sunscreen is still sensible, since the acids are usually used on skin with pigment marks or acne scars.
Why does a 10 percent acid sometimes sting less than a 5 percent one?
Because pH decides how much of the acid is in its active un-ionised form. Cumulative testing found 13 percent glycolic acid at pH 4.4 less irritating than 8 percent at pH 3.25, and one 8 percent product at pH 3.6 was milder than several 4 percent products at pH 3.7.
Is salicylic acid safe to use in pregnancy?
Salicylic acid peels are not recommended during pregnancy, and EU law separately bars salicylic acid from products for children under 3 years of age. A low strength wash is a different exposure from a 20 percent peel, but the decision belongs with your midwife or doctor rather than a label.
Are these acids riskier on darker skin?
The risk shifts from redness to pigment, and the evidence is thinner: every photosensitivity trial cited here recruited phototypes 1 to 3 only. In one salicylic acid peel study in a group of 25 people with darker skin, 16 percent had mild side effects, including one case of temporary crusting and pale patches that cleared in 7 days.
Sources
- Scientific Committee on Cosmetic Products and Non-Food Products intended for Consumers, position paper on the safety of alpha-hydroxy acids, SCCNFP/0370/00, adopted 28 June 2000 · European Commission committee document, read in full. Source of the European advice figures (glycolic acid 4 percent at pH 3.8 or above, lactic acid 2.5 percent at pH 5 or above), the 1997 Cosmetic Ingredient Review limits of 10 percent at pH 3.5 for consumers and 30 percent at pH 3.0 for salons, the turnover optimum at pH 3 and the band of pH 2.8 to 4.8, the product survey figures, the irritation data (13 percent at pH 4.4 against 8 percent at pH 3.25; the panel of 100 people with 26 percent reporting irritation), and the full results of the regulator initiated photosensitivity studies in 16 and 12 subjects, including the 4 percent and 1.4 percent follow up studies.
- Scientific Committee on Consumer Safety, opinion on salicylic acid (CAS No. 69-72-7, EC No. 200-712-3), SCCS/1646/22, adopted 6 and 7 June 2023 · Final opinion of the EU scientific committee, 75 pages, read for the physicochemical and regulatory sections. Source of the molecular weight 138.12 g per mol, the octanol water partition coefficient of 2.25, the pKa of 2.9, the assumed dermal absorption of 60 percent, the Annex III limits of 3.0, 2.0 and 0.5 percent by product type, the ban on preparations for children under 3 years, and the note that toxic effects have followed application to extensive areas of diseased skin.
- Kornhauser A, Coelho SG, Hearing VJ. Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clin Cosmet Investig Dermatol. 2010;3:135 to 142 · Open access review by regulatory and cancer institute scientists, read in full. Source of the 2 percent to 70 percent range across dermatology, the 4 to 10 percent cosmetic band and the peel threshold above 20 percent, the chemistry point that salicylic acid does not act physiologically as a beta hydroxy acid, the 14 subject study in which 10 percent glycolic acid raised sensitivity to simulated solar radiation while 2 percent salicylic acid did not, and the authors' own observation that all these trials used phototypes 1 to 3.
- Arif T. Salicylic acid as a peeling agent: a comprehensive review. Clin Cosmet Investig Dermatol. 2015;8:455 to 461 · Peer reviewed review, read in full. Source of the acne range of 0.5 to 10 percent and the superficial peel range of 20 to 30 percent, the contact time of 3 to 5 minutes and the schedule of 3 to 6 peels at 2 to 4 week intervals, the side effect rates in a series of 35 Korean patients and, separately, in a series of 25 people with darker skin where 16 percent had mild side effects, the salicylate toxicity threshold of 35 mg per decilitre and the case after 20 percent on half the body surface, and the advice against salicylic acid peels in pregnancy.
- Almeman AA. Evaluating the Efficacy and Safety of Alpha-Hydroxy Acids in Dermatological Practice. Clin Cosmet Investig Dermatol. 2024;17:1661 to 1685 · Open access clinical and regulatory review, read for the chemistry table and the comparison of national limits. Source of the molecular weights of lactic acid (90), mandelic acid (152) and citric acid (192), the point that a larger molecule such as mandelic acid penetrates more slowly and irritates less, the side by side summary of the United States and European limits, the note that the European position of 2000 was reaffirmed in 2004, and the report of mild post-inflammatory hyperpigmentation that resolved without treatment in a glycolic acid group.
Read on
Skincare
How often to exfoliate: by skin type and acid
Aesthetics
Chemical peels: superficial, medium and deep
Skincare
A damaged skin barrier: signs, causes and repairTools that fit
This guide is for general information. It does not replace advice from a dermatologist, physician or qualified practitioner, or an examination.