AHA, BHA, PHA: the complete 2026 guide to exfoliating acids in skincare

What is chemical exfoliation?

Quick answer: Chemical exfoliation uses acids to break the molecular bonds (desmosomes) that hold dead cells to the surface of the epidermis, so they shed naturally without rubbing or abrasion.

Your epidermis renews itself constantly: every 28 to 40 days in a young adult, a new generation of keratinocytes migrates from the deep layers to the surface. With age, the process slows. By 40, the cycle can reach 50 to 60 days, and dead cells accumulate, dulling your complexion, enlarging the look of pores, and blocking cosmetic actives from absorbing.

Physical exfoliation (grainy scrubs, brushes) removes these cells by mechanical abrasion. Chemical exfoliation works differently: at an acidic pH, the acid molecules dissolve intercellular bonds evenly across the whole treated surface. The result is more regular, with no risk of micro-tears, and you can adjust concentrations precisely to the result you want.

The three acid families differ on three key parameters: their molecular size (which sets how deep they penetrate), their solubility (water-soluble for AHAs/PHAs, oil-soluble for BHA), and their specific mechanism of action . These differences decide which one suits which skin type and which skincare goal.

AHAs: alpha hydroxy acids

Quick answer: AHAs are water-soluble acids that exfoliate the surface of the epidermis, stimulate cell turnover, and boost collagen synthesis. Recommended for dry, dull, or photoaged skin, they are the most scientifically documented anti-aging acids.

The main AHAs and what sets them apart

• Glycolic acid — Extracted from sugarcane. The smallest AHA molecule (76 Da), the deepest penetration, a powerful exfoliating and anti-aging effect. A study by Stojiljkovic et al. (2021) shows that at pH 4, it stimulates collagen production and speeds up epidermal renewal without raising pro-inflammatory markers (TNF-α). Recommended concentration for daily use: 5-10%; for professional peels: 20-70%.
• Lactic acid — From milk fermentation. A larger molecule than glycolic acid, a more superficial action, better skin tolerance. It does double duty, exfoliating and humectant, because it draws water into the stratum corneum. Ideal for starting chemical exfoliation, or for skin that combines dryness and blemishes.
• Mandelic acid — Derived from bitter almonds. A large molecule, slow and progressive penetration. Antibacterial properties that help combination, acne-prone skin. Well tolerated by darker skin tones, with less photosensitization than glycolic acid.

What clinical studies show

A double-blind, placebo-controlled study (Javaheri et al., 1996) showed that glycolic acid peels significantly improve the signs of photoaging: fewer fine wrinkles, a more even complexion, softer actinic keratoses. You see these effects within 4 weeks with 5% formulas applied daily (Smith, 1997).

Watch out: AHAs raise your skin's photosensitivity for up to 7 days after application. SPF 30 minimum in the morning is mandatory. Without sun protection, the anti-aging benefits cancel out and the risk of dark spots goes up. Read our guide on sun protection as your first anti-aging step to go further on this point.

BHA: salicylic acid, the pore expert

Quick answer: Salicylic acid is the only BHA used in cosmetics. Its distinctive property: it's oil-soluble . It gets into sebum-filled pores and cleans them from the inside, where water-soluble AHAs can't reach. Recommended for oily, acne-prone skin and visibly enlarged pores.

That oil solubility is the key to its unique action. Salicylic acid slips into the follicular canal, dissolves sebum plugs (open and closed comedones), and reduces local inflammation thanks to its anti-inflammatory properties. It belongs to the same chemical family as aspirin (acetylsalicylic acid). This double mechanism, keratolytic and anti-inflammatory, makes it the reference active for mild to moderate acne-prone skin.

Clinically validated effectiveness

A 2024 prospective study (PMC12274963) evaluating a salicylic acid gel on acne-prone skin found a significant reduction in inflammatory lesions, better skin hydration, and a stronger barrier. It shows that salicylic acid doesn't weaken skin when properly formulated. A multicenter study (Dreno et al., 2021) confirms that combining salicylic-based products reduces comedonal lesions by 76% after 12 weeks of regular use.

PHAs: the gentle option for sensitive skin

Quick answer: PHAs (polyhydroxy acids) like gluconolactone, gluconic acid, and lactobionic acid are the gentlest chemical exfoliants. Their large molecular size keeps their action at the surface of the epidermis, which greatly reduces the risk of irritation. They suit sensitive, reactive skin, or skin with mild rosacea.

The main difference from AHAs lies in their structure: PHAs contain several hydroxyl groups (hence the "poly" prefix), which also gives them humectant properties. They draw water into the upper layers of the skin and hold it there, exfoliating and hydrating at the same time. That dual action makes them especially suited to dry, sensitive skin that can't tolerate classic AHAs.

Another distinctive advantage: PHAs cause no significant photosensitization, unlike AHAs. That's a real asset for sun-exposed skin, or for anyone who doesn't apply SPF every single morning. Their tolerance profile makes them the active of choice to introduce chemical exfoliation to reactive skin, or to transition out of an irritating dermatological treatment (retinol, topical antibiotics). If you have sensitive skin with rosacea, read our dedicated article on LED light therapy and rosacea.

Which acid for your skin type?

Quick answer: Your choice of acid depends above all on your skin type and your goals. Here's the match dermatologists recommend, with suggested starting concentrations.

For skin with dark spots (melasma, sun spots), glycolic acid at progressive concentrations is clinically validated to even out the complexion: it speeds up the shedding of melanin-loaded keratinocytes. Pair it with a brightening active like niacinamide and you boost the effect. Our guide on niacinamide covers this synergy in detail.

Building acids into your skincare routine

Quick answer: Apply exfoliating acids after cleansing, on dry skin, before your hydrating products. Use AHAs and BHA in the evening only (they're photosensitizing), and never on the same evening as retinol or another irritating treatment.

The optimal order in your evening routine

1. Double cleanse — Cleansing oil, then foaming cleanser, to remove every trace of SPF, makeup, and sebum
2. Balancing toner (optional) — Alcohol-free, to prep your skin's pH before the acid
3. Acid exfoliant (AHA / BHA / PHA) — On clean, dry skin; let it sit 20-30 minutes if the concentration is high (>10% for AHA)
4. Treatment serum — Niacinamide, peptides, hyaluronic acid (skip vitamin C on the same evening as concentrated AHAs)
5. Moisturizer — Emollient, to close the exfoliated barrier and prevent water loss
6. Oil or occlusive balm (optional) — To seal everything in and maximize overnight recovery

Recommended progression: Start with 1 application a week for the first 2 weeks. Move up gradually to 2-3x/week as your skin tolerates it. Your skin should never show persistent tightness, redness, or excessive flaking. Those signs mean over-exfoliation, which weakens your skin barrier.

Exfoliating acids and LED light therapy: effective prep

A little-known but clinically logical synergy: chemical exfoliation the day before, or two days before, a light therapy session improves how red light penetrates the dermis. Remove the layer of dead cells that blocks photons, and your skin receives more effective irradiance. Professional photobiomodulation protocols regularly include this epidermal prep. On the flip side, don't exfoliate right before or right after an LED session, so you don't stack two stimulations at once. To understand how light therapy works, our photobiomodulation guide gives you the complete picture.

Precautions and common mistakes to avoid

Quick answer: The main mistake is over-exfoliation: using concentrations that are too high, too often, until you compromise your skin barrier. The warning signs: persistent redness, a burning sensation, excessive flaking, skin that looks unusually shiny or feels tight. If these symptoms appear, pause all exfoliation for 1 to 2 weeks and step up hydration.

The 5 most common mistakes

1. Skipping SPF in the morning — Mandatory with AHAs and BHA. These acids raise photosensitivity by removing the surface cells that naturally filter UV. Without sun protection, the anti-aging benefits cancel out and the risk of post-inflammatory pigmentation rises.
2. Layering two strong exfoliants on the same evening — AHA + retinol together, or BHA + glycolic acid in successive layers. You get excessive irritation and no extra benefit. Alternate on separate evenings.
3. Applying acid to damp skin — Skin still wet after cleansing lets the acid penetrate more and raises the risk of irritation. Wait until your skin is completely dry (5 minutes minimum after rinsing).
4. Ignoring the product's pH — Exfoliating acids are active at pH 3-4. A product formulated at neutral pH (6-7) doesn't really exfoliate, even if the advertised acid concentration is high. Favor brands that disclose their formulation pH.
5. Starting with concentrations that are too high — Jumping straight to 15-20% glycolic acid with no adjustment is pointless, even counterproductive. Start at 5%, let your barrier adapt for 4 weeks, then increase gradually. Skin that tolerates well delivers better long-term results than irritated skin.

Absolute contraindications: pregnancy (avoid high concentrations of glycolic acid), skin with open wounds, or eczema in an acute phase. If you're on prescribed retinol or topical antibiotics, ask your dermatologist before adding exfoliating acids to your routine.

Frequently asked questions

Scientific sources

1. Stojiljkovic et al., 2021 — Glycolic acid (pH 4) stimulates collagen production and epidermal renewal without increasing pro-inflammatory TNF-α
2. Javaheri et al., 1996 — Clinical improvement of photoaged skin with 50% glycolic acid. A double-blind vehicle-controlled study
3. Smith WP, 1997 — Effectiveness of daily 5% glycolic acid formulation in treating photoaging (randomized double-blind trial)
4. PMC12274963, 2024 — Clinical efficacy of a salicylic acid-containing gel on acne management and skin barrier function
5. Dreno et al., 2021 — Effectiveness of salicylic acid-based products for mild comedonal-papular acne (multicenter observational study)