Chemical exfoliants are among the most effective actives in skincare — but choosing between AHA and BHA confuses a lot of people, and some of the advice repeated about them (including the sun-sensitivity claim) doesn't hold up equally for both. Both dissolve the bonds holding dead skin cells together, but they do so through genuinely different mechanisms and reach different skin layers. Here's exactly how to choose, and what the evidence actually supports.
- AHAs are water-soluble and work on the skin's surface; BHA (salicylic acid) is oil-soluble and penetrates into sebum-filled pores — that solubility difference explains almost every practical difference between them.
- Only AHAs meaningfully increase sun sensitivity. FDA-backed research found AHA use raises UV-redness sensitivity by about 18% and roughly doubles sensitivity to UV-induced cell damage — salicylic acid does not carry the same risk, and animal studies even suggest a mild photoprotective effect.
- AHA suits dry, dull or pigmented skin; BHA suits oily, acne- and blackhead-prone skin — combination skin can reasonably use both, just not layered in the same step.
- Over-exfoliating, not choosing the 'wrong' acid, is the most common real mistake — start at 1–2 times a week regardless of which one you pick.
- Daily sunscreen matters most while using AHA specifically, though it's good practice with any exfoliant since fresh skin underneath is always somewhat more exposed.
AHA — alpha hydroxy acid
AHAs are water-soluble acids derived mostly from natural sources. The most common are glycolic acid (from sugarcane, the smallest molecule and most potent), lactic acid (from milk, gentler), mandelic acid (from almonds, larger molecule and slower to penetrate), and malic acid.
Because they are water-soluble, AHAs work on the skin's surface, breaking the bonds between dead skin cells so they shed more evenly rather than building up. The result is smoother texture, a brighter tone, and reduced fine lines — but being water-soluble also means AHAs don't penetrate into oil-filled pores the way an oil-soluble acid can.
BHA — beta hydroxy acid
The main BHA used in skincare is salicylic acid. Unlike AHAs, it's oil-soluble, meaning it can travel through sebum and reach deep into a clogged pore rather than staying on the skin's surface — this is the single mechanical reason BHA is uniquely effective against blackheads, whiteheads and acne where AHA generally isn't.
Salicylic acid also carries genuine anti-inflammatory properties independent of its exfoliating action, which is part of why it tends to be better tolerated by acne-prone and reactive skin than an AHA at comparable strength. Capryloyl salicylic acid (LHA) is a gentler, slower-releasing derivative worth considering for sensitive skin that still wants BHA's pore-clearing benefit.
Key differences at a glance
- Solubility: AHA is water-soluble and works on the surface; BHA is oil-soluble and penetrates into pores.
- Best for: AHA suits dry, dull or aging skin; BHA suits oily, acne-prone and clogged-pore skin.
- Strongest use case: AHA for hyperpigmentation and surface texture; BHA for blackheads and active breakouts.
- Sun sensitivity: AHA meaningfully increases it (always pair with SPF); BHA does not carry the same documented risk.
- Irritation profile: AHA tends to be more irritating at equal efficacy; BHA's anti-inflammatory action buffers some of its own irritation.
Which should you use?
Dry or mature skin with texture or pigmentation concerns generally does best with an AHA — lactic acid is the gentlest starting point, glycolic acid the most potent. Oily, acne-prone or clogged-pore skin does better with a BHA, typically salicylic acid at 0.5–2%. Combination skin can reasonably use both, just on different days rather than stacked in one routine. Sensitive skin should start with low-concentration lactic acid, or mandelic acid, whose larger molecule penetrates more slowly and tends to be the gentlest AHA option available.
Frequently asked questions
What's the main difference between AHA and BHA?
Solubility. AHA is water-soluble and works on the skin's surface, making it better for texture and pigmentation. BHA (salicylic acid) is oil-soluble and can penetrate into sebum-filled pores, making it uniquely effective against blackheads and acne. That single difference explains almost every practical distinction between them.
Does BHA increase sun sensitivity the same way AHA does?
No — this is a common misconception. FDA-backed research found AHA use increases UV-redness sensitivity by about 18% and roughly doubles sensitivity to UV cell damage, which is why AHA products carry a mandated sunburn warning. Comparative studies found salicylic acid did not increase UV sensitivity the same way, and some research even suggests a mild photoprotective effect. Sunscreen is still good practice with any exfoliant, but the sun-sensitivity caution applies specifically to AHA.
Which is better for acne — AHA or BHA?
BHA (salicylic acid) is generally the better choice for acne, because its oil solubility lets it penetrate into clogged, sebum-filled pores where AHA's water solubility can't reach. AHA is more effective for surface texture and post-acne pigmentation than for active breakouts themselves.
Can I use AHA and BHA in the same routine?
You can, and some products combine them deliberately, but avoid layering two separate AHA and BHA products in the same routine — double exfoliation raises irritation risk. Alternating days, or using one in the morning and the other at night, gets the benefit of both without stacking the irritation.
How often should I use a chemical exfoliant?
Start at once or twice a week regardless of whether you choose AHA or BHA, and build up gradually from there based on how your skin tolerates it. Over-exfoliating — not picking the 'wrong' acid — is the most common mistake, and it strips the barrier rather than improving texture.
Sources & further reading
- "Evaluating the Efficacy and Safety of Alpha-Hydroxy Acids in Dermatological Practice: A Comprehensive Clinical and Legal Review" — PubMed, 2024
- Liu C, et al. "Clinical Efficacy of a Salicylic Acid-Containing Gel on Acne Management and Skin Barrier Function" — Journal of Cosmetic Dermatology, 2025
- FDA — "Alpha Hydroxy Acids" (UV sensitivity research and mandated labeling)
- FDA — "Beta Hydroxy Acids"
This article is written and edited by the Pure Moonlight team, grounded in the peer-reviewed and clinical sources listed above and cross-checked against our ingredient analyzer's database. It is not a substitute for advice from a dermatologist or doctor.