9 min read
Chemical Exfoliation Explained: AHA vs BHA vs PHA
Chemical exfoliants dissolve the bonds holding dead cells to the surface of your skin, rather than scraping them off. Done correctly, that means smoother texture, clearer pores, better product absorption, and faster fading of marks. Done badly — too strong, too often, stacked with a retinoid — it means a stripped barrier that takes months to rebuild. The difference is almost entirely in choosing the right acid family and the right frequency.
AHAs: surface texture and tone
Alpha hydroxy acids are water-soluble, so they work on the skin's surface. They are the right choice for dryness, dullness, uneven tone, fine lines, and rough texture — not for congestion deep in the pore.
- Glycolic acid — the smallest molecule, deepest penetration, strongest results and strongest sting. Best for normal to oily, resilient, non-sensitive skin. 5–10% for home use.
- Lactic acid — larger molecule, gentler, and humectant, so it hydrates while it exfoliates. The best AHA for dry skin. 5–10%.
- Mandelic acid — largest molecule, slowest and gentlest penetration, with mild antibacterial action. The safest AHA for sensitive skin and for deeper skin tones, where aggressive exfoliation risks post-inflammatory hyperpigmentation. 5–10%.
BHA: inside the pore
Salicylic acid is oil-soluble, which is the entire point — it can travel through sebum into the follicle and clear it from the inside. That makes it the correct choice for blackheads, closed comedones, oily skin, and mild inflammatory acne. It is also anti-inflammatory, related structurally to aspirin, so it calms as well as clears.
Effective home strengths are 0.5% to 2%. Two percent leave-on liquids are the classic format; 0.5% to 2% cleansers give a lower-contact option for sensitive or dry skin that still gets congested. It is not the right pick for dry skin with no congestion — you will get flaking without benefit.
PHA: the gentle option that still works
Polyhydroxy acids — gluconolactone and lactobionic acid — are chemically similar to AHAs but with much larger molecules and additional hydroxyl groups. They exfoliate more slowly and superficially, hold water at the surface, and have antioxidant properties.
They are the reasonable choice for rosacea-prone, eczema-prone, very sensitive, or freshly retinized skin, and for anyone who wants to exfoliate more frequently without cumulative irritation. Expect subtler results over a longer period; that trade is the point.
Matching acid to skin type
Match the acid family to the problem you actually have:
- Oily, congested, blackheads — salicylic acid 2%, two to three times a week.
- Dry, flaky, dull — lactic acid 5–10%, one to two times a week.
- Combination — salicylic in the T-zone, lactic on the cheeks, or alternate nights.
- Sensitive or rosacea-prone — PHA, up to three times a week, or mandelic acid once weekly.
- Deeper skin tones with dark marks — mandelic or PHA first; aggressive glycolic peels can worsen pigmentation.
- Mature skin with fine lines — glycolic 5–8% if tolerated, otherwise lactic.
How often, and how to build up
Start once a week, on clean dry skin, at night, followed by moisturizer. Hold that frequency for two weeks. If there is no tightness, stinging, or flaking, move to twice a week. Two to three times a week is the ceiling for nearly everyone, and once a week is a perfectly effective long-term routine.
Never exfoliate and use a retinoid on the same night unless you have used both for months without issue. Never combine two exfoliating products on the same night. Do not exfoliate at all in the 48 hours before or after waxing, threading, laser, or shaving irritated skin.
Signs you are over-exfoliating
Stop immediately if you notice any of these:
- A tight, shiny, waxy look rather than a smooth one.
- Stinging from products that used to feel neutral, including plain moisturizer.
- Increased redness, or small bumpy dermatitis around the mouth and nose.
- Paradoxical oiliness as the barrier tries to compensate.
- New breakouts in unusual places, or marks that fade more slowly than before.
The non-negotiable: sunscreen
Exfoliation removes part of the layer that scatters UV and increases photosensitivity for days afterwards. Using AHAs without daily broad-spectrum SPF 30+ will make pigmentation worse, not better, and is the single most common reason people conclude that acids 'did nothing' for their dark marks.
If you are not willing to wear sunscreen every day, use a BHA cleanser or a PHA instead of a leave-on AHA, and rely on retinoids and niacinamide for tone.
Frequently asked
- What is the difference between AHA and BHA?
- AHAs are water-soluble and work on the skin's surface for tone and texture. BHA (salicylic acid) is oil-soluble and penetrates into the pore, so it suits congestion and acne.
- Which acid is best for sensitive skin?
- PHAs such as gluconolactone, or mandelic acid. Both use large molecules that penetrate slowly, so they exfoliate with much less irritation.
- How often should you chemically exfoliate?
- Start once a week and build to a maximum of two or three times a week. More frequent use damages the barrier for most skin types.
- Can I use an acid and retinol on the same night?
- Not until both are well tolerated separately over several months. Alternating nights is safer and just as effective.