AHA vs BHA: what chemical exfoliants do, which to choose, and how to use them

Chemical exfoliants are among the most effective at-home skincare tools available — and among the most misused. Understanding what AHAs and BHAs actually do, and which one fits your skin concern, makes the difference between clear skin and chronic irritation.

What chemical exfoliants do

Physical exfoliants (scrubs, brushes) mechanically remove dead skin cells. Chemical exfoliants work differently: they break the bonds holding dead skin cells together, allowing the outer layer to shed more rapidly and uniformly.

AHAs (alpha-hydroxy acids): Water-soluble acids that work on the skin surface, dissolving the bonds between dead corneocytes (surface skin cells). Primary effect is on the epidermis. Slightly hygroscopic — they also attract water to the skin.

BHAs (beta-hydroxy acids): Oil-soluble acids that penetrate through sebum into the pore lining. Salicylic acid is the main BHA used in skincare. Works both on the skin surface and inside the follicle, making it uniquely effective for comedones and acne.

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AHAs in detail

Glycolic acid
Molecule size: Smallest AHA — penetrates deepest of the AHAs
Effect: Strongest exfoliation among AHAs; most evidence for collagen stimulation, fine line reduction, and pigmentation improvement
Concentration range: 5–10% in leave-on products; 20–70% in professional peel formulations
Best for: Normal to oily skin with concerns about aging, texture, uneven tone
Caution: Most irritating of the common AHAs; can be sensitizing in high-sun environments

Lactic acid
Molecule size: Larger than glycolic; shallower penetration
Effect: Gentler exfoliation; additional humectant properties (attracts water). FDA-reviewed evidence for fine line improvement.
Concentration range: 5–15% in leave-on products
Best for: Dry, sensitive, or combination skin; beginners to chemical exfoliation; anyone who found glycolic too irritating
Notable: Good choice for patients with dry skin as it exfoliates while maintaining hydration

Mandelic acid
Molecule size: Largest of the common AHAs; shallowest penetration; slowest absorption
Effect: Gentlest AHA; antibacterial properties; suited for acne in dry or sensitive skin
Concentration range: 5–15% in leave-on products
Best for: Sensitive skin, acne-prone dry skin, rosacea-adjacent skin, darker skin tones (less risk of post-exfoliation irritation triggering PIH)

Malic, tartaric, and citric acids
Less commonly featured as solo actives in skincare; appear in blends. Malic and tartaric are often combined with glycolic or lactic to moderate the formula.

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BHAs: salicylic acid

Salicylic acid is the dominant BHA in skincare. Its oil solubility is the defining characteristic — it penetrates sebum, making it the right choice for oily skin and comedones.

What salicylic acid does:
Dissolves surface dead skin cells (exfoliation)
Penetrates into the pore lining to dissolve the sebum plug
Anti-inflammatory properties that reduce redness in active acne
Mildly antibacterial

Concentration range: 0.5–2% in leave-on products; 10–30% in professional treatments
Best for: Oily/combination skin; blackheads and whiteheads (comedones); acne; enlarged pores

Limitation: The oil-solubility that makes salicylic acid effective for pores can over-dry very dry or mature skin.

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Which to choose: skin type and concern

| Concern | Recommended |
|---|---|
| Blackheads / whiteheads | BHA (salicylic acid) |
| Acne / oily skin | BHA (salicylic acid) |
| Dry, flaky skin | AHA (lactic acid) |
| Hyperpigmentation / uneven tone | AHA (glycolic or lactic) |
| Fine lines / anti-aging | AHA (glycolic acid) |
| Sensitive skin / rosacea-adjacent | AHA (mandelic or lactic) |
| Texture + occasional breakouts | AHA + BHA combination |
| Darker skin tones (PIH risk) | Mandelic acid or lactic acid |

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How often to use chemical exfoliants

Beginners: Start 1–2x per week. The most common mistake is over-exfoliating immediately.

Signs of over-exfoliation: Redness that doesn't resolve within hours, stinging that persists, flaking without improvement in skin quality, increased breakouts (paradoxical purging beyond the first 2–4 weeks), sensitivity to moisturizer or sunscreen.

Established routine: 2–4x per week is common for leave-on products at 5–10%. Daily use is possible with lower concentrations (2–3% lactic acid, 0.5% salicylic acid) designed for daily application.

Professional peels: Monthly or every 4–6 weeks for medium-depth peels (20–40%); deeper peels require longer intervals.

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Layering with other actives

AHA/BHA + SPF (essential): Chemical exfoliants increase photosensitivity — the freshly exfoliated skin has less UV protection. Daily SPF 30+ is non-negotiable when using these acids.

AHA/BHA + retinoid: Both are potent actives. Using them simultaneously increases irritation risk without a clear benefit. Standard approach: retinoid at night, AHA/BHA on alternate nights. Or: AHA/BHA in the morning (if a morning routine), retinoid at night.

AHA/BHA + vitamin C: Both are acidic; layering can cause over-irritation. Common approach: vitamin C in the morning, acid exfoliant at night.

AHA/BHA + niacinamide: Niacinamide can buffer the low-pH effect of acids somewhat; many products combine them intentionally. Layering them back-to-back in a routine is generally fine.

AHA/BHA + physical sunscreen (immediately after treatment): If using at night only, no conflict. If using a peel or high-concentration acid treatment, wait 20–30 minutes before applying anything else.

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When not to use chemical exfoliants

Active skin conditions: Don't use AHAs/BHAs on broken skin, active eczema flares, severe sunburns, open acne sores, or post-procedure skin in the acute healing phase.

Post-procedure timing:
Post-laser resurfacing: avoid for 7–14 days minimum (follow provider guidance)
Post-microneedling: avoid for 5–7 days
Post-chemical peel: avoid until fully healed; then resume gently
Post-Botox/filler: not contraindicated directly, but avoid active lesion treatment within 24–48 hours

Pregnancy: Glycolic and lactic acid at typical OTC concentrations are generally considered safe topically. Salicylic acid at high concentrations (leave-on 2% daily) is sometimes listed as a concern during pregnancy. At low concentrations (0.5% wash-off), the risk is considered very low. When in doubt, defer to your OB.

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Professional treatments vs. at-home

Professional chemical peels use much higher concentrations (20–70% glycolic, 10–30% salicylic, 15–35% TCA) with controlled application and neutralization. The results are more dramatic with a single treatment; the procedure carries more risk and requires provider experience for safe application.

At-home products (5–15% AHA, 0.5–2% BHA) are calibrated for unsupervised daily use with a much lower risk profile. The difference isn't just concentration — professional peels also have lower pH (more acidic), increasing penetration depth.

The relationship is cumulative: at-home exfoliants maintain the gains from professional treatments and slowly improve texture over months of consistent use. Professional peels produce step-change improvements that at-home products can't replicate from a single session.

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Common mistakes

Using too many exfoliants simultaneously: Combining glycolic acid toner + salicylic acid treatment + a retinoid in the same routine produces irritation, not better results. Pick the right acid for your concern and use one at a time.

Using acids every day immediately: Starting daily with a 10% glycolic acid formula will disrupt the skin barrier before it's adapted. Start 2x/week, increase gradually.

Skipping sunscreen: Chemical exfoliants and photosensitivity are inseparable. If you won't wear sunscreen daily, chemical exfoliation is not advisable.

Peeling = working: Visible skin peeling is not the measure of effectiveness. Well-formulated at-home acids at appropriate concentrations produce results without peeling. Peeling indicates you've over-exfoliated or the concentration is too high for your current barrier tolerance.

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