Cluster 5 · #44Phase 1Volume: HighDifficulty: Medium
AHA vs BHA vs PHA: The Complete Exfoliant Guide
Chemical exfoliants are among the most impactful ingredients you can add to a skincare routine — but the differences between AHAs, BHAs, and PHAs are clinically meaningful, not just marketing distinctions. The wrong choice for your skin type can cause irritation or fail to address your concerns; the right choice can transform skin texture, clarity, and tone within weeks.
Quick Answer
AHAs (glycolic, lactic acid) exfoliate the skin surface — best for texture, brightness, and anti-ageing. BHAs (salicylic acid) are oil-soluble and penetrate pores — best for oily, acne-prone, and congested skin. PHAs (gluconolactone) work gently at the surface with minimal irritation — best for sensitive skin and beginners.
AHAs: Alpha-Hydroxy Acids
AHAs are water-soluble acids that work by breaking the bonds between dead skin cells at the skin surface, allowing them to shed more rapidly. The most commonly used AHAs in skincare:
Glycolic acid — the smallest AHA molecule, deepest penetration, highest potency. Excellent for anti-ageing (stimulates collagen synthesis at higher concentrations), texture refinement, and brightening. The most likely to cause irritation; best for experienced users or those without sensitive skin.
Lactic acid — larger molecule, more superficial action, gentler than glycolic. Also acts as a humectant, hydrating while it exfoliates. Ideal first AHA for most people. Effective for brightness and texture at 5–10%.
Mandelic acid — the largest AHA, gentlest, and particularly useful for darker skin tones because it carries a lower risk of post-inflammatory hyperpigmentation than glycolic acid.
Malic acid and tartaric acid — less commonly used as standalone actives; often found in combination formulas.
AHAs increase UV sensitivity for 24–48 hours after use. Always wear SPF the morning after applying an AHA — and every morning regardless.
BHAs: Beta-Hydroxy Acids
The primary BHA used in skincare is salicylic acid, a naturally occurring compound derived from willow bark. Unlike AHAs, salicylic acid is oil-soluble — meaning it can penetrate through the sebum in pores rather than being limited to the skin surface. This makes it uniquely effective for:
Clearing blackheads and whiteheads by exfoliating inside the pore lining
Reducing excess sebum and shine
Anti-inflammatory effects (salicylic acid is related to aspirin and has demonstrable anti-inflammatory activity at the skin)
Addressing mild to moderate acne
Effective concentrations range from 0.5% (gentle daily use) to 2% (targeted treatment). BHAs are less likely to cause post-inflammatory hyperpigmentation than AHAs, making them suitable for a broader range of skin tones. Less ideal for dry skin as they can be dehydrating with frequent use.
PHAs: Polyhydroxy Acids
PHAs include gluconolactone, lactobionic acid, and galactose. Their larger molecular size means they penetrate only the very outermost layers of the stratum corneum, delivering exfoliating benefits with significantly reduced irritation. PHAs also have humectant properties and some antioxidant activity. Key attributes:
Compatible with sensitive and rosacea-prone skin that cannot tolerate AHAs or BHAs
Gentler than AHAs at equivalent concentrations
Less dramatically effective for deep texture refinement, but deliver real brightening and smoothing benefits
Less UV sensitisation than AHAs — though SPF should still be worn daily
Can You Use AHAs and BHAs Together?
Combination AHA/BHA products exist and can be effective — but they carry a higher irritation risk than either acid alone. For most people, using an AHA on some nights and a BHA on others (never the same night as retinol) is a safer and equally effective approach. Beginners should master one acid type before introducing a second.
The Bottom Line
Oily, acne-prone, or congested skin → BHA (salicylic acid 0.5–1%). Dull, uneven texture → lactic acid 5–10%. Sensitive skin → PHA. Anti-ageing and brightening with experience → glycolic acid 5–10%.
Decode any exfoliating product and check it's compatible with your routine using Skin Stacker's free ingredient analyser.
Choosing the Right AHA: Glycolic vs Lactic vs Mandelic
Within the AHA family, the choice of acid matters as much as choosing between AHAs and BHAs. The key variable is molecular size, which determines penetration depth, potency, and irritation risk.
Glycolic acid (76 Da) is the smallest AHA and penetrates most deeply. At 5–10%, it is the most potent OTC exfoliant, with the strongest evidence for collagen stimulation, wrinkle reduction, and pigmentation improvement — and the highest irritation risk. Best for experienced users with resilient skin. Lactic acid (90 Da) penetrates more superficially with significantly reduced irritation risk, and doubles as a humectant — hydrating as it exfoliates. The right first AHA for most people. Effective for brightness and texture at 5–10%. Mandelic acid (152 Da) is the gentlest common AHA, with the most superficial action and the lowest risk of post-inflammatory hyperpigmentation — making it particularly appropriate for darker skin tones where glycolic acid carries meaningful PIH risk. Also has mild antibacterial properties relevant for acne-prone skin.
Concentration and pH: What the Label Does Not Tell You
Acid concentration alone does not determine efficacy. The pH of the formula is equally important — and it is almost never disclosed on packaging. AHAs require a pH below approximately 4.0 to be meaningfully active; at higher pH values, the acid is largely ionised and cannot penetrate the stratum corneum effectively. This means a 10% glycolic acid product at pH 5.5 may deliver less exfoliation than a well-formulated 5% glycolic acid at pH 3.5.
For BHAs, salicylic acid remains active up to approximately pH 4.5 — giving it more formulation flexibility than AHAs. PHAs, because their mechanism is less pH-dependent, are more forgiving of higher-pH formulations. The practical implication: if a product marketed as a chemical exfoliant causes no noticeable effect despite regular use, inadequate pH is often the explanation — particularly in "gentle daily exfoliants" that sacrifice efficacy to reduce irritation risk.
Building an Acid Routine: Frequency and Sequencing
The most common mistake with chemical exfoliants is over-use. Daily exfoliation with a potent AHA or BHA chronically disrupts the barrier faster than it can repair between uses — leading to sensitisation, redness, and paradoxically more congestion as the damaged barrier becomes more reactive.
Evidence-based frequency guidelines: Beginners should start once per week for four weeks, then twice per week, assessing skin response before increasing further. Experienced users can use AHAs two to three times per week; BHAs can be used more frequently (daily in some cases) because salicylic acid's anti-inflammatory properties partially offset its barrier-disrupting effect. PHAs can be used daily by most skin types, including sensitive skin.
Sequencing within a routine: acids belong after cleansing and before other actives, applied to dry skin — applying to damp skin dilutes the acid and raises the effective pH, reducing efficacy. Wait two minutes before the next step. Never apply acids and retinoids on the same night during the first three to six months of using either ingredient.
Common Questions About Chemical Exfoliants
Do you need to rinse off AHAs and BHAs?
It depends on the product format. Leave-on serums and toners are formulated for sustained contact at safe concentrations. Rinse-off peels and masks use higher concentrations appropriate only for the specified contact time. Using a rinse-off peel as a leave-on product is a common cause of chemical burns.
Can chemical exfoliants cause more breakouts?
Purging — a temporary increase in breakouts during the first four to six weeks — is a recognised phenomenon caused by accelerated cell turnover pushing existing microcomedones to the surface faster than they would naturally surface. True purging resolves within six weeks; breakouts that persist beyond this point, or appear in areas where you don't normally break out, suggest a reaction to an ingredient rather than genuine purging.
Is SPF really necessary after using AHAs?
Yes — non-negotiable. AHAs remove the outermost protective layer of the stratum corneum, increasing UV sensitivity for 24–48 hours after application. Studies show a measurable increase in UV-induced erythema following AHA use. This increased sensitivity compounds with regular exfoliant use if SPF is not worn consistently. Daily broad-spectrum SPF 30 or above is the essential companion to any acid routine.
🔗
Check your ingredient interactions Use the Interaction Map to see a colour-coded matrix of which ingredients pair well, clash, or need caution.
🔍
Decode any product Search, scan, or paste any skincare product into the Ingredient Decoder to see what's actually in it — with a transparency score and personalised match rating.
