What Are AHA, BHA, and PHA?
AHA (alpha-hydroxy acids), BHA (beta-hydroxy acid), and PHA (polyhydroxy acids) are the three families of chemical exfoliants. Unlike physical scrubs, they chemically dissolve the bonds between dead skin cells at an acidic pH, enabling controlled desquamation without mechanical abrasion. Van Scott and Yu first documented the clinical potential of AHAs in the 1970s; more than three decades of subsequent research has clarified the mechanisms and safety profiles of all three families.
Molecular Structure and Depth of Action
The most fundamental differences between the three families lie in molecular weight and solubility.
Molecular weight: small → medium
Glycolic acid · Lactic acid · Mandelic acid
Molecular weight: medium
Salicylic acid — the only acid family that penetrates sebum-filled pores
Molecular weight: large
Gluconolactone · Lactobionic acid — minimal irritation, humectant properties
Glycolic acid has the smallest molecular weight among AHAs (76 Da), giving it the deepest epidermal penetration. Salicylic acid is the only lipid-soluble option, allowing it to penetrate pores filled with sebum. Gluconolactone stays near the skin surface due to its large molecular size, making it the gentlest of the three.
AHA (Alpha-Hydroxy Acids)
Mechanism of Action
AHAs work by weakening desmoglein bonds in the lower stratum corneum, reducing cohesion between corneocytes. Tang & Yang (2018) identify two simultaneous pathways:
pH reduction interferes with calcium ion activity in the stratum corneum → weakens intercellular bonds → dissolves the outermost dead skin layer
Fibroblast stimulation → increased collagen and hyaluronic acid synthesis → improved skin thickness and elasticity
Clinical Evidence — Ditre et al., 1996
Subjects using glycolic acid cream over 12 months showed histologically confirmed, significant increases in dermal glycosaminoglycan (GAG) density — the first tissue-level evidence that AHAs remodel the dermis, not just the surface.
Key AHA Acids
OTC 5–15%
Professional peels 20–70%
Strongest keratolytic action, melanin inhibition, and dermal remodeling. Highest photosensitivity — SPF is mandatory.
Typical concentration 5–12%
Exfoliation plus NMF (natural moisturizing factor) production — delivers a dual moisturizing benefit. A gentler alternative to glycolic acid for dry or sensitive skin.
Typical concentration 5–10%
Antimicrobial (acne-reducing) and brightening. Larger molecular size slows penetration, minimizing irritation. Lower risk of post-inflammatory hyperpigmentation in deeper skin tones (Fitzpatrick IV–VI).
Clinical Evidence
Clinical Evidence — Sharad, 2013
Meta-analysis confirmed that 20–70% glycolic acid peels produced statistically significant improvements across acne, pigmentation, and photoaging endpoints.
Clinical Evidence — Van Scott & Yu, 1989
AHAs thin the stratum corneum, improving the absorption of subsequently applied moisturizers by over 60%. AHAs function not only as exfoliants but as absorption enhancers for other actives.
BHA (Beta-Hydroxy Acid)
Mechanism of Action
Salicylic acid’s defining property is its lipophilicity. Its structural affinity for sebum allows it to penetrate into sebum-filled pores — dissolving plugs from the inside out. This is fundamentally different from AHAs, which act at the epidermal surface.
Per Arif’s 2015 comprehensive review, salicylic acid operates through four simultaneous pathways:
Disrupts lipid bonds between corneocytes → epidermal exfoliation
Dissolves sebum inside follicular openings → clears blackheads and whiteheads
Inhibits Cutibacterium acnes proliferation → directly targets the primary acne-causing bacterium
Suppresses the Cox-2 pathway → reduces prostaglandin synthesis → calms acne redness and swelling (same mechanism as aspirin)
Clinical Evidence — Decker & Graber, 2012
2% salicylic acid achieved equivalent reduction in acne lesion count compared to benzoyl peroxide, with significantly less skin dryness reported.
Concentration-Effect Relationship
When BHA Is the Right Choice
- Oily, congestion-prone, large-pore skin
- Recurring blackheads and whiteheads
- Acne-prone skin (anti-inflammatory co-benefit)
- Body acne (back, chest)
PHA (Polyhydroxy Acids)
Mechanism of Action
PHAs have higher molecular weights than AHAs and carry multiple hydroxyl (-OH) groups. This structure produces three distinct advantages:
Large molecular size slows epidermal permeation → suitable for sensitive and atopic skin types
Multiple hydroxyl groups bind water molecules → exfoliates while actively hydrating the skin
Metal ion chelation → neutralizes free radicals → suppresses oxidative stress
Clinical Evidence — Becker et al., 2012
PHAs showed significantly lower Irritation Index scores than AHAs in sensitive and atopic skin, and were the only acid family confirmed not to worsen transepidermal water loss (TEWL) — a key marker of barrier integrity.
Key PHA Acids
Exfoliation combined with potent antioxidant activity via metal chelation. Safe for rosacea and reactive skin types.
Derived from milk sugar (lactose); strongest humectant capacity among PHAs. Ideal for dry, aging, and atopic skin.
Choosing Between AHA, BHA, and PHA
By Skin Type
Salicylic acid (BHA) first — pore purification + anti-inflammation. Glycolic acid (AHA) can be added as a secondary
Minimized irritation with simultaneous hydration benefit
BHA on T-zone + AHA on U-zone — targeted care by area
Glycolic or lactic acid — epidermal renewal + dermal stimulation
The only acid family confirmed to preserve barrier integrity
Combinations to Avoid
- AHA/BHA + Retinol · Vitamin A: Concurrent use causes excessive exfoliation and irritation → use in separate routines (AM/PM)
- AHA/BHA + Vitamin C (L-ascorbic acid): pH conflict may reduce efficacy of both — layer them 20–30 minutes apart
- AHA/BHA + Niacinamide: High-strength AHAs applied simultaneously may reduce niacinamide efficacy — sequential application is preferred
→ Ingredient Combination Complete Guide — acid conflict and synergy strategies
Clinical Usage Protocol
Starting with a sensitization-prevention approach is recommended for all new users.
Safety — Green et al., 2009
SPF 50 for at least 2 weeks is recommended after high-concentration AHA peels. Freshly exposed epidermal cells are substantially more vulnerable to UV-induced damage.
FDA OTC Guideline
Maximum AHA concentration of 10% at pH ≥ 3.5 for consumer safety in over-the-counter products.
Multi-Acid Combination Products
An increasing number of products now combine AHA+BHA or AHA+PHA.
Clinical Evidence — Kornhauser et al., 2010
Low-concentration multi-acid formulations achieved comparable exfoliation efficacy to single high-concentration acids, with significantly reduced irritation scores. Component acids must share compatible pH requirements for synergy.