PHA guide: the next-generation acid safe for sensitive and eczema-prone skin

Polyhydroxy acids (PHAs) are the next generation of chemical exfoliants — structurally related to alpha-hydroxy acids (AHAs) but with multiple hydroxyl groups that change their behavior on skin. They exfoliate comparably to AHAs at equivalent concentrations but with dramatically lower irritation potential, making them the preferred exfoliant for sensitive, eczema-prone, rosacea, and post-procedure skin. Here is the complete guide.

What makes PHAs different from AHAs

Structural distinction

AHAs (glycolic, lactic): One hydroxyl group (–OH) on the alpha carbon adjacent to the carboxylic acid group.

PHAs: Two or more hydroxyl groups distributed along the molecular chain.

The key PHAs:
Gluconolactone: Cyclic ester of gluconic acid; 178 Da; five hydroxyl groups
Lactobionic acid: Disaccharide-derived; 358 Da; eight hydroxyl groups; the largest common chemical exfoliant
Galactose: Less commonly used; similar structure

These multiple hydroxyl groups produce three key behavioral differences compared to AHAs:

Larger molecular weight = slower penetration:
Gluconolactone at 178 Da and lactobionic acid at 358 Da penetrate the stratum corneum far more slowly than glycolic (76 Da) or lactic (90 Da). This produces a surface-level exfoliation profile with minimal penetration to deeper epidermal layers — the mechanism that makes PHAs gentler.

Humectant activity:
The multiple hydroxyl groups attract and bind water — PHAs are potent humectants. Where AHAs are purely exfoliating (with lactic acid having modest NMF humectancy), PHAs simultaneously exfoliate the surface and hydrate the stratum corneum. This is particularly relevant for dry and eczema-prone skin where maintaining hydration during exfoliation is critical.

Chelating activity:
PHAs chelate metal ions (Fe³⁺, Cu²⁺) that catalyze free radical formation — providing mild antioxidant-adjacent protection beyond their exfoliating role.

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The exfoliation mechanism

PHAs share the core AHA mechanism:
Reduce corneocyte cohesion by activating endogenous serine proteases (kallikreins) through pH reduction
Chelate calcium ions required for corneocyte adhesion
Accelerate desquamation → smoother texture, improved tone, mild brightening

How it differs from AHAs: The exfoliation occurs primarily in the outer stratum corneum rather than reaching the lower epidermis or papillary dermis. This produces:
Less cytokine release from keratinocytes
Less inflammatory signaling
Less erythema and stinging on application
Lower PIH risk (relevant for Fitzpatrick types IV–VI)

The trade-off: PHAs produce less significant collagen stimulation than glycolic acid (which reaches the dermis), and their anti-aging benefit operates primarily through surface exfoliation + hydration rather than dermal remodeling.

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Evidence: comparable exfoliation, less irritation

Green et al. — PHA tolerance studies

Multiple studies comparing gluconolactone to glycolic acid have demonstrated:
Equivalent keratolytic activity (stratum corneum thinning, desquamation index improvement) at comparable concentrations in non-sensitive populations
Significantly lower irritation scores in sensitive and eczema-prone populations — subjects rated PHA stinging, burning, and redness substantially lower than AHA at equivalent concentrations
No increase in UV sensitivity in some PHA studies (a difference from AHAs, which consistently increase photosensitivity)

Eczema-specific evidence: PHAs are used in clinical atopic dermatitis protocols as a keratolytic adjunct because they exfoliate the thickened, scaling skin of eczema without disrupting the already-compromised barrier. Gluconolactone at 8–12% has been shown to reduce scaling and improve skin texture in AD patients without the barrier impairment that AHAs would cause.

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Concentrations and formulations

Leave-on products (5–15%)

5–8%: Very gentle; appropriate for daily use in the most sensitive and reactive skin types, including active eczema management, rosacea, and post-procedure recovery.

10–12%: Standard effective PHA concentration. Produces visible improvement in texture and tone over 8–12 weeks of regular use. The reference range for clinical use.

15%: Upper consumer concentration. Equivalent to a mild AHA in exfoliant effect with significantly lower irritation — appropriate for experienced exfoliant users who cannot tolerate AHAs at equivalent concentrations.

Formulation types: Toners, serums, and moisturizers. Unlike AHAs, PHAs can be delivered in slightly higher pH formulations while maintaining activity because the multiple hydroxyl groups provide exfoliant activity across a broader pH range.

Professional use

PHAs are less commonly used as professional-concentration peels than AHAs. Their primary professional application is:
Post-procedure maintenance (post-laser, post-peel recovery)
Sensitive skin management protocols combining gentle exfoliation with barrier repair
Eczema management as an adjunct to topical corticosteroids

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Ideal candidates for PHAs

Primary indications

Sensitive, reactive skin: The defining indication. Patients who have never been able to tolerate AHAs — finding them too stinging, too irritating, or too drying — are the core PHA user. PHAs deliver measurable exfoliation benefits without the barriers to tolerability.

Eczema (atopic dermatitis): PHAs are the only chemical exfoliant class with positive evidence in AD skin. They reduce scaling and improve texture without the barrier disruption that would worsen AD. Gluconolactone specifically is used in dermatological protocols for scaling management.

Rosacea-prone skin: The combination of gentle exfoliation and humectancy — without the erythema trigger of AHAs — makes PHAs manageable for rosacea skin that still requires surface exfoliation for texture improvement.

Post-procedure recovery: After laser resurfacing, chemical peels, or microneedling — once the acute healing phase is complete — PHAs provide surface exfoliation to prevent hyperkeratotic buildup during healing without risking barrier disruption or PIH in healing skin.

Fitzpatrick types IV–VI: Like mandelic acid, PHAs offer a lower-PIH-risk exfoliation option. Their gentle penetration profile produces minimal inflammatory response.

Not the first choice for

Maximum anti-aging: Glycolic acid or tretinoin for patients who can tolerate them.

Severe acne with comedonal involvement: BHA (salicylic acid) for follicular penetration; AHAs for faster keratolytic effect.

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PHAs in a routine

Compatibility: PHAs are compatible with all other skincare actives. Unlike AHAs with vitamin C (potential irritation if layered at low pH), gluconolactone's pH range makes it less likely to conflict with other actives.

With retinoids: PHAs can be used on the same night as retinoids in tolerant skin (or on alternate nights). The humectant activity of PHAs complements the potential dryness of retinoid use.

With niacinamide: Excellent compatibility. Apply niacinamide after PHA (5-minute gap) for combined barrier support + exfoliation.

SPF: As with all exfoliants, morning SPF 30+ is standard practice. The photosensitivity increase from PHAs is smaller than from AHAs in most studies, but SPF is non-negotiable regardless.

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