Polyhydroxy acids (PHAs) — the gentlest exfoliants, and when they make sense.

Polyhydroxy acids — gluconolactone, lactobionic acid, and galactose — are the third generation of hydroxy acid exfoliants, following the established AHAs (glycolic, lactic, mandelic) and BHA (salicylic acid). They share the exfoliating mechanism of their predecessors but behave significantly differently on the skin. Understanding why helps clarify when they are the correct choice.

How PHAs differ from AHAs

The defining difference between PHAs and AHAs is molecular size. AHAs — particularly glycolic acid, the smallest — have low molecular weight that allows them to penetrate quickly and deeply into the skin, where they achieve their exfoliating and cell-turnover effects but also, in sensitive or barrier-compromised skin, their irritating ones.

PHAs have a larger molecular structure. This limits their rate of penetration. The practical result is exfoliation that occurs more slowly and at a more superficial level — which means less irritation, less photosensitivity, and meaningful tolerance by skin types that cannot use conventional AHAs.

The trade-off is efficacy at shorter timeframes. PHAs produce slower, less aggressive exfoliation than glycolic acid at equivalent concentrations. For skin that tolerates AHAs well, the faster-acting AHAs remain appropriate. For skin that does not, PHAs deliver exfoliation without the irritation cost.

Humectant properties

A notable property that sets PHAs apart from AHAs is that gluconolactone and lactobionic acid function as humectants in addition to exfoliants. They attract and bind water in the skin, contributing to surface hydration alongside the exfoliating effect. This dual function — exfoliate and hydrate simultaneously — is unusual in the acid category and particularly relevant for dry and dehydrated skin that needs resurfacing without additional water loss.

Lactobionic acid, specifically, has been studied for its antioxidant properties in addition to its exfoliating and humectant activity. It chelates metal ions that catalyse free radical formation — which provides modest antioxidant contribution alongside the primary exfoliating function.

Who PHAs are best suited for

Sensitive and rosacea-prone skin that has attempted AHAs and experienced irritation, flushing, or prolonged stinging are the primary candidates for PHAs. The slower penetration and smaller magnitude of skin response mean that PHAs can provide the texture and tone benefits of acid exfoliation without triggering the vascular response that people with rosacea in particular must avoid.

Barrier-compromised skin — whether from over-exfoliation, environmental damage, or active dermatitis — can use PHAs as a way to gently address surface cell buildup without the risk of exacerbating barrier disruption. The humectant properties support rather than undermine hydration.

Post-procedure skin — following laser treatments, chemical peels, or microneedling — may tolerate PHAs during recovery where AHAs are contraindicated. The exfoliating activity helps prevent post-procedure congestion and promotes even healing without aggressive re-exposure of recently treated skin.

Beginners to acid exfoliation can use PHAs to introduce their skin to chemical exfoliation before progressing to AHAs if they choose to. The lower irritation risk provides a useful entry point.

Evidence

Clinical evidence for PHAs is available, though the body of research is smaller than for glycolic acid.

Studies on gluconolactone have demonstrated reduced fine line depth, improved skin texture, and measurable improvements in post-inflammatory hyperpigmentation over eight to twelve weeks of consistent use. Comparative studies against glycolic acid have generally found PHAs less effective at equivalent timeframes but significantly better tolerated — particularly on sensitive and dry skin types.

A study in patients with rosacea found that a gluconolactone-based formulation significantly improved multiple rosacea parameters over 12 weeks, including erythema and skin texture, without triggering flares. This is clinically meaningful for a skin condition that excludes most exfoliating options.

How to use PHAs

PHAs are formulated in toners, serums, and moisturisers at concentrations typically between 5% and 15%. They are applied after cleansing, before heavier products. They are used in the same step as other exfoliating acids — typically in the evening, though their reduced photosensitivity compared to AHAs makes morning use more tolerable for most people.

PHAs do not require the same strict morning-avoidance that high-concentration glycolic acid products do, but daily SPF remains appropriate when using any exfoliating acid — UV exposure stimulates the pigmentation production that exfoliation is working to reduce.

There is no benefit in combining PHAs with AHAs in the same step — the combined acid load produces irritation without proportionate additional benefit. Use one or the other on a given evening.

The Lux & Glo position

The foundational three-step ritual does not include a chemical exfoliant. It is built for consistent use across a wide range of skin types — including reactive and sensitive skin that needs the barrier-first foundation before introducing any active that increases cell turnover.

PHAs are a logical first choice when adding exfoliation to an established, functioning baseline. For skin that has been through barrier recovery and wants to address surface texture or pigmentation without the sensitivity risk of glycolic acid, gluconolactone or lactobionic acid at 5–10%, used two to three evenings per week, is a measured and evidence-supported approach.