Everything you need to know about PHAs.

Regarded as the new generation of hydroxy acids, the PHAs (poly-hydroxy acids) differ from AHAs (alpha-hydroxy acids) and BHAs (beta-hydroxy acids) by their distinctive chemical structure: they are carboxylic acids carrying two or more hydroxyl (–OH) groups attached to carbon atoms or to a cyclic aliphatic chain, whereas AHAs and BHAs bear only one. Moreover, unlike AHAs, PHAs are bulkier molecules, which limit their penetration into the epidermis. Their action is thus more superficial but also gentler.

Three PHAs are primarily used in cosmetics:

• Gluconolactone (or gluconic acid): Naturally present in skin cells, gluconolactone is the most popular polyhydroxy acid. In addition to its exfoliating properties, it exhibits antioxidant activity and anti-inflammatory effects, beneficial for redness-prone skin.

• Galactose: This simple sugar occurs naturally in the body. In cosmetics, it is prized for its exfoliating properties, as well as for its ability to stimulate the regeneration of skin tissues.

• Lactobionic acid: Derived from the oxidation of lactose, it belongs to the subcategory of bionic polyhydroxy acids (PHBAs). Its unique chemical structure, enriched with a sugar, makes it a highly effective humectant: it attracts and holds water within the skin, thereby enhancing epidermal hydration.

The polyhydroxy acids are versatile active ingredients that can be used on both the skin and the scalp. They possess a combination of exfoliating, moisturising, soothing and antioxidant properties, which explains their growing success in cosmetics.

• PHAs exhibit a mild keratolytic effect.

  Like AHAs, PHAs exert a keratolytic action in a more gradual and better-tolerated manner. They dissolve the bonds between corneocytes, thereby promoting the detachment of dead cells accumulated on the skin’s surface. This process of chemical exfoliation refines skin texture, reduces pore blockages and limits blackhead formation. By stimulating cell renewal, PHAs also help to diminish the appearance of fine lines and combat the dull complexion, caused by an accumulation of dead skin cells on the face that prevents light from reflecting.

• PHAs are hydrating.

  Unlike AHAs and BHAs, PHAs have multiple hydroxyl groups that confer a strong hygroscopic capacity. They absorb water present in the skin and retain it in the epidermis, which helps restore hydration and improve skin elasticity. This ability to limit dehydration explains why polyhydroxy acids are particularly well suited for exfoliating dry skin.

  Clinical studies have confirmed this property. In 16 women with dry skin, the effect of applying a skincare product containing 10% gluconolactone was evaluated using corneometric measurements. After two weeks of use, a significant increase in skin hydration was observed, as shown in the table below.

• PHAs are believed to have anti-inflammatory properties.

  Several studies suggest that PHAs also have an anti-inflammatory effect. By reducing the production of pro-inflammatory mediators, they may help limit redness and irritation. This property, still under investigation, complements the exfoliating role of PHAs in caring for acne-prone skin.

• PHAs have antioxidant properties.

  PHAs can also protect the skin against oxidative stress. Indeed, their chemical structure, rich in hydroxyl groups, enables them to chelate free radicals, which are unstable molecules that attack cellular components. By neutralising these reactive species, PHAs help to shield the skin from environmental aggressors such as pollution or UV radiation. This antioxidant effect also contributes to slowing down the processes associated with skin ageing.

• PHAs protect the skin from photo-ageing.

  Beyond the antioxidant activity of polyhydroxyl acids, certain studies have highlighted their potential to protect the skin from damage caused by UV rays. For example, gluconolactone applied at 7.5% has been shown in vitro to reduce UV-induced activation of the elastin promoter by 50%, without encouraging the formation of so-called “sunburn” cells, which indicate UV-related structural damage.

  Moreover, a clinical study in which a cream containing 8% lactobionic acid was applied topically for 12 weeks showed, on histological analysis, a significant reduction in the expression of matrix metalloproteinase 9 (MMP-9), an enzyme that degrades elastin and contributes to skin laxity. These results suggest that PHAs not only protect against oxidative damage but also preserve the dermal extracellular matrix and limit photoageing.

PHAs are considered safe and well tolerated, even by sensitive skin.

Their distinctive feature lies in their larger molecular size compared with AHAs and BHAs. This structure limits their capacity to penetrate deeply into the epidermis, thus reducing the risk of irritation, stinging or excessive flaking. It is for this reason that polyhydroxy acids provide an interesting alternative for exfoliating sensitive skin. Furthermore, unlike AHAs and BHAs, PHAs are not photosensitising. They can therefore be used without concern in summer or during the morning, although they of course do not eliminate the need to apply sun protection. Moreover, PHAs may be used during pregnancy or breastfeeding.

That said, even though these are gentle actives, some individuals may be sensitive to them and develop slight redness upon contact. Therefore, as a precautionary measure, as with any new active ingredient, we recommend that you perform a tolerance test on first use. To do so, apply a small amount of product to the inside of your arm and observe the area for 24 hours. In the absence of any reaction (redness, itching or irritation), application to the face or scalp can be considered safe.

PHAs are most commonly found in exfoliating serums, toning lotions, cleansing gels and facial masks. Typology has developed three products incorporating gluconolactone:

• The exfoliating face gel cleanser : With 5% gluconolactone and aloe vera, this formulation is composed of 98% naturally derived ingredients. It unclogs pores, evens out skin tone and smoothes skin texture. It is particularly recommended for dull complexions and those prone to blackheads.

• The mask for enlarged pores and blackheads : It combines 10% of an AHA complex (glycolic acid, lactic acid, tartaric acid, mandelic acid) and 10% PHA (gluconolactone) to act at different depths of the epidermis and eliminate dead skin cells. It is used in the evening, once or twice weekly, and helps even out the complexion and smooth the skin's texture.

• The mark-fading serum : Enriched with 14% PHA and also containing Centella asiatica, this serum works to fade acne scars. It is applied locally to the targeted areas, only in the evening.

• The scalp scrub : It is an exfoliating gel to be used before shampooing that gently removes dead skin cells for a healthy, rebalanced scalp. It delivers both chemical and mechanical exfoliation thanks to gluconolactone and jojoba beads.

• BARNETSON R. & al. A comparative study of gluconolactone versus benzoyl peroxide in the treatment of acne. Australasian Journal of Dermatology (1992).

• KSENZENKO S. M. & al. The polyhydroxy acid gluconolactone protects against ultraviolet radiation in an in vitro model of cutaneous photoaging. Dermatologic Surgery  (2004).

• EDISON B. L. & al. The use of polyhydroxy acids (PHAs) in photoaged skin. Cutis (2004).

• VAN SCOTT E. J. & al. Clinical and cosmeceutical uses of hydroxyacids. Clinics in Dermatology (2009).

• HEARING V. J. & al. Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clinical, Cosmetic and Investigational Dermatology (2010).

• ROTSZTEJN H. & al. Lactic and lactobionic acids as typically moisturizing compounds. International Journal of Dermatology (2019).

• ROTSZTEJN H. & al. Corneometric evaluation of skin moisture after application of 10% and 30% gluconolactone. Skin Research and Technology (2021).