AHAs, BHAs and PHAs
The three families of acid exfoliant: AHAs smooth and brighten the surface, BHAs help keep pores clear, and PHAs suit sensitive skin.
hydroxy acids, alpha hydroxy acids, beta hydroxy acids, polyhydroxy acids, AHA, BHA, PHA, chemical exfoliants, acid exfoliants, lactobionic acid
Texture & large pores · Dull skin · Blemish-prone skin · Pigmentation & dark spots · Ageing skin
What it does
Hydroxy acids are the backbone of chemical exfoliation, and they are not interchangeable. AHAs, such as glycolic, lactic and mandelic acid, work on the surface to soften rough texture, brighten dullness and, over time, even the look of pigmentation and fine lines; some also hydrate. BHAs, chiefly salicylic acid, work inside the pore and suit oily, congested skin. PHAs, such as gluconolactone and lactobionic acid, are the gentlest, and suit sensitive, reactive or dry skin.
How it works
All three loosen the bonds that hold dead cells together on the skin's surface, so they shed more evenly. AHAs are water-soluble, and their size sets their strength: glycolic acid is the smallest and most active, lactic acid is larger and gentler, and mandelic acid, the largest, is the gentlest. BHAs are oil-soluble, so they can travel into the pore to dissolve oil and debris. PHAs are larger still, so they exfoliate slowly and gently while drawing in moisture. The whole formula matters too: its strength, pH and soothing partners all change how well an acid is tolerated.
MG says
In the studio, The Désincrustation Facial pairs diamond microdermabrasion with a personalised infusion of enzymes and AHAs, and our Organic Radiance and Hydrating Glow facials use a custom AHA or enzyme peel. At home, I like Eighth Day's The Resurfacing Tonic in the evening, after cleansing and before serum and moisturiser. Whichever acid you choose, start low and go slow.
Cautions
All hydroxy acids can make skin more sensitive to the sun, so use a broad-spectrum sunscreen daily and for a week after use. Start with a low strength once or twice a week and build up as your skin adapts; cut back if you notice persistent redness, stinging, tightness or flaking. Don't layer several acids, or acids and retinoids, on the same night. Avoid BHAs if you're allergic to aspirin or salicylates. If you're pregnant, trying to conceive or breastfeeding, check with your doctor, midwife or pharmacist before using acids; in pregnancy, keep to everyday-strength AHAs, use salicylic acid only at 2% or less in rinse-off products, and avoid professional peels.
Regional notes
Australia: Glycolic acid in retail skincare is limited to 5%, or 20% at pH 3.5 or higher, unless the product carries poison labelling; salon-only professional products are exempt.
United States: The FDA recommends that cosmetics containing AHAs carry a “Sunburn Alert” label; the advice to wear sunscreen applies everywhere.
United Kingdom: In the UK and EU, salicylic acid in cosmetics is capped at 2% (3% in rinse-off hair products), and it must not be used in products for children under 3.
