Serums with AHAs & BHAs for Gentle Winter Exfoliation

AHAs, BHAs and PHAs all exfoliate, but they do it differently, and winter changes which one makes sense. Here is how to tell them apart and use them without wrecking your barrier.

AHA, BHA, PHA: What the Letters Mean

Hydroxy acids are a family of exfoliating ingredients. The main groups are alpha-hydroxy acids (AHAs), beta-hydroxy acids (BHAs), polyhydroxy acids (PHAs) and bionic acids^[1]. Glycolic acid is the most commonly used AHA^[2], and lactic acid is another. Salicylic acid is the best-known BHA, and its use as a peeling agent is well documented^[3].

What AHAs Do

AHAs loosen surface cells and smooth the skin. In a three-month trial, volunteers who applied 5% or 12% lactic acid twice a day saw improved smoothness, texture and lines. The 12% strength also increased epidermal and dermal firmness and thickness, while 5% produced similar surface and epidermal changes without changes in the dermis^[4]. A higher strength does more, but it also asks more of your skin, which is worth remembering in winter.

What BHA Does

Salicylic acid exfoliates the outer layer of skin, and its comedolytic property (it helps clear clogged pores) is why it is used for acne. Its action is described as disrupting the junctions between cells^[3]. That makes it a natural choice for oily or congested areas, and a sensible one for combination skin where the T-zone is oily while the cheeks are dry.

The Trade-Off: Sun Sensitivity

The most important thing to know about AHAs is that they can make skin more vulnerable to UV. In a randomized, double-blind trial, 10% glycolic acid at pH 3.5 applied six days a week for four weeks increased sunburn cells and lowered the UV dose needed to redden the skin. A week after stopping, the sensitivity was gone^[5]. Reviews of hydroxy acids put special emphasis on safety with prolonged use on sun-exposed skin^[1]. In practice: use these acids with daily sunscreen, every day.

A Gentler Route: PHAs

PHAs, such as gluconolactone and lactobionic acid, were developed as a new generation of AHAs that give similar exfoliating and smoothing effects without the sensory irritation that can limit classical AHAs. Reviews report that they are compatible with sensitive skin, including rosacea and atopic dermatitis, and that they add humectant properties^[6][2].

A note on the sources: the two PHA reviews were written by authors closely tied to the people who developed these ingredients, so treat the tolerability claims as promising rather than independent.

Choosing and Using One in Winter

• Start low and slow: a low-strength AHA or PHA once or twice a week, and build only if your skin stays calm.
• One acid at a time, and not on the same night as retinol or other strong actives.
• If you are acne-prone, a BHA is the more targeted choice for clogged pores, applied only to the areas that need it.
• If your skin is sensitive or reactive, a PHA is the gentler place to start.
• If you tend to get dark marks, be especially careful: irritation can worsen post-inflammatory hyperpigmentation^[7].
• Sunscreen every morning, and see a dermatologist if skin stings, burns or breaks out.