AHA BHA PHA Formulation: Exfoliating Acid Chemistry for Skincare

1. AHA BHA PHA Formulation Basics: The Desquamation Mechanism

Healthy skin sheds its outermost layer continuously through a process called desquamation, in which corneocytes — the flattened, dead keratinocytes that make up the stratum corneum — detach from the skin surface as the desmosomal proteins holding them together break down naturally. Chemical exfoliants don't create a new mechanism; they accelerate one the skin already performs, by weakening those same desmosomal adhesions faster than the skin's baseline turnover rate would achieve on its own. That acceleration is genuinely useful — it can smooth visible texture, improve the appearance of hyperpigmentation and fine lines, and help unclog pores — but it also means every exfoliating acid is, by design, thinning the outermost protective barrier of the skin, which is exactly why formulation precision matters more here than in a typical cosmetic active.

Three chemical families dominate the cosmetic exfoliant category, and each interacts with that desmosomal breakdown mechanism differently based on a single defining property: how the molecule partitions between water and lipid, and how large it is.

• Alpha hydroxy acids (AHAs) — small, water-soluble molecules such as glycolic and lactic acid that act primarily at and just below the skin surface.
• Beta hydroxy acid (BHA) — salicylic acid, the only cosmetically significant BHA, which is lipid-soluble and can penetrate into sebum-filled pores.
• Polyhydroxy acids (PHAs) — larger molecules such as gluconolactone and lactobionic acid that exfoliate more gradually due to slower skin penetration.

That size-and-solubility distinction is the single most useful mental model for predicting how any given exfoliating acid will behave in a formulation, and it's the thread that runs through every section that follows.

2. AHAs: Water-Soluble Surface Exfoliants

Alpha hydroxy acids are defined by a hydroxyl group on the carbon adjacent to the carboxylic acid group, and within the cosmetic category, glycolic acid and lactic acid account for the overwhelming majority of formulated products. Both are small, highly water-soluble molecules, and glycolic acid in particular has the smallest molecular size of any commonly used AHA, which lets it penetrate the stratum corneum more readily than its chemical relatives. That combination of small size and high water solubility is precisely why AHAs are formulated as leave-on serums, toners, and peel solutions rather than oil-based products — they need direct, sustained contact with the skin's aqueous environment to work.

Lactic acid shares glycolic acid's basic mechanism but is a slightly larger molecule, which generally translates to somewhat gentler surface action alongside a genuinely useful secondary property: lactic acid is also a natural component of the skin's own moisturizing factor, giving it humectant behavior that glycolic acid lacks to the same degree. This dual exfoliant-humectant behavior is one reason lactic acid has become the more common choice for sensitive-skin AHA formulations, even though glycolic acid generally produces more pronounced surface renewal at an equivalent formulation strength.

That surface-acting, water-soluble behavior is also exactly what AHAs can't do: reach the sebum-filled environment inside a pore, which is the gap salicylic acid's very different chemistry was built to fill.

3. BHA: Salicylic Acid's Lipophilic, Pore-Penetrating Action

Salicylic acid stands alone as the cosmetically relevant beta hydroxy acid, and its defining formulation property is the opposite of the AHAs: it is lipid-soluble rather than water-soluble. That single difference in solubility explains almost everything about why salicylic acid behaves so differently on skin and why it dominates acne-focused and oily-skin formulations rather than general anti-aging exfoliation.

Because salicylic acid partitions into lipids rather than staying at the water-based skin surface, it can travel down into sebum-filled hair follicles and pores in a way that water-soluble AHAs simply cannot, giving it a genuine comedolytic effect — the ability to help clear the follicular blockages that contribute to acne. Salicylic acid also carries anti-inflammatory activity related to its structural similarity to aspirin, a secondary benefit that further supports its use in blemish-prone skin formulations beyond exfoliation alone.

Where AHAs and BHA differ by solubility and reach, PHAs differ mainly by scale, and that scale difference is what makes them the exfoliant class formulators reach for when a client's skin can't tolerate either of the first two.

4. PHAs: Larger-Molecule, Gentler Exfoliation Chemistry

Polyhydroxy acids work through essentially the same desmosomal-loosening mechanism as AHAs, but their molecules are meaningfully larger — gluconolactone and lactobionic acid both carry multiple hydroxyl groups that increase molecular size and slow the rate at which the compound penetrates the stratum corneum. That slower penetration is the entire basis of the PHA gentleness claim: it isn't a weaker acid in a chemical sense, it's an acid that physically can't reach deeper skin layers as quickly, which produces a more gradual exfoliation with a meaningfully lower irritation profile in most clinical comparisons.

PHAs also bring genuine secondary functionality that AHAs and BHA don't share to the same degree: their multiple hydroxyl groups give them humectant, water-binding behavior, and compounds like lactobionic acid function as effective metal-ion chelators, which can help protect other actives in the formulation from oxidative degradation catalyzed by trace metals. This combination of gentle exfoliation, moisture retention, and antioxidant-supporting chelation is why PHAs have become the default recommendation for rosacea-prone, post-procedure, and generally reactive skin that can't tolerate a standard AHA or BHA regimen.

None of that gentleness matters, though, if the finished formulation's pH doesn't put enough of the acid in its active, unionized form, which is where formulation chemistry, not molecule choice, becomes the deciding factor in whether any of these three acid classes actually exfoliate.

5. pH, Free Acid Content & Formulation Stability

An exfoliating acid's effectiveness depends on more than which molecule is chosen — the finished product's pH determines what fraction of that acid exists in its active, protonated form versus its inactive, ionized salt form, and getting this wrong can render an otherwise well-designed formulation essentially non-functional. This relationship, governed by each acid's specific dissociation constant, is why two products listing the same acid at the same nominal concentration can perform completely differently on skin depending on their formulated pH.

• pH buffering systems — AHA and BHA formulations typically require a buffering system to hold pH within the acid's effective range across the product's shelf life, since drift toward neutral pH as the formula ages reduces the fraction of active, unionized acid.
• PHA pH tolerance — because PHAs rely on molecular size rather than a strongly acidic environment for their gentler action, they can be formulated at a comparatively higher, skin-friendlier pH without losing as much efficacy, one more reason they suit sensitive-skin products.
• Packaging and light exposure — several exfoliating acid actives are sensitive to light and oxygen exposure over time, making opaque or UV-protective packaging a genuine stability consideration rather than a marketing preference.
• Preservation compatibility — the low pH environment AHA and BHA formulations require can limit which preservative systems remain effective, since many common preservatives have a narrower optimal pH window than the exfoliant itself.

Because pH interacts with efficacy, irritation risk, and preservation simultaneously, exfoliating acid formulation is fundamentally a pH-engineering exercise as much as an ingredient-selection one, a discipline that becomes even more important once other actives enter the same formulation.

6. Regulatory Considerations & Combining Exfoliants with Other Actives

Regulatory bodies treat AHAs with more formal scrutiny than most cosmetic actives because of their well-documented photosensitizing effect and their history of consumer complaints when used at aggressive strengths without adequate guidance. The FDA has issued specific labeling guidance for AHA-containing cosmetics in the United States, and the EU's Scientific Committee on Consumer Safety has published opinions addressing AHA concentration and pH limits for the European market, so formulators need to consult current guidance for each target market rather than assuming a single global standard applies.

Combining exfoliating acids with other active ingredients introduces its own compatibility questions, and getting this wrong is one of the most common sources of consumer-reported irritation. Retinoids and AHA or BHA exfoliants both increase skin cell turnover and barrier sensitivity through different mechanisms, and layering them without careful formulation or usage guidance meaningfully raises irritation risk, a consideration explored further in our guide to retinoid encapsulation and stabilization technology. Niacinamide is generally considered more compatible with exfoliating acids than retinoids are, though formulators still need to account for niacinamide's own pH sensitivity, a topic covered in depth in our niacinamide stability and pH compatibility guide.

Bringing an AHA, BHA, or PHA product to market safely and effectively means resolving molecule selection, pH engineering, stability, and regulatory compliance together rather than in isolation, exactly the kind of integrated cosmetics and personal care formulation work that turns a promising exfoliant concept into a product that performs consistently batch after batch.