| AHA water solubility | Water-soluble; works at skin surface |
| BHA oil solubility | Oil-soluble; penetrates pores |
| PHA molecular size | Larger than AHAs — slower, gentler penetration |
| Optimal pH range for activity | 3.0–4.0 for most chemical exfoliants (Cosmetic chemistry literature) |
| Common AHA examples | Glycolic acid, lactic acid, mandelic acid |
| Common BHA example | Salicylic acid (most widely used) |
| Common PHA examples | Gluconolactone, lactobionic acid |
| Sun sensitivity risk | Increased with AHA/BHA use; SPF is essential (FDA consumer guidance) |
The Three Classes at a Glance
Chemical exfoliants work by breaking the bonds between dead skin cells rather than physically buffing them away. The three main classes — AHAs, BHAs, and PHAs — differ in their solubility, molecular size, and the depth at which they act. Choosing between them starts with understanding those structural differences.
| AHA water solubility | Water-soluble; works at skin surface |
| BHA oil solubility | Oil-soluble; penetrates pores |
| PHA molecular size | Larger than AHAs — slower, gentler penetration |
| Optimal pH range for activity | 3.0–4.0 for most chemical exfoliants (Cosmetic chemistry literature) |
| Common AHA examples | Glycolic acid, lactic acid, mandelic acid |
| Common BHA example | Salicylic acid (most widely used) |
| Common PHA examples | Gluconolactone, lactobionic acid |
| Sun sensitivity risk | Increased with AHA/BHA use; SPF is essential (FDA consumer guidance) |
For a broader look at how these ingredients appear on product labels, see our guide to reading a skincare ingredient list.
AHAs: Surface Renewal for Texture and Tone
Alpha hydroxy acids are water-soluble, which means they work primarily at the skin's outer layers. Glycolic acid has the smallest molecular size among common AHAs, allowing relatively deep surface penetration — making it effective for uneven tone, fine lines, and dullness. Lactic acid is larger, penetrates more slowly, and carries mild humectant (moisture-attracting) properties, often making it better tolerated by drier skin types. Mandelic acid, derived from almonds, has the largest molecule of the three and is frequently recommended for sensitive or darker skin tones where post-inflammatory hyperpigmentation is a concern.
AHAs are well-studied for improving surface texture and supporting collagen synthesis over time, though individual results vary. They are also associated with increased photosensitivity, so consistent daily SPF use is essential.
Sun Protection Is Non-Negotiable
AHAs in particular are known to increase photosensitivity, meaning skin can burn more easily after use. The FDA recommends applying broad-spectrum SPF 30 or higher daily when using AHA-containing products. This applies to BHAs and PHAs as well, though their photosensitizing effect is generally considered lower. Consult a dermatologist if you have questions specific to your skin.
BHAs: Pore-Penetrating Action for Oily and Acne-Prone Skin
Salicylic acid is the dominant BHA in skincare. Unlike AHAs, it is oil-soluble — meaning it can travel through sebum into the pore lining, dissolving the mix of dead cells and oil that causes comedones (blackheads and whiteheads). This makes BHAs particularly suited to oily, acne-prone, or congestion-prone skin.
Salicylic acid also has anti-inflammatory properties, which can help calm the redness associated with breakouts. In the United States, over-the-counter formulations are capped at 2% concentration by the FDA. If your skin sits on the drier or more sensitive end, BHAs may be too stripping unless used sparingly or in low concentrations. Understanding your baseline skin type is key — our article on what your skin type means for your routine covers this in detail.
PHAs: A Gentler Entry Point
Polyhydroxy acids share a similar exfoliating mechanism with AHAs but have a larger molecular structure. That size means they penetrate skin more slowly, producing a milder effect with a lower likelihood of redness, stinging, or irritation. Research suggests PHAs — including gluconolactone and lactobionic acid — may also offer antioxidant and humectant benefits alongside exfoliation.
PHAs are often a practical starting point for those new to chemical exfoliants, for sensitive or reactive skin types, or for skin that has not tolerated AHAs well. They are also considered compatible with rosacea-prone skin in many cases, though a dermatologist's input is advisable before introducing any active ingredient to reactive skin.
Chemical exfoliant
A skincare ingredient that dissolves the bonds between dead skin cells, encouraging them to shed without physical scrubbing. Common types include AHAs, BHAs, and PHAs.
AHA (Alpha Hydroxy Acid)
A class of water-soluble acids derived primarily from natural sources such as fruit or milk. AHAs work on the skin's surface to improve texture, tone, and brightness.
BHA (Beta Hydroxy Acid)
An oil-soluble acid — salicylic acid being the primary example — that can penetrate into pores to dissolve sebum and dead cell buildup. Particularly effective for acne-prone and oily skin.
PHA (Polyhydroxy Acid)
A newer generation of chemical exfoliants with larger molecular structures than AHAs, meaning they penetrate more slowly and cause less irritation. Well-suited to sensitive or reactive skin.
Keratolytic
A term describing an ingredient's ability to break down keratin, the protein that holds dead skin cells together, thereby facilitating exfoliation.
pH
A scale from 0 to 14 measuring acidity or alkalinity. Chemical exfoliants are most effective at lower pH levels (typically 3–4), as this activates their keratolytic properties.
Fitting Exfoliants Into Your Routine
Chemical exfoliants are typically applied after cleansing and before heavier products such as serums, moisturizers, and oils. Frequency matters: starting with two to three applications per week allows skin to adjust before increasing use. Combining multiple exfoliant types — for example, layering an AHA with a BHA — is not generally recommended without dermatological guidance, as it increases irritation risk.
For guidance on sequencing exfoliants with other actives like niacinamide or vitamin C, see our field guide to popular actives. And for a practical look at product layering order overall, our skincare layering guide explains the logic step by step.
This article is for general informational purposes only and does not constitute medical or dermatological advice. If you have a skin condition or concerns about introducing new active ingredients, consult a qualified dermatologist before making changes to your skincare routine.