Unit 1 · Chemical exfoliation & the biology
Why concentration alone never tells you how active an acid is
This unit sets the biological foundation for every acid decision that follows. By the end you should be able to explain why the three acid families work differently at a mechanistic level, and why the percentage on a product label is only half the story of how active it actually is.
Framing
Chemical exfoliation in aesthetic medicine
Chemical exfoliation uses acids to accelerate the natural desquamation process — the shedding of corneocytes from the skin surface. Done correctly, it improves texture, reduces comedonal congestion, evens pigmentation and, with regular use, stimulates epidermal renewal and dermal remodelling.
In aesthetic medicine, chemical exfoliants exist on a spectrum from gentle daily-use retail products to professional chemical peels administered in clinic. The same ingredients appear at both ends of this spectrum — it is concentration, pH and application technique that determine clinical depth and associated risk.
The three acid families
- AHAs — alpha-hydroxy acids
- Water-soluble. Act at the skin surface and into the upper epidermis. Best suited to photoageing, texture and dry-to-normal skin.
- BHAs — beta-hydroxy acids
- Lipid-soluble. Penetrate into the sebaceous follicle, where AHAs cannot reach effectively. Best suited to oily, acne-prone and comedonal skin.
- PHAs — polyhydroxy acids
- Larger molecules with slower, gentler penetration. Best suited to sensitive, barrier-compromised and post-procedure skin.
A patient presents with oily, congested, comedonal skin and asks which acid family will make the biggest difference. Based on solubility and mechanism, the correct first-choice family is:
Select an option to commit. The reasoning appears afterwards.
BHAs are lipid-soluble, which lets them penetrate the lipid-rich lining of the sebaceous follicle — the site where comedones actually form. AHAs, being water-soluble, cannot reach this environment as efficiently; they work well at the skin surface but leave follicular congestion largely untouched.
This is the deciding question for acid selection: is the primary concern oily or congested skin? If so, a BHA is the first-choice family — not because it is the strongest acid overall, but because it is the only one of the three families built to reach the follicle.
Learn · The biology of exfoliation
How acids accelerate shedding
Normal desquamation is regulated by serine proteases — enzymes that cleave the protein bridges holding corneocytes together in the stratum corneum. Two of these enzymes, kallikrein-5 and kallikrein-7, drive the process. Both are pH-sensitive: most active at a slightly acidic pH of 5.5–6, and less active outside that range.
Corneodesmosomes — protein bridges containing desmoglein-1 and corneodesmosin — hold adjacent corneocytes together. Enzymatic cleavage of these bridges is what releases corneocytes at the skin surface.
Two mechanisms, two acid classes
AHAs lower the local pH at the skin surface. This activates kallikrein-5 and kallikrein-7 and simultaneously weakens the ionic and hydrogen bonds between corneocytes — loosening the stratum corneum and accelerating shedding.
Salicylic acid follows a different path entirely. Being lipophilic, it penetrates the lipid-rich lining of the sebaceous follicle, dissolving sebaceous plugs and desquamating cells within the follicle itself — not at the surface.
Regular chemical exfoliation also sends a mild wounding signal to the basal epidermis, stimulating keratinocyte proliferation and, over time, fibroblast activity and mild collagen synthesis. This secondary effect is why consistent AHA use produces improvements in fine lines that go beyond simple exfoliation.
The protein bridges that hold adjacent corneocytes together in the stratum corneum — and which must be enzymatically cleaved for normal desquamation to occur — are called:
Select an option to commit. The reasoning appears afterwards.
Corneodesmosomes — protein bridges containing desmoglein-1 and corneodesmosin — are what hold adjacent corneocytes together in the stratum corneum. Serine proteases, principally kallikrein-5 and kallikrein-7, cleave these bridges, releasing corneocytes at the surface.
This is the target every acid ultimately works toward, whether directly — AHAs lowering surface pH to activate the kallikreins — or indirectly, BHAs penetrating the follicle to desquamate cells within it. Understanding the mechanism is what lets you reason about why a product works, rather than simply reciting a marketing claim.
Predict · Free acid value
Concentration and pH together determine activity
Not every product with a stated acid percentage is equally active. The biologically available acid — the free acid — depends on both the concentration and the formulation pH. Before reading on, commit to a position.
Product A is 10% glycolic acid at pH 3.8. Product B is 15% glycolic acid at pH 5.0. Which one delivers more active, unionised acid to the skin?
Hold your answer before you open this. The value is in having committed to a position on concentration versus pH first.
Product A — the 10% product at pH 3.8 — has the higher free acid value (FAV), despite the lower stated concentration. At a higher pH, more of the acid sits in its ionised, conjugate-base form, which does not penetrate the stratum corneum.
Free acid value is the proportion of acid in its non-ionised, active form, and it depends on concentration and pH together. A label percentage alone tells you less than the label percentage read alongside the formulation pH.
This is why two products can share an identical percentage on the label yet perform very differently in practice. The manufacturer's buffering and pH choices are doing as much clinical work as the stated concentration — which is why pH on the label is as clinically significant as concentration itself.
Free acid value — the proportion of an acid that is biologically active — is determined by:
Select an option to commit. The reasoning appears afterwards.
Free acid value depends on concentration and pH together, not either variable alone. At a higher pH, more of the acid sits in its ionised form, which does not penetrate the stratum corneum — so a higher percentage on the label does not guarantee greater activity.
This is the reasoning behind the earlier comparison: a 10% glycolic acid product at pH 3.8 delivers more active acid than a 15% product at pH 5.0. Reading a label for percentage alone, without checking pH, tells you less than it appears to.
Unit 1 summary
Clinical takeaways
- Acid family follows mechanism, not just strength. AHAs are water-soluble and act at the surface and upper epidermis; BHAs are lipid-soluble and reach the sebaceous follicle; PHAs are larger and penetrate slowly. Oily, congested skin needs a BHA specifically because it is the only family that reaches the follicle.
- Desquamation is enzyme-driven, and acids work with that system, not around it. Serine proteases — kallikrein-5 and kallikrein-7 — cleave the corneodesmosomes holding corneocytes together. AHAs accelerate this by lowering surface pH; BHAs bypass it by penetrating the follicle directly.
- Concentration alone does not describe an acid's activity. Free acid value depends on concentration and pH together — a lower percentage at a lower pH can outperform a higher percentage at a higher pH.
- Regular exfoliation does more than shed cells. Over time it sends a mild wounding signal to the basal epidermis, stimulating keratinocyte proliferation and fibroblast collagen synthesis — producing benefits beyond simple surface renewal.