AHA in Korean Skincare: Glycolic, Lactic, and Mandelic Acid Compared
What it is
Alpha hydroxy acids (AHAs) are water-soluble acids that exfoliate the skin surface by breaking the ionic bonds (desmosomes) between corneocytes in the stratum corneum. The three most common AHAs in K-beauty are glycolic acid (from sugarcane, smallest molecule at 76 Da), lactic acid (from fermented milk, 90 Da), and mandelic acid (from bitter almonds, 152 Da). Molecule size determines penetration depth and irritation potential — smaller molecules penetrate faster and irritate more.
How it works
AHAs lower the pH of the skin surface and chelate calcium ions that hold desmosomal bonds together between corneocytes. When these bonds dissolve, dead cells on the surface shed more quickly, revealing newer cells underneath. At higher concentrations (above 10%), AHAs also stimulate glycosaminoglycan and collagen production in the dermis through fibroblast activation. The exfoliation effect is pH-dependent: AHAs work best below pH 4, with an effective free acid concentration above 4%.
Clinical benefits
Accelerated cell turnover and smoother texture
A 22-week double-blind study of 74 women found that 8% glycolic acid cream produced statistically significant improvements in skin smoothness, fine wrinkle reduction, and overall photoaging scores compared to vehicle. Improvement was progressive — results at 22 weeks exceeded those at 10 weeks.
Stiller et al., 1996 — Journal of the American Academy of Dermatology
Hyperpigmentation reduction
AHAs fade dark spots by accelerating the shedding of melanin-loaded keratinocytes. A 12-week study of lactic acid at 8% showed measurable reduction in melasma severity index in 25 Indian women, with lactic acid performing comparably to hydroquinone 4% for superficial epidermal pigmentation.
Sharquie et al., 2012 — Journal of Cosmetic Dermatology
Dermal collagen stimulation
Glycolic acid at concentrations of 20-70% in clinical peels increased dermal collagen density measured by ultrasound in a 6-month study. Even at lower daily-use concentrations (8-12%), AHAs upregulate hyaluronic acid and collagen production through fibroblast stimulation, though the effect is slower.
Bernstein et al., 2001 — Dermatologic Surgery
Improved absorption of other actives
By removing the outer layer of compacted dead cells, AHAs reduce the barrier thickness that other actives must penetrate. A study found that glycolic acid pre-treatment increased the absorption of topical hydroquinone and vitamin C by up to 2x compared to untreated skin.
Tung et al., 2000 — Dermatologic Surgery
Products with aha
Bio-Peel Gauze Peeling Lemon
Neogen
AHA BHA PHA 30 Days Miracle Acne Clear Foam
SOME BY MI
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Skin types
Oily and combination skin tolerate AHAs well and benefit from the oil-clearing exfoliation. Mandelic acid is the best choice for darker skin tones (Fitzpatrick IV-VI) because its larger molecular size reduces the risk of post-inflammatory hyperpigmentation from over-exfoliation. Lactic acid is the gentlest option for dry or sensitive skin — it doubles as a humectant. Glycolic acid is the strongest and most studied but carries the highest irritation risk for reactive skin.
Effective concentrations
Daily-use products: 5-10% AHA at pH 3-4 for gradual exfoliation. Weekly treatments: 10-15% for more noticeable resurfacing. Professional peels: 20-70% (dermatologist-administered only). Below pH 3.5, more free acid is available and irritation risk increases. Most K-beauty AHA toners sit at 5-8% with a pH of 3.5-4.0 — a balance between efficacy and daily tolerability. Start with 5% used 2-3 times per week and increase frequency before increasing concentration.
Pairs well with
Niacinamide
Apply AHA first, wait 15-20 minutes, then apply niacinamide. Niacinamide calms any irritation from the acid and strengthens the barrier while AHA exfoliates. Do not apply them simultaneously — niacinamide's neutral pH reduces free acid availability.
Hyaluronic acid
AHA exfoliation increases TEWL temporarily by thinning the corneocyte layer. Applying HA after AHA has absorbed replenishes surface hydration. The combination prevents the tight, dry feeling that AHAs sometimes cause.
Avoid combining with
Retinol in the same routine step
Both AHAs and retinol increase cell turnover and thin the stratum corneum. Using both in the same application can cause excessive peeling, redness, and barrier damage. Alternate nights: AHA on one evening, retinol on the next.
Vitamin C (L-ascorbic acid) at the same time
Both require a low pH to work, but layering two low-pH actives multiplies irritation without proportional benefit. Use vitamin C in the morning and AHA in the evening.
Other exfoliants (BHA, PHA, enzymes) in the same step
Stacking exfoliants accelerates barrier disruption beyond what the skin can repair overnight. Pick one exfoliant per routine. If you use BHA for pores and AHA for texture, alternate them on different nights.
Common questions
- What is the difference between glycolic acid, lactic acid, and mandelic acid?
- Glycolic acid is the smallest AHA molecule (76 Da) — it penetrates deepest and exfoliates most aggressively. Best for experienced users targeting wrinkles and thick, sun-damaged skin. Lactic acid (90 Da) penetrates more slowly, causes less irritation, and has humectant properties that hydrate while exfoliating. Good for dry or sensitive skin. Mandelic acid (152 Da) is the largest and gentlest. It exfoliates the surface without penetrating deep enough to trigger post-inflammatory hyperpigmentation, which makes it the safest choice for darker skin tones.
- Do AHAs cause sun sensitivity, and for how long?
- Yes. AHAs thin the stratum corneum, exposing newer cells that have less melanin and are more vulnerable to UV damage. A study by the FDA found that glycolic acid increased UV sensitivity by 18% at 4% concentration. This sensitivity persists for about one week after you stop using the AHA. Wear SPF 30+ every morning when using any AHA product, even on overcast days.
- Can AHAs cause purging, and how do I tell purging from a breakout?
- AHAs accelerate cell turnover, which pushes existing microcomedones (clogs you cannot see) to the surface faster. This purging looks like small whiteheads or closed comedones in areas where you normally break out. It should resolve within 4-6 weeks. If you see inflamed cysts, breakouts in areas where you never had acne, or irritation lasting beyond 6 weeks, that is a reaction to the product — not purging. Stop use and reassess.
- How often should I use AHA if I have never exfoliated before?
- Start at 5% concentration, twice per week, in the evening. Leave the product on for the recommended contact time (varies by formula — some are rinse-off, some are leave-on). After two weeks with no irritation, increase to three times per week. After a month, you can move to every other night if your skin tolerates it. Do not jump to daily use in the first month. The goal is consistent, gentle exfoliation, not maximum acid exposure.
- Should I rinse off my AHA toner or leave it on?
- Check the product instructions. Leave-on AHA toners (most K-beauty options at 5-8%) are designed to stay on the skin and continue working as you apply subsequent products. Rinse-off AHA treatments (peeling gels, high-concentration pads above 10%) should be washed off after the specified contact time. If a leave-on product stings for more than 30 seconds, rinse it off — your skin is reacting to the acid concentration.
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