Lactic Acid

Overview

Lactic acid is the alpha-hydroxy acid (AHA) naturally produced in sour milk, yogurt, and fermented vegetables. Cosmetic-grade lactic acid is made by bacterial fermentation of sugar (most often from corn) and comes as a clear, water-thin liquid at 88-90% active concentration. It is one of the oldest documented skincare ingredients — Cleopatra reportedly bathed in sour donkey milk, which is essentially a low-concentration lactic-acid soak.

It is the friendliest of the AHAs for DIY use. It is gentler than glycolic acid, more hydrating than mandelic acid, and easier to formulate than complex fruit-acid blends. The molecule is mid-sized — bigger than glycolic, smaller than mandelic — which means it penetrates the skin at a moderate, manageable pace.

Important note: lactic acid is also a natural component of the skin’s NMF (natural moisturizing factor), which is why it has a dual identity in skincare. At higher percentages (5-10%) it exfoliates. At lower percentages (1-3%) it primarily hydrates.

What it does in a formula

Primary roles:

• Surface exfoliation — dissolves the bonds between dead skin cells in the stratum corneum, accelerating turnover
• Brightening — reduces hyperpigmentation and uneven tone over 8-12 weeks of consistent use
• Humectant — at lower percentages, it functions as part of the NMF and increases skin hydration
• Mild collagen stimulation at sustained higher percentages (8-10%)

Secondary roles: improves skin texture, softens keratosis pilaris on body areas, helps with mild post-acne marks, and acts as a pH-adjusting acid (useful in formulations that need to be brought down to acidic pH).

How to use

Add to the water phase. Heat-stable up to boiling; can go in either heated water phase or cool-down. Most formulators add it cool-down to avoid issues with other heat-sensitive actives in the same batch.

Usage range:

• As a hydrating active: 1-3% (no exfoliation, but plenty of moisture)
• For gentle exfoliation: 3-5% (mild peeling, suitable for daily use after build-up)
• For active resurfacing: 5-10% (stronger effect, 2-3x per week starting frequency)
• As a pH adjuster only: 0.1-0.5%

The 88% liquid form is what most suppliers sell. To convert: a 5% finished lactic-acid serum from an 88%-active liquid requires 5.7% of the liquid (5 / 0.88 = 5.68%).

pH is critical. For exfoliation, the final formula needs to be at pH 3.5-4. The protonated (free acid) form is what does the exfoliation; above pH 4 it converts to its salt (lactate) and stops working as an exfoliant. Test with a calibrated meter, not strips.

Photosensitivity: lactic acid increases sun sensitivity for up to a week after use. Daily SPF 30+ is mandatory.

EU regulation: lactic acid is permitted up to 10% in leave-on cosmetics (above this, it crosses into “professional product” or “drug” territory).

Best for / Worst for

Best for: dry skin (the hydration component is a real benefit), mature skin needing gentle exfoliation, sensitive skin starting AHAs (start at 3-5%), keratosis pilaris on arms and thighs, post-acne marks, low-grade hyperpigmentation, brightening serums for dull complexions.

Worst for: very thin or compromised skin barriers, eye-area products (sting risk), pregnancy (most dermatologists allow AHAs but recommend caution above 5%; consult a doctor), formulas where photosensitivity would be a deal-breaker (no SPF in routine), people with active dermatitis or eczema flares.

Common pitfalls

Wrong pH. A 5% lactic acid serum at pH 5 is just a mild humectant — it does not exfoliate. Test and adjust to 3.5-4 with citric acid solution or sodium hydroxide to neutralize partway.

Overusing. Daily 10% lactic acid is too much for almost everyone. Start at 3-5% three times a week and build tolerance over 4-6 weeks.

Stacking with other acids in the same routine. A 5% lactic acid serum plus a 2% salicylic acid plus a 0.5% retinol the next morning is a recipe for barrier damage. Pick one resurfacing active and rotate.

Skipping SPF the next day. Lactic acid increases UV sensitivity. Skipping sunscreen after an AHA night cancels most of the brightening benefit.

Forgetting the dual identity. A 2% lactic acid in a face cream is not “exfoliating” — it is just hydrating. Adjust expectations to the percentage.

Substitutes

• Mandelic acid — larger molecule, slower-penetrating, gentler for sensitive skin and darker skin tones.
• Glycolic acid — smallest AHA, fastest-penetrating, strongest effect. Less hydrating, more irritating.
• Salicylic acid — oil-soluble BHA, exfoliates inside pores rather than the surface.
• Polyhydroxy acids (PHAs) — gluconolactone and lactobionic acid, larger and gentler than lactic; best for very sensitive skin.
• Sodium lactate — the salt form, used at higher percentages as a pure humectant without exfoliation.
• Lactobionic acid — gentle PHA cousin of lactic; good for very sensitive AHA-curious skin.