Lactic acid explained — the gentlest AHA and why it is often the right starting point.

Lactic acid is an alpha-hydroxy acid originally derived from fermented milk — the compound responsible for the skin-smoothing reputation of the historical milk baths that eventually became a biochemical curiosity and then a cosmetic active. Modern cosmetic lactic acid is biosynthetically produced rather than dairy-derived, but the molecule is identical.

It is the gentlest of the commonly used AHAs. Glycolic acid has smaller molecules and penetrates more deeply and more rapidly. Mandelic acid is slower still. Lactic acid sits between them in practice: effective enough to produce consistent exfoliating and brightening results, gentle enough to be a realistic starting point for skin new to acids or for daily use in lower concentrations.

How it works

Alpha-hydroxy acids work by weakening the desmosomes — protein structures that hold corneocytes (dead surface skin cells) together in the stratum corneum. Lactic acid disrupts the ionic and hydrogen bonds in these connections, causing the outermost layer of bound cells to shed. What follows is a thinner, smoother surface, a more even texture, and — over time — stimulation of the skin renewal processes in the layers below.

The mechanism is pH-dependent. Lactic acid needs to be formulated below approximately pH 4 to be meaningfully active as an exfoliant. Above pH 4, the acid is predominantly in its dissociated salt form and loses the proton it needs to interact with the desmosomes. This is why the pH of an AHA formulation matters as much as the concentration, and why a product with a high lactic acid percentage but a high pH may be less active than a product with a lower percentage at the correct pH.

The humectant difference

Lactic acid has a property that glycolic acid lacks: it functions as a humectant. Its molecular structure allows it to attract and bind water in the stratum corneum, increasing surface hydration alongside the exfoliating function.

This dual action — exfoliate and hydrate simultaneously — makes lactic acid formulations more forgiving in use than glycolic acid at comparable exfoliating strengths. The surface hydration partially offsets the potential for post-exfoliation dryness. For skin that tends toward dehydration or that finds glycolic acid consistently stripping, lactic acid is the more practical option.

Concentrations

The exfoliating effect becomes clinically relevant at around 5% at the appropriate pH. Most OTC leave-on exfoliating treatments use 5–12% lactic acid. Concentrations above 12% are typically professional-only or are lower-frequency treatments (masks, targeted spot applications) rather than daily use formulations.

At concentrations below 5%, the exfoliating function is minimal and the humectant and surface-softening effects predominate. Products using lactic acid at 1–3% in moisturisers, toners, and cleansers are functioning primarily as hydrating and smoothing agents rather than as exfoliants in the traditional sense.

Evidence

The clinical evidence for lactic acid's efficacy as a chemical exfoliant is substantial. Studies at 5–12% concentrations show consistent improvement in surface texture, pore appearance, and mild hyperpigmentation over 8–12 weeks of regular use. The hyperpigmentation benefit is partly secondary — exfoliation removes the surface cells that contain the deposited melanin — and partly direct, as some evidence suggests AHAs may mildly inhibit tyrosinase activity.

There is also reasonably good evidence for lactic acid's effect on photodamage and fine lines. Regular use of 5–8% lactic acid reduces the appearance of fine lines and improves the uniformity of skin tone in studies of mild-to-moderate photodamage. This is partly exfoliation and partly the collagen-stimulating effect associated with regular mild acid exfoliation in the dermis.

Photosensitivity

Lactic acid is a photosensitiser. Chemical exfoliation removes the outermost protective cells and thins the cornified layer that contributes to UV resistance, leaving the skin more vulnerable to UV damage in the days following use. This does not mean avoiding the sun entirely, but it does mean SPF is important when using AHAs regularly — both to protect the newly exposed skin and to prevent the re-stimulation of melanin production that would undermine any brightening benefit.

The photosensitivity effect lasts roughly five to seven days after AHA use for most formulations. Regular daily use creates ongoing increased photosensitivity. The practical implication: if lactic acid is in the routine, daily SPF is not optional.

Introducing lactic acid

For skin new to chemical exfoliation, starting with a 5% lactic acid product two to three evenings per week is standard. Most people can progress to every-other-day use over a month or two, and some to daily use with a lower-concentration formulation. Sensitive, reactive, or already-compromised skin may need to start even more slowly.

AHAs are not compatible with all actives in the same application. Layering lactic acid with other AHAs or BHAs in the same step adds acid load without a proportional benefit and increases the risk of barrier disruption. Retinoids and AHAs applied simultaneously also increase the risk of irritation — most clinicians recommend using them on alternating evenings rather than in the same step.

Lactic acid is compatible with niacinamide, hyaluronic acid, panthenol, and ceramides. Applying these afterwards supports barrier recovery after exfoliation and reduces post-acid surface sensitivity.

The Lux & Glo position

The foundational three-step ritual does not include a chemical exfoliant. The baseline is built for tolerance across a wide range of skin types, and chemical exfoliation introduces variables — photosensitivity, potential for irritation, the need for careful sequencing — that belong in the next layer of the routine rather than the foundation.

For skin that has established the baseline and is looking to address surface texture, mild hyperpigmentation, or dullness, a 5% lactic acid formulation used two to three evenings per week is a logical first step into acid exfoliation. Begin with a barrier that is functioning. Exfoliate on top of it, not in spite of it.