What Is Lactic Acid? Exfoliating and Moisturising Effect, Concentrations
Lactic acid, from the AHA family, is an acid that helps dead cells slough off and is also among the skin's own moisture-holding components. It is used at low concentrations for roughness and dryness; as the concentration rises, its effect deepens. Being an AHA, it requires sun protection and can sting on sensitive skin.
At a glance
- INCI name
- Lactic Acid
- Type
- Alpha hydroxy acid, molecular weight about 90
- What it does
- Mild exfoliation, moisture, support for roughness and dryness
- Typical concentration
- In a US market scan, 1-9%, average 3,7%
- Concentration framework
- US 10% and pH 3,5; EU committee as a precaution 2,5% and pH 5
- Caution
- Sun sensitivity, stinging on sensitive skin
What's on this page?
Lactic acid is not foreign to the skin: lactate is one of the components of the natural moisturising factor (NMF) that holds water within dead cells. A reduced lactate level within the NMF has also been measured in skin with mild atopic dermatitis. Known as milk acid, it is found in fermented foods. With a molecular weight of about 90, it comes right after glycolic acid in the AHA family.
How does lactic acid both exfoliate and give moisture?
It does two separate jobs at once. First, as an AHA, it loosens the bonds between cells in the upper layer and eases shedding. Second, its water-attracting structure holds moisture. There is a third finding from a study dated 1996: in cell culture, keratinocytes, the main cells of the epidermis, increased ceramide production fourfold with the L form and twofold with the D form. In volunteers, a lotion with L-lactic acid raised ceramide in the upper layer by 48%, and the mixed form by 25%, while the D form had no effect. Barrier measurements came out better with the first two forms.
On the label you cannot see this distinction, because the INCI name "Lactic Acid" does not specify the isomer. Moreover, this data is old and rests on a single study. It is not enough to say "lactic acid repairs the barrier", but it helps explain why this acid is used in moisturising products too.
What changes between 5% and 12%?
| Concentration | Where is it seen? | What was shown? |
|---|---|---|
| 1-9%, average 3,7% | US scan of 1999-2000 cosmetic products | Real-world range on the market |
| %5 | Twice daily, 3 months | Improvement in the surface and upper skin, no change in the lower skin |
| %12 | Twice daily, 3 months | Changes in the upper and lower skin including firmness and skin thickness, reduction in lines |
| 12% ammonium lactate, pH 4,4 | Ichthyosis and severe dryness, 10 people | Water loss acceptable after 3 weeks, no barrier impairment seen |
| %30 | Clinical peel, once every 2 weeks | Work of a physician or trained practitioner |
Reading the 5% and 12% comparison as a firm threshold is wrong, because it rests on a single old study. For 8% lactic acid's 22-week data on the appearance of sun damage, see the AHA article, where the effect is also described as "modest". So a deeper effect requires higher concentrations and longer use, and the risk of irritation rises in parallel.
Why are there two different frameworks for a safe concentration?
Here the sources do not say the same thing. The independent US expert panel considers glycolic and lactic acid safe in consumer products up to 10%, at pH 3,5 and above, with a daily sun protection instruction. The EU's scientific committee, citing insufficient data in 2000, proposed at most 2,5% and pH 5 as a precaution for lactic acid; in its 2004 update it kept this view because it did not find the data presented sufficient.
The EU cosmetics regulation's restricted substances annexes have no separate concentration record for lactic acid. So the committee's proposal did not turn into a binding limit; even the 3,7% average in the US scan is above that proposal. The practical result for you: pH matters together with concentration. The same concentration can be far more active at low pH. If you do not know the pH, use the product less often.
Is it really gentle on sensitive skin?
The phrase you hear most is "lactic acid suits sensitive skin". But in skin research, one of the standard tests used to separate people who feel stinging is exactly this: in 1977 at 5%, and in the method corrected in 1996 at 10%, lactic acid is applied to the side of the nose or the cheek and the stinging is scored. Even today this same test is used to select volunteers for sensitive-skin studies.
This does not mean lactic acid is bad. The word "gentle" only holds when compared with glycolic acid; it is not an absolute property. When adding it to your routine, pay attention to the following:
- Start with a low-concentration or rinse-off product, 1-2 evenings a week. This is a starting example derived from the established recommendation for AHAs.
- Before applying it to your face, try it on the inner arm or elbow crease, twice a day, for 7-10 days.
- Mild stinging that passes is not the same as persistent burning. If burning, redness or peeling continues, stop. The sensitive skin article can help during this period.
- If you have rosacea or eczema, ask your dermatologist first.
Does lactic acid cause sun sensitivity?
Yes, this warning applies to the AHA class of products. Although the controlled studies measuring increased sun sensitivity have mostly been done with glycolic acid, the label warning is recommended for all AHAs and covers the week after you stop the product. In the morning apply a broad-spectrum sunscreen of at least SPF 30; if you are outdoors, renew it every 2 hours and after sweating or swimming.
Do not forget body lotions. Lactic acid is often used on the small follicular bumps of the arms (keratosis pilaris) and covers sun-exposed arms too. If you are pregnant or breastfeeding, talk to your doctor before adding this acid to your routine.
This page is for general information only and does not replace medical diagnosis or treatment. If you have an ongoing skin or scalp problem, consult a dermatologist.
