Exfoliating Serum: How to Choose the Right Acid for Your Skin

An exfoliating serum is a leave-on liquid that uses acids to loosen the dead cells on the surface of your skin. Done well, it leaves skin smoother, brighter and better at taking in whatever comes next. Done badly, it leaves skin red, tight and stinging.

The difference between the two outcomes is rarely the brand. It is the acid, the strength, the pH and how often you use it. This guide walks through each one so you can match a serum to your skin.

How acid exfoliation works

The outer layer of skin is a wall of flat, dead cells held together by protein links. As skin ages, those links let go more slowly, so dead cells pile up and the surface turns rough and dull. Exfoliating acids loosen the links so the cells shed evenly instead of in flakes.

The acids in exfoliating serums fall into three families: alpha hydroxy acids such as glycolic, lactic and mandelic acid; beta hydroxy acid, which in skincare means salicylic acid; and polyhydroxy acids such as gluconolactone [1]. They do the same basic job, but they differ in molecule size, in where they work and in how much they sting.

Your skin renews its surface in about 4 weeks in early adulthood but takes noticeably longer than that after 50, so dullness is often a backlog of old cells.

The main acids, compared

Glycolic acid: the strongest for texture and photoaging

Glycolic acid is the smallest acid molecule, so it penetrates quickly and produces the most visible smoothing. It also has the most evidence for deeper change. In a study where people applied a 25% lotion of glycolic, lactic or citric acid to one forearm for about 6 months, the treated skin was roughly 25% thicker, with more collagen and better-quality elastic fibres than the placebo arm [2]. That was a much higher strength than a facial serum, but it shows that acids can reach below the surface.

The trade-off is irritation. Glycolic acid is the acid most likely to sting, and it suits normal, oily or sun-damaged skin that is already used to actives. Our guide to glycolic acid serum covers strengths and how to start.

Your skin renews its surface in about 4 weeks in early adulthood but takes noticeably longer than that after 50, so dullness is often a backlog of old cells.

Lactic acid: the gentle all-rounder

Lactic acid is a larger molecule than glycolic acid, so it works more slowly and stings less. It is also a natural part of the skin’s own moisturising factor, so it tends to leave skin feeling softer rather than stripped. In a 3-month study, both 5% and 12% lactic acid improved smoothness, texture and the look of fine lines, and only the 12% strength also thickened and firmed the dermis [3].

That makes lactic acid the best first exfoliating serum for dry, mature or easily irritated skin. Our lactic acid serum guide goes deeper into concentrations.

Mandelic acid: for sensitive and darker skin tones

Mandelic acid is the largest of the common alpha hydroxy acids, so it penetrates slowly and evenly. That slower uptake is why it is often recommended for sensitive skin and for deeper skin tones that are prone to dark marks after irritation. Expect gentler, slower results than glycolic acid.

Salicylic acid: for clogged pores and oily skin

Salicylic acid is oil-soluble, so unlike the water-soluble acids it can work inside the oily lining of a pore. It loosens the bonds between skin cells and clears the plugs that become blackheads and breakouts, which is why it is the standard choice for acne-prone skin [4]. It is also calming, so it rarely stings at serum strength. If clogged pores are your main concern, read our guide to salicylic acid serum.

Polyhydroxy acids: for the most sensitive skin

Polyhydroxy acids such as gluconolactone are large molecules that exfoliate very gently. They also draw water into the skin and have antioxidant effects, which makes them suitable for sensitive skin, rosacea-prone skin and skin recovering from procedures [5]. They are the slowest of the group, but the easiest to live with.

The number that matters more than the percentage

A serum’s acid percentage tells you only half the story. The other half is pH. An acid exfoliates when it is in its “free” form, and how much of it is free depends on how acidic the formula is. A 10% glycolic serum at pH 4 can be gentler than a 5% serum at pH 3.

Your skin often needs 1 to 2 weeks off acids to calm down after over-exfoliation, far longer than the few nights it took to overdo it.

Many brands do not print the pH. A rough guide: if a serum tingles for a few seconds and then settles, it is working. If it burns, or your skin stays pink for an hour, it is too strong for you.

Strength also brings sun sensitivity. In a 4-week study, people who applied 10% glycolic acid six days a week burned more easily under UV light than those using a placebo, with more sun-damaged cells in their skin; the effect was gone a week after they stopped [6]. Daily sunscreen is not optional with any exfoliating serum.

How often to use an exfoliating serum

Start with two evenings a week, and add a third after a fortnight if your skin is comfortable. Most people do best at two to four nights a week; very few need daily acids. Our guide on how often to exfoliate your face breaks it down by skin type.

Apply to clean, dry skin, wait a minute, then follow with a plain moisturiser. Keep it away from the eye area and the corners of the nose and mouth, where skin is thinner.

Stop and give your skin a break if you notice:

• stinging from products that never stung before,
• a tight, shiny, “plastic” look,
• new redness or patches of flaking,
• breakouts that appear after you increased the frequency.

Your skin often needs 1 to 2 weeks off acids to calm down after over-exfoliation, far longer than the few nights it took to overdo it.

Exfoliation is the surface; retinoids work underneath

An exfoliating serum is excellent at what it does: a smoother, brighter surface and clearer pores. But wrinkles, loss of firmness and crepey texture start deeper, in the collagen and elastin of the dermis, and most daily serums are gentler than the strengths that changed the dermis in studies.

Retinoids work at that deeper level. In a 24-week trial in older adults, 0.4% retinol improved fine wrinkling and increased collagen production compared with the vehicle [7]. The usual advice is to alternate rather than layer: an acid serum on two or three nights, a retinoid on the others, never both on the same night.

The hurdle with retinol is irritation, and an acid routine makes that hurdle higher. Nanoretinol was designed for exactly that problem. It carries 0.2% retinol inside biomimetic lipid nanoparticles in a light, water-based gel suitable for all skin types, including sensitive skin. In North Biomedical’s clinical study it was 232% more effective in collagen recovery and 73% more effective in elastin recovery than conventional retinol, and users saw a 61% increase in skin firmness and a 56% increase in elasticity after 56 days, with milder side effects than conventional retinol [8].

Pair your exfoliating serum with Nanoretinol on alternate nights, moisturise well, and wear sunscreen every morning. The acid smooths the surface while the retinol works on the layer underneath it.

References

1. Kornhauser A, Coelho SG, Hearing VJ. “Applications of Hydroxy Acids: Classification, Mechanisms, and Photoactivity.” Clinical, Cosmetic and Investigational Dermatology. 2010;3:135-142. doi:10.2147/CCID.S9042
2. Ditre CM, Griffin TD, Murphy GF, Sueki H, Telegan B, Johnson WC, et al. “Effects of Alpha-Hydroxy Acids on Photoaged Skin: A Pilot Clinical, Histologic, and Ultrastructural Study.” Journal of the American Academy of Dermatology. 1996;34(2 Pt 1):187-195. PubMed: 8642081
3. Smith WP. “Epidermal and Dermal Effects of Topical Lactic Acid.” Journal of the American Academy of Dermatology. 1996;35(3 Pt 1):388-391. PubMed: 8784274
4. Arif T. “Salicylic Acid as a Peeling Agent: A Comprehensive Review.” Clinical, Cosmetic and Investigational Dermatology. 2015;8:455-461. doi:10.2147/CCID.S84765
5. Green BA, Yu RJ, Van Scott EJ. “Clinical and Cosmeceutical Uses of Hydroxyacids.” Clinics in Dermatology. 2009;27(5):495-501. doi:10.1016/j.clindermatol.2009.06.023
6. Kaidbey K, Sutherland B, Bennett P, Wamer WG, Barton C, Dennis D, Kornhauser A. “Topical Glycolic Acid Enhances Photodamage by Ultraviolet Light.” Photodermatology, Photoimmunology & Photomedicine. 2003;19(1):21-27. doi:10.1034/j.1600-0781.2003.00013.x
7. Kafi R, Kwak HS, Schumacher WE, Cho S, Hanft VN, Hamilton TA, et al. “Improvement of Naturally Aged Skin with Vitamin A (Retinol).” Archives of Dermatology. 2007;143(5):606-612. doi:10.1001/archderm.143.5.606
8. North Biomedical LLC. “Nanoretinol vs. Conventional Retinol: Efficacy in Collagen and Elastin Recovery.” Clinical Study Summary, 2024. Study summary