AHA: dangers, limitations, and tolerability

AHAs are not dangerous; they are misused. These fruit acids are among the most studied cosmetic active ingredients, and their tolerance profile is good at the concentrations sold in France. Almost all reported incidents come from too frequent use, too concentrated use, or use without sun protection. Here's how to gauge what you're experiencing, in four levels.

Level 1 — the normal reaction to these acids

A tingling sensation lasting a few seconds upon application. This is the expected reaction of these acids on healthy skin, and it disappears in less than a minute.

Slight redness that fades in ten minutes. Exfoliation briefly increases surface circulation, without consequence.

Skin that appears smoother the next day. This is surface cell renewal, and it's precisely what we're looking for.

At this stage, the routine is well calibrated. Continue using these acids at the same rhythm for three to four weeks before considering increasing the frequency.

A word on what these acids are. Alpha-hydroxy acids include glycolic acid, lactic acid, mandelic acid, and malic acid. They are also called fruit acids because several of them are derived from fruits.

Their action is the same: they break the bonds between dead cells on the surface, which accelerates their elimination. This action remains superficial, which is what makes these acids generally safe.

Glycolic acid is the smallest of these acids, making it the most penetrating and effective — but also the most irritating. Mandelic acid, being larger, is the gentlest.

Level 2 — to monitor: slow down the acids

A persistent feeling of tightness between two applications. This indicates that exfoliation exceeds the skin's ability to regenerate.

Slightly peeling skin. Contrary to popular belief, this is not a sign of successful exfoliation: well-dosed acids do not cause anything to flake off.

Tingling that lasts several minutes, or that appears with a product that was previously well-tolerated. The skin barrier has become weakened.

At this stage, space out applications: go from three applications to one per week, and increase hydration. A rich cream and a soap-free cleanser are generally enough to remedy the situation in about ten days.

The use of these acids requires progression, never a fixed rhythm from the outset. Many skin types only tolerate weekly application, and this use yields perfectly good results.

Accompanying care is as important as frequency. A soap-free cleanser, a repairing cream, and no other exfoliating active ingredients in the same week.

Level 3 — stop the acids

Redness that persists upon waking, a burning sensation, skin that becomes reactive to previously neutral treatments. The barrier is damaged, and continuing worsens it.

Stop exfoliation completely. Two to four weeks of break, with a routine reduced to gentle cleansing, a repairing cream, and sunscreen.

Do not replace the acids with another active ingredient during this period. This is the mistake that prolongs the problem: repairing skin needs to be left alone.

Resumption then occurs at half the previous frequency, with a lower concentration. Many skin types only tolerate weekly application, and that is perfectly sufficient.

Level 4 — medical advice is necessary

Blisters, oozing, severe pain, swelling. These signs fall outside the scope of cosmetics and require a consultation.

A reaction that extends beyond the application area, or that is accompanied by intense itching. Contact allergy is possible, and only patch tests can confirm it.

Hyperpigmentation that appeared after use, especially on medium or dark skin. Acids can trigger post-inflammatory pigmentation, and early management prevents it from becoming permanent.

The real risk of fruit acids: the sun

This is the main limitation of these acids, and the one most often overlooked. Exfoliation removes part of the stratum corneum, which increases sensitivity to UV for several days.

Use without daily sun protection produces the opposite of the desired effect: spots worsen, complexion becomes uneven, and the benefit of several months disappears in one summer.

This rule applies even when applying in the evening. Photosensitivity is not limited to the day of application.

In practice: no fruit acids without SPF 50 sunscreen in the morning, and a break during sun holidays.

Who should refrain

Skin with active rosacea. Glycolic acid and other alpha-hydroxy acids worsen redness in this condition.

Skin undergoing isotretinoin treatment, or within six months following it. The combination is not recommended and dermatologist's advice takes precedence.

Skin with an active flare-up of atopic dermatitis. Exfoliation on an already compromised barrier makes no sense.

During pregnancy, cosmetic use of these low-concentration acids poses no known problem, but medical advice remains the reference.

On medium and dark skin, the use of these acids requires more caution. The risk is not burning but the hyperpigmentation that follows irritation, and it can become permanent.

These skin types still benefit significantly from these acids. The rule is simply to go slower and never let irritation set in.

What regulations say about these acids

In mass-market cosmetics, the concentration of glycolic acid is regulated, and products sold in France remain at levels where the risk is low.

High-concentration peels are professional procedures. Buying a highly concentrated product online to use alone at home is the scenario that produces actual burns.

The mention of an exfoliating action also requires a warning about sun protection. A product that does not carry one deserves suspicion.

What AHAs do not do

They do not permanently thin the skin. The idea of skin "worn out" by acids corresponds to no data: the stratum corneum regenerates continuously.

They do not create addiction. You can stop and restart them without a rebound effect.

They do not treat inflammatory acne or rosacea. In these conditions, they are at best useless, at worst aggravating.

They do not replace other active ingredients in a routine. An exfoliating acid works on the surface; it does not do what a retinoid does, nor what ceramides do.

Their action on existing irritations is nil, even negative. On already irritated skin, these acids worsen instead of improving.

And none of these acids has a proven effect on enlarged pores. Exfoliation temporarily makes them less visible, which is not the same thing.

To summarize

Brief tingling is normal. Persistent tightness requires slowing down. Redness upon waking requires stopping. A blister or pain requires a doctor.

And in all cases, sunscreen. This is the only non-negotiable rule for using these acids, and the one that makes the difference between an improved complexion and a damaged one.