AHA: what concentration and how much to apply

5 to 8% to start — and this number means nothing without the name of the acid. An 8% glycolic acid and an 8% mandelic acid do not produce the same effect on the skin, because their molecule size differs by a factor of two. This page addresses the three acids one by one.

What really matters: molecule size

Alpha-hydroxy acids act by breaking down the bonds between dead cells on the surface. Their action remains superficial.

The smaller the molecule, the faster and deeper it penetrates. Glycolic acid is the smallest of all, mandelic acid the largest.

At an equal dose, glycolic acid therefore acts more strongly and is more irritating. This is the information that packaging never highlights.

pH also matters: these acids only work in an acidic environment, around 3.5 to 4. A poorly buffered formula yields no results, regardless of its dosage.

Glycolic acid: 5 to 8% to start

It is the most potent, the most studied, and the one that requires the greatest caution. It is derived from sugarcane.

Start at 5%, two evenings a week. Moving to 8 or 10% happens after two months, if the skin tolerates it.

Higher dosages exist in cosmetics up to a regulatory ceiling, but they don't offer much for home use. Professional peels work between 20 and 70%, with preparation and follow-up.

For oily and thick skin, it's the best choice. For thin or reactive skin, another acid is more suitable.

Lactic acid: 5 to 10%, gentler

Its molecule is larger, so its action is less deep. It also has a useful peculiarity: it is a humectant, which retains water while exfoliating.

It is the acid of choice for dry or dehydrated skin, which poorly tolerates the drying effect of exfoliation. At 5%, it is suitable for almost all skin types.

At 10%, complexion and texture significantly improve after six weeks. It's a comfortable dosage for regular use.

It is also found in "night" treatments at low doses, around 2%, for daily use. This approach is valid and simply requires more time.

Mandelic acid: 5 to 10%, the best tolerated

The largest molecule of the three, derived from bitter almonds. It penetrates slowly, which distributes its action over time and calms reactions.

It's the right choice for sensitive skin that wants to try an exfoliant without taking risks. Many reactive skin types tolerate it where glycolic acid fails.

It is particularly beneficial for dark and brown skin types, where the risk of post-inflammatory hyperpigmentation is higher. Its slow action limits this risk.

At 10%, it acts more modestly than an 8% glycolic acid. This is the trade-off for tolerance.

How much to apply

Three to four drops of serum for the face, or the equivalent of a pea-sized amount for a cream. The quantity does not need to be generous.

A toner should be applied by hand rather than with cotton, which absorbs half of it and rubs unnecessarily.

Avoid the eye contour, the sides of the nose, and the corners of the mouth. These areas concentrate reactions.

A moisturizer should follow ten minutes later. This interval significantly reduces irritation without diminishing the effect.

Frequency matters more than concentration

Two weekly applications at 5% are more effective than four at 10% for skin that is just starting out. This is the most useful rule on this page.

Many skin types stick to twice a week indefinitely and achieve perfectly satisfactory results. Regularity outweighs intensity.

Peeling or redness upon waking signals overuse. Space out applications and resume when comfort returns.

Combining acids: the total dose

A glycolic acid cleanser, an exfoliating toner, and an AHA serum add up. Count the total dose, not the percentage of each product taken in isolation.

One acid at a time is sufficient. Combining an AHA and a salicylic acid on the same evening increases irritation without improving the result.

Retinol should be reserved for another evening. These active ingredients affect the skin differently and their effects are cumulative.

What concentration does not change

The need for daily sun protection. These acids increase UV sensitivity for several days, and without sunscreen, the benefit is nullified.

The timeframe. Allow six to eight weeks before judging complexion and texture, regardless of the dose.

And the nature of the active ingredient. For inflammatory acne or rosacea, these acids are not a treatment: in these cases, they are at best useless.

Frequency outweighs percentage

A moderate dosage applied regularly yields more than a strong dosage used sporadically, because the skin surface renews continuously. A lactic acid peel is at the higher end of the lactic acid range, which means once or twice a week, no more.