AHA storage: shelf life and signs of oxidation

Alpha-hydroxy acids do not oxidize; their pH rises. A bottle opened a year ago may look exactly the same but exfoliate half as effectively.

Why pH changes

An AHA is only active in its free form, and the free portion depends on the formula's pH. At pH 3.5, most of the acid is active; at pH 4.5, most of it is neutralized. This is a difference of one unit, invisible to the eye.

However, the pH of a formula slowly drifts: through contact with air, exchange with the container, or evaporation. The glycolic acid or lactic acid molecule itself remains intact. It is not the active ingredient that degrades, but its environment.

The most reliable sign: tingling

A correctly dosed AHA produces a brief tingling sensation upon application, lasting a few seconds to a minute. Its gradual disappearance in a product used for months is the best indicator of a loss of activity.

Be careful not to confuse this with habituation: a accustomed skin tingles less, which is normal and desirable. The difference lies in the result — if skin texture and complexion stop improving at the same time as the tingling disappears, it's the product that has changed.

How long they last

Six to twelve months after opening. The printed information on the packaging provides the guideline, and it is reliable. After this period, the product remains safe but its effectiveness is no longer guaranteed.

At room temperature, away from light, with the bottle tightly closed after each use. A pump dispenser is preferable to a wide-mouthed bottle, less for light than for limiting air exchange.

What accelerates the loss

A bottle often opened and left open for too long. A very humid bathroom, where evaporation alters the concentration. Heat, which accelerates all reactions. Transferring to another container, which reintroduces pH instability.

A one-time exposure does nothing. It's the accumulation, over months, that matters — and that's why the period after opening is a better indicator than the manufacturing date.

When should you change the bottle

When the tingling has disappeared and results have stagnated for six weeks. When the texture has changed. When the "period after opening" date has passed, even if everything seems normal — for this active ingredient, appearance is not an indicator.

There's no need to change it sooner. An acid used twice a week consumes a thirty-milliliter bottle in five to six months, which falls perfectly within the useful window. Buying small is the right reflex here.

The case of formulas combining several acids

A product that combines glycolic acid and lactic acid, or adds citric acid, should be stored like the least stable of the three. In practice, the stated shelf life on the packaging takes this combination into account.

Our lactic acid AHA peel combines the acid with a humectant, and the radiance toner with rose water offers a lower concentration for daily use. The lactic acid page reviews the role of pH.

What loss of activity changes about skin texture

An active AHA regulates skin texture in three to four weeks and brightens the complexion in six to eight, by accelerating the shedding of dead cells from the surface. An acid whose pH has risen leaves these cells in place.

The result is not an aggravation, but a halt in progress. The skin slowly returns to its initial state, which is primarily seen in spots and radiance — the two parameters that took the longest to improve.

The rhythm, and why it also protects the bottle

Two applications per week initially, three at most thereafter. This rhythm is not only a rule of tolerance: it determines how quickly the bottle empties, and therefore the proportion of the product that will be consumed while it is still active.

An acid used twice a week consumes thirty milliliters in five to six months, which falls within the useful window. Used once every two weeks, the same bottle will be inactive before it's finished.

Glycolic acid or lactic acid, the same rule

Both belong to the alpha-hydroxy acid family and behave similarly over time. Glycolic acid, being smaller, penetrates faster; lactic acid, being larger, is better tolerated and is part of the skin's natural moisturizing factors.

This difference concerns intensity, not preservation. A bottle of lactic acid does not last longer than a bottle of glycolic acid, and the tingling indicator works for both.

Application and protection

In the evening, on clean, dry skin, followed by a moisturizing cream. Sun protection the next morning is a prerequisite for any exfoliation, whether with a fresh or aged product.

It is not advisable to increase the concentration when results stagnate. The right question is first about the age of the bottle, then about the frequency, and only lastly about the dose. Our glycolic acid and lactic acid fact sheets detail the concentrations suitable for each skin type.