Podcast: Episode 35, AHAs: exfoliating acids

Listen to the audio episode to understand everything about the different types of AHA!

The gif:

Summary of the episode:

  • Explanation of the different exfoliation techniques (their forms, the gestures, their classifications)
  • Their modes of action : the advantages and disadvantages.

What are AHAs?

The different exfoliation techniques

There are different ways to exfoliate the skin to remove dead cells and improve the appearance and texture of the skin:
  • chemical exfoliants (the subject of this article)
  • physical scrubs
  • Konjac sponge-type brushes, Kessa Moroccan gloves
  • enzymatic exfoliants
  • retinoids

There are two specific categories: chemical exfoliants (AHAs, retinoids, enzymes) and physical techniques (scrubs, brushes, sponges, towels or others).
They work in different ways and can be adapted to different skin types.

Physical scrubs
The topic of the day is AHAs (or Alpha-hydroxy acids) , with many comparisons with physical scrubs.

Forms of exfoliants

Exfoliating acids

Exfoliating acids break the bonds between dead surface skin cells to help remove them.
They are often available as a toner, serum or mask and can contain different types of acids, such as glycolic acid, salicylic acid or lactic acid.
Chemical exfoliants are often recommended for skin looking for radiance and uniformity (because the exfoliating acids act in depth).

Physical scrubs

Physical scrubs, also called mechanical scrubs, use grains or crystals to remove dead skin cells by rubbing the skin.
They are often available in cream or gel form and may contain ingredients such as sugar, ground coffee beans or ground fruit seeds, for example.

Different gestures

The use between a physical and chemical scrub is very different:

  • Physical scrubs require mechanical action, and therefore friction. They are often used on damp skin and applied in circular movements, then rinsed with water.
  • Chemical exfoliants are used on clean, dry or damp skin. Depending on the type of exfoliator, it may need to be left on the skin for a while before rinsing off.

The classification / nomenclature of acid hydroxides

There are different major groups of exfoliating acids in cosmetics:

  • AHAs (which can also be BHAs)
  • salicylic acid
  • PHAs

AHA is an acronym for acid alpha-hydroxide, BHA stands for acid beta-hydroxide, PHA stands for acid poly-hydroxide and salicylic acid is separate.

“Hydroxy-acid” is the term that comes up.
They are therefore molecules of the class of carboxylic acids, therefore -COOH, and which contain a hydroxyl group, of formulas -OH, which correspond to alcohols.

The location and number of these hydroxyl groups chemically differentiate AHAs from BHAs and PHAs. An acid alpha-hydroxide is a carboxylic acid that has a hydroxyl group on its alpha carbon, i.e. on the carbon attached to the carboxyl group –COOH.
Similarly, a BHA has its hydroxyl group on the beta carbon so the carbon bonded to that which is bonded to the carboxyl group -COOH.

PHAs are carboxylic acids that have multiple hydroxyl groups.

Here is the GIF that sums it all up:

GIF explaining the different types of acids

The different modes of action

The mode of action of the physical scrub

Physical scrubs work through their mechanical actions, i.e. they contain small physical particles that rub, scrape and remove dead cells from the surface of the skin, they help with flaking.

💡 As a reminder from episode 9 📎 , dead cells on the surface of the skin are called scales and exfoliation, removing these scales is called desquamation.

Influence of particle size

The size of the particles in a physical scrub can influence the effectiveness of the product and how it is used.

The finer the particles, the less harsh they are on the skin, but they may be less effective at removing dead skin cells. Conversely, the larger the particles, the more effective they are at removing dead skin cells, but they can be more aggressive and irritating to the skin.

It is important to choose a scrub with an appropriate particle size for your skin type and needs. If you have sensitive skin, it is recommended to choose a scrub with finer particles to avoid irritation. If you have thicker or less sensitive skin, you can use a scrub with larger particles for better results.

Following this logic, a body scrub will have larger particles than a face scrub.

It is therefore necessary to follow the instructions for use of the product carefully and not to apply the scrub too often or with too much pressure, which could irritate the skin and create microlesions. These small lesions would then be the gateway for bacteria and what would create inflammation.

In general, it's better to under-exfoliate than to over-exfoliate. Preserve your skin barrier!

Exfoliation with salt - physical scrub

The mode of action of AHAs

Wang's theory on the mechanism of action of AHAs suggests that AHAs reduce the quantity of calcium ions in the epidermis and in particular at the level of cell adhesion. The cells then adhere less well to each other, which helps with desquamation, therefore with the elimination of scales, which are the corneocytes, the dead cells on the surface of the skin.

Influence of molecular weight of acids

AHAs are a type of molecule, but each AHA has its specificities. Among their specificities, a parameter to note is the size of their molecule, because they influence their exfoliating powers. The smaller the molecule, the more it is able to slip between skin cells and penetrate deeply. Exfoliation is therefore greater, but in return, the risk of irritation is also greater.

The size of the molecule influences its molecular weight, it is said that the higher the molecular weight of AHAs, the softer they are. Exfoliation is less important and there is less risk of irritation.

Conversely, low molecular weight AHAs, such as glycolic acid and lactic acid, penetrate deeper into the skin and therefore have a greater exfoliating effect than high molecular weight AHAs, such as malic acid. and citric acid.


Although AHAs have many benefits for the skin: they refine skin texture, they can reduce the appearance of wrinkles, improve skin luminosity, they also have some risks:

  • Skin irritation: AHAs can cause skin irritation, especially if the skin is sensitive or if high concentrations of AHA are used. It is important to start with a low concentration and test it on a small area of ​​skin before using it all over your face.
  • Photosensitization: AHAs can increase the skin's sensitivity to UV rays, so it's important to use sunscreen with a high SPF when using products containing AHAs.

These risks are easy to anticipate, because AHAs increase desquamation, therefore the elimination of dead skin cells, which refines the stratum corneum, the upper layer of the epidermis.

They then directly impact the barrier function of the skin. This can lead to a temporary reduction in the skin's ability to retain water and protect against external aggressions.

It is therefore important to take precautions when using products containing AHAs:

  • Use moisturizing products to help the skin retain water.
  • Apply a sunscreen with a high sun protection factor to protect the skin against UV rays, as AHAs can increase the skin's sensitivity to UV rays.
  • Avoid overuse of products containing AHAs and do not use them more often than recommended.
  • Use products with appropriate AHA concentrations for your skin type, always starting low to avoid irritation.


Tang S.-C., Yang J.-H. Dual Effects of Alpha-Hydroxy Acids on the Skin . 2018. Available at: https://doi.org/10.3390/molecules23040863

Kornhauser A., ​​Coelho SG, Hearing VJ Applications of hydroxy acids: classification, mechanisms, and photoactivity . 2010. Available at: https://doi.org/10.2147/CCID.S9042

Wang X. A theory for the mechanism of action of the α-hydroxy acids applied to the skin . 1999. Available at: https://doi.org/10.1054/mehy.1998.0788 .

Smith WP. Comparative effectiveness of alpha-hydroxy acids on skin properties . 1996 . Available at: https://doi.org/10.1111/j.1467-2494.1996.tb00137.x

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A chemist in the cosmetics industry for more than 5 years and a graduate of the Natural Raw Materials in Cosmetics Master's degree from ISIPCA, Julie is an expert in the development of natural cosmetic products.