The name may sound scary but you’ve probably seen hydroxy acids on the labels of skincare products without realising it. Or not, depending on how bored you’ve been in the bathroom when you forget your phone and resort to reading labels…
Regardless, I’m going to go through some different categories, uses and examples because apparently these are one of the heavy weights of skincare science. They are sometimes referred to as “fruit acids” as some of them are found naturally in fruit or plant material like sugar cane.
There are two main categories, and a third which, sort of, scoops up any leftovers:
- alpha-hydroxy acids (AHA)
- beta-hydroxy acids (BHA)
- polyhydroxy acids (PHA)
Each category has distinctive chemical similarities which define their properties and therefore their uses.
alpha-hydroxy acids (AHA)
The chemical structure of these compounds all have the following functional group/moiety: they have a carboxylic acid group (pink) and a hydroxy group (blue) separated by one single carbon (black).
In general, chemical structures don’t show carbons out of convenience, so just assume everything is a carbon in the backbone unless shown otherwise. For bigger compounds, this makes it easier to see what’s happening and also chemists are lazy – work smart, not hard.
Most are found in nature but in cosmetics, they generally tend to be synthetic for economic and purity reasons. Click here to read what this means and why you shouldn’t be scared of synthetic ingredients.
Because they are small and are water soluble, they tend to be able to penetrate the skin’s uppermost barrier (stratum corneum) and remove dead skin cells by dissolving the bonds that hold the dead cells in place – chemical exfoliation. There are a few theories on the mechanism but regardless, the effects are well studied.
This layer of skin is mainly made up of dead cells but too many may make your skin look dull. You know when people comment on how tired and drained you look, even though you remembered to put mascara on this time? Ja, better go exfoliate. Also sleep but exfoliate first.
AHAs are water soluble because of their short carbon chains and the amount of oxygens in relation to that, they are more polar (meaning more likely to dissolve in water or alcohol solvents) than BHAs.
This exfoliation helps your skin recover better from sun damage and promotes skin renewal, collagen production, and hydration (humectant); giving you that glowing dewy look we all strive for. For this reason, it is sometimes marketed as anti-aging.
Examples of AHAs
The bold compounds are the two main top dogs.
- Glycolic acid – the smallest AHA, most deeply penetrating
- Lactic acid
- Malic acid (technically also a BHA – but skincare people are trolling us)
- Citric acid (same)
- Tartaric acid
- Mandelic acid (larger with a ring side group – making it less polar)
- Benzilic acid (similar to above)
Application and precautions for AHAs
AHAs are generally good for dry skin as the removal of the dead skin cells allows the skin to regenerate and, provided you moisturise properly, hydrate better. Ane prone skin can also benefit as this removes any debris and dead skin, thereby inhibiting bacteria growth.
AHAs can be found in a range of product types: Toners, serums, masks, moisturisers and cleansers. These will usually have a low concentration of AHAs and are safe to use every other day or daily if very dilute.
The high concentration products are designed to be used not as regularly, usually in the form of serums or chemical peel masks (these should only be carried out by professionals!).
AHAs can cause your skin to be sensitive and may cause some irritation, which is why you can’t use high concentrations every day. Some studies show that the pH of the product is actually a bigger factor to the irritation, rather than the concentration, as AHAs work better at lower pH (more acidic). They also promote photosensitivity, so sunscreen is important if you use it on the regular.
beta-hydroxy acids (BHA)
BHAs have an extra carbon (black) between the carboxylic acid (pink) and hydroxy group (blue). They have a higher percentage of neutral carbon sections. This makes them less polar than the above AHAs so they are more likely to be oil compatible.
General uses of BHAs
Because BHAs are oil soluble (similar to oil, birds of a feather and all that), so they can get into the oil filled pores of your skin and break it up. Handy for acne-prone or oily skin. It seems there actually aren’t any real BHAs in skincare, only the one impostor, see below.
Examples of BHAs
- Salicylic acid* – most common and only one used in skincare products
* Salicylic acid is considered a BHA in the skincare/cosmetic community, although from a chemist’s point of view it isn’t actually one. The carboxylic acid and hydroxy groups are directly attached to a benzene ring so they actually behave slightly differently. I’m triggered if you haven’t noticed, and we wonder why people don’t trust the professionals these days.
Application and precautions of BHAs
Because BHAs can break up oil and build up in pores, they tend to be formulated into “for oily skin” products. Next time you buy one (or are sitting on the toilet reading labels), check for salicylic acid, it’ll most likely be there.
Again, like AHAs they are found in many different types of products but the most concentrated types are best left to your facials and chemical peels by the professionals.
BHAs, like their sister AHAs, can cause the skin to be photo-sensitive and irritated. So wear that sunscreen honey! Or stay indoors and be a hermit… Either works fine.
polyhydroxy acids (PHA)
The new kid on the block! PHAs have the same carboxylic acid and hydroxy groups as above but they also have additional hydroxy groups. There is also a subcategory where the PHA has an extra sugar molecule attached which are called bionic acids.
General uses of PHAs
PHAs are a good alternative to AHAs as they have the same effect with little irritation. They are generally larger so they take longer to penetrate the skin and won’t get as deep into the barrier which causes less irritation. Hola all my sensitive-skinned peeps, I see you and validate your feelings.
- Lactobionic acid (sounds like a little nano-robot right?!)
Application and precautions of PHAs
This is the new buzzword for hydroxy acids because of the lack of irritation it causes without losing the benefits. AHAs have been extensively studied and that’s why they are preferred at the moment. PHA studies are starting to come through, now showing that PHAs may also have humectant properties (enhances moisturisation) like AHAs. It also has shown some antioxidantative properties. See this post for a very quick overview of an antioxidant mechanism presented in an (slightly obnoxious) analogy.
Take Home Message
Stay tuned! I will take a closer look at glycolic acid, lactic acid, salicylic acid and gluconolactone in the next few weeks. A series of sorts. A montage. A collage. Wistful research into the science of skincare.
- FDA website
- Lab Muffin
- Clinical and cosmeceutical uses of hydroxyacids – Green et al. in Clinics in Dermatology
- Applications of hydroxy acids: classification, mechanisms, and photoactivity – Kornhauser et al. in Clinical, Cosmetic and Investigational Dermatology
- The use of polyhydroxy acids (PHAs) in photoaged skin. – Grimes et al. in Curtis
- Hydroxy Acids and Retanoids in Cosmetics – Ramos-Silca et al. in Clinics in Dermatology
- Feature image – https://www.buzzfeed.com/natashaumer/this-is-what-the-human-body-really-looks-like-under-a-micros