The primary difference between Alpha Hydroxy Acids (AHAs), Beta Hydroxy Acids (BHAs), and Polyhydroxy Acids (PHAs) lies in their molecular structure, solubility, and depth of penetration. AHAs, such as glycolic and lactic acid, are water-soluble molecules that work by interfering with ionic bonds between keratinocytes, promoting the shedding of the stratum corneum to improve surface texture and hydration [1]. Because they are relatively small molecules, they penetrate deeply, making them highly effective for treating photodamage and hyperpigmentation, though they can increase UV sensitivity [2].
In contrast, BHAs (most commonly salicylic acid) are lipid-soluble, allowing them to bypass skin oils and penetrate the sebaceous follicle. This makes BHAs uniquely suited for oily and acne-prone skin, as they can dissolve the 'glue' holding debris inside pores to reduce comedones [3]. PHAs, such as gluconolactone and lactobionic acid, are considered the 'next generation' of AHAs. They possess a larger molecular weight, which results in slower skin penetration and significantly less irritation. Unlike AHAs, PHAs are humectants that strengthen the skin barrier and do not increase photosensitivity, making them ideal for sensitive skin types or those with rosacea [4].
Chemical exfoliation is governed by the process of desquamation, where acids lower the skin's pH to catalyse the breakdown of desmosomes—the protein structures that hold skin cells together. While physical scrubs can cause micro-tears, chemical exfoliants offer a controlled method to accelerate cell turnover and optimise the skin's natural regenerative cycle [1]. The efficacy of these acids is heavily dependent on the final formulation pH and the pKa of the acid, which determines the free acid concentration available to interact with the biological tissue [5].
Beyond simple exfoliation, these hydroxy acids provide secondary physiological benefits. AHAs have been shown to stimulate the synthesis of glycosaminoglycans and collagen in the dermis, providing anti-ageing benefits [2]. Meanwhile, PHAs offer antioxidant properties by chelating metals, which helps protect the skin against oxidative stress and the degradation of structural proteins [4]. Understanding these biochemical pathways allows for a more specialised approach to skincare, ensuring that the chosen acid aligns with the individual's barrier function and specific dermatological concerns.
For those looking to integrate these principles into their daily ritual, our Surface Renew AHA Enzyme Cleanser was formulated with a specialised fruit complex to gently encourage cell turnover and luminosity. Alternatively, if your skin is prone to congestion, you may recognise the benefits of Surface Purify, which includes salicylic acid (BHA) to assist in clarifying the pores for a more balanced complexion.
FAQ
Can I use AHAs and BHAs together in the same routine?
Yes, but it must be approached with caution to avoid compromising the skin barrier. Using them together—often referred to as 'multi-acid' formulation—can target both surface texture and pore congestion simultaneously [3]. However, for most Australian skin types exposed to high UV levels, it is often safer to alternate nights or use a pre-blended product formulated at a safe pH to minimise the risk of irritant contact dermatitis [5].
Do I need to wear sunscreen when using these acids?
Strict sun protection is essential, particularly when using AHAs. Research indicates that AHAs can increase sun sensitivity and the risk of sunburn by up to 50% by thinning the protective layer of dead skin cells [2]. While PHAs do not appear to increase photosensitivity, the general consensus in Australian clinical practice is to apply a broad-spectrum SPF 50+ daily to protect the fresh, vulnerable cells revealed by any exfoliation process [4].
Which acid is best for sensitive or rosacea-prone skin?
PHAs are the gold standard for sensitive skin. Due to their large molecular structure and multiple hydroxyl groups, they provide gentle exfoliation while actively hydrating the skin [1]. Studies have shown that PHAs like gluconolactone are as effective as AHAs for treating photo-ageing but are significantly better tolerated by individuals with clinically sensitive skin or inflammatory conditions like rosacea [4].
References:
[1] Babilas P, et al. Cosmetic and dermatologic use of alpha hydroxy acids. Journal of the German Society of Dermatology. 2012;10(7):488-491. doi:10.1111/j.1610-0387.2012.07931.x
[2] Kornhauser A, et al. Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clinical, Cosmetic and Investigational Dermatology. 2010;3:135-142. doi:10.2147/CCID.S10464
[3] Arif T. Salicylic acid as a peeling agent: a comprehensive review. Clinical, Cosmetic and Investigational Dermatology. 2015;8:455-461. doi:10.2147/CCID.S84766
[4] Grimes PE, et al. The use of polyhydroxy acids (PHAs) in photoaged skin. Dermatologic Surgery. 2004;30(2):189-195. doi:10.1111/j.1524-4725.2004.30056.x
[5] Smith WP. Epidermal and dermal effects of topical lactic acid. Journal of the American Academy of Dermatology. 1996;35(3):388-391. doi:10.1016/S0190-9622(96)90602-X
Medical Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any new skincare regimen. Content reviewed by a biomedical scientist.
