Hydroxy acids are popular components in dermatology and cosmetic science, renowned for their exfoliative properties and skin rejuvenation capabilities. This article delves into their mechanisms of action at the cellular level, evaluates clinical evidence supporting their efficacy, examines their regulatory status concerning safety, and discusses formulation considerations for optimal use.
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What are hydroxy acids?
Hydroxy acids are a group of chemicals which are composed of both a hydroxyl (OH) and carboxylic acid (COOH) function group. Differentiation between these acids occurs due to differences in chain lengths (number of carbons), position of the hydroxyl group and their structures between the hydroxyl and carboxylic acid groups.
There are three main categories of hydroxy acids.
Alpha Hydroxy Acids (AHAs)
αHAs are carboxylic acids with one hydroxyl group attached to the α-position of the carboxyl group. The simplest representative of αHA is glycolic acid (chemical name, hydroxyacetic acid), which was the first of this class of compounds to be introduced into skin care products.
Glycolic Acid
Lactic Acid
Malic Acid
Tartaric Acid
Citric Acid
Function Groups
Pros of AHAs
- Brighten and even out skin tone.
- Promote skin cell turnover.
- Increase skin permeability.
Cons of AHAs
- Only exfoliate the surface of the skin.
- Excess use leads to photosensitisation.
- May causes skin irritation and sensitisation.
Beta Hydroxy Acids (BHAs)
β-Hydroxy acids (βHAs) are carboxylic acids having one hydroxyl group attached to the β-position of the carboxyl group.
Some βHAs are also considered αHAs as they contain a hydroxyl group in the α-position to one carboxyl group and in the β-position to the other carboxyl group. Malic acid and citric acid are prominent representatives in this category.
SA is often described as a βHA, but that classification is incorrect.In SA, both the hydroxyl and the carboxyl groups are directly attached to an aromatic benzene ring and both exhibit acidic properties. In contrast, the hydroxyl groups in αHAs, βHAs, and PHAs are neutral under the conditions used in clinical and cosmetic settings.
Salicylic Acid
Beta Hydroxybutonic
Tropic Acid
Trethocanic Acid
Functional Groups
Pros of BHAs
- Exfoliate beneath the skin surface.
- Help reduce breakouts.
- Provide anti-inflammatory effects.
Cons of BHAs
- Can cause prolonged erythema.
- Excess use leads to photosensitisation.
- May causes skin irritation and sensitisation.
Poly Hydroxy Acids (pHAs)
PHAs are carboxylic acids with two or more hydroxyl groups attached to carbon atoms or an alicyclic chain. It is essential that at least one hydroxyl group be attached to the α-position. Attaching a sugar molecule to the PHA structure results in a polysaccharide known as bionic acid.
PHAs are αHAs containing multiple hydroxyl groups, while PHBAs are PHAs with an additional sugar molecule attached to the PHA structure.
Galactose
Gluconolactone
Lactobionic Acid
Functional Groups
Poly hydroxy acids act in a similar way to AHAs, function as a chelating agent and exhibit potency in scavenging free radicals. Due to their larger size, they are less potent and do not penetrate the epidermis.
PHAs and PHBAs act in a very similar way to AHAs, except that they have a larger structure and bigger molecular weight, which means that they are unable to penetrate even the outermost layers of the stratum corneum. This property means that they are much gentler exfoliants compared to both BHAs and AHAs, making them more suitable for people with sensitive skin. Their action should not go dismissed though, as studies show how with prolonged use, PHAs help overall skin health without damaging the skin.
Pros of PHAs
- Extremely gentle.
- Suitable for sensitive skin.
- Provide anti-inflammatory effects.
Cons of PHAs
- Prolonged use required to notice effects.
- Less effective than AHAs and BHAs.
- Excess use can still cause photosensitisation.
Hydroxy Acids Mechanism of Action
At the cellular level, AHAs, such as glycolic and lactic acids, diminish the ionic bonding between desmosomal proteins, leading to the desquamation of superficial skin cells. This process not only enhances epidermal renewal but also stimulates fibroblast activity in the dermis, resulting in increased collagen and glycosaminoglycan synthesis, which contribute to improved skin elasticity and hydration.
BHAs, notably salicylic acid, are lipid-soluble, enabling them to penetrate sebaceous follicles effectively. They exert comedolytic effects by dissolving intra-follicular keratin plugs and exhibit anti-inflammatory properties through the inhibition of cyclooxygenase activity, thereby reducing prostaglandin synthesis. This dual action makes BHAs particularly effective in managing acne vulgaris and related disorders.
Clinical Evidence
Extensive clinical research substantiates the efficacy of hydroxy acids in dermatological applications.
AHAs have demonstrated significant improvements in photodamaged skin, manifesting as reduced fine lines, enhanced skin texture, and diminished hyperpigmentation. Glycolic acid peels have been shown to increase epidermal thickness and density of collagen fibers in the papillary dermis.
BHAs, particularly salicylic acid, have proven effective in treating acne vulgaris due to their ability to penetrate and exfoliate within the follicular unit, coupled with anti-inflammatory effects that mitigate lesion formation and erythema.
Hydroxy Acids Regulatory Status
Food and Drug Administration
The safety profiles of alpha hydroxy acids and beta hydroxy acids have been rigorously evaluated by regulatory agencies. The U.S. Food and Drug Administration (FDA) acknowledges the widespread use of AHAs in cosmetics for their exfoliative benefits but advises caution due to potential increased photosensitivity. Consequently, the FDA recommends that AHA-containing products include directions of use to include sun protection in skincare routines to mitigate risks of photo-damage.
FDA received a total of 114 adverse dermatologic experience reports for AHA-containing skin care products between 1992 and February 2004, with the maximum number in 1994. The reported adverse experiences included burning (45), dermatitis or rash (35), swelling (29), pigmentary changes (15), blisters or welts (14), skin peeling (13), itching (12), irritation or tenderness (8), chemical burns (6), and increased sunburn (3). The frequency of such reports for skin exfoliating products that contain AHAs has been considerably lower in subsequent years.
Scientific Committee on Consumer Safety
Alpha Hydroxy Acids
The SCCS opinion on alpha hydroxy acids is as follows:
- Glycolic acid may be used safely at a level of up to 4% and a pH ≥ 3.8
- Lactic acid up to a maximum level of 2.5% and a pH ≥ 5
Further, it is recommended that there should be appropriate warnings to the consumer of :
- Avoiding contact with the eyes.
- Avoiding / or affording protection from UV whilst using products containing AHAs.
Salicylic Acid
The SCCS opinion on salicylic acid is as follows:
The use of salicylic acid as a restricted ingredient for purposes other than inhibiting the development of micro-organisms is safe at a concentration:
- Up to 3.0 % for the cosmetic rinse-off hair products
- Up to 2.0 % for other products, except body lotion, eye shadow, mascara, eyeliner lipstick, non-spray deodorant, and
- Up to 0.5% for body lotion, eye shadow, mascara, eyeliner, lipstick, oral products and non-spray deodorant.
This Opinion is not applicable to any sprayable product (including mouth spray) that may lead to exposure of the end-user’s lungs by inhalation.
Cosmetic Ingredient Review Opinion
The CIR Expert Panel has assessed salicylic acid, concluding that it is safe for use in cosmetics when formulated to avoid irritation and accompanied by sun protection measures. Manufacturers are advised to test their products to determine potential increases in UV sensitivity and to provide appropriate labeling to inform consumers.
The Panel concluded that the following ingredients are safe in cosmetics in the present practices of use and concentration described in the safety assessment, when formulated to be non-irritating and non-sensitizing, which may be based on a quantitative risk assessment (QRA):
Salicylic Acid
Amyl Salicylate
Butyloctyl Salicylate
Calcium Salicylate*
C12-15 Alkyl Salicylate*
Ethylhexyl Salicylate
Hexyl Salicylate
Hexyldodecyl Salicylate*
Isocetyl Salicylate*
Isodecyl Salicylate
Isotridecyl Salicylate*
Magnesium Salicylate
Methyl Salicylate
Myristyl Salicylate*
Potassium Salicylate*
Sodium Salicylate
TEA-Salicylate
Tridecyl Salicylate
Formulation Considerations
Incorporating hydroxy acids into cosmetic formulations requires attention to pH levels, concentration, and overall product stability to ensure both efficacy and safety. AHAs are most effective at lower pH levels (around 3.5), where they remain in their free acid form, facilitating optimal skin penetration. However, such acidic environments require the use of compatible preservatives and stabilizers to maintain product integrity and prevent microbial growth.
Additionally, formulating with BHAs like salicylic acid demands solubilization techniques, often involving solvents such as ethanol or propylene glycol, to enhance bioavailability. It is imperative for formulators to adhere to regional regulatory guidelines regarding hydroxy acid concentrations and to conduct thorough stability testing to ensure product safety and quality.
key Considerations:
- pH
- Preservative system
- Stability system
- Solubilization techniques
- Restricted concentrations
