Dear Valerie: I was told by another chemist that 10% L-ascorbic acid is too high for a serum and will cause irritation on skin. I’ve seen that most products in the market contain at least 10% L-ascorbic acid, but I haven’t seen any warnings about potential irritation or burning. What’—C What I Mean

Dear C:
The reason ascorbic acid is used at such high concentrations is likely due to peer-reviewed literature citing ascorbic acid’s minimum efficacious use level for skin brightening to be more than 5%.¹ It’s also important to note studies show no benefit of ascorbic acid over 20%, but that doesn’t stop brands from using more than that! Ascorbic acid must also have a pH of 3.5 of lower to be bioavailable to skin. The low pH and high concentration are usually a recipe for irritation.

You can formulate an ascorbic acid serum to be less irritating by targeting a pH higher than 3.5, but the ascorbic acid will be virtually useless. Not to mention, ascorbic acid at these quantities is difficult to stabilize for long periods of time. It oxidizes readily² and the oxidation products can contribute to skin irritation.

Brands still launch these products and consumers still demand them despite knowing the challenges of high ascorbic acid concentrations with sensitive skin. This partially drives the demand for less irritating alternatives with lower use levels, like 3-o-ethyl ascorbic acid and tetrahexyldecyl ascorbate.

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Valerie George
[email protected]
 
Valerie George is a cosmetic chemist, science communicator, educator, leader, and avid proponent of transparency in the beauty industry. She works on the latest research in hair color and hair care at her company, Simply Formulas, and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist or showcasing her favorite ingredients to small brands and home formulators at simply-ingredients.com


References:

1. Pinnell, Sheldon R., et al. “Topical L-ascorbic acid: Percutaneous absorption studies.” Dermatologic Surgery, vol. 27, no. 2, Feb. 2001, pp. 137–142, https://doi.org/10.1046/j.1524-4725.2001.00264.x. 
2. Gallarate, M et al. “On the stability of ascorbic acid in emulsified systems for topical and cosmetic use.” International journal of pharmaceutics vol. 188,2 (1999): 233-41. doi:10.1016/s0378-5173(99)00228-8