AHA Exfoliants and Sun Sensitivity, Explained: What the FDA Sunburn Alert and Its One-Week Window Rest On
Glycolic and lactic acid products often carry a line most people skim: use sunscreen while using this product and for a week afterwards. That week is not folklore. It traces to a four-week clinical trial and a 2005 FDA guidance.
What the sunburn alert says
In January 2005 the FDA recommended that cosmetics containing an alpha hydroxy acid (AHA) carry a "Sunburn Alert": the product "may increase your skin's sensitivity to the sun and particularly the possibility of sunburn," so use sunscreen and limit sun exposure "while using this product and for a week afterwards." It is non-binding guidance, and products pairing an AHA with sunscreen actives are exempt.
The trial behind the wording
Kaidbey and colleagues (Photodermatology, Photoimmunology and Photomedicine, 2003) treated the backs of 29 Caucasian volunteers daily, six days a week, with 10% glycolic acid at pH 3.5 or a placebo. After four weeks, skin received 1.5 times each person's minimal erythema dose (MED). Glycolic sites showed more sunburn cells and lower MEDs. DNA photodamage rose, but not significantly.
How big was the effect?
The FDA summarizes it this way: sensitivity to UV-induced reddening rose 18 percent, and sensitivity to UV-induced cellular damage doubled on average, "with considerable differences among individuals." Twenty-nine backs cannot say how a face, or a darker skin tone, behaves.
Why a week afterwards
One week after treatment stopped, sunburn cells and MEDs showed no remaining difference between treated and untreated sites. That single result is the origin of the week. It shows sensitivity had gone by then in this study, not that it ends on day seven for everyone.
What the EU panel found
The EU's SCCNFP reviewed more industry data in 2004 (SCCNFP/0799/04). It concluded that AHAs do increase UV damage: glycolic formulations at pH 3.5 and 2.5-4.0% may lower MED or raise sunburn cells, and no no-effect level could be defined. It kept its 2000 precautionary suggestion of glycolic acid up to 4% at pH 3.8 or higher and lactic acid up to 2.5% at pH 5.0 or higher, with a UV warning.
Two ceilings, one condition
The FDA page also cites the Cosmetic Ingredient Review finding that glycolic and lactic acid are safe at 10% or less with a final pH of 3.5 or higher, if the formula protects against sun sensitivity or directs daily sun protection. Both bodies tie the limit to sun protection. See the glycolic acid article and the lactic acid article on pH.
Can built-in sunscreen offset it?
The SCCNFP also saw manufacturer studies of moisturizers combining 2-8% AHA with a low SPF (2.9-3.5). Sunburn cells did not rise versus untreated skin. The groups were tiny (4 or 5 people) and the full formulas unavailable, so treat it as a hint, not a substitute for a separate broad-spectrum sunscreen.
What it means for your routine
Look for the alert on acid products. Wear sunscreen every morning while using one and for about a week after stopping, in the amount in the 2 mg/cm² sunscreen guide. These studies did not compare morning with evening use, so none favors a time of day. For stinging or peeling, see the over-exfoliation guide.
This article summarizes published research and is not medical advice. Patch test any new product first, and ask a dermatologist before using acids on irritated or sun-damaged skin.
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