Reactions and irritation

Reactions, patch testing and when to stop.

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Close crop of four teal body cream jars with gold screw lids, one carrying a printed TESTER sticker — A tester jar on the shelf is the cheapest way to judge texture and scent before paying for a full size.Photo: Pittigrilli · CC BY-SA 4.0 · Wikimedia Commons · Open source page

Two faces of a triangular pottery fragment, pale outside and orange inside, beside a centimetre scale bar

Acids and sun sensitivity: introducing an active without a reaction

FDA-reported studies put AHA sun sensitivity at 18 percent after four weeks, lasting a week after you stop. Here is how that shapes a sensible introduction.

Updated 11/3/2026 · 8 min

Photo: LCC, Lisa Keys, 2005-12-09 14:51:24 · CC BY-SA 4.0 · Wikimedia Commons