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Retinoids are among the best-evidenced skincare ingredients, and a large share of people give up in the first month because they started too strong, too often.
SPF 50 is tested at 2 mg per square centimeter. Studies of real use repeatedly find people apply a quarter to a half of that — and protection falls at least as fast as the amount does.
The underlying science is real and the consumer devices are frequently far weaker than anything tested. The gap between the study and the mask is where the money goes.
