Health & beauty
SPF - can you mix sunscreens?
There are many different types of sunscreen (different active ingredients) and mixing them--even two products with the same SPF but different ingredients--can create an ineffective mixture that provides inadequate protection. And the actual SPF of two combined sunscreens is something that has to be measured as a chemical compound, not an easy algebraic equation. The best solution is to find a sunscreen that both moisturizes and provides the desired amount of SPF, to avoid possible interactions. But if you want both, The Beauty Brains says it's best to allow 10-15 minutes between products, so it can dry properly [source].
@volta made a very good point about using ENOUGH sunscreen to actually get the amount of SPF protection shown on the bottle. An excerpt from the Herald Tribune: ""It turns out that if you apply half the amount, you get the protection of only the square root of the SPF," said Dr. Darrell S. Rigel, a clinical professor of dermatology at New York University, who has done efficacy testing for Johnson & Johnson and the Procter & Gamble Co. So applying a half-ounce of SPF 70 will not give you the protection of SPF 35, but 8.4, Rigel" [source]
Bottom line: mixing SPF products together isn't a good idea. Sunscreen contains chemicals that are complicated. Leave it to the scientists.