First, as SPF measures a product's ability to protect the skin only from UVB rays, that's only half the story. It quantifies the amount of time you can be in the sun wearing sunscreen before you are burned. So if you are fair-skinned and would begin to burn in ten minutes without sun protection, wearing an SPF of 15 will protect you fifteen times as long - for 150 minutes (10 x 15 = 150). SPF math is also deceptive because the numbers don't add up. SPF 30 does not double the protection of SPF 15, for example. It would figure that you should be able to stay in the sun thirty times longer, but that's not the case. An SPF 15 allegedly blocks 93 to 95 percent of UVB rays, while an SPF 30 supplies 97 percent coverage. So bumping your SPF to over 50 doesn't make it that much more protective. It may be advantageous to check the percentages of active ingredients on the back of the label, in addition to the SPF number on the front. One top brand increases the amount of active ingredients, almost doubling the percentage of some, from an SPF 45 sunblock to an SPF 55. Those numbers do add up, especially since both products contain UVA filters too. Don't let SPF numbers give you a false sense of security. It's more important to look for "broad-spectrum" UVA and UVB protection and check exactly how much of the active ingredients you're getting from one sunscreen to another.
Also, don't assume that applying an SPF 15 tinted moisturizer over an SPF 15 facial lotion will magically turn into SPF 30. You still have on SPF 15. Imagine two stereo speakers; one is set to volume 5 and another is set at volume 5. You're not at volume level 10. However, because you're doubling the amount of SPF product, you are probably getting more protection, just by virtue of the quantity you're applying. When sunscreens are tested, they are applied as thick as icing to get that SPF rating. We usually don't put on a thick coat of sunscreen, so we're getting only a fraction of the protection we think we are. When you use an SPF 15 on your face, you usually rub on a very thin layer and the SPF probably drops down to 5 or less. So doubling up your SPF won't hurt, although it won't add up to twice as much protection either. Bottom line: buy sunblock/sunscreen SPF 55 or less, and reapply every hour when outside in the sun.
More Answers from Ellen Marmur, MD