1. The SPF on the bottle is not a reliable number. The problem? Most people do not apply enough to get the amount of protection the sunscreen claims. You should apply about 1 -2oz of sunscreen - (1oz = 1shot glass)
2. Environmental Working Group has come out with some new information suggesting that Vitamin A (or its derivatives ie. retinol) can actually promote burning. This is very preliminary information and the studies have only done on mice.
However, it is something to think about when buying sun screen
3. Even in the shade you can be reached by UV rays - so put sunscreen on either way. Same goes for sheer or thin clothing.
Now that its getting sunny and hot out we are spending more time outside. It is important to apply sunscreen before you go out, after swimming or sweating, and apply liberally. Most dermatologists (mine included) suggest anything below SPF 15 is useless. Use broad-spectrum sunscreen so it blocks burning and cancer causing rays as well as those that cause tanning and wrinkling.
-remember to use enough of your spf containing makeup to be protected. - re-read this post