The American Melanoma Foundation defines SPF as the product's ability to screen or block out the sun's harmful rays. For example, if you use a sunscreen with an SPF 15, you can be in the sun 15 times longer that you can without sunscreen before burning. Consumers need to be aware that SPF protection does not increase proportionally with an increased SPF number.
I would urge checking out sunscreens being used for the ingredients' toxins. A good place to start might be http:www.ewg.org/skindeep