I'm with Traveler,
I was listening to a dr talking on the radio this morning, about sunscreens. The factors mentionned on the bottles are those factors only if you apply it in a thick layer. Most people apply it in a thinner (what we think is normal, like applying a body lotion) layer, which means it'll in fact be only half the factor it mentions.
Be sure to use a sunscreen with as less harmfull ingredients as possible. A lot of sunscreens contain a lot of harmfull ingredients.
MarieLS