To find out if sunscreens with higher SPF offer more protection
According to WebMD, SPF refers to the ability of a sunscreen to block UVB rays, which cause sunburns, but not UVA rays, which are linked to skin damage. The number doesn't indicate how long a person can be outside in the sun before suffering from a burn. To give you an idea of what the numbers mean, an SPF 15 sunscreen blocks 94 percent of UVB rays, SPF 30 blocks 97 percent, SPF 50 blocks 98 percent, and SPF 100 blocks 99 percent. No sunscreen is a 100 percent guarantee, and some doctors argue that ditching your bottle of 30 or 50 for a higher SPF could lead to a false sense of security about staying in the sun longer without reapplying.
Dermatologists suggest that rather than becoming obsessed about the SPF number, it's more important to choose a broad spectrum sunscreen (one that protects against both UVB and UVA rays), and to focus on wearing enough. A full shot glass of the stuff should do the trick. Lube up 20 to 30 minutes before heading out and be sure to reapply every two hours.