An SPF of 15 screens 93 percent of UVB rays, while a 30 screens 97, just four percent more.
Dr. Elizabeth Hale of Skin Cancer Foundation says, "UVB protection does not increase proportionally with the SPF number."
According to the American Academy of Dermatology and the Skin Cancer Foundation, SPF 2 screens 50 percent of UVB rays, SPF 15 screens 93 percent, SPF 30 screens 97 percent and an SPF 50 screens 99 percent.
Leading dermatologists say a SPF 15 should do the trick day-to- day for most people.
"This would be for casual wear, you're going to and from your car, you're putting out the garbage, you're picking up the kids at the bus stop," said Dr. Zoe Draelos of the American Academy of Dermatology.
However, doctors warn the numbers mean nothing if the product is not used properly.
"Studies have definitely shown that people are not using enough sunscreen. In fact, the effective SPF might be far less than what is actually seen on the bottle because people are not using enough," Hale said.
You're supposed to use a shot glass full of sunscreen on all exposed areas to get the full benefit. Doctors recommend if you're going to be out in the sun all day, you should definitely use an SPF of 30 or higher.
Don't forget about UVA protection. There are no ratings for UVA protection like SPF, so look for the words 'broad spectrum' and specific ingredients like avobenzone, titanium dioxide and zinc oxide.
Also, remember sunscreen is only good for about two to three years.
"If you're really using your sunscreen properly, that eight ounce bottle should not last more than a few weeks. If it's still on your shelf, you weren't using enough in the first place," Hale said.
So, who should consider using an SPF 50 or higher? Dermatologists recommend them for those who are sensitive to the sun, are taking medications that make them more prone to sunburn, or those who have a history of skin cancer.
When picking a sunscreen, should you use a cream, gel, or spray? Dermatologists say it's a matter of personal preference, but specific types can have benefits for certain parts of the body.
For instance, gels dry quicker, which can be great for hairy areas, and sprays can be beneficial for tough to reach areas or for balding heads. Creams, doctors say, are still the standby for most body areas.
Of course, all the standard sunscreen rules apply: apply 15 to 30 minutes before you go outside, and waterproof doesn't mean you can swim all day and assume you'll still be protected. Everyone should reapply every two hours.
For more information, visit:
- The American Academy of Dermatology
- The Web site for Melanoma/Skin Cancer Detection and Prevention Month
- The Web site for the Skin Cancer Foundation