What do SPF numbers mean?

UVB Absorption

So, now we know that a higher SPF number means more sun-exposure time. It also indicates the level of UVB absorption, but this number doesn't increase exponentially, which can be confusing. For example, an SPF of 15 absorbs 93.3 percent of UVB rays, but an SPF of 30 absorbs 96.7 percent. The SPF number has doubled, but the absorption rate has increased by only 3.4 percent.

sunscreen kid
© Photographer: Photoeuphoria | Agency: Dreamstime
Avoid sunburn by reapplying sunscreen every two hours.

Because of the confusion about UVB absorption, the FDA proposed a cap on SPF numbers. Any sunscreen higher than 30 SPF would be a "30-plus." Thirty was the decided cap because above that, the percentage of UVB absorbed and overall protection of the skin increases only slightly, but people may misinterpret these higher SPF numbers as a much higher level of protection or even a guarantee of all-day protection.

SPF % UV absorbed
2 50

As helpful as the FDA was trying to be, the cap is clearly not in practice: Neutrogena and Hawaiian Tropic recently released sunscreens boasting an SPF of 70. With that number, a person who normally burns within 10 minutes can now enjoy 700 minutes -- almost 12 hours -- in the sun without any damage. Right?

­Well, not exactly. We now know that most of us apply only about half the required amount of sunscreen. Also, despite waterproof or sweatproof labels, all sunscreens decrease in effectiveness when exposed to water or sweat. If you don't apply the correct amount and then reapply after exposure to water, a 12-hour bake in the sun could give you a very serious sunburn.