Summertime, skin damage time. If you spend any significant time outdoors in the blazing sun, you should be wearing sunscreen to protect yourself from skin damage and increased skin cancer risk.
These days, a range of sunscreens are available touting SPF protection levels of 50 or more, with some options going as high as 100-plus. While these high SPF numbers may seem to offer outsize protection against the sun, the truth is otherwise—especially if you fall into the trap of two popular misconceptions about what these numbers mean. Here are the key facts to understand.
Fact #1: Double the SPF does not mean double the protection
As you can see in the chart below, once you get above SPF 30 the amount of extra protection from sunburn-causing UVB rays is so marginal as to be close to meaningless. For example, a sunscreen with SPF 50 blocks 98 percent of sunburn-causing UVB rays, while an SPF 100 sunscreen blocks 99 percent. (SPF 30 sunscreens block 96.7 percent.)
Also consider this: SPF is designed to indicate how much longer you can stay in the sun without getting burned. So, for example, if you sunburn in 15 minutes without sunscreen, an SPF 10 sunscreen should in theory protect you from sunburn for 2.5 hours (150 minutes). So, also in theory, a SPF 100 sunscreen would allow you to stay safely protected for 25 hours of continuous exposure.
Which, of course, is bogus. Sunscreens, regardless of SPF, dissipate steadily from sweat, activity, and water, which means that even if you’ve slathered on SPF 100 sunscreen, it almost certainly won’t provide maximum protection after a few hours. (It’s recommended that you regularly re-apply sunscreen every two hours.)
Fact #2: A high SPF rating only ensures protection from sunburn-inducing UVB rays, not from damaging, skin-aging UVA.
|The skin-aging effects of sustained UVA
exposure are apparent on the left side of
this truck driver’s face. Learn more.
UV rays come in two primary varieties—UVB and UVA. The SPF rating, no matter how high, only indicates the level of protection from UVB, which causes sunburn, but indicates absolutely nothing about the level of protection from UVA, which causes long-term skin damage and premature wrinkling. (To keep this straight, think B for “Burn” and A for “Aging.”)
Plenty of sunscreens are available that protect from UVA as well, but unlike SPF, there is no labeling standard to indicate whether, or how much, this is true for any given sunscreen. While reading labels closely for sunscreens that offer “broad-spectrum” coverage is a good indication of UVA protection, it’s still worthwhile doing some research in advance. (The Environmental Working Group’s 2014 Guide to Sunscreens is a great place to start.)