What SPF Does And Doesn't Tell You About Your Sunscreen

Consumers generally look to the SPF rating to determine the efficacy of a sunscreen product. What the deceptively simple index means, and how much it misses


Milwaukee Brewers' infielder Brooks Conrad sprays his face with sunscreen (Darryl Webb/Reuters)

Not long ago, plain old SPF 30 seemed like all we needed, the formula de rigueur. Now SPF 50, 70, even 100 are everywhere. The SPF rating system made its debut in the early 1960s, but the numbers game has since become something of an arms race; the Food and Drug Administration even proposed in 2007 to cap the SPF system at 50 to keep things from spiraling out of control. What happened to the sunscreen industry, and what does it mean that SPF values have inflated over time?

It's hard to know what to make of SPF inflation without first going into how the system works. The number that makes up the SPF rating is based on a ratio. If you take an average person with untreated skin and calculate how long it takes for them to develop a sunburn -- about 15 minutes -- then the SPF value tells you how many times weaker that person's UV exposure would be after applying the sunscreen. So, using sunscreen with an SPF of 15 means that an average person would be protected 15 times more than if they'd gone without.

Based on this logic, someone using SPF 15 who would ordinarily burn after 15 minutes of exposure would instead take 3 hours and 45 minutes to be affected the same extent. And an SPF of 100 implies that that same person would be protected for more than 24 hours. Here's the formula:

SPF x the time it takes to burn = time needed to receive the same dose of UV that you'd have gotten if you hadn't used sunscreen.

But as scientists are beginning to point out, SPF is a rather incomplete and somewhat misleading index. The key factor you want to pay attention to isn't really time -- it's the completeness of your protection.

One example of how this perspective on sunscreen tampers with SPF is that as the numbers increase, you're not really buying substantially better protection. As our own Derek Thompson explained previously:

Each minute wearing SPF 30 sunscreen lotion, you get a 1/30th, or 3.33%, of UV exposure that you would get without the lotion. Subtracting that 3.33 percentage points from 100, you're protected from 97% of UVB rays with SPF 30 sunscreen.

Buying SPF 80 improves on SPF 30, but not by much. The stronger product blocks another 1.75 percentage points of UVB radiation. Upgrading again to SPF 100 blocks 99 percent of UVB rays, but compared to SPF 80, it's really only a quarter of a percentage point better.

When all these products are functionally the same, it becomes absurd to say that one will wear out within hours while the other gives users license to spend the whole day outdoors on a single application. Yet that's precisely what manufacturers imply, with routine claims that their products are waterproof or that they offer "all-day protection."

And it's not just how much of the sun's ultraviolet radiation gets blocked, either -- it also matters what kind. We know that UV comes in three different flavors so far: there's UVC, which is filtered out naturally by the earth's atmosphere; there's UVB, the kind that causes sunburns, melanoma, and basal cell skin cancers; and then there's UVA, which ages your skin and causes most cases of squamous cell carcinoma (the second most common form of skin cancer).

In the United States, commercial sunscreens have generally done a great job at blocking UVB rays. But they've been less effective at stopping UVA. The FDA has approved only one major ingredient, avobenzone, that's been shown to combat UVA radiation. There are others, such as a compound called Tinosorb, but it isn't available here.

These limitations haven't stopped sunscreen manufacturers from liberally pasting "broad spectrum protection" on many of their product labels, which tends to mislead consumers into thinking that they're safer than they really are. It creates a kind of moral hazard where people who don't understand or who failed to follow the instructions potentially wind up taking even greater risks with their bodies than they would if the labels were more measured.

The FDA put in place new rules on sunscreen labeling that went into effect in June. These are some of the big ones (you can get the full readout here):

  • Sunscreens that want to brand themselves as "broad spectrum" must pass a new test that demonstrates protection against both UVA and UVB to a proportional degree. 
  • SPF labels on broad spectrum sunscreens must reflect the overall level of protection, not just the magnitude of protection against UVB. These products may be labeled as "Broad Spectrum SPF [value]"
  • The only sunscreens that can claim to reduce the risk of skin cancer and skin aging will be broad spectrum sunscreens with SPFs of 15 or higher, as long as they also say "if used as directed with other sun protection measures." All other types of sunscreen can only claim to prevent sunburn.
  • Sunscreens can no longer claim to be waterproof, though they can still claim water resistance at grades of either 40 minutes or 80 minutes depending on testing.

Sunscreen manufacturers have until December to comply with all the new rules. That means, at least for the remainder of this summer, you'll still want to exercise caution when you're lathering up.