Sunscreen Math: Is SPF 30 Twice as Good as SPF 15?

Summer is coming, and if you’re as pasty as I am you may have already gotten your first hint of sunburn this year (since after all, the days are just as long now as in mid-August, though it likely doesn’t feel anywhere near as hot yet). It’s time to go stock up on sunscreen, but you may notice that the labels look a bit different this year. Here’s why.

The SPF (Sun Protection Factor) numbers on sunscreen bottles have been confusing people for years. For one thing, the SPF number only refers to the sunburn-causing UVB rays that are blocked, not the UVA rays that can cause deeper skin damage. Also, logic would dictate that SPF 30 would block twice the amount of ultraviolet rays as SPF 15, and that SPF 60 or 75 would be four or five times as powerful, but that’s not how it works. The actual percentages blocked are as follows:

  • SPF 2 – 50%
  • SPF 4 – 75%
  • SPF 8 – 87%
  • SPF 15 – 93%
  • SPF 30 – 97%
  • SPF 50 – 98%
  • SPF 100 – 99%

The numbers actually refer to the multiplier of the extra time you can spend in the sun before burning. An SPF 15 sunscreen will let you stay outside for 15 times as long, so if you start to burn after only 10 minutes unprotected, the sunscreen will protect you for 150 minutes (2½ hours), while SPF 30 will protect you for 300 minutes (5 hours). Of course, if you have darker skin and don’t burn as quickly, the sunscreens will last even longer.

Of course, the times above only work if you actually apply the proper amount of sunscreen. For your face alone you probably need about 1.25g (1.25mL) or ¼ teaspoon. The average recommendation for your body is 1 ounce, the amount that would fill a regular shotglass, but of course you may need more or less than this amount depending on your height/body size and how much skin you’re actually exposing. If you use a spray, foam, or sunscreen wipe, this amount may be harder to calculate as you apply but you may be able to figure out if you’re using enough by how many applications per container you’re getting. Most bottles hold 5-6oz., so if you’re applying sunscreen to your full body ten times before you use it all up, you’re probably only using about half as much as you should (and thus getting half the protection).

In 2011, the FDA approved new regulations on sunscreen labeling to help alleviate some of the confusion about how well different formulas work. The regulations were initially slated to go into effect on June 18, 2012, but fears of a summer shortage while manufacturers performed new tests and switched to new packaging led the FDA to delay implementation until December 17, 2012 (with a further extension to December 17, 2013 for products with less than $25,000 in annual sales). The highest rating now allowed on packaging is SPF 50+, since the difference in protection above that level is negligible. There’s also a new “Broad Spectrum” label that indicates that a product protects you from UVA rays in addition to UVB. The regulations also stipulate that:

  • If a product does not protect against UVA rays or has an SPF of only 2-14, it must carry a warning that reads, “Skin Cancer/Skin Aging Alert:  Spending time in the sun increases your risk of skin cancer and early skin aging. This product has been shown only to help prevent sunburn, not skin cancer or early skin aging.”
  • On the contrary, if a product does provide broad spectrum SPF 15+ coverage, the label on the back will inform consumers that, if used as directed in conjunction with other sun protection measures, it can reduce the risk of skin cancer and premature aging in addition to preventing sunburns.
  • Sunscreens cannot be labelled as “waterproof” or “sweatproof,” but instead can tell you if they offer protection in the water for 40 or 80 minutes before needing to be reapplied.
  • Products cannot be called “sunblocks,” since that gives the false impression that the sun’s effects can be totally blocked.
  • FDA approval is needed before a product can claim that it gives full protection immediately upon application or that it lasts longer than two hours before needing reapplication.

So which sunscreen is right for you? For most people, SPF 15 or 30 will be just fine assuming you diligently reapply every two hours or after swimming or sweating profusely. Even if you have extremely sensitive skin and start to burn after only 5 minutes in the sun, SPF 30 should protect you for two and a half hours if you use the full recommended amount. Realistically, though, most of us don’t apply enough in the first place, get distracted and forget to reapply as often as we should, and are likely to miss a spot here or there, so I don’t blame you at all if you still grab the highest protection on the shelf. I certainly will!

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[E] Hillary

Hillary is a giant nerd and former Mathlete. She once read large swaths of "Why Evolution is True" and a geology book aloud to her infant daughter, in the hopes of a) instilling a love of science in her from a very young age and b) boring her to sleep. After escaping the wilds of Waco, Texas and spending the next decade in NYC, she currently lives in upstate New York, where she misses being able to get decent pizza and Chinese takeout delivered to her house. She lost on Jeopardy.

7 thoughts on “Sunscreen Math: Is SPF 30 Twice as Good as SPF 15?”

    1. Right there with you, pasty sister! I have the neutrogena SPF 70, which makes me feel better even if it doesn’t provide much of a difference in protection. My mom, grandma, and brother have all had skin cancer cut out of their faces, and I am way too vain to run the risk of a scar. I’m pale and I like it that way.

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