What Is SPF? The Numbers, Explained

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It’s that time of year again: You’re at your local drugstore, and you’re looking to stock up on sunblock for the summer (or for year-long use, as many dermatologists recommend). You’re trying to avoid that super greasy, heavy feel and off-putting odor of the sunblock you went with last year. Despite how little you liked that sunblock, you know it worked well because you didn’t get a sunburn whenever you put it on. It was an SPF 50 sunblock, but in front of you at the drugstore, you see far more options with much lower numbers that hover more around 15, 30, or 35 SPF.

What exactly is SPF? Most people know what SPF represents without understanding the science and math behind it — read on to learn about how it’s calculated and what it means for you.

What SPF measures

The abbreviation SPF stands for Sun Protection Factor. This term represents how well the sunscreen protects the skin from the ultraviolet light that the sun emits. Ultraviolet comes in two types, UVA and UVB, but only exposure to the latter causes sunburn (though the former does pose harmful long-term effects). SPF measures how well a sunblock blocks the skin from absorbing UVB rays.

So how is it calculated?

SPF is the ratio of how long a person’s skin will take to burn with sunscreen applied to how long that person’s skin will take to burn with no sunscreen applied. If a person’s skin burns in burns in 20 minutes without sunblock and 5 hours (300 minutes) with sunblock, then the sunblock’s SPF is 15, because 300 divided by 20 is 15. Using that calculation, SPF 50 sunblock would theoretically offer 20 * 50 = 1,000 minutes (nearly 17 hours) of protection.

What more can SPF tell me about a sunscreen?

If you were to plot SPF values against sun protection length values, you would be able to draw a straight line through the points. However, the relationship between SPF and the percentage of incoming UVB rays blocked isn’t so simple. This relationship also helps to explain why even people who apply high-SPF sunblock can still be sunburnt.

An SPF 15 sunblock protects the skin from 93 percent of incoming UVB rays. SPF 30 protects from 97 percent, SPF 50 from 98 percent, and SPF 100 from 99 percent. In other words, although SPF expresses a simple ratio, even the sunblocks that offer the longest UVB protection can’t stop every UVB ray from interacting with the skin. This is, in part, why sunblock requires regular reapplication.

What SPF do I need?

The American Academy of Dermatology, which represents a large number of American dermatologists, advises against using SPF 15. Instead, they suggest sticking to SPF 30 or higher

Studies have also found that, in practice, even high-SPF sunblocks only offer a fraction of the protection they mathematically should. Experts thus often conclude that any sunblock SPF 30 or higher should protect even those with sensitive or fair skin, but perhaps more important than SPF is being sure to reapply sunblock every two hours. Additionally, wearing proper clothing and seeking out shade are vital for proper sun protection, no matter your SPF.

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