There’s a figure that has been circulating on the internet for years, stating that SPF 50 provides only about 1% more protection than SPF 30. While that figure is factual, people have been doing the SPF math all wrong. Although the percentage is small, it has a much bigger impact than we can imagine.

HuffPost spoke to three board-certified dermatologists, including two Mohs surgeons (specialists in skin cancer), to explain what that means and help determine the type of sunscreen we should use and why.

What does SPF 50 do?

SPF stands for “sun protection factor” and refers to the amount of protection against UVB rays. Before picking a sunscreen, “You need to know what percentage of the sun’s UVB rays are being filtered. For instance, SPF 15 is 93%, SPF 30 is 97%, SPF 50 is 98% and SPF 100 is 99%,” said Dr. Kenneth Mark,a board-certified dermatologist and Mohs surgeon.

“Numerically, there isn’t much of a difference between SPF 50 and SPF 30 but in real-world use, it is significantly better” said Dr. Margarita Lolis, a board-certified dermatologist and Mohs surgeon at Schweiger Dermatology Group in New Jersey. “Most people under-apply sunscreen, which lowers the actual protection against UVB rays. SPF 50 gives a better buffer in my opinion.”

The protection against UVA rays isn’t measured in the same way. “The SPF number has nothing to do with UVA protection,” Mark said.

In the U.S, the Food and Drug Administration mainly considers zinc and titanium dioxide-based sunscreens as offering adequate protection against both UVA and UVB rays. However, a broad-spectrum sunscreen is what you should be on the lookout for. Outside the U.S., you can look for a “PA rating,” which is common in Japan, or a “star rating” in Europe.

“While SPF 50 offers only 1% extra filtering of........

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