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Dec 03, 2023

What Does SPF Stand For?

Understanding how sunscreen works helps you get the best protection

Understanding how sunscreen works helps you get the best protection

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SPF, short for sun protection factor, represents how well a product guards against damage from the sun's ultraviolet B rays, which are strongest between 10 a.m. and 4 p.m. The chief cause of reddening and sunburn, UVB rays tend to damage the epidermis, the skin's outer layers, where basal cell carcinoma and squamous cell carcinoma—the most common (but least dangerous) forms of skin cancer—can occur. Those cancers are linked to sun accumulation over the years. Another type of skin cancer, melanoma, is thought to be mostly caused by brief, intense exposures, such as a blistering sunburn. Melanoma is more likely than other types of skin cancer to grow and spread to other areas of the body, and it can be deadly.

SPF is usually explained as the amount of time it takes for skin to turn red. For instance, if you burn in 15 minutes, an SPF 30 would give you 450 minutes of protection (15 x 30 = 450). That's true up to a point. The UV dose is a big factor. The same 15 minutes of sun will result in a very different UVB amount depending on location (sea level vs. high altitude, for instance), time of day, time of year, the amount of cloud cover, and the color of your skin.

And that calculation doesn't apply to UVA rays, which are present as long as there's daylight. These rays are long enough to reach the skin's dermal layer, damaging collagen and elastin, which give skin its elasticity. (See the image below.) Injury to this layer can lead to sagging and wrinkling skin over time. The dermis is also where the cells that stimulate skin darkening are found; that's why UVA rays are considered the dominant tanning rays. Though many people still think a tan looks healthy, it's actually a sign of DNA damage. The skin darkens in an imperfect attempt to prevent further injury, which can lead to the cell mutations that trigger skin cancer.

To get the most protection, you want a broad-spectrum sunscreen. They protect against UVB and UVA rays. But as the chart below shows, double the SPF doesn't mean the sunscreen blocks double the UVB rays. For example, an SPF 30 isn't twice as protective as SPF 15, says Mona Gohara, MD, an associate clinical professor of dermatology at Yale University. And no sunscreen, not even one with an SPF 100, screens out all UVB rays.

No matter what type of sunscreen you choose, you need to use it properly to get the most protection. Apply about an ounce to cover your body if you’re wearing a bathing suit. (That's about the amount that would fit in a shot glass.) If not, use about a teaspoon per body part that's exposed to the sun—for example, a teaspoon for each arm, each leg, and for your face and neck. Apply it at least 15 minutes before you go outside. And diligently reapply. "Higher numbers don't absolve you from the rules of how much and how often to apply sunscreen," Gohara says. Reapply the same amounts every 2 hours you’re out in the sun no matter what SPF you’re using, and sooner if you’ve gone into the water or you’re sweating excessively. Finally, using sunscreen shouldn't be the only sun protection measure you take. You also need to cover up with clothing and a hat, and seek shade.

These products, listed in alphabetical order, will help protect you from the sun's UVB and UVA rays.

Sally Wadyka

Sally Wadyka is a freelance writer who contributes to Consumer Reports, Real Simple, Martha Stewart Living, Yoga Journal, and the Food Network on topics such as health, nutrition, and wellness.