What Does SPF Mean?

SPF—or sun protection factor—is a measure of how well a sunscreen guards against sunburn. The SPF number is usually explained as the amount of time it takes for skin to turn red while wearing sunscreen. For instance, if you burn in 15 minutes, an SPF 30 would theoretically give you 450 minutes of protection from the sun (15 minutes, multiplied by 30 SPF, equals 450 minutes). 

While that’s true to a point, the UV dose plays 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—UVB rays are the strongest between 10 a.m. and 4 p.m.—time of year, and the amount of cloud cover.

An SPF 30 isn’t twice as protective as SPF 15, says Mona Gohara, MD, an associate clinical professor of dermatology at Yale University. In fact, it only blocks 4 percent more rays than an SPF 15 sunscreen, as shown in our chart. And no sunscreen, not even one with an SPF 100, screens out all UVB rays.

The chief cause of reddening and sunburn, UVB rays tend to damage the epidermis, the skin’s outer layers. This is where basal cell carcinoma and squamous cell carcinoma—the most common (but least dangerous) forms of skin cancer—can form. 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.

UVA rays 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. 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.

No matter what type of sunscreen you choose, you need to use it properly to get the most protection. 

Make sure you use the right amount. 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. 

Use sunscreen in advance. Apply it at least 15 minutes before you go outside. 

Reapply diligently. 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. “Higher numbers don’t absolve you from the rules of how much and how often to apply sunscreen,” Gohara says. 

Don’t forget about other sun protection measures. Physical protection, like covering up with clothing and a hat, as well as seeking shade, will also help reduce exposure to UV radiation.

In our labs, we test sunscreens to see how well they guard against UVA and UVB rays, including how accurate their labeled SPFs are. These products, which come out at the top of our ratings, will help protect you from the sun’s UVB and UVA rays.