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SUN SAFETY

What does SPF stand for?

Understanding how a sunscreen works helps you get the best protection

Published: May 15, 2015 06:00 AM

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SPF (sun protection factor) is a relative measure of how long a sunscreen will protect you from ultraviolet (UV) B rays. The chief cause of reddening and sunburn, UVB rays tend to damage the epidermis, skin’s outer layers, where the most common (and least dangerous) forms of skin cancer occur. Those cancers are linked to sun-accumulation over the years. Another type of skin cancer, melanoma, is thought to be caused by brief, intense exposures, such as a blistering sunburn.

Assuming you use it correctly, if you’d burn after 20 minutes in the sun, an SPF 30 sunscreen protects for about 10 hours. But intensity and wavelength distribution of UVB rays vary throughout the day and by location. And that calculation does not apply to UVA rays.

UVA rays are long enough to reach skin’s dermal layer, damaging collagen and elastic tissue. That layer is also where the cells that stimulate skin darkening are found; that’s why UVA rays are considered the dominant tanning rays. (UVA rays are also used in tanning beds.) 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 the further injury, which can lead to the cell mutations that trigger skin cancer.

Get more sun-protection questions answered in our guide, Stay Safe in the Sun and learn 5 things you must know about sunscreen.

To get the most protection, you want a broad spectrum sunscreen—these protect against UVB and UVA rays. But it's important to note that no sunscreen blocks 100 percent of UVB rays, and ultrahigh SPFs are not much more protective than SPFs of 30 or 50. SPF 15 blocks 93 percent of UVB rays. SPF 30 blocks 97 percent. The increase in protection is even more gradual after that, 98 percent for SPF 50 and 99 percent for SPF 100. Sunscreen should be just one of the sun protection strategies you use to protect your skin—you also need to cover up with clothing and a hat, seek the shade, and try to stay out of the sun during peak hours 10:00 am to 4:00 pm.

Editor's Note:

This article also appeared in the July 2015 issue of Consumer Reports magazine.


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