Sunscreens: UVA vs. UVB

by Food and Drug Administration (FDA)

(Page 2 of 4)

The complexities of light quickly overwhelm freshmen physics students, but some basic principles can be readily understood. In one model of how light works, the electromagnetic radiation can be thought of as a series of waves, like ocean waves at the beach, steadily marching toward shore. At the beach, the wind makes the waves by transferring kinetic or mechanical energy into the water. The harder the wind blows, the more energy in the water and the higher and closer together the ocean waves. On a calm summer day, widely spaced waves lap mildly against the shore. During a hurricane, the wave action intensifies, pounding the sand with closely packed wave after wave of crashing white foam strong enough to wipe away the beach.

The electromagnetic energy in sunlight works much the same way: The higher the energy of the light, the closer together its waves. Some types of light have waves that are far apart — like ocean waves on a calm day. Other types of light have waves that are packed closely together, like ocean waves on a windy day.

This difference in closeness of a light's waves, its wavelength, gives different parts of the electromagnetic spectrum its characteristics, such as the colors of visible light and the destructive capabilities of x-rays and ultraviolet light.

Physicists classify ultraviolet light into three types, by its wavelengths: UVA, UVB and UVC. The dimensions of their wavelengths are roughly 400 to 320 nanometers (nm) for UVA, 320 to 290 nm for UVB, and 290 to 200 nm for UVC. Although it may seem backwards, the shorter the wavelength and the lower the number, the greater the energy level of the light and the more damage it can do. For example, direct exposure to UVC for a length of time would destroy the skin. Fortunately, UVC is completely absorbed by gases in the atmosphere before it reaches the ground.

The longer wavelengths of UVB and UVA, however, pass right through the atmosphere, even on a cloudy day. That's why you can still get sunburned on a cloudy or hazy day. The molecules in sunscreens absorb most UVB and prevent it from reaching the skin just as the molecules of the atmosphere absorb UVC and prevent it from reaching the ground.

UVA, however, is another story.

According to a 1998 review article, most sunscreens do not protect the skin from the longer UVA wavelengths. And that may be critical to the creation of skin cancer. Approximately 65 percent of melanomas and 90 percent of basal and squamous cell skin cancers are attributed to UV exposure.

The precise wavelengths of ultraviolet that contribute to the formation of skin cancer still need to be sorted out. And scientists must still figure out how best to formulate sunscreens to provide effective protection against these wavelengths.

Scientists use a number of techniques to measure the UV-blocking ability of a sunscreen. Some rely on electronic laboratory equipment, some on living tissue or live animals. Some testing procedures even use human volunteers.

"We have a good way of measuring UVB protection with a sunburn or erythema test in humans," says Sharon Miller, an optical engineer in FDA's Center for Devices and Radiological Health. But scientists lack a simple measure of UVA's impact on the skin, she says. That makes it difficult to determine how much UVA protection a sunscreen provides.

That leaves FDA with an unresolved technical dilemma that it is trying to resolve through additional research. "We are trying to determine a testing method that will demonstrate that a sunscreen is providing UVA protection," Lipnicki says. A claim such as "broad spectrum" on a sunscreen label needs to be supported by evidence that the product provides significant and meaningful protection across the entire UVB/UVA spectrum.

To Australia's Robin Marks, however, the issue is not UVA vs. UVB or even UVA combined with UVB. "The most common skin cancers seen in humans are related to sunlight, not to a limited band of the solar spectrum," Marks says. "It is the whole of all light coming from the sun. Don't concentrate on one band, but the entire spectrum. Keep it off the skin."

The SPF Debate

To figure out how much protection a sunscreen provides, most consumers turn to a simple number: the SPF, or sun protection factor, listed on the label. Studies show that most consumers understand that the higher the number, the more the product protects the skin.

Unfortunately, studies also show that people often have the mistaken notion that the higher the SPF number of the sunscreen they use, the longer they can stay — and will stay — in the sun. In August 1999, the Journal of the National Cancer Institute published a study showing that use of higher-SPF sunscreens led to increased sun exposure. Two groups of French and Swiss volunteers used unlabeled sunscreen during their vacations. One group used SPF 10 and the other group used SPF 30. The group using the higher-SPF sunscreen spent 20 percent more time in the sun (72.6 hours vs. 58.2 hours) than the group using the lower-SPF sunscreen.

"Because of variations between individuals, products, exposures, and conditions of use, there is no really easy way to explain SPF in a few words," says FDA's Lipnicki. "In the past, it was explained in terms of the amount of time you could stay in the sun longer with sunscreen than without it before getting 'burned'. We have gotten away from that. Sunscreen should not be used to prolong time spent in the sun. Even with a sunscreen, you are not going to prevent all the possible damage from the sun. Some of the newer research in the last several years shows that the sub-erythemal doses [exposure to the sun that does not cause reddening of the skin], as little as one-tenth the energy needed to get a sunburn, start the process of skin damage of one sort or another."

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.