UVA, UVB and SPF: How does sunscreen work?

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Protecting him from the sun
Lindsay Walters, of Minneapolis, adjusted her 15-month-old son Julian's hat after putting sunscreen on him July 15, 2015 at Lake Nokomis.
Jennifer Simonson | MPR News 2015

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Sunscreen is a cream or spray-on substance that is used to protect our skin from the sun. That's cool, but how does it really work?

There are two kinds of sunscreens: physical sunBLOCKS and chemical sunSCREENS. These products use different methods to protect the skin from the sun's powerful rays: Sunblocks protect the skin from UVB rays, while sunscreens protect the skin from UVA.

"With a physical sunblock, there's a product called zinc oxide. And it works by physically reflecting the UV rays. I usually tell [my clients] to get physical block with at least 5 percent zinc oxide," said Dr. Anudeep Rahil, a dermatologist at North Metro Dermatology in North Oaks, Minn.

Chemical sunSCREEN, on the other hand, absorbs the UV radiation and dissipates it into the skin as heat.

The ABC's of radiation

The sun emits three kinds of UV radiation: UVA, UVB and UVC. UVC rays only penetrate up to the ozone layer and do not affect the skin. But UVA and UVB radiation can cause reasonable damage to the skin.

UVA is the most common type of UV ray to hit the Earth's surface. UVA penetrates the skin, but doesn't burn it. UVB rays hit the top layer of the skin and cause it to turn red and peel: That's sunburn.

When the sun burns skin with UVB rays, the top layers of skin release chemicals that make blood vessels swell and leak fluids. The skin then turns red and feels hot and painful. After a sunburn, your skin begins to peel, removing the damaged cells. It then builds another layer to protect itself.

Since UVA rays penetrate the skin, they can cause mutations in skin cells. These mutated cells can accumulate over time to form skin cancer or cause wrinkles.

The power of SPF

The power of a sunscreen is measured by the Sun Protection Factor (SPF).

"SPF basically measures how long you can be outdoors before getting a burn," Rahil said. "SPF just reflects the UVB radiation, it doesn't take into account the UVA radiation. If something says SPF 15, it means that you can stay outside fifteen times longer before burning."

Before sunscreen was created in the early 1930s by Milton Blake, people used a variety of natural products to protect their skin. Ancient Egyptians used a combination of rice bran and jasmine, which are still used in some sunscreens today.

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