Screening the Sun: The Science of Sunscreen

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Fireworks, concerts, barbecues, picnics, parades, sandy beaches and swimming are just a few of the many ways we celebrate the Fourth of July holiday. Each of these fun outdoor activities increases our sun exposure, which can increase our risk of melanoma or skin cancer.

Sunlight consists of infrared, visible and ultraviolet (UV) radiation. There are two types of UV radiation from sunlight that largely contribute to skin damage: UVA, which we are exposed to year-round and which tans our skin, and UVB, which is highest in the summer months and leads to sunburn. Luckily, there are UV filtering agents, also known as sunscreen, that can help protect our skin from the sun’s harmful UV radiation. Let’s look at how sunscreens work.

Sunscreens or ultraviolet filters protect our skin from harmful UV rays by blocking, reflecting, absorbing or scattering the sunlight. Ultimately, sunscreens can be classified into one of two categories based on how they act:

  • Mineral or inorganic sunscreens, also called sunblock, protect our skin by scattering and reflecting the UV energy from the skin surface. They essentially act as a physical barrier (think of the lifeguard with the white nose).
  • Chemical or organic sunscreens act by absorbing UV light and converting it into heat energy that is released back through the skin. Organic sunscreens can be further classified based on their range of protection: UVA, UVB or both, which is called broad-spectrum sunscreen.

The effectiveness of a sunscreen is determined by various factors, including the sun protection factor (SPF). SPF refers to the ability of sunscreen to prevent damage when we’re exposed to UVB radiation. There is a range of SPF values and these numbers represent a ratio. The ratio is the amount of UV radiation that will burn the area of protected skin when we’re wearing sunscreen to the amount of UV radiation exposure needed to burn the same area unprotected. For example, a sunscreen with a SPF of 50 will protect our skin until it is exposed to 50 times more UVB light than when it’s unprotected.

Ideal sunscreen agents are safe, nontoxic and aren’t irritating to the skin. Sunscreen that won’t break down when exposed to light, doesn’t react with other chemicals and can withstand swimming or sweating is also important. Most sunscreens are not waterproof or sweatproof, but many are water resistant for a period of time after water exposure.

Most importantly, sunscreen is only effective if we use it correctly. The U.S. Food and Drug Administration recommends applying 2 milligrams of sunscreen per square centimeter of your skin (think of a shot glass amount) and to put more on every two hours or after we sweat, towel off, bath or swim.

As you head out the door for the lake or that family barbecue this summer, be sure to lather on your sunscreen and throw an extra bottle in your bag.

Casey Derella, PhD, is a postdoctoral associate in the Department of Kinesiology at the University of Virginia. Her research interests primarily focuses on how sex and disease alter the microcirculation and skeletal muscle, contributing to cardiovascular disease.

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