Skin cancer is the most common cancer in the United States. It affects up to 1 in 5 Americans, according to statistics from the American Academy of Dermatology.

Sun exposure is the single largest preventable risk factor for skin cancer. Yet many Americans don’t exercise necessary precautions to protect their skin.

“Skin cancer is a serious, potentially deadly cancer,” says Charles Yoon, MD, PhD, a surgical oncologist at Brigham and Women’s Hospital. “Many types of skin cancer are just as aggressive as colorectal, breast, or lung cancer.”

Debunking sunscreen myths

Despite the well-documented benefits of sunscreen to prevent skin cancer, like those reported in the Canadian Medical Association Journal, some people still question its importance. In part, this is because of several common misconceptions:

Myth: You won’t get sun damage if you reapply sunscreen every few hours.

You’ve likely heard that you should reapply sunscreen every few hours when spending time outside. What many people don’t understand, however, is that sunscreen simply delays how quickly you’ll experience sun damage. Reapplying every few hours isn’t enough to bring your risk to zero.

Yoon uses the analogy of an oven mitt to explain sunscreen application. Wearing an oven mitt offers an extra layer of protection so that you don’t burn your hand when you pick up a hot pan. However, if you hold the hot pan for an hour, the heat will eventually go through the oven mitt and damage your hand.

Reapplying sunscreen after an hour is like switching out a fresh oven mitt. It may delay the time it takes to burn, but it doesn’t prevent damage entirely. It also doesn’t undo the hour of damage that’s already occurred. For true protection, even with sunscreen, you still need to limit your total time spent in the sun.

Myth: You don’t need sunscreen on a cloudy day or during winter.

Some people think that they won’t get sun damage if they can’t see the sun, such as during a rainy or gray winter day. But clouds generally provide little protection from the sun.

Sun damages your skin because it gives off ultraviolet (UV) radiation, specifically UVA and UVB rays. The UV index rates the intensity of those rays at a given time. Higher UV indexes are associated with a higher risk of burning. A UV index of 6 or 7 is considered a high risk of sun damage.

“Even with cloud cover, the UV index often exceeds 6 or 7,” explains Yoon. “It would take a hurricane or other significant storm to provide real protection from cloud cover—and you likely wouldn’t want to be outside in that.”

Fact: Sunscreen lowers vitamin D production.

It’s true that sunscreen lowers the efficiency of vitamin D production. This is because our bodies make vitamin D through UV exposure. Vitamin D is an important nutrient that helps your body absorb calcium to keep your bones healthy and strong.

Wearing sunscreen or having darker skin causes your body to make vitamin D from sun exposure less efficiently. Luckily, we have vitamin D supplements to compensate for any deficiencies.

Read more facts about skin cancer prevention at massgeneralbrigham.org.