How Often Do You Need to Reapply Sunscreen?

Here's how to use sunscreen the right way, including how to tell the difference in types, as well as when and how to apply and reapply.

You need sunscreen for many reasons, from stopping sun damage, which helps healthy skin aging, to preventing skin cancer, according to the Skin Cancer Foundation. In fact, using a sunscreen every day can reduce your risk of skin cancer by as much as 50%. And using that sunscreen properly—plus choosing the best formula for you—will offer the most protection. So, to help you understand the steps to take to slather on sunscreen the right way, including how to tell the difference in types, as well as when and how to apply and reapply, we turned to experts in dermatology. Here’s everything you need to know. (For starters, check out these nine best sunscreens for every activity.)

What’s the difference between chemical and physical sunscreens?

There are two types of sunscreen: physical (or mineral) and chemical. Physical sunscreens contain zinc oxide alone or with titanium dioxide, explains Joshua Zeichner, MD, director of cosmetic and clinical research in the department of dermatology at The Mount Sinai Hospital in New York City. “They are effective but tend to be heavier on the skin and may leave behind a white discoloration,” he says. “They are gentle and can be used across a variety of skin types, even in those with sensitive skin or in children.”

Physical sunscreens work to reflect the light away from the skin, says Bob Hsia, MD, assistant professor at the Medical University of South Carolina Department of Dermatology and Dermatologic Surgery. Chemical sunscreens work by blocking UV radiation via a chemical reaction.

Chemical sunscreens, compared with physical ones, offer a lighter feel and a sheer color when applied, Dr. Zeichner says, explaining that ultra-high chemical SPFs won’t leave any white coloring behind. (Check out the 12 sunscreens dermatologists use on themselves.)

Are all sunscreens safe?

This has been a hot topic for years and last year, the U.S. Food and Drug Administration (FDA) issued a statement that said we need more research into ingredients commonly used in chemical sunscreens. Currently, there’s no evidence to date that shows chemical sunscreens to be harmful, says Dr. Hsia, unless you have an allergy. However, there is some evidence that shows that your body might absorb some ingredients, but it’s not known whether or how much that matters.

When it comes down to choosing which way to go, physical or chemical, Dr. Hsia says it’s a personal decision about what feels nice and easy to apply. Dr. Zeichner agrees, saying, “The best sunscreen is the one that you’re actually using.”

What’s more important than type: choosing an SPF of at least 30 and a broad-spectrum sunscreen, meaning it blocks both UVA and UVB rays, Dr.Hsia says.

How often should I apply and reapply sunscreen?

“You only get the level of protection if you apply enough,” says Dr. Zeichner. Avoid shorting yourself by applying 15 minutes before you go outside, as suggested by the FDA. Then, make sure you put on enough of the lotion, stick, or spray sunscreen.

For your full face, squeeze about a quarter-sized dollop of sunscreen in your hand to smear it on, Dr. Zeichner says. For sticks, do four back-and-forth passes. And for sprays, hold them one inch from the skin and spray until the skin glistens. Then—a crucial step—make sure you rub it in.

The general rule for reapplication: Do it every two hours, following the same rules above in regards to amount. “Chemical sunscreen ingredients are inactivated by UV light over time. Physical sunscreen ingredients tend to clump on the skin over time,” Dr. Zeichner says.

mature woman applying sunscreen to husband's back at the beachdavidf/Getty Images

Do I need to reapply more often if I’m swimming or sweating?

If you know you’ll be swimming, it’s probably smart to choose a sunscreen labeled “water-resistant.” (Also, note, there are no waterproof sunscreens; they’ll all come off eventually, says the FDA.) Water-resistant varieties will tell you on the label whether they last 40 or 80 minutes, meaning if you apply before swimming they’ll stay active for that duration, Dr. Zeichner says. “I typically recommend reapplying sunscreen after drying off from your swim or after heavy sweating,” he says.

For those exercising outdoors, playing volleyball, or say, working construction and sweating a lot, Dr. Zeichner says it’s probably smart to reapply even more frequently as sweat can wash off the sunscreen too.

Should I follow the same sunscreen application rules for my face?

Dr. Hsia recommends SPF 30 for all areas of the body—including the face—but if you’re sitting at the pool or on the beach, you probably want to go higher, especially for the face. For everyday use, he recommends a daily moisturizer with sunscreen in it, with an SPF of at least 15. (This is for those who don’t have outdoor activities planned for the day.) “Those tend to be designed to wear every single day, like your typical daily moisturizer and so they’re good for skin and keep skin healthy,” he says. Check out the safest sunscreens you can buy.

Are there any spots I should pay particular attention to?

In short, yes. A few common areas the FDA calls out that people tend to miss include ears, nose, lips, back of the neck, hands, tops of feet, along the hairline, and any area exposed by balding or thinning hair. Make sure you hit these areas on the first application and each reapplication after that.