• A sunscreen's sun protection factor (SPF) is only fully effective for two hours after you put it on. 
  • Experts recommend carrying a bottle of SPF 30 to SPF 50 sunscreen around with you, even on cloudy or rainy summer days, so you can throw some on if the sun comes out.
  • You can still get a tan with sunscreen on, because while SPF blocks damaging UVB rays, it doesn't prevent deep-seeping UVA rays from penetrating your skin and giving it color. 


Sun-soaking season is here, and that means that if you don't lather up with sunscreen, you could be doing serious damage to your skin cells' DNA, setting yourself up to develop a potentially deadly form of cancer.

Melanoma is typically caused by UV radiation you get from either baking in the sunshine, or lounging in a tanning bed.

Indulge in either of those often enough, and you may start to develop cancerous moles. If those marks aren't caught in time, the melanoma can spread to other areas of your body, and become tough to control, even deadly cancer. 

So it's important to lather up with some protective sunscreen if you're going to be spending time in the sun —especially during warmer summer months, when the UV Index rises and the sun becomes more potent.

We enlisted the help of two skin experts to explain just how long sunscreen lasts and how to re-apply it correctly. Here's their best advice: 

You sunscreen's SPF is only 100% guaranteed for two hours after you put it on.

"When we talk about reapplying sunscreen every two hours, that number comes from how SPF is tested," John Zampella, a dermatologist at the New York University Langone Health, said. He explained that SPF numbers are based on how much protection a certain sunscreen will give you against the sun for two hours. 

When you're active and enjoying the sun, like at the beach or a ballgame, it's important to reapply after that. 

If you put on an SPF of 50 at 10 a.m., by noon, "you're probably still getting some protection, but your SPF 50 is no longer accurate," Zampella said. "Maybe you're getting SPF 10 at that point."

For full-fledged protection, use at least SPF 30.

That's what the American Academy of Dermatologists suggests.

SPF 30 also happens to be the dose that skin expert David Leffell, author of "Total Skin: The Definitive Guide to Whole Skin Care For Life," and chief of dermatological surgery at Yale School of Medicine, keeps on hand for himself at home. For most people, that protection factor gives enough of a shield to fight off about 96% of the damaging, burn-inducing UVB solar rays outside, whether you're at the beach or on the golf course.

Sensitive and fairer skin tones may opt for SPF 50 to get a little extra boost of protection, but Leffell says any SPF higher than 50 only offers minimal extra benefit.

It's especially essential to "refresh" sunscreen in certain burn- and skin cancer-prone areas of the body and face.

That includes the top of the ears, the forehead, the cheeks, and the nose.

Carry sunscreen with you during the day.  

Zampella says he totes a bottle of sunscreen with him every day. 

"Do I apply it every two hours? Probably not," he said. Realistically, he's probably applying twice a day while in the city. But he's more vigilant when he's spending the entire day outside. "At the beach, I do try and get it on every two hours." 

Make it a daily habit.

According to Leffell, you should make putting on sunscreen everyday a part of your morning routine, "just like brushing your teeth." 

"By making it a regular behavior, you have a much increased chance that you won't see it as an annoyance, and that you won't forget to do it," he said.

Zampella agrees. He says making sun care a daily reflex will "give you the best long term benefits of both skin cancer prevention and also anti-aging protection." 

Women should pay extra attention to their chest, while men should watch out on top of their head. 

Leffell says women should be especially careful to protect the "v" of their chest from getting burned, since damage there is "very hard to reverse cosmetically with lasers."

For men, he recommends anyone who's lost a bit of hair should liberally apply sunscreen to their bald spots and their neck, and probably add a hat.

"We see a lot of skin cancer on the scalp," he said.

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