The numbers that signal summer’s approach are here: The countdown to the last day of school. A 90-degree forecast. And, of course, SPF 30. Or 50? Or maybe it’s time to break out SPF 100?

SPF stands for sun protection factor. That sounds simple enough, but what these ubiquitous figures mean in practice can be complicated.

We asked two board-certified dermatologists to answer our burning SPF questions.

What do different SPF numbers actually mean? 

The numbers tell you how well a sunscreen protects your skin against ultraviolet B, or UVB, rays — the type of ultraviolet radiation most responsible for causing sunburns. 

The higher the SPF, the more these UVB rays are blocked, said Dr. Adewole Adamson, a dermatologist and internal medicine professor at the University of Texas at Austin. 

Doubling the SPF doesn’t mean the sunscreen protects twice as well, however. 

SPF 30 sunscreen filters about 97% of UVB rays, for example, and SPF 50 filters about 98%, said Dr. Marisa Garshick, a dermatologist based in New York and clinical dermatology professor at New York Presbyterian-Weill Cornell Medicine. 

You can think of SPF as a measure of how much solar energy it takes to burn skin with sunscreen compared to the solar energy needed to burn skin without sunscreen, according to the U.S. Food and Drug Administration.

What SPF is high enough?

Use at least SPF 30, but even higher is better.  

That’s partly because most people don’t apply enough sunscreen, which means they get less protection than what the label advertises.

“Lower SPF sunscreens may feel lighter or be easier to wear, but they provide less protection,” Adamson said. “The higher the number, the more margin of error you may have if you under apply.”

What else should I look for when selecting sunscreen? 

Buy sunscreen with a “broad spectrum” label, which means it protects against UVA and UVB rays, dermatologists said. 

SPF can tell you only how well a sunscreen blocks UVB rays, not ultraviolet A or UVA rays that are also harmful. Much like UVB rays, UVA rays have been linked to sunburns, wrinkles and some skin cancers. 

If you’ll be sweating or spending time in the water, you should use sunscreen that is also labeled “water resistant.” 

“Most of all, find a sunscreen that feels good on your skin because sunscreen will only work if you use it,” Adamson said. 

Does SPF 100 block 100% of UVB rays?

No. In fact, the FDA banned the term “sunblock” in 2011 in part because no sunscreen can block all UVB rays. 

SPF 100 filters about 99% of the sun’s UVB rays, Garshick said. 

It provides slightly more UVB protection than lower SPF options, but the added protection gets smaller as the numbers go up. Plus, higher SPF sunscreen might cost more, UC Davis Health reported. 

If sunscreen is applied correctly — and remember, that’s a big qualifier — the increase in UVB protection from SPF 50 to SPF 100 might be underwhelming. 

Higher SPF sunscreens can also create a false sense of security, Garshick said. “Someone wearing SPF 100 may think they can stay in the sun significantly longer without reapplying, which is not the case.” 

Whether you’re reaching for SPF 50 or SPF 80, it’s important to use enough sunscreen and reapply frequently, she said. Slather on another coat at least every two hours or right after getting out of the pool, participating in water activities or doing anything that makes you sweat a lot.

What else is important when it comes to sun protection? 

While properly and repeatedly applying at least SPF 30 sunscreen is important, it’s just one part of what should be a multi-pronged approach to protecting yourself from the sun’s harmful rays. 

It’s best to stay out of the sun during peak UV hours — 10 a.m. and 4 p.m. 

And when you are outside, experts said, try wearing wide-brimmed hats, sunglasses and sun-protective clothing. 

RELATED FACT-CHECK: Wear your sunscreen. The sun does not prevent skin cancer.