Ever wonder how a cream can magically shield from the sun’s invisible rays? Here’s how our spiffy SPF protects our freckled skin.

Who Was the First to Use SPF?

In the loosest sense of sun protection, SPF use goes back to ancient times. The Egyptians, ancient Greeks, and some Native American tribes used local ingredients to protect from the sun, repair sun damage, and sometimes to lighten skin for cosmetic reasons.

One of the pioneers of sunscreen development was chemist Franz Greiter. His sunscreen was commercially marketed in 1938 and popular among mountain climbers and hikers. Greiter also introduced the SPF concept to the world in the 60s.

What Does That SPF Number Mean?

Every bottle of sunscreen has a little number associated with it, I’ve seen it range from 10 to 150. For longer than I’d like to admit, I thought that number meant how many minutes I could stay outside without worrying about getting sunburned. WRONG. I have all the sunburns to prove it.

I was wrong, but the SPF number does relate to time and sun rays. According to Steven Wang, MD., An SPF of 30 will block about 97 percent of UVB rays, whereas SPF 50 will block about 98 percent. He also states, “Ideally, with SPF 30 it would take you 30 times longer to burn than if you weren’t wearing sunscreen.”

How Is the SPF Number Determined?

For every bottle of sunscreen you buy, there’s a burned butt somewhere in the world. Just kidding. Kind of.

In order to protect your skin, someone had to expose a very vulnerable part of their body to some ultraviolet rays.

Sunscreen is tested and created in a lab, where a volunteer exposes a part of their body that the sun doesn’t shine on—typically their butt.

It’s a simple observational experiment. Scientists test how long it takes for the patch of skin to redden after being slathered in sunscreen, then test again for how long it takes that patch to redden with no sunscreen applied. The final step is to divide the number of seconds it took the sunscreened butt to burn by the number of seconds it took the no sunscreen butt to burn. Voilà – SPF.

What’s My Ideal SPF Number?

It’s tempting to buy the highest SPF number out there, slather it on, and go about your business in the sunshine. But this can be one of the worst mistakes sunscreen wearers make. When too much confidence is placed in that high SPF number, people forget to reapply. And keep in mind, these sunscreens are made in a controlled lab setting, while in reality the sun’s intensity varies, like if you’re near the Equator, at the top of a mountain, or in reflective waters.

The Environmental Working Group shed light on several reasons why higher SPFs may not be worth the extra cost and confidence. For example, in an experiment conducted by Procter and Gamble, the same brand of 100 SPF sunscreen was sent to five different labs for testing. The SPF results from the labs came back in a range of SPF 37 to SPF 75.

The point is, don’t put all your trust in that SPF number, and reapply as often as possible if you’re out in the sun for long periods of time.

Sunscreen isn’t perfect, but it’s a huge help to a freckled persons health. Slather on (repeatedly) and enjoy not being sunburned!