It Started in the Jungle, Not on the Beach
The image most Americans have of sunscreen is inseparable from summer: a bottle on a beach towel, a white streak on someone's nose, a warning from a dermatologist before a vacation. But sunscreen didn't begin as a lifestyle product. It began as a wartime problem.
During World War II, American soldiers fighting in the Pacific theater were getting badly sunburned — not just uncomfortably, but to the point where it was affecting their ability to operate. A pharmacist named Benjamin Green, who was serving as an airman, developed a solution: a thick, red, petroleum-based substance that sat on the skin and physically blocked UV rays. It wasn't elegant. Soldiers described it as resembling red veterinary petrolatum. But it worked well enough that after the war, Green refined the formula, added cocoa butter and jasmine to make it more appealing, and sold it commercially under the name Coppertone.
Photo: Benjamin Green, via yt3.googleusercontent.com
For the next few decades, sunscreen existed in a kind of regulatory gray zone. Products made claims about sun protection, but there was no standardized way to measure or compare them. One brand's 'high protection' might mean something completely different from another's. Consumers had no reliable way to know what they were actually buying.
The SPF System Was Supposed to Fix That
In 1974, a scientist named Franz Greiter — who had actually been developing sun protection formulas since the 1940s — proposed a standardized measurement he called the Sun Protection Factor, or SPF. The FDA eventually adopted a version of this framework, and by the early 1980s, SPF numbers were appearing on sunscreen labels across the United States.
Photo: Franz Greiter, via 0901.static.prezi.com
The idea was straightforward: give consumers a number they could compare across products. A higher number means more protection. Simple enough.
Except the way SPF is calculated doesn't match the way most people intuitively read those numbers — and that gap has caused decades of confusion.
What SPF Actually Measures
Here's what the number actually means. SPF measures how much longer you can stay in the sun before experiencing sunburn compared to wearing no protection at all. It's specifically measuring protection against UVB rays, which are the ones primarily responsible for burning.
SPF 30 means that, theoretically, it would take you 30 times longer to burn than if you had nothing on your skin. If unprotected skin starts to burn after 10 minutes in the sun, SPF 30 would theoretically extend that to 300 minutes — about five hours.
But here's where the math gets counterintuitive. The protection isn't linear. SPF 15 blocks about 93% of UVB rays. SPF 30 blocks about 97%. SPF 50 blocks about 98%. SPF 100 blocks about 99%.
Read that again. SPF 100 does not give you twice the protection of SPF 50. The gap between SPF 50 and SPF 100 is approximately one additional percentage point of UVB blockage. Meanwhile, most consumers assume the numbers work like a simple scale — that SPF 100 is literally twice as protective as SPF 50 — which is why higher-SPF products sell so well and command premium prices.
The FDA's Long Frustration With Sunscreen Marketing
Dermatologists and the FDA have been trying to correct this misunderstanding for years, with limited success.
In 2011, the FDA introduced new regulations requiring sunscreen labels to be clearer about what they do and don't protect against. One of the more significant changes was around the term 'broad spectrum' — which indicates that a product protects against both UVB and UVA rays. (The SPF number only measures UVB protection; UVA rays, which penetrate more deeply into the skin and contribute to aging and certain skin cancers, had long been underemphasized on labels.)
The FDA also proposed capping SPF claims at '50+' on labels, arguing that numbers above 50 gave consumers a misleading impression of dramatically superior protection. That proposal met significant pushback from the sunscreen industry and has remained in regulatory limbo. In the meantime, you can still walk into any drugstore and find SPF 100 products marketed with language implying extraordinary protection.
The American Academy of Dermatology's official guidance recommends SPF 30 or higher as sufficient for most people — not because higher isn't marginally better, but because the practical difference is small, and no number compensates for applying too little or not reapplying often enough.
The Reapplication Problem No Number Fixes
This is the part of the sunscreen conversation that gets the least attention. SPF ratings are calculated under laboratory conditions, applied at a specific thickness — about a teaspoon for the face alone, which most people dramatically underapply in real life. Studies consistently show that people apply roughly 25 to 50 percent of the recommended amount, which means the effective SPF they're getting is significantly lower than what's on the label.
And then there's water, sweat, and time. Even water-resistant sunscreens degrade with exposure, which is why the recommendation to reapply every two hours exists. Most people don't do it.
The uncomfortable truth is that the difference between SPF 50 and SPF 100 matters far less than whether you're applying enough and reapplying consistently. A well-applied SPF 30 outperforms a poorly applied SPF 100 every time.
The Takeaway
Sunscreen went from a wartime improvisation to a multibillion-dollar industry, and the SPF system that was designed to bring clarity to consumer choices has mostly added confusion. The numbers aren't meaningless — but they work on a curve, not a straight line, and the gap between SPF 50 and SPF 100 is a fraction of what most people assume. What actually determines how well you're protected has less to do with the number on the bottle and more to do with how much you use and how often you reapply. That's the real story behind the number you've been reading your whole life.
