You can buy a product that is labeled as higher than SPF 30, but it’s almost always a waste, and potentially harmful. SPF 15 filters out about 93 percent of UV-B rays. SPF 30 filters out approximately 97 percent. SPF 50 filters out approximately 98 percent. SPF 100 might get you to 99. The problem, though, is the psychology of the larger number. Above 30, the difference is essentially meaningless, and higher SPFs do not last longer than lower SPFs — but they do tend to make people feel invincible. We put on the "more powerful" sunscreens and then suddenly think we're Batman or some other superhero who can stay out in the sun indefinitely. But no sunscreen is meant to facilitate prolonged exposure of bare skin to direct sunlight.

A recent report by the Environmental Working Group notes that several years ago the FDA deemed SPF labels above 50 to be “inherently misleading” but has not prohibited the practice, even while European and Japanese regulators do not allow products to be labeled above 50, and in Canada the maximum labeling possibility is 50+.

That’s not even accounting for the fact that SPF is an outdated term, in that it only refers to protection from UV-B rays, not UV-A, which don't cause sunburns but are no less dangerous in terms of skin cancer. I bet if the inventor of SPF, Austrian mountaineer and photobiologist Franz Greiter were alive today, he would say “I don’t regret what I did, because it made sense at the time. Heck, I invented wearable sunscreen. Give me some credit. But, I will say, knowing what we know now, there is clearly a more effective way to communicate information than the SPF system. We could use a more comprehensive, intuitive metric.”

UV-A coverage not only protects against DNA damage and cancer, but also protects against premature skin aging. The only way that UV-A coverage is communicated on a label is with the words broad spectrum. Unless a sunscreen says that, it’s almost sunscreen in name only. And there is no metric for just how much UV-A coverage is provided by a broad-spectrum product. The Northwestern dermatologists tested out a five-star rating system for UV-A coverage, and their patients took to it well. But given that the SPF system has been around for decades and most people still don’t understand it, there’s clearly benefit in keeping things simple.

"There's only so much sunscreen information that people can process," Kundu said when we spoke this week. “I emphasize broad-spectrum, SPF 30, and water resistance. And that, like any tool, you have to use it correctly.”

A good metaphor might be another tool, a hammer. You can't just throw a hammer at a nail and expect the nail to be driven into a nearby wall. That would be an amazing throw. No, you have to use the hammer carefully, in a hammering motion, again and again, over a nail carefully pinned between two fingers, thunk, thunk, until the nail is "in the wall." Sunscreen only works when reapplied every two hours, and after venturing into water, and in large quantities—“about a shot-glass worth,” Kundu recommends. But you probably also shouldn’t have a shot glass at the beach, because glass is usually not allowed.