In addition to leaching into the marketing of cosmetics and clothing in recent years, SPF is the primary factor in people’s sunscreen purchasing behavior. According to the Northwestern study, SPF is even more important to consumers than the cost of a sunscreen product. All despite the fact that most don’t know what it really means.

A majority of patients at the clinic falsely believed that SPF 30 was twice as protective as SPF 15. That does seem like it should be true. But it’s nowhere close, according to the American Academy of Dermatology. SPF 30 is roughly four percent more protective than SPF 15—SPF 15 filters out around 93 percent of UV-B rays, and SPF 30 filters out around 97 percent.

That difference is not nothing, but it is so small that lower SPF values may even be more effective in the long run, once behavioral psychology comes into play. Because the real factors that determine how well a sunscreen works is how much we use and how often we reapply. When we feel very well protected, we’re less likely to reapply frequently and to seek shade. It’s the same effect that led researchers at the National Bureau of Economic Research to warn that the overall safety benefit of bike helmets can sometimes be mitigated if helmeted bikers, feeling protected, ride less cautiously.

That psychological paradox of high SPF numbers stands to get worse as the number grows. The marginal gains in buying a product labelled SPF 50 or 100 are tiny, as they block 98 and 99 percent of of UV-B rays, respectively.

And even these numbers may not be reliable, as the Environmental Working Group notes. Testing by Procter & Gamble has found that small differences in light conditions and sunscreen application techniques (especially thickness) have dramatic effects on the value of any particular product. The company warned in a 2011 letter to the FDA that the lab conditions in which sun-protecting products are tested and graded tend to be so different from the real world—and the many variables of actual usage of the product by humans—that the value the SPF system is “at best misleading to consumers” and that “SPF 50+” should be the highest allowable claim on any label. The Environmental Working Group contends that “manufacturers should stop selling high-SPF products altogether.”

Where the story of SPF gets weird to me, though, is when authorities on the matter contradict one another in consumer-facing information. The American Academy of Dermatology and the Northwestern dermatologists, among others, refer to SPF as “sun protection factor.” According to the FDA, it’s “sunburn protection factor”—which seems like a not-trivial distinction, as the agency’s definition is “a measure of how much solar energy (UV radiation) is required to produce sunburn on protected skin (i.e., in the presence of sunscreen) relative to the amount of solar energy required to produce sunburn on unprotected skin.” That’s at odds with American Academy of Dermatology’s explanation of SPF as a matter of UV protection percentages.