If I ever get skin cancer, I’ll blame it on the time I burned myself so badly I thought my face was going to peel off. Hiking high up on Mount Rainier, in Washington, where the snow reflects light onto one’s face from all directions and the frigid air keeps skin numb to the injury, I put on SPF 100 at dawn and didn’t think about it again. The next morning I was so inflamed it hurt to smile.

To understand why people like me make such stupid and consequential mistakes—one in five of us will get skin cancer, despite most cases being preventable—last year a trio of dermatologists at Northwestern University asked its patients to take a quiz about sunscreen. The doctors, who spend much of their careers sampling and extracting what they call “suspicious lesions” from people’s faces (among other places), needed to know why we’re not better at preventing cancers. Because even though cancer is often listed among the most terrifying words in the English language, and one of the most common types (skin cancer) can be prevented through the simple measure of avoiding excessive ultraviolet radiation, these cancer rates aren’t falling.

A big part of that discrepancy, the dermatologists found, seems to come down to misinformation about how to use sunscreen. The fact that the quiz takers were patients at a dermatology clinic means they cared at least somewhat about skin to begin with. Even still, they knew little about how to use the tool that keeps that skin from breaking down, wrinkling, and mutating. The doctors reported in JAMA Dermatology that fewer than half of people could define SPF (sun protection factor). Fewer still knew how to use the number.

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.

The FDA clarifies, too, that SPF is not directly related to time of solar exposure but to amount of solar exposure. On a bright, snowy day on Mount Rainier, you burn much faster than on a cloudy hike through Shenandoah. The agency cites a common misconception: “Many consumers believe that, if they normally get sunburn in one hour, then an SPF 15 sunscreen allows them to stay in the sun 15 hours (i.e., 15 times longer) without getting sunburn. This is not true ...”

Meanwhile, one of the Northwestern dermatologists who did the study about misconceptions expressed that exact misconception in a press release for the study, as an example of consumer ignorance: “Only 43 percent [of people’ understood that if you apply SPF 30 sunscreen to skin 15 minutes before going outdoors, you can stay outside 30 times longer without getting a sunburn.”

Even amid the misconceptions and unreliability, possibly the biggest downside to SPF is that it only refers to UV-B rays. It does not address UV-A rays, which are also carcinogenic. To make sure that you’re protected from UV-A, you have to use a sunscreen that’s labeled “broad spectrum.” For UV-A, there is no SPF system, only this binary indicator of presence or absence.

UV-A is also a major factor aging of the skin, as I discuss in this week’s episode of If Our Bodies Could Talk, in response to a reader question about preventing wrinkles as a behavioral motivator for sunscreen use (however perverse that might seem if you think about it too long).

James Hamblin responds to a reader question about how sunscreen prevents wrinkling of the skin.

None of this is meant to question from the potential value of sunscreen, used lavishly and frequently. I’ve suggested before that sunscreen might do well to be publicly available in parks and on beaches, as it has recently become in Miami Beach as several other areas. As cancer goes, using sunscreen is among the least technical approaches to winning the war, and it is not sexy enough to warrant mention in the moonshot, despite evidence that there is much ground to be gained here.

While many cancers are preventable, it is usually through a lifelong pattern of eating well and moving, deciding not to inhale smoke and to generally adopt patterns of moderation. For many people, these things are not straightforward or easy. One of the few simple methods of preventing cancer in one of its commonest forms is to block and avoid excessive exposure to sunlight. But relying so heavily on the SPF value system that can so readily be misleading or misunderstood—using it at all—could be doing more harm than good.