Skin cancer is the most common form of cancer, and largely benign in most instances – but it can still be deadly. That’s why millions of people slather on sunscreen, that gloppy, foul-smelling wonder cream that magically protects us from the Sun’s harmful rays. That so many people use sunscreen is good, but a study headed by Northwestern University found that no one seems to understand what things like SPF and UV protection actually mean.

“Despite the recent changes in labeling mandated by the U.S. Food and Drug Administration, this survey study suggests that the terminology on sunscreen labels may still be confusing to consumers,” Dr. Roopal Kundu of Northwestern University Feinberg School of Medicine (and colleagues) wrote.

The changes to which the authors refer concern labeling regarding UV-A and UV-B protection, which the FDA mandated in 2011. UV-A rays contribute to aging, while UV-B rays are associated with sunburns. Both, however, contribute to skin cancer, making so-called “broad-spectrum” protection important.

In the survey of 114 patients at a dermatology clinic, the researchers found that 93% had purchased sunscreen since 2013. most (75%) said they did so to prevent sunburns, and two-thirds said they were aiming to prevent skin cancer. SPF was the driving factor for purchase decisions, although only 43% actually understood the meaning of SPF.

Speaking with skin cancer experts, the Chicago Tribune found that even most physicians underestimate the importance of sunscreen. Many people still believe that sunscreen is for the beach or any prolonged outdoor activity. In reality, sunscreen should be worn at all times, as sun exposure is cumulative.

When selecting a sunscreen, experts say that sunbathers should look for an SPF of at least 30, broad-spectrum protection, and water/sweat resistance. An SPF of 30 means that if someone would normally burn after 10 minutes of exposure, the sunscreen will give them 300 minutes. Per FDA regulations, sunscreens labeled “water resistant” must maintain their SPF after 40 minutes in water.

And, yes, you can still get a tan while wearing sunscreen. Melanin enhancement is the skin’s natural reaction to sunlight exposure, while burning (and cancer) are the result of UV damage. What most people think of as initial suntans are really just mild sunburns.