Many people don’t understand SPF numbers on product labels, and some may not be using sunscreen properly, which could increase their skin cancer risk, the American Academy of Dermatology reports.

In a 2016 survey, only 32 percent of respondents knew that an SPF 30 sunscreen does not provide twice as much protection as an SPF 15 sunscreen. Moreover, only 45 percent knew that a higher-SPF sunscreen does not protect you from the sun longer than a lower-SPF sunscreen.

And while 85 percent of participants in the AAD survey did know that sunscreen needs to be reapplied after swimming, a new study from the Johns Hopkins University School of Medicine in Baltimore, published in the Journal of the American Academy of Dermatology, suggests that some people may not be using sunscreen correctly.

In the study of 758 people with a history of nonmelanoma skin cancer (NMSC) and 34,161 control subjects, those individuals with a history of NMSC were more likely to seek shade, wear protective clothing and apply sunscreen, but they still received sunburns as often as those without a history of NMSC. While seeking shade and wearing protective clothing were associated with lower odds of sunburn, sunscreen use was not, the study showed.

“While it makes sense that people with a history of skin cancer were more likely to practice sun protection, we were surprised to see that their methods were not always effective,” says dermatologist Anna L. Chien, MD, FAAD, one of the study’s co-authors, in a news release. “Our results reinforce the importance of everyone using multiple types of sun protection; people who rely only on sunscreen may not be applying enough, covering all their exposed skin or reapplying often enough to shield themselves from the sun’s  harmful UV rays.”