Most people know that they should apply sunscreen when they are outdoors, but many do not apply it properly, leaving themselves at risk for developing skin cancer, a new study shows.

Researchers from Johns Hopkins University of Medicine wanted to learn if people, particularly those who had dealt with non-melanoma skin cancer, knew how to get the best protection from their sunscreen.

Non-melanoma skin cancer is the most common form of cancer, with more than 3 million people diagnosed per year.  Although this type of cancer is not as deadly as melanoma, which spreads within the body, a small number of people do die from it each year, and it can be disfiguring.

Their study, published in the Journal of the American Academy of Dermatology, found that even people who have been treated for this type of skin cancer do not know how to properly understand the SPF designation on sunscreen labels.

They queried 758 people with a history of non-melanoma skin cancer and compared their answers with those of 34,161 people who acted as controls.  They found that, although 85 percent of the respondents knew that sunscreen should be reapplied after swimming, a large number of people were confused as to what the SPF designations on sunscreen labels mean.

For example, only 32 percent of respondents knew that an SPF 30 sunscreen does not provide twice as much protection as an SPF sunscreen, and only 45 percent knew that a higher-SPF sunscreen does not protect you from the sun longer than a lower-SPF sunscreen, the researchers aid.

“It’s important that everyone understands what they are seeing on a sunscreen label,” says Dr. Abel Torres, president of the American Academy of Dermatology (AAD). A sunscreen with an SPF of 30 blocks up to 97 percent of the sun’s rays. Higher SPFs block slightly more rays, but a higher-number SPF does not allow you to spend more time outdoors without reapplication; all sunscreens should be reapplied every two hours, or after swimming or sweating.”

The AAD recommends that everyone protect themselves from the sun by seeking shade; wearing protective clothing, such as a long-sleeved shirt, pants, a wide- brimmed hat and sunglasses; and using a broad-spectrum, water-resistant sunscreen with an SPF of 30 or higher, applying enough to cover all exposed skin — for most adults, this is about 1 ounce, or enough to fill a shot glass. Also, sunscreen should be applied 15 minutes before sun exposure and reapplied every two hours, or after swimming or sweating, the organization says.