Sunscreen is important for protecting against serious conditions such as skin cancer, but a new study suggests many people are confused by the terminology on product labels.

In a recent survey conducted by Northwestern University, only 43 percent of participants understood the definition of sun factor protection (SPF), and only 7 percent knew what to look for on labels if they wished to protect their skin against early aging.

"We need to do a better job of educating people about sun protection and make it easier for them to understand labels," said Roopal Kundu, lead author of the study.

Sunscreens with SPF work to protect the skin from ultraviolet B (UV-B) rays, which are the primary cause of sunburns, but both ultraviolet A (UV-A) and UV-B  have been linked to premature skin aging and cancer.

"We recommend you buy a sunscreen lotion labeled 'broad spectrum protection' -- which helps to protect against both types of UV rays -- with an SPF of 30 or higher that is also water resistant," Kundu said. "SPF 30 blocks 97 percent of the UVB radiation. But, you need to reapply it every two hours, using about a shot glass full of lotion over your exposed skin, for the best results."

To make their findings, the researchers surveyed 114 participants who attended the Northwestern Medicine dermatology clinic. About 80 percent of the survey participants had purchased sunscreen in 2013, and 75 percent said the main reason they applied sunscreen was to prevent sunburn. Almost 66 percent cited the prevention of skin cancer as a major reason for using sunscreen.

The three top factors influencing their decision to purchase a particular sunscreen were: " highest SPF value, sensitive skin formulation and water and sweat resistance."

"Just because you buy SPF 100 doesn't mean you are 100 percent protected," Kundu said. "Staying out of the sun is the only way to guarantee 100 percent protection."

The participants were also shown images of the front and back of a common sunscreen with an SPF of 30, and many proved to have trouble deciphering the terminology they saw. Only about 38 percent of the participants correctly identified terminology associated with skin cancer prevention; 23 percent correctly identified how efficiently the sunscreen would protect against sunburn; and 7 percent identified how well the product protected against premature skin aging. Almost 80 percent were able to determine the level of UV-A protection and close to 90 percent could determine UV-B protection, suggesting this could be a more effective labeling method.

"A lot of people seem unsure about the definition of SPF, too," Kundu said. "Only 43 percent 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."

The findings were reported in a recent edition of the journal JAMA Dermatology.