These days, many people incorporate sunblock into their everyday skin care routines (public awareness around skin cancer prevention has gotten pretty good, after all). But are we wearing enough of it? Or, alternatively, is it giving us all cancer, as some rumors would suggest?

To set the record straight on the effectiveness of SPF, we turned to the experts and asked them to dispel some myths.

Here’s how sunscreen works.

Sunlight produces two types of UV rays that we have to protect against: UVA, which penetrates to the middle layer of the skin, and UVB, which reaches the outer layer of the skin.

There are two types of sunscreens. Chemical sunscreens include active ingredients such as oxybenzone, avobenzone, octisalate, octocrylene, homosalate, and octinoxate, and work by absorbing UV rays’ chance of penetrating the skin and changing UV energy into imperceptible heat, says Jackie Dosal, M.D., a dermatologist at Skin Associates of South Florida and the University of Miami.

Physical (or mineral) sunscreens, on the other hand, block UV rays from penetrating the skin. These include active ingredients such as zinc oxide or titanium dioxide.

What exactly do those SPF numbers mean, anyway?

“The sun protection factor [SPF] is a measure of the fraction of sunburn-producing UV rays that reach the skin,” says Laura Ferris, M.D., Ph.D., associate professor of dermatology at the University of Pittsburgh. “For example, SPF 15 means that one-fifteenth of the burning radiation will reach the skin.”

A certain SPF number won’t work the same for everyone—it depends on how likely you are to burn. If your skin begins to burn after 10 minutes in full sun without any protection, a sunscreen of SPF 15 would provide 15 times the protection of no sunscreen.

SPF only indicates how much protection you’re getting from UVB rays, since they’re what cause you to burn. The only way to know a sunscreen also protects against UVA rays is if the label says “broad spectrum,” which means it protects against both.

How much sunscreen is enough sunscreen?

Studies show most people don’t apply enough sunscreen to achieve the full SPF of the product they’re using. You need at least a shot glass full of sunscreen for the whole body, Ferris says, following a recommendation by the American Academy of Dermatology. Sunscreen should be applied liberally, all over exposed skin.