Sun protection factor (SPF) is a number on sunscreen labels that indicates how long skin can be in the sun and maintain a low risk for sunburn. The higher the SPF number, the longer it protects a person from burning rays.

For example, an SPF of 8 protects twice as long as an SPF of 4. Most sunburns can be prevented by using a sunscreen with an SPF of 15.

The SPF rating applies only to protection from ultraviolet B (UVB) rays. Some sunscreens protect against UVA rays, but they have no SPF rating. No sunscreen gives total protection, but “broad-spectrum” sunscreens that contain ingredients such as avobenzone, benzophenones, cinnamates, salicylates, sulisobenzone, titanium dioxide, and zinc oxide usually protect from UVA and UVB rays.

Sweating heavily, swimming, or doing other water activities reduces the SPF because sweat or water on the skin will reduce the amount of protection the sunscreen provides. Sunscreen needs to be reapplied more frequently during these activities.