Sun protection factor (SPF) is a number on sunscreen labels that shows 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 the sun's burning rays.
It is important to read the information on the sunscreen label about the SPF factor listed on the label and how much protection it gives the skin. The sunscreen should be applied according to the directions on the label so it is most effective in protecting the skin from the sun's ultraviolet rays.
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 ultraviolet A and B (UVA and UVB) rays. The label of the sunscreen product will say what types of UV rays it protects the skin from.
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.
Medical Review:William H. Blahd Jr. MD, FACEP - Emergency Medicine & Adam Husney MD - Family Medicine