SPF - Sun Protection Factor

Definition:

SPF stands for Sun Protection Factor. It is a measure of UVB protection and ranges from 1 to 90.

How SPF is Determined

The actual SPF number is determined experimentally indoors by exposing people to a light spectrum similar to noontime sun. Some people wear sunscreen and others do not. The amount of light that causes redness in sunscreen-protected skin, divided by the amount of light that causes redness in unprotected skin is the SPF.

Practical SPF

Here is an easy way to remember what information SPF gives you (but not a suggestion to take sun exposure lightly):

A sunscreen with an SPF of X allows you to stay out in the sun X times longer without burning.

SPF - Only Part of the Picture

The SPF of a sunscreen only measures the UVB component. Another important component of a sunscreen is its ability to filter UVA radiation. A good broad-spectrum sunscreen will filter both UVA and UVB.