SPF stands for Sun Protection Factor. The number is determined experimentally indoors by exposing human subjects to a light spectrum meant to mimic noontime sun. Some subjects wear sunscreen and others do not. The amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in unprotected skin is the SPF. It is mainly a measure of UVB protection and ranges from 1 to 45 or above.
A sunscreen with an SPF of 15 filters 92% of the UVB. Put another way, a sunscreen with an SPF of 15 will delay the onset of a sunburn in a person who would otherwise burn in 10 minutes to burn in 150 minutes. The SPF 15 sunscreen allows a person to stay out in the sun 15 times longer.
There is currently no uniform measure of UVA absorption. There are broad-spectrum sunscreens that protect against UVA and UVB radiation although it is important to remember that the SPF does not predict UVA protection. The UV Index is a public health education tool reported by meteorologists in 58 U.S. cities. It offers a daily report of UV light levels on a scale from 1-10+.
The "protectiveness" of clothing can also be measured by SPF. The following are SPF's of various types of clothing:
- Nylon Stockings - SPF 2
- Hats - SPF 3-6
- Summer-weight clothing - SPF 6.5
- Sun-protective clothing - up to SPF 30