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What is SPF?
SPF stands for Sun Protection Factor. It is determined experimentally indoors by exposing human beings to a light spectrum which mimics the afternoon sun. Some subjects wear sunscreen and others do not.
The number of the SPF represents the level of sunburn protection provided by the sunscreen. 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. The protection is not proportional to SPF factor. While SPF 15 will give about 93% protection, SPF 30 will give 97% protection.
A sunscreen with SPF 15 filters 92% of the UVB. In other words, a sunscreen with SPF 15 will delay 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 and so on.
UVA absorption has no uniform measure currently. There are broad-spectrum sunscreens that protect against UVA and UVB radiation. It is important to remember that the SPF does not predict UVA protection. The US FDA is currently reviewing ways to measure how much protection, sunscreen products provide in the UVA range.
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