Physics trivia

# Sun Protection Factor (SPF)

The Sun Protection Factor (SPF) is an index that establishes the maximum time of exposure to the Sun when the skin is protected by some type of sunscreen.

The Sun Protection Factor (SPF) is an index that determines the time of exposure to the sun’s rays , with the skin properly protected, without erythema, that is, the redness generated by the dilation of blood vessels .

Imagine that the time of exposure to the Sun capable of generating erythema in an individual with unprotected skin is five minutes. With the use of an SPF 20 sunscreen, this time will be twenty times longer, so the individual can remain exposed to the sun’s rays for up to 100 minutes without suffering damage.

The SPF is calculated from the Minimum Erythema Dose (DME), which is the minimum time of exposure to sunlight capable of generating redness in the skin. The ratio of DME to skin protected by the DME of unprotected skin provides the Sun Protection Factor (SPF) for a protective substance.

It is worth mentioning that the DME values ​​for unprotected skin depend on the type of skin exposed to radiation. Thus, the darker the skin, the longer the time of exposure to sunlight without the occurrence of erythema.

When exposing itself to solar radiation, the skin will be hit by ultraviolet rays , types of high frequency electromagnetic wave . Ultraviolet radiations are divided into three groups, according to their physical characteristics and ability to penetrate human skin.

1. UVA: Type of ultraviolet radiation witha wavelengthbetween 315 nm and 400 nm (1 nm = 1 x 10– 9m). This radiation completely crosses theozone layerand, upon reachinghuman skin, penetrates even the deepest layers.
2. UVB: Type of ultraviolet radiation with a wavelength between 280 nm and 315 nm. This radiation is partially absorbed by the ozone layer and does not reach the deeper layers of the skin, but it can cause erythema.
3. UVC: Type of ultraviolet radiation with a wavelength between 100 nm and 280 nm. This radiation is completely absorbed by the ozone layer.