Sun protection factor
Sunscreens are usually rated with a sun protection factor (SPF). SPF measures the fraction of sunburn-producing UV rays that reach the skin. For example, “SPF 15” means that 1⁄15 of the burning radiation reaches the skin through the recommended thickness of sunscreen. Sunscreens are designed to remain effective at original strength for up to three years. UPF (ultraviolet protection factor) is a similar scale developed for rating fabrics for sun protective clothing. UPF ~30+ is typical for protective fabrics, while UPF ~20 is typical for standard summer fabrics. Mathematically, the SPF (or the UPF) is calculated from measured data as
where E(λ) is the solar irradiance spectrum, A (λ) the erythemal action spectrum and MPF (λ) the monochromatic protection factor, all functions of the wavelength (λ) . The MPF is roughly the inverse of the transmittance at a given wavelength.
Everyday usage of sunscreen doesn’t lead to vitamin D insufficiency
People who use sunscreen daily can maintain their vitamin D levels. High-SPF sunscreens are designed to filter out most of the sun’s UVB radiation, since UVB damage is the major cause of sunburn and can lead to skin cancers. UVB wavelengths happen to be the specific wavelengths that trigger vitamin D production in the skin. Nonetheless, clinical studies have never found that everyday sunscreen use leads to vitamin D insufficiency. In fact, the prevailing studies show that people who use sunscreen daily can maintain their vitamin D level.
One of the explanations for this may be that no matter how much sunscreen you use or how high the SPF, some of the sun’s UV rays reach your skin. An SPF 15 sunscreen filters out 93 percent of UVB rays, SPF 30 keeps out 97 percent, and SPF 50 filters out 98 percent. This leaves anywhere from 2 to 7 percent of solar UVB reaching your skin, even with high-SPF sunscreens. And that’s if you use them perfectly.
So, Go outside and get some sunshine, but after wearing sunscreen!
