Exposure of 10 to 15 Minutes Adequate for Vitamin D Production
As winter wanes and sunny days return, it’s a great shift from being cooped up at home to basking in the sun. Before going outside, however, it’s better to be safe than sorry from the effects of UV light exposure that will follow. Here are some facts about UV light.
The sun is a natural source of UV radiation, while artificial sources include tanning beds; mercury vapor lighting (as in stadiums and school gyms); some halogen, fluorescent, and incandescent lights; and some types of lasers.
UV radiation is categorized into ultraviolet A (UV-A) from 315 to 400 nanometers (nm), ultraviolet B (UV-B) from 280 to 315 nm, and ultraviolet C (UV-C) from 100 to 280 nm. UV-A is not absorbed by the ozone layer and atmosphere, while UV-B is mostly absorbed, and UV-C is completely absorbed.
Overexposure to UV-A is linked to cataracts, skin cancer, and retinal burns, while overexposure to UV-B and UV-C are linked to corneal injuries, photokeratitis (sunburned eyes), erythema (sunburn), and skin cancer. In the case of photokeratitis, contact lenses on one’s eyes should be removed immediately.
The amount of exposure to UV-B for adequate vitamin D depends on skin exposure, time of day, season, and latitude of one’s location. In a 2019 Nature study, for example, Swiss researchers determined that 10 to 15 minutes of sunlight exposure with 22% of uncovered skin in adults was sufficient for 1,000 IU of vitamin D production in summer and spring. However, this increased to 6.5 hours with 8% to 10% of uncovered skin in autumn and winter, which is unachievable without sunburn risks.
According to the International Ultraviolet Association, plain window glass allows UV-A to pass through but almost completely blocks UV-B and UV-C light below 330 nm. This means that going outside to take in the sunlight is best way to take in UV-B for vitamin D production.
The World Health Organization has a UV index (a measurement of the level of UV radiation) based on the following categories: low (1 and 2), moderate (3, 4, and 5), high (6 and 7), very high (8, 9, 10), and extreme (11 or more). A UV index of 0 to 2 is considered safe for being outside without sun protection. Sunscreen is recommended for a UV index of 3 to 7, and avoiding the outdoor midday sun is recommended for very high (8 and above) categories.
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