UV-B

What does UV-B mean?

UV-B is the band of ultraviolet light that skin can use to make vitamin D. It spans roughly 290 to 320 nanometres. Visible light cannot start that process, and neither can the warmth you feel in sunlight. UV-B also does not pass through ordinary window glass, so sitting beside a bright, warm window produces no vitamin D in skin. That is the useful distinction: the room may look flooded with daylight while the relevant ultraviolet band stops at the glass. Outdoors, whether UV-B reaches the ground depends strongly on how high the sun is, because a low sun sends those wavelengths through a longer path in the atmosphere. That is why the band is strongest around solar noon and weaker toward either end of the day. This is a mechanism, not a dose. Ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer, and sunscreen used correctly still allows vitamin D synthesis while preventing burning, so it is never something to skip for vitamin D. No forecast can tell you how much vitamin D your skin will make.

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