Ultraviolet Radiation: UVA, UVB, UVC and What Reaches the Ground
Ultraviolet light spans 100 to 400 nanometers and is divided into UVA, UVB, and UVC. The ozone layer blocks all UVC and most UVB, so more than 95% of the UV reaching Earth's surface is UVA. UV has both beneficial effects (vitamin D synthesis) and harmful ones (sunburn, eye damage, skin cancer).
Ultraviolet radiation (UV) is electromagnetic radiation with wavelengths from about 100 to 400 nanometers, shorter than visible light but longer than X-rays. It is conventionally split into three bands by wavelength: - **UVA** (315 to 400 nm, long-wave): penetrates deepest, reaches the retina, drives skin aging. - **UVB** (280 to 315 nm, medium-wave): the band most responsible for sunburn, photokeratitis, and cataract formation; also triggers vitamin D synthesis in skin. - **UVC** (100 to 280 nm, short-wave): the most energetic and biologically damaging, but absorbed almost entirely by the atmosphere. The Sun emits all three, but Earth's ozone layer blocks essentially all UVC and most UVB. As a result, more than 95% of the UV that reaches the ground is UVA, with a small remainder of UVB. UV makes up roughly 10% of total solar output. Artificial UV sources include welding arcs, mercury-vapor and germicidal lamps, and tanning beds, all of which can deliver intense UV at close range. UV exposure has a dual nature. UVB stimulates the skin to produce vitamin D, a clear benefit in moderation. But excess exposure causes sunburn, accelerates skin aging through collagen damage, raises skin-cancer risk, and damages the eyes (see UV Eye Damage: Why Sunglasses Are Medical Equipment, Not Just an Accessory). Surface UV intensity is not constant: it rises with altitude (roughly 4% per 300 meters), peaks near the equator and at solar noon, and is amplified by reflective surfaces such as snow and water. The UV index is the standard public scale for forecasting this risk. UV's energy is also exploited deliberately, for example in UV Flashlights for Fluorescent Mineral Collecting: Wavelength Guide and germicidal disinfection.