Ultraviolet Radiation

Ultraviolet (UV) radiation is electromagnetic radiation with wavelengths shorter than visible light and longer than X-rays, roughly 10 to 400 nanometers. About a tenth of the Sun's output is UV, and its energetic photons can break chemical bonds, which makes it important for life, climate and health.

UVA (315 to 400 nanometers) reaches the ground in large amounts. UVB (280 to 315 nanometers) is partly absorbed, and UVC (100 to 280 nanometers) is absorbed completely by the atmosphere. The absorption is done mainly by oxygen and by the ozone layer, which is why ozone depletion raised concern about extra UVB reaching the surface.

UVB is absorbed by DNA, where it joins neighboring pyrimidine bases into abnormal links called cyclobutane dimers. Cells fix this damage with DNA repair pathways, but errors cause mutations that can lead to skin cancer. UVB is also needed for the skin to make vitamin D, so the effect is a balance of benefit and risk.

UVC lamps at 254 nanometers destroy microbes and are used to disinfect water, air and surfaces. UV also drives atmospheric chemistry, making ozone and radicals, and it is a part of the Sun's spectrum in the solar radiation that enters the Earth's energy budget.

• Ozone Layer — the atmospheric UV shield
• DNA Repair — how cells fix UV damage
• Solar Radiation — the source of UV at Earth