This leads to frameshifting during genetic replication and protein synthesis, usually killing the cell.

UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. Gas leaks, water leaks, urine stains, authenticating identification documents, and cash all benefit from its use. Vacuum UV, or VUV, wavelengths (shorter than 200nm) are strongly absorbed by molecular oxygen in the air, though the longer wavelengths around 150–200nm can propagate through nitrogen. In ultraviolet astronomy, measurements are used to discern the chemical composition of the interstellar medium, and the temperature and composition of stars. Ultraviolet lasers have applications in industry ( laser engraving), medicine ( dermatology, and keratectomy), chemistry ( MALDI), free-air secure communications, computing ( optical storage), and manufacture of integrated circuits.Other black lights use plain glass instead of the more expensive Wood's glass, so they appear light-blue to the eye when operating. Many insects use the ultraviolet wavelength emissions from celestial objects as references for flight navigation. UVB causes thymine base pairs next to each other in genetic sequences to bond together into thymine dimers, a disruption in the strand that reproductive enzymes cannot copy.

For humans, suntan and sunburn are familiar effects of exposure of the skin to UV light, along with an increased risk of skin cancer.Mercury-vapor black lights in ratings up to 1kW with UV-emitting phosphor and an envelope of Wood's glass are used for theatrical and concert displays. The big problem with mass produced UV flashlights is that they're just too underpowered, and with a bad quality UV beam things won't show up. The WHO-standard ultraviolet index is a widely publicized measurement of total strength of UV wavelengths that cause sunburn on human skin, by weighting UV exposure for action spectrum effects at a given time and location. A suntan occurs when melanin pigments in cells in the deeper tissue portion of the skin are activated by ultraviolet radiation, and the cells migrate to the surface of the skin.