Ultraviolet B (UVB) with wavelengths between 290 and 320 nm, comprising less than 1 per cent of the ultraviolet radiation that reaches the earth's surface. Ultraviolet B causes sunburn and a number of damaging photochemical changes within cells, including damage to DNA, leading to premature aging of the skin and malignant changes, and a variety of photosensitivity reactions; it is also used therapeutically for treatment of skin disorders.