UV-B is expressed as a percentage. UV-B radiation is found in the wavelength range of 280 - 320 nm and is responsible for indirect pigmentation.
The role of UV-B radiation is to stimulate the pigment in skin cells (melanocytes) to produce melanin, or "dye", dimming later by UV-A. UV-B is responsible for the formation of erythema.
The UV-B/UV-A ratio of tanning lamps ranges from 0,7-3,3%. The higher the ratio, the more agressive the light, causing a higher chance of erythema.