Ultraviolet A is one of the three types of invisible light rays (together with ultraviolet B and ultraviolet C) given off by the sun. Although ultraviolet C is the most dangerous type of ultraviolet light in terms of its potential to harm life on earth, it cannot penetrate earth's protective ozone layer.
Therefore, it poses no threat to human, animal or plant life on earth. Ultraviolet A and ultraviolet B, on the other hand, do penetrate the ozone layer in attenuated form and reach the surface of the planet. Because ultraviolet A is weaker than ultraviolet B, scientists long blamed ultraviolet B as the sole culprit in causing skin cancer in persons with a history of sunburn and repeated overexposure to ultraviolet radiation. Recent research, however, has also implicated ultraviolet A as a possible cause of skin cancer. In addition to natural light from the sun, artificial light from tanning lamps contains ultraviolet A and ultraviolet B. Electric arc lamps can also generate ultraviolet light to heat furnaces for melting and to enable motion-picture projectors to show movies.