UV-A

where S and E are the ratios of the test over calibration source for the total signal measured and the actual UV-A light from the sources, respectively.

A second form of ultraviolet light reaches Earth's surface in lesser amounts than UV-A. Ultraviolet-B light (UV-B), with wavelengths of 290 to 320 nm, doesn't penetrate skin as deeply as UV-A light does, but it's more damaging to the cells that it reaches.

The tests, conducted at the FDA laboratory in Rockville by Victoria Hitchins, exposed the cells to pure UV-A, using only wavelengths in excess of 340 nanometers.