ABSTRACT
Sunlight is composed of a continuous spectrum of electromagnetic radiation that
is divided into three main regions of wavelengths (Fig. 1): ultraviolet (UV),
visible, and infrared. UV radiation comprises the wavelengths from 200 to
400 nm, the span of wavelengths just shorter than those of visible light (400-
700 nm). UV radiation is further divided into three sections, each of which has
distinct biological effects: UVA (320-400 nm), UVB (280-320 nm), and
UVC (200-280 nm). UVC is effectively blocked from reaching the earth’s
surface by the stratospheric ozone layer. UVA and UVB radiation both reach
the earth’s surface in amounts sufficient to have important biological conse-
quences to the skin and eyes. Wavelengths in the UVB region of the solar spec-
trum are absorbed into the skin, producing erythema, burns, and, eventually, skin
cancer. Although UVA is the predominant component of solar UV radiation to
which we are exposed, it is supposed to be weakly carcinogenic and to cause
aging and wrinkling of the skin. In addition, the important role of UVA radiation
in the induction of systemic immunosuppression is becoming a major concern.
