**1.1 UV irradiation**

The spectrum of solar electromagnetic radiation striking the earth's atmosphere ranges from 100 nm to 1 mm. This includes the UV spectrum (100-400 nm), visible spectrum (380-780 nm) and infrared spectrum (700 nm-1 mm). The UV spectrum is further subdivided into three catogories: UV-C (100-280 nm), UV-B (280-315 nm), and UV-A (315-400 nm) (Ballaré, 2003). The shortest of these wavelengths, UV-C, is blocked completely by the ozone layer and atmospheric oxygen. In contrast, UV-A is weakly absorbed and directly transmitted to the earth's surface. Wavelengths in the UV-B range are absorbed efficiently though not completely by ozone, as a very small percentage may pass through holes in the ozone. UV-C is extremely harmful, followed by UV-B, while UV-A has milder effects (Batschauer et al., 1999).
