The earth is continuosly showered with solar radiation: the electromagnetic energy emitted by the sun. This energy is the source of all that we know and yet, like many other good things, too much can be harmful. Among the spectrum of radiation that the sun emits, Infrared, Visible and Ultraviolet radiation command most of our attention. Visible light is generally considered begnin and won't be discussed here.
Ultraviolet Radiation (UVR)
The solar spectrum is divided into various portions according to wavelenght. UVR covers from 200 nanometers (nm) to 400nm. UVR is, in turn, divided into UVC (200-
UVC, also known as GERMICIDAL UV, is very toxic. As this implies, it is lethal to many microorganism as well as to most plant life. In addition, it is carcinogenic to humans.Fortunately, virtually all UVC is filtered out by the ozone layer.
UVB makes up about 18% of the solar UV spectrum (prior to attenuation by the earth's atmosphere) but only about 1% of the UVR that gets to the earth's surface because it is largely filtered out by the ozone layer. Despite its relatively meager presence however UVB is associated with much of the damage caused to humans by sun exposure. Traditionally, UVB was credited as being the sole cause of sunburn and various skin cancers. Although still considered a major cause of skin cancers, UVB is no longer though to be acting alone. It now seems likely that other wavelengths (UVA) will be found to be involved in tumor formation, and perhaps, in some cases, even as the primary agent.
UVA makes up about 75% of the solar UV spectrum and about 99% of the earthly spectrum. This is because UVA is largely unaffected by the ozone layer: Much more abundant, UVA is also much less energetic than UVB and thus traditionally has been thought to be biologically less significant. UVA is certainly a causative factor in photo-
Infrared radiation (IR)
In descending wavelenght order IR (>700 nm) precedes visible light (400-