There are three categories of UV radiation:
UV-A, between 320 and 400 nm
UV-B, between 280 and 320 nm
UV-C, between 200 and 280 nm

Generally, the shorter the wavelength, the more biologically damaging UV radiation can be if it reaches the Earth in sufficient quantities. UV-A is the least damaging (longest wavelength) form of UV radiation and reaches the Earth in greatest quantity. Most UV-A rays pass right through the ozone layer in the stratosphere. UV-B radiation can be very harmful. Fortunately, most of the Sun's UV-B radiation is absorbed by ozone in the stratosphere. UV-C radiation is potentially the most damaging because it is very energetic. Fortunately, all UV-C is absorbed by oxygen and ozone in the stratosphere and never reaches the Earth's surface.

Although the ozone layer is the one constant defence against UV penetration, several other factors can effect the amount of UV radiation arriving at the Earth's surface. The Sun�s rays are the most intense near the equator where they impact the Earth�s surface at the most direct angle. At higher latitudes, the Sun is lower in the sky during daylight hours and the strength of radiation is weaker. Furthermore, during the winter months, the Sun's rays strike at a more oblique angle than they do in the summer. This means that all solar radiation travels a longer path through the atmosphere to reach the Earth, and is therefore less intense. Daily changes in the angle of the Sun influence the amount of UV radiation that passes through the atmosphere. When the Sun is low in the sky, its rays must travel a greater distance through the atmosphere and may be scattered and absorbed by water vapour and other atmospheric components. The greatest amount of UV reaches the Earth around midday when the Sun is at its highest point.