The three types of UV radiation are classified according to their wavelength. They differ in their biological activity and the extent to which they can penetrate the skin. The shorter the wavelength, the more harmful the UV radiation. However, shorter wavelength UV radiation is less able to penetrate the skin.
The UV radiation spectrum is invisible & divided into three regions called UVA, UVB and UVC. UV-A 320 to 400 nm, UV-B 280 to 320 nm, UV-C 100 to 280 nm.
The UV-C band contains the wavelengths (250-270nm) which are effective in destroying many microorganisms (optimum wavelength is 254nm). This range is also called the germicidal range and primarily has a surface effect.
As sunlight passes through the atmosphere, all UVC and most UVB is absorbed by ozone, water vapour, oxygen and carbon dioxide. UVA is not filtered as significantly by the atmosphere.
Short-wavelength UVC is the most damaging type of UV radiation. However, it is completely filtered by the atmosphere and does not reach the earth’s surface.
Medium-wavelength UVB is very biologically active but cannot penetrate beyond the superficial skin layers. It is responsible for delayed tanning and burning; in addition to these short-term effects, it leads to skin ageing and significantly promotes the development of skin cancer. Most solar UVB is filtered by the atmosphere.
The relatively long-wavelength UVA accounts for approximately 95 percent of the UV radiation reaching the Earth’s surface. It can penetrate into the deeper layers of the skin and is responsible for the immediate tanning effect. Furthermore, it also contributes to skin ageing and wrinkling. For a long time, it was thought that UVA could not cause any lasting damage. Recent studies strongly suggest that it may also enhance the development of skin cancers