While there are many artificial sources of ultraviolet light, there is only one important natural source: the sun. The ultraviolet light from the sun accounts for most of that type of radiation that we experience on Earth. Both necessary and dangerous to human health, ultraviolet light from its natural source is an important concern to the world's population.
Physicists define ultraviolet light as light whose waves fall within the range of 10 nanometres (nm) and 100 nm in wavelength. On the spectrum, this area corresponds to wavelengths shorter than visible light (whose wavelengths range between about 100 and 1000 nm). The name "ultraviolet" actually means "beyond violet."
There are three main types of ultraviolet radiation from the sun. UVA radiation has wavelengths between 315 and 400 nm, UVB radiation ranges from 280 to 315 nm and UVC radiation has the shortest wavelengths of between 100 and 280 nm.
The earth's atmosphere effectively blocks a large part of all of the ultraviolet light coming from the sun. The upper atmospheric gases ozone, water vapour, carbon dioxide and oxygen absorb almost all of the UVC and about 90 per cent of the UVB light before it can reach the earth's surface. Most UVA radiation can, however, pass through, with only about 10 per cent of light at those wavelengths stopped in the upper atmosphere.
Despite its bad reputation, ultraviolet light actually does have some beneficial health properties. Ultraviolet light from the sun is necessary for the production of vitamin D, which maintains bone health by promoting calcium production. Additionally, doctors consider ultraviolet light helpful in the treatment of such diseases as rickets, jaundice, psoriasis and eczema. Artificial sources of ultraviolet light can compensate partially for a lack of sunlight (a common problem in Arctic regions), but only the natural ultraviolet light of the sun meets all health requirements over the long term.
Overexposure to ultraviolet light can have many serious and dangerous health implications. The most well-known short-term effect of overexposure to ultraviolet light is sunburn. Long-term overexposure to the ultraviolet light of the sun has much more serious health effects. Premature ageing of the skin results from the degeneration of skin cells induced by ultraviolet radiation. Similar degenerative effects on the eyes can lead to cataracts over time. The most dangerous effect of ultraviolet radiation on the body is skin cancer, as the damage from ultraviolet light can lead to melanomas on the skin.