The sun's UV rays are part of the solar electromagnetic spectrum which cover a broad wavelength range. The shorter the wavelength, the greater the radiation energy and the capability to produce chemical and biological reactions.
Solar UV radiation is a relatively small part of the spectrum and may be further divided into three bands: UV-C (100-280 nm), UV-B (280-315 nm), and UV-A (315-400 nm). Different groups of scientists apply different ranges for each band.
Most UV-C is absorbed by ozone in our atmosphere and only very little reaches us at the Earth's surface. UV-B and UV-A are the radiation bands that we have to be concerned about because they do reach the Earth's surface, and they have the potential to do us harm.
Solar UV radiation comes from the sun. Other ways to be exposed to UV radiation are by using tanning beds and some types of welding equipment.
For humans, exposure to solar UV radiation has both positive and negative effects. Just the right amount of sun exposure helps our bodies produce sufficient vitamin D to maintain healthy bones, to provide us with a feeling of well-being and to help us fight against certain diseases.
Too little solar UV radiation exposure means our bodies might not be able to produce enough vitamin D which sometimes leads to developing rickets and osteoporosis.
Our biggest concern in South Africa is when we "get too much sun" - the harmful effects of excess solar UV radiation exposure affect our skin, eyes and immune system.
Sunburn and skin cancer are probably the two most commonly experienced adverse effects of too much sun exposure. Sunburn is described as a photo-injury. While we cannot see or feel solar UV radiation, we can see the effects it has when it reaches our skin - blood rushes to the damaged area and makes our skin feel warm and look red.
The relationship between sun exposure and skin cancer is more complex. Non-melanoma skin cancers, such as squamous cell carcinoma and basal cell carcinoma, are seldom fatal; however, they may be extremely disfiguring and cause pain. Melanoma skin cancers are fatal, in other words, if they are not detected early enough and treated accordingly, they can cause death.
Photo-ageing is another consequence from spending too much time in the sun without adequate sun protection. Wrinkling, fine lines, dryness and discolouration are some of the irreversible effects you face as you get older if you have spent too much time outdoors.
The 'skin you're in' provides you with some protection against sunburn and skin cancer. This depends on how much naturally occurring pigment or melanin you have. The darker your skin, the more melanin you have and the greater your natural protection against the sun's potentially damaging UV radiation.