Ultraviolet radiation is part of the sun's rays that reach the earth. About ten percent of that, roughly. A distinction is made between UVA, UVB and UVC radiation. The latter is the most energetic, but for us the least relevant radiation, as it is mostly filtered by the atmosphere and the oxygen in the air. The UVB rays are short-wave and only penetrate the upper layers of the skin. It is they who cause sunburn.

The ozone layer filters most of these rays, but a lot still gets through. The UVB rays accelerate skin aging and can cause skin cancer.

The long-wave UVA rays penetrate the deeper layers of our skin. They are responsible for tanning and also cause our skin to age. For a long time it was believed that they had no negative effects. However, more recent studies suggest that they too have a major influence on whether we develop skin cancer.