The sun and artificial sources produce ultraviolet radiation or UVR. It is classified as UVA, UVB and UVC. Many people consider that UVR damages the skin because its absorption leads to primary and secondary photochemical reactions. These reactions bring about changes in cell biochemistry and tissue physiology which can lead initially to inflammation responses (acute photo-damage), and later to the manifestations of photo-ageing, (chronic photo-damage), and sometimes skin cancer. Free radicals and reactive oxygen species (ROS), generated by ultraviolet radiation as a secondary reaction, can alter elastin and collagen fibres, leading to premature wrinkling. Moreover, DNA can be damaged by ROS and  can  lead to carcinogenesis.
- UVC (100 - 280nm) is the most damaging for the  skin. UVC in sunlight is absorbed by the ozone layer in the atmosphere and does not reach the surface of the earth.
- UVB (280 - 315nm) causes reddening (erythema) and burning of the skin. It is also responsible for the so-called delayed tanning reaction. It causes skin ageing, wrinkling, and thickening. UVB may also produce more serious effects such as non-melanoma skin cancer.
- UVA (315 - 400nm) can produce erythema and sunburn but to a lesser extent than UVB. It causes immediate tanning by activating the melanin pigment already present in the skin. UVA penetrates more deeply into the skin than UVB and contributes to its photo-ageing. There is currently no consensus on which type of ultraviolet causes melanoma although many suspect this to be ultraviolet A.
The sun is generally the main source of UV radiation exposure, but individuals can be exposed to ultraviolet radiation in industry, medical treatment, or when using sunbeds. Sunbeds emit mainly UVA radiation, which has in the past been promoted as the 'safe' UVR. However, there is a wide variety of emission spectra and newer lamps emit a higher proportion of UVB to accelerate tanning. The harmful effects of UVA are also increasingly recognized.
When ultraviolet radiation is used under supervision, its health benefits can outweigh its harmful effects. UVR, given as both narrowband and broadband UVB, and as PUVA (ultraviolet A with a photo-sensitizing psoralen) are in regular therapeutic use by dermatologists for the treatment of psoriasis and eczema.
When given by a dermatologist, the dose after each treatment is noted and a lifetime total is recorded as a safety measure. prolonged exposure to harmful radiation can increase the risk of skin damage and skin cancer. This careful dosing is not a feature of private sunbed use; doses are often unknown and/or not recorded, and little restriction is placed upon the user. This means that the user can carry on for a long time . This can cause damage from long term UV exposure and is associated with skin ageing and increased risk of skin cancer.
Sunbed use is popular with young people. The most common reason given for sunbed use is a desire to look better. The British Photodermatology Group (BPG) recommends that should people choose to use sunbeds, they should not exceed two courses a year of no more than 10 sessions each.