An Ultraviolet Protection Factor (UPF) shows how much solar ultraviolet radiation (UVR) is reduced when passing through fabric of clothing. It is similar to the sun protection factor (SPF) used for sunscreens. The higher the UPF value, the lower the UVR penetration and the better is the protection against sunburn. For example, a garment with UPF label of 20 will reduce the UVR reaching the covered skin to 1/20 of that reaching bare skin. Different wavelengths of radiation in the UVR spectrum have different effects on human skin and this is taken into consideration when calculating the UPF rating.
An UPF is assessed in the laboratory under controlled conditions using an optical instrument called a spectrophotometer, which provides a direct measurement of UVR penetration. A British Standard (BS EN 13758-1:2002) describes the test method for clothing fabrics; another (BS EN 13758-2: 2003) describes the classification and marking of sun-protective clothing. Only clothing with UPF of at least 40 can be labelled as solar UV-protective according to the standard. Factors that contribute to the UPF rating are as follows.
Ultraviolet radiation (UVR) is strongly linked to the induction of skin cancers, and probably to eye disorders, in particular cataract, and suppression of the body's immune system. We live in an environment where UVR is usually present, the main source of exposure coming from the sun. It is believed that any UVR exposure is associated with an increased individual risk of these health effects. Scientific studies cannot demonstrate that there is a completely safe level of UVR exposure. At work, guidelines are set for UVR exposure to the skin and the eyes based on acute effects, such as skin redness and burning and inflammation in the outer eye. These guidelines are prepared assuming that all individuals are repeatedly exposed. Their aim is to prevent acute effects in this worst case situation, but they do not represent a line between safe and hazardous levels. UVR exposure does have beneficial effects, such as in vitamin D synthesis or in medical treatments for skin and other diseases. In the former, relatively low levels of UVR exposure are required and, in the latter, the exposures are controlled to maximise the beneficial effects of treatment.