To demystify some well known abbreviations:
UV is Ultraviolet light.
It is electromagnetic radiation with a wavelength shorter than that of visible light but longer than X Rays. It is named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet. There are three familiar types:
UVA long wave
UVB medium wave
UVC short wave
SPF sun protection factor a laboratory measurement of the effectiveness of sunscreen
UV is a major cause of sunburn but the UV spectrum has other effects, both beneficial and damaging, to human health such as UVB exposure inducing the absorption of vitamin D. Too much however, may lead to direct DNA damage resulting in sunburn and skin cancer.
On April 13, 2011 the International Agency for Research on Cancer of the World Health Organization classified all categories and wavelengths of Ultraviolet Radiation as a Group 1 carcinogen. This is the highest level designation for carcinogens and means
"There is enough evidence to conclude that it can cause cancer in humans".
There is a popular oversimplification of how SPF determines how long one can stay in the sun. For example, many users believe that, if they normally get sunburn in one hour, then an SPF 15 sunscreen allows them to stay in the sun fifteen hours (i.e. fifteen times longer) without getting sunburn. This would be true if the intensity of UV radiation were the same for the whole fifteen hours as in the one hour, but this is not normally the case.
Intensity of solar radiation varies considerably with time of day. During early morning and late afternoon, the sun's radiation intensity is diminished since it must pass through more of the Earth's atmosphere while it is near the horizon.
In practice, the protection from a particular sunscreen depends, besides on SPF, on factors such as:
▪ The skin type of the user.
▪ The amount applied and frequency of re-application.
▪ Activities in which one engages (for example, swimming leads to a loss of sunscreen from the skin).