As a period of extreme heat is forecast across the UK this week, UV levels are expected to be high in many areas. Some exposure to UV is essential for our wellbeing, but too much is damaging and can cause skin cancer. UV radiation is emitted by the Sun and penetrates the Earth's atmosphere.
It enables our skin to produce essential vitamin D, which is important for the function of bones, blood cells and our immune system. It can lead to skin cancer by damaging DNA in skin cells. UV also plays a substantial role in skin ageing, contributing to wrinkles and loosened folds.
It has also been linked to eye problems, including cataracts. "Every exposure to UV, especially every sunburn, increases our risk of skin cancer," says Prof Dorothy Bennett, from St George's, University of London. "Melanoma, the most dangerous skin cancer, is now the fifth commonest cancer in the UK." Levels of UV radiation vary throughout the day.
The highest readings are in the four-hour period around "solar noon", when the sun is at its highest - usually from late morning to early afternoon. The UV Index (or UVI) is a measure of ultraviolet radiation used around the world. Values start at zero and can rise above 10.
The higher the number, the greater the potential for damage to the skin and eyes, and the less time it takes for harm to occur. In the UK, the UV index would typically be around 5-6 during the summer, with a maximum of 8 only in exceptional circumstances. Countries close to the equator can experience very high UV levels in the middle of the day, throughout the year.
Nairobi in Kenya, for example, can have UV levels above 10 all year, according to the World Health Organization (WHO). Majorca in Spain normally hits nine in June and July. Some sun protection is required when UK levels are medium (3-5) or high (6-7), the WHO says.
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