Ultraviolet (UV) levels in the UK are expected to be high in Friday’s sunshine.

UV radiation is emitted by the Sun and penetrates the Earth’s atmosphere.

Some UV radiation is essential for our wellbeing, and the sun’s rays provide warmth and light.

But balance is key as UV exposure can also cause dangerous skin damage.

We need to control our exposure, says Prof Dorothy Bennett, from St George’s, University of London.

UV is beneficial because it enables our skin to produce essential vitamin D.

This is important for the function of bones, blood cells and our immune system.

“But UV is also dangerous because every exposure to UV, especially every sunburn, increases our risk of skin cancer,” she said.

“Melanoma, the most dangerous skin cancer, is now the fifth commonest cancer in the UK, the ongoing rise being attributed to sunbathing.”

UV radiation promotes skin cancer by damaging DNA in skin cells.

It has also been linked to eye problems, including cataracts.

And there is growing evidence that UV light may reduce the body’s ability to defend itself against certain diseases.

Levels of UV radiation vary throughout the day.

The highest readings occur in the four-hour period around “solar noon”, which is when the sun is at its highest spot in the sky – usually from late morning to early afternoon.

The UV Index (or UVI) is a standard, international measure of ultraviolet radiation.

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.

Countries close to the equator can experience very high UV levels in the middle of the day, throughout the year.

Nairobi in Kenya can have UV levels above 10 all year, according to the World Health Organization (WHO).

Majorca in Spain, will normally hit nine in June and July.

But the Falkland Islands in the South Atlantic never usually gets above five in December and January (when it is summer in the southern hemisphere).

According to the WHO, extra sun protection is required when UV levels are:

• 11+ (extremely high)
• 8-10 (very high)

Protection is required when levels are:

No protection is required when levels are:

There is no safe or healthy way to get a tan, according to the NHS.

If you want one, Dr Bav Shergill, of the British Association of Dermatologists, says the safest way to tan is “out of a bottle” – using self-tan.

“When you tan, ultraviolet light stimulates your skin cells to produce pigment to try and protect the DNA of skin cells – but that protection is minimal – the equivalent of SP4.

“That is not much protection at all – so you can still burn very early,” he warns.

BBC Weather’s Helen Willetts says: “Your skin can burn just as quickly whether it’s 30C or 20C.

“Don’t be caught out on cloudy days. UV will still penetrate thin clouds – so even if you don’t think it’s that sunny, you can still burn.”

The amount of UV reaching your skin is not driven by the daily temperature, Dr Michaela Hegglin, from the University of Reading’s Department of Meteorology, says.

“UV levels on a bright and breezy late April day in the UK will be about the same as a warm sunny day in August.”

Unprotected exposure to UV rays plays a substantial role in skin ageing – breaking down collagen and elastin fibres in healthy skin.

This contributes to wrinkles and loosened folds.

The sun’s rays also dry out skin, making it coarse and leathery.

Tips from the NHS, include:

• Spend time in the shade when the sun is strongest (in the UK that is between 11:00 and 15:00 from March to October)
• never burn
• cover up with suitable clothing and do not forget sunglasses
• use at least factor 30 sunscreen
• reapply sun screen every two hours
• take extra care with children

Global research shows people often miss parts of their bodies when putting on sunscreen, Dr Shergill warns.

“People often forget include side of nose by eye – where I have seen a lot of skin cancer,” he says.

Other areas include the groove by the side of the nose and into the cheek, the temples and the upper chest.

As a guide, adults should aim to apply about six to eight teaspoons of sunscreen if covering the entire body.

Yes.

“I have, for example, seen South Asian people with skin cancer and I have seen people with dual-heritage get skin cancer.

“The skin may look darker, but it doesn’t always behave that way from a protection point of view – because there are more genes at play than we think about, ” Dr Shergill says.

Regardless of skin colour – the risk of eye damage and of potential harmful effects on the immune system remains.