Start wearing sunglasses early, and they may help to preserve your sight in later life

For decades, doctors have warned about the dangers of going out in the sun. Slap on a hat, slip on a shirt and slop on the suncream to protect your skin from harmful rays and avoid getting cancer, they say.

But what about the eyes? If sunlight burns skin, what does it do to the cornea, lens and retina? Most people wear sunglasses for comfort or to look cool. But perhaps there is a more pressing reason – to save our sight.

Should the advice be; wear shades or risk going blind? One of the world's best-known scientists, James Watson, a Nobel prizewinner and the discoverer of the DNA double helix, seems to think that is exactly what the risk is.

At the opening of the Wellcome Medical Museum in London last month, he was extolling the benefits of genetic research. In May, he became the first person to be presented with his entire DNA sequence on a disc, potentially allowing him to identify genetic defects that could put him at risk of disease.

Asked if there were any downsides to such research – something he is notably reluctant to admit – he had his answer ready. Analysis of his genome might have revealed a vulnerability to, say, macular degeneration, which is the most common cause of blindness.

"I could have seen the [gene] sequence when I was in my fifties and worn dark glasses for the rest of my life. But I am now 79, and I don't have macular degeneration. So it would have been unnecessary – based on incomplete information," he said.

It was a deliberately frivolous point, intended to suggest that the worst the new genetics could foist upon us was the inconvenience of wearing dark glasses. But the assumption that lay behind it was that dark glasses can protect eyes from premature ageing, in the same way that suncream can protect the skin. They may even, Watson implied, prevent vulnerable people from losing their sight.

The facts about blindness are chilling. By the age of 80, more than half of Americans either have a cataract or have had cataract surgery. A cataract occurs when the lens in the eye becomes cloudy, blurring the vision. Treatment is by surgery, to replace the lens with an artificial one made from plastic.

Age-related macular degeneration affects about 500,000 people in the UK. It occurs when cells in the centre of the retina at the back of the eye become damaged. Symptoms are the loss of central vision and visual distortion. Both conditions are most common in the elderly – and as we live longer, the numbers affected are growing. The eyes, in common with other organs, need protection if they are to last. Some eye specialists say that protecting the eyes of children is the most effective way to prolong 20-20 vision into old age.

Ian Anderson, an optometrist and the chairman of the Eyecare Trust, a charity devoted to promoting eye health, said: "Your eyes can be damaged by ultraviolet light. There are two types in sunlight – UVA and UVB. UVA sunlight penetrates quite deeply and can damage the lens and the retina. People should be aware that they need to wear dark glasses and do more to protect their eyes."

People who have fair skin are at greatest risk. They have less pigment and their eyes are thus most vulnerable to UV light, while dark-skinned people are better protected.

But it is a myth that blue-eyed people are more sensitive to light and therefore more vulnerable to eye damage. The iris is almost opaque, although there are differences in the amount of pigment in the retina, Anderson said.

Children are worse off because their eyes are young and the lens and vitreous – the fluid behind the lens – are clearer, so the light goes straight through and goes on to hit the retina.

"Children need sunglasses, but parents need to be careful that they are not toys with tinted lenses. That causes the iris to open and let more light through. Parents need to be very careful to buy sunglasses with the right CE marking to show that they filter out UV light. It is more important to wear sunglasses when young to protect the eyes."

In older people, as the lens of the eye ages, it creates more glare. "It becomes like a frosted window – this is called 'veiling glare'. A lot of older people have incredible problems driving when the sun is low or it is reflecting off wet roads," Anderson says.

If sunglasses are necessary to protect the eyes from damage, why are they not the subject of health promotion campaigns? The answer, according to Andrew Lotery, professor of ophthalmology and a specialist in macular degeneration at Southampton University, is that the case for shades is unproven.

"It has been a hypothesis for decades [that exposure to sunlight damages the eye]. There have been large-scale epidemiological studies; for example, of fishermen who are exposed to a lot of light reflected off the sea. There is no evidence of an increase in macular degeneration. That link has been looked for, but it has not been found."

Some experts say that blue light is more damaging to eyes, but this too has not been proved, Lotery says. "One concern was that when cataracts were removed, the blue light filters present in the natural lens were removed also. Now, all replacement lenses have blue light filters," he says.

For certain people with rare eye conditions, such as retinitis pigmentosa (an inherited condition that causes degeneration of the retina), excess light can be damaging. "I advise these patients to avoid sunlight and wear dark glasses," Lotery says.

A juvenile form of macular degeneration called Stargardt's disease, which affects one in 10,000 children, is also affected by light. Experiments in mice show that they are protected when raised in the dark. Lotery says: "Most ophthalmologists would recommend sunglasses for this group." But he's sceptical about suggestions that we all need protection: "I don't think, for the general population, that the evidence is there."