Nearly 3m people in this country use contact lenses. For anyone who doesn't mind sticking their paws in their eyes, there are clear advantages: they correct vision better than glasses, are less often crushed underfoot, don't make you look like a geek, and come in a huge variety of forms - soft, rigid, disposable, coloured, ultraviolet-proof and even theatrical. (For a while after the September 11 attacks, special stars-and-stripes lenses became popular in the US.)
But are they safe? This week the British Journal of Ophthalmologists reports research from Austria which shows that not all disinfectant solutions are capable of killing all germs - and some of these germs can cause blindness in just 24 hours.
While most problems with contacts can still be traced back to poor hygiene or over-use, not all potentially sight-threatening conditions are confined to those who rinse their lenses under the tap or decide, while tired and emotional, to sleep in their lenses for the third night running.
Two of the biggest eye-damaging culprits - protozoan acanthamoeba and bacteriapseudomonas - are all around us, in our homes and water supplies. This apparently innocuous pair can cause blindness through keratitis (swelling of the cornea). "The cornea is like a five-layered ham sandwich," says Raymond Curran, an optometrist from Omagh, Northern Ireland. "Once the bacteria get into the layer of ham they can replicate there and turn the cornea opaque."
A high-street optician may see one potentially sight-threatening case each month, but at eye hospital casualty departments there may be several each week. Acanthamoeba, often apparently mild, can be misdiagnosed until it is too late, while pseudomonas, more dramatically, can cause increasing sight loss with each hour that passes. If not treated immediately, patients may need a corneal transplant.
And the really bad news, according to Professor Roger Buckley, a consultant ophthalmologist at the Moorfields eye hospital in London, is that acanthamoeba is "one of the hardiest organisms on Earth" - it can survive at the North Pole.
So how do you make sure that you don't pick up one of these infections?
The fact is that lenses can never be 100% safe. Even if you use daily disposables, you must make sure that your hands are spotlessly clean - and not wet with tap water, a potential germ carrier. If you don't use disposables, the Austrian research suggests that of all cleaning systems (all-in-one, one-step peroxide and two-step), only the two-step system (overnight soaking in hydrogen peroxide followed by soaking with a neutraliser) is fully effective in killing acanthamoeba.
Unfortunately, the two-step method is relatively unpopular, because it requires more effort. It carries its own risks, too - users tend to cut corners because the procedure is time-consuming. "People do not realise what terrible risks they are taking with a primary sense organ, and people who break the rules are usually the ones who get into trouble," says Buckley.
A scrupulous cleaning regimen is vital because lenses are an ideal breeding ground for infection. The cornea, which has no blood vessels of its own, relies on oxygen being supplied and waste products removed by the water that covers it. All lenses - particularly low-water-content soft lenses - reduce this level of gas exchange.
Hot, smoky or airless environments, such as pubs or aeroplanes, make things worse - as does staring for hours at a computer screen, which dries out the eyes as people blink less. Leaving lenses in for longer than 14 hours can waterlog the eye and make it swell.
"Broadly speaking, contact lenses are safer than ever, and only a small minority of people have problems," says Curran. "But it's crucial to seek help if they suspect anything slightly unusual. A problem is that people may have some pain but because they have two eyes they don't realise that they are losing vision in one of them. Delaying treatment can mean a much worse prognosis."
A growing danger, according to the College of Optometrists, is that people now buy contact lenses from internet warehouses and market stalls, especially cosmetic lenses (which do not correct vision). These are often bought by people who don't normally use lenses and sold by sales assistants who are not aware of the importance of hygiene.
"People need professional care for their eyes," says Curran. "You won't do much harm by using a lens that is too strong or weak, but you could be in trouble if it's the wrong shape - it could stick to the eyeball or otherwise damage it."
Experts say it is vital to be on the look-out for symptoms of eye infection - changes in vision, pain, eye redness or excessive tearing - and to obey the basic rules of lens hygiene.
A brief guide to contacts
Rigid gas-permeable lenses
Made from silicone and fluorine, these account for 15% of the UK market. They allow more oxygen through them than soft lenses, are easier to handle and designed to fit an individual's eye. They suffer less protein build-up than soft lenses, and can even be professionally polished. However, it can take several weeks to get used to them.
Cost: Around £140.
Soft lenses
It may take only a few days to get used to soft lenses. They are also less likely to fall out or get dust trapped beneath them than rigid ones. However, they give less clear vision and are more fragile. Mostly made of water, they act like sponges and can absorb damaging residues such as soap or moisturiser from your hands.
Cost: From around £50.
Disposable lenses
Ready-sterilised daily disposable lenses are the safest - but can be expensive. An important minority of lens users with certain eye conditions cannot use these. Other lenses are thrown away after a fortnight or a month, but the problem with these is that while you have them they are generally cleaned with an all-in-one solution. It is easier than a two-step system, but may not kill all the germs.
Cost: Around £1 a day.
Extended-wear contact lenses
These are designed to be left in for 30 days and not removed at all. After a controversial start in the 80s (it was shown that leaving lenses in for even one night could increase the risk of infection by up to 30 times) they are now made from super-permeable silicone hydrogel, which allows up to six times more oxygen to get through. However, it is still safest to use them overnight only occasionally.
Cost: £200 for a six-month supply.
Hard lenses
Hardly anyone still uses these, which do not allow gas through to the eye.
Coloured lenses
These may be normal lenses with a light tint applied so that they can be found easily if dropped - or strong tints that change your eye colour dramatically, say from brown to blue. Light-filtering tints are good for sports players as they mute all colours except, for example, yellow - the colour of tennis balls. Cosmetic lenses may be fun, but teenagers have been known to try out each other's reptile or alien eye lenses at parties - swapping around their germs at the same time. Professor Buckley calls them "a disaster zone".
Cost: From £20.
Toric lenses
Unlike normal spherical lenses, toric lenses are specially shaped to correct the distorted vision caused by astigmatism (an irregular cornea).
Orthokeratology lenses
Hard lenses used to reshape a short-sighted person's cornea so that they eventually won't need lenses or glasses at all.
Bifocal lenses
Like bifocal glasses, these have two powers in one lens to correct both distance and near vision. These may be divided between top and bottom, or in concentric rings like an archery target.
Implanted lenses
Infection-free option currently being studied in the United States. Small lenses made of collagen are inserted inside the eye during a 10-minute operation. They can improve vision more than contact lenses as they are further back in the head. However, they could cause an added risk of cataract as they actually touch the natural lens.
Lens care
Always follow these rules:
· Never lick your lenses - this can transmit germs from the mouth to the eye. Never use tap water, distilled water or home-made saline solution on your lenses.
· Don't touch the tips of your solution bottles - you could contaminate them. Don't wear lenses while swimming. Don't let cosmetics come into contact with your lenses.
· Clean, rinse and disinfect the lenses every time you remove them.
· Clean and air-dry the lens case, and replace it every three months. Do not boil or microwave it - you could damage it.
