Leonardo Da Vinci may have been the original inventor of contact lenses - in 1508 he made several sketches of what looked like an early prototype.

Today there are three million contact lens wearers in the UK and presumably Da Vinci, if he were alive, would marvel at the array.

Although for most people lenses are safe and effective, there are risks if they are not used correctly.

Here are the unexpected ways you could be putting yourself at risk.

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SPRING CLEAN YOUR SOLUTION
Always use the lens solution - the liquid used to disinfect and clean contact lenses before re-use - advised by your optician, says Ceri Smith-Jaynes, spokeswoman for the Association of Optometrists, and check the expiry date.

"If the solution is out of date, don't use it," she advises.

"It may no longer be sterile and can leave your eyes open to bacterial infections and eye irritation."

Contact lens cases - used by monthly and fortnightly lens wearers to store their lenses when they"re not wearing them - can harbour a host of horrors.

"You'd be surprised at the number of people who use the same case to store their lenses for months on end," she says. "I've seen debris floating in them which can be transferred to the eye and lead to infections."

She advises changing your case once a month, pouring away the old solution and putting in fresh.

Rinse your case in fresh solution every time you store your lenses, then let it air-dry for a few hours, says Ms Smith-Jaynes.

If you forget, or run out of, the solution that came with the lenses, don't buy simple saline solution from the pharmacist as a replacement, as this will not disinfect your lenses.

GOT A COLD? WEAR GLASSES
If you're a hay fever sufferer, you may find contact lenses reduce the symptoms.
"Sometimes daily disposable lenses can act like a barrier that stops pollen sticking to the eye," says Ms Smith-Jaynes.

"However, if your hay fever symptoms are especially bad, wear your glasses, as you may need eye drops several times a day and you can't take your lenses out and keep putting them back in."

Hay fever can also make your eyes feel itchy, and excessively rubbing your eyes while wearing lenses can cause an abrasion on the cornea, leaving it open to infection, says Dr Cindy Tromans, a consultant optometrist at Manchester Royal Eye Hospital.

If you have a cold, it's also a good idea to forgo your lenses and wear your glasses instead.

"The risk of viral conjunctivitis [where the eye becomes red and inflamed as a result of a virus] is increased when people have a cold, as your immune system will be low, and wearing contacts when you have this condition can damage the cornea," adds Ms Smith-Jaynes.

Always use new lenses once conjunctivitis has cleared up, to prevent re-infection.

TAKE THEM OUT BEFORE A DIP
Swimming and even showering in lenses is an "absolute no-no", says Dr Tromans.

"There is an organism - a type of amoeba called acanthamoeba - that lives in the domestic water supply, and if you shower or swim in contact lenses it can get stuck behind the lens and begin to destroy the eye."

Contact lens wearers are at raised risk because the amoeba can survive in the space between lens and cornea, then get into the eye.

Although rare, the infection is hard to tackle, says Dr Tromans. "The treatment is multiple drugs taken for a long time, and some patients even have to have a corneal transplant."

The condition, characterised by a painful, pink eye and blurred vision, can be mistaken for more common eye problems such as conjunctivitis.

Early detection is key: the infection is usually diagnosed only after antibiotics for other conditions have not worked. A swab of the eye will then be taken to test for the amoeba.

"Don't wear your lenses in the shower - and if you go swimming, use prescription goggles: the amoeba can survive in chlorine," adds Ms Smith-Jaynes.

"If you really have to wear lenses, wear decent goggles over the top and take the lenses out as soon as you leave the water. It's the same for hot tubs - even the steam rising above the water can be a hazard."

For the same reason, never wash your lenses in tap water.

GIVE YOUR EYES A BREAK
The length of time you should wear your lenses varies from person to person. Those with dry eyes may only be able to tolerate three to four hours' wear, as lenses can absorb water from the eye surface.

The newer silicone hydrogel disposable lenses allow more oxygen to the cornea, so they can be worn for about 16 hours, according to Ms Smith-Jaynes.

However, extensive wear of all types of lens carries the risk of corneal neovascularisation -where the cornea is starved of oxygen and new blood vessels start to grow there in an attempt to compensate.

"They start in the corner and move into the centre of the eye," Ms Smith-Jaynes says. "If this goes unchecked the cornea can become opaque, leading to the possibility of blindness."

It is important to have regular checks, as this condition can go unnoticed for months or years until it causes symptoms such as red, irritated eyes.

It is best to alternate lenses with glasses, and take them out at home in the evenings.

DON'T SLEEP WITH LENSES IN
When your eyes are closed, contact lenses prevent oxygen in tears reaching the cornea, the transparent layer at the front of the eye.

"This can lead to nasty corneal ulcers and bacterial infections, as without oxygen the cornea swells up," says Dr Tromans.

"Bacteria can then sneak in, one of the most dangerous being a pseudomonas infection that can turn the eye luminous green and lead to blindness."

A 2012 study in the journal Ophthalmology showed that the risk of developing keratitis - inflammation of the cornea, which can be sight-threatening - was raised 6.5 times by sleeping in contact lenses, even only occasionally.

"When you sleep in them, the lens can become tight because you're not blinking, so you're not lubricating the eye," says Ms Smith-Jaynes.

"This can make the lenses harder to remove once you wake up."

BEWARE WHEN BUYING ONLINE
If you wear contact lenses, have an eye check once a year, says Dr Tromans. "If you've worn the same lenses for a couple of years it could well be time for a change, as technology is advancing all the time."

This could mean lenses made of a more comfortable material, or with a higher water content to allow more oxygen through.

More people are ordering contact lenses online as it can be cheaper, but regular eye checks are still important so you know that you have the right prescription.

Dr Tromans advises checking you are buying from a reputable website, too.

"In the UK, contact lenses have to go through rigorous checks. It is a very highly regulated sector," adds Simon Rodwell, secretary general of the Association of Contact Lens Manufacturers.

"There's no harm in trying to save money if you know your prescription - but check the site you're buying from. If it's not based in the UK, the lenses could be unsafe."

Poorly fitting lenses are not only uncomfortable but can heighten your chances of getting infections.

This article was first published in the Daily Mail.