Unless your lenses have been fitted for overnight use (and most are not), you should never sleep with them on, says Andy Meau, an optometrist at iSight Optometric Eye Care Centre in Central. It limits the amount of oxygen entering the eyes and can lead to dryness, eye infections and corneal ulcers, which in severe cases can cause blindness.

2. Wearing contact lenses when your eyes are red

Redness is a symptom of a problem, such as an allergy, an eye infection like conjunctivitis (pink-eye), eye fatigue, or other, more serious eye conditions.

Whatever the cause, Meau advises against wearing your lenses, especially if the redness has lasted for a few days, as this may worsen the redness and cause bigger problems. See an optometrist as soon as possible.

3. Cleaning or rinsing your lenses with tap water

Tap water contains microorganisms like bacteria and amoeba, and there’s a risk of transferring these to your contact lenses. Dr Arthur Cheng, an ophthalmologist at Hong Kong Sanatorium & Hospital says to use fresh contact lens cleaning solution (as recommended by your optometrist) every time you need to disinfect or rinse your lenses.

To clean them, place the lenses in the palm of your hand, apply a few drops of cleaning solution, rub each lens individually on both sides for about 20 seconds, and then rinse them with the solution. And remember to always wash your hands before and after handling your lenses.

4. Wearing your lenses when your eyes are dry

When your eyes are dry you may not only experience discomfort, but also damage the surface of your eyes. “When the corneas are dry they lose their protective ability and thus become more vulnerable to infection,” says Cheng.

In the long run, persistent wear despite severely dry eyes may cause (abnormal) corneal vascularisation [development of capillaries] and/or corneal decompensation, says Professor Alvin Young, from the Department of Ophthalmology & Visual Sciences at the Chinese University of Hong Kong.

5. Using saliva to rehydrate your contact lenses

Your lenses feel dry but you don’t have eye drops or lens lubricant handy so what do you do? You remove your lenses, lick them to rehydrate them and insert them back into your eyes. This is a no-no, says Meau. “Your mouth is full of bacteria that you really don’t want to be exposing your eyes to.”

6. Buying contact lenses online without a prescription

The danger here is that the lenses may not be the right fit. “Improperly fitted contact lenses may lead to a potentially serious eye infection,” says Meau. “Most online retailers do not require proof of a proper prescription from an optometrist. This means that the buyer can get away with not getting their eyes examined for corneal health and a proper contact lens fit. I’ve come across people who have bought the wrong lenses online for many years, with the incorrect prescription or lens type. Incorrect prescription may cause headaches and eye strain, while ill-fitted lenses may lead to lens wear discomfort and also affect the health of the eyes.”

Why we must learn from 1918 Spanish flu to fight the next pandemic

Before buying online, make sure you have the correct prescription from an optometrist or ophthalmologist. A valid prescription lasts one to two years, depending on the condition of your eyes.

7. Not cleaning your contact lens case

Cheng says that bacteria and proteins can build up in the case and under the caps, potentially contaminating your contact lenses. Be sure to clean the case thoroughly, rubbing it with contact lens cleaning solution for several seconds before wiping it dry with a clean tissue and air-drying it with the caps off, face-down on a tissue.

8. Using expired products

Reusing daily disposable contact lenses and using expired cleaning solution may increase your risk of a corneal infection. Young says that you may also develop an allergic or immune reaction. Be sure to take note of when these products must be discarded.

9. Sharing contact lenses with friends

To save money on coloured lenses, it’s not uncommon for groups of friends to share colours with one another. Cheng says that this is unhygienic and may lead to cross-infections.

10. Not seeing (or delaying seeing) your ophthalmologist when things go wrong

If you experience any pain, discomfort, redness or vision problems, see your ophthalmologist right away, says Young. The sooner the issue is dealt with the better.