There are two main lens types - RGP (rigid gas permeable lenses) and soft contact lenses. Here we will talk about soft contact lenses, since they are the most popular type.

The quality of your contact lenses, how much oxygen they let through to your eyes and how high is the percentage of water in the lens should all be considered. It is important to select lenses made from high quality polymer with a high (more than 50%) water content. Also the lenses should "breathe" - in other words, they should let enough oxygen through. A good choice would be Acuvue Advance lenses or any of the Focus contacts. Also see more suggestions on .

Another consideration is the thickness of the lens. The more corrective power your eyes require, the thicker the lens is. A thick lens presents a higher risk of infection. If your eyesight is very bad, RGP lenses might be a better choice for you, instead of soft contact lenses.

Third, having your lens properly fitted is especially important. The lens shouldn't be too tight across the surface of your eye. There should be space for tears between the lens and your eye. Only an experienced eye doctor can fit your lenses properly. You should never buy contact lenses without a prescription, on the assumption that they will fit you. Most likely they won't, and you can do irreparable damage to your eyes.

Another problem is exposure to pollution, smoke, dust or pollen. Lenses tend to collect even the smallest particles from the air. This causes discomfort and, quite often, inflammation of the cornea. If you feel that there is something stuck between your eye and the lens, take your lens off immediately and wash both the lens and your eye. See additional .

Sleeping in your contact lenses isn't recommended. Contact lens manufacturers realize that people would very much like to have a lens that they can wear continuously for several days. They put a lot of research into producing such lenses: for example, Focus Night and Day is said to be safe for 30 days of continuous wear, and Acuvue is designed to be worn for a week.

However, from your health point of view it is better to be safe than sorry; don't sleep in your lenses. Of course, if you live in the mountains where the air is crystal clear, wearing your lenses for several days might be all right. But most of us live in polluted cities and travel a couple hours a day through peak hour traffic. Just imagine all that dust collecting between the delicate surface of your eye and a lens for a week, or a month … It's not a pleasant thought, is it?

Early symptoms of keratitis are blurry vision (while wearing lenses) and discomfort when exposed to light. If you notice these signs, don't wear your lenses and see a doctor.

After reading this article you probably think that soft contact lenses are dangerous and you are better off with your old glasses. Please don't. Modern brand-name contact lenses are designed to minimize the risks and, if they are properly fitted, worn according to the schedule and properly cared for, they are quite safe. The most important thing is to follow the advice given above and see your eye doctor immediately, if you start to feel discomfort.