All contact lenses reduce the amount of oxygen that reaches the front surface of the eye to some degree. When the cornea's oxygen supply is significantly reduced — a condition called hypoxia — a number of problems such as red eyes, corneal swelling, blurred vision and eye discomfort can occur. Hypoxia also can increase contact lens wearers' risk for a number of eye infections.
Hypoxia-related eye problems became a significant issue in the 1990s when extended wear contact lenses first gained popularity. As the number of people who wore contacts overnight and continuously for several days increased, so did the number of contact lens-related eye infections.