Why is there a limit to the length of wearing time ?
The cornea the "clear part of the eye", is avascular or without a blood supply. It is avascular otherwise it wouldnt transmit light without distortion. As a result of this living tissue being avascular it is necessary to obtain oxygen from the atmosphere. The wearing of a contact lens interrupts the flow of oxygen to the cornea and due to changes in the metabolic pump of the corneal cells the tissue thickens, called oedema. Contact lenses, as described in Sections 3.3 and 3.4, are manufactured from material that allows maximum oxygen transmission. But this is still not exactly the same as the 20.4% therefore the lens wearing time must be controlled to reduce oxygen deprivation to the cornea.