Hypoxia and hypercapnia
Under natural circumstances, the cornea accepts oxygen from the environment and excretes its metabolic product, carbon dioxide, into the environment.
An applied contact lens represents a bigger or smaller barrier in the process of exchanging gases with the environment. The consequence is a reduction in oxygen supply (hypoxia) and accumulation of carbon dioxide (hypercapnia).
Nowadays, all contact lenses (except PMMA) provide a certain degree of transmissibility allowing gas exchange through the lens.