Corneal thickness changes were measured on human subjects who wore gel lenses that varied in center thickness. Using these measurements and earlier studies in which changes in corneal thickness were monitored on human corneas exposed to O2 tensions below that in air, the O2 tension under most contact lenses varied from 0 to 25 mm Hg, which produced a corresponding O2 flux into the cornea of 0-6 .mu.l cm2 h. A critical O2 tension and flux under the lens was 10 mm Hg and 2 .mu.l cm2 h, respectively, below which corneal swelling occurs. To maintain these critical levels of tension and flux, the minimum O2 transmissibility of a stationary lens on the cornea was 5 .times. 10-9 and 15 .times. 10-9 (cm .times. ml O2)/(s .times. ml .times. mm Hg) for the open and closed eye conditions, respectively.