The presumed percentage value of oxygen pressure beneath a contact lens. An avascular cornea needs oxygen from the atmosphere; otherwise, inflammation will occur and cause clouding of the cornea, discomfort, reduced visual acuity and long-term adverse consequences. The cornea requires three and ten microliters of oxygen per square millimeter per hour (3- 10 µl O2/mm2 cornea/hour) and an oxygen pressure close to 155 millimeters of mercury (155 mmHg).