Human corneas in vivo were exposed to a reduced partial pressure of atmospheric oxygen in order to determine the minimum oxygen for normal corneal hydration. Below a critical range of 11 to 19 mm Hg, the cornea will hydrate and swell. The rate of swelling increases as the oxygen tension decreases. The oxygen required by the epithelium is about one tenth of that available from the atmosphere when the eyes are open and about one fourth of that available from the palpebral conjunctiva when the eyes are closed.
Polse KA, Mandell RB. Critical Oxygen Tension at the Corneal Surface. Arch Ophthalmol. 1970;84(4):505-508. doi:10.1001/archopht.1970.00990040507021