Humidity is of critical importance in contact lens performance since it directly contributes to a drying of the ocular environment, which can affect the quality of vision, the amount of oxygen reaching the cornea, and the wearer's ability to tolerate the lens. This has implications for cockpit use of hydrogels, since the low humidity found there will contribute to water loss in the lens, a phenomenon known to decrease its ability to transmit oxygen (O'Neal, 1990). Thus, oxygen transmissibility is directly related to water content, with higher water content translating to higher levels of oxygen available to the eye. Rather, oxygen reaches the cornea by diffusing through the water in the lens itself. Hydrogel lenses do not allow substantial tear pumping under the lens. In RGP lenses, additional oxygen diffuses through the lens itself, the amount depending on the exact chemistry and thickness of the lens. With rigid lenses, some oxygen is supplied in the tears pumped under the lens with each blink. The amount of oxygen available under a lens varies with lens material and thickness. In the absence of contact lenses, sleep-induced edema normally requires only a few hours to disappear once the eyes are opened. The closed-eye environment offers perhaps only a third of the oxygen available to the open eye (Mertz, 1990) and has been used as a guide for lens designers in determining tolerable levels of corneal edema. Low levels of corneal swelling-under 5 percent-are experienced regularly during sleep when the eye is closed.

The cornea can apparently tolerate moderate levels of hypoxia-induced edema without adverse affects. In addition, hypoxia depletes the cornea's energy reserves in the form of glycogen-another indication that the cornea is under stress. This buildup of lactic acid creates an osmotic load, drawing water into the cornea faster than it can be removed and resulting in corneal swelling or edema (Mertz, 1990) (see discussion below). When sufficient oxygen is not available, the cornea's metabolism is stressed and a buildup of lactic acid occurs as the cornea begins to respire anaerobically. Normally, the cornea consumes from 3 to 10 µl O 2/cm 2/hr (Larke et al., 1981 Lowther, 1990). A pilot flying at altitude also experiences a reduced level of oxygen in the ambient air relative to sea level, and this reduction, compounded by the action of contact lenses, results in a hypoxic condition whose effects vary with the degree of oxygen deprivation and the length of exposure time. By modifying access to this ambient air, most contact lenses reduce the oxygen available to the cornea. The cornea does not have blood vessels (except near its edges) and must obtain the oxygen it requires for metabolism from the surrounding air.

Double pupil contact lense