Effect of Aircraft Cabin Altitude and Humidity on Oxygen Tension Under Soft and Hard Gas-Permeable Contact Lenses

Melvin R. O'Neal

The uninitiated reader on contact lenses may wish to start with an encyclopedia overview of contact lens types and lens wear modalities and complications (O'Neal, 1988). The primary source of oxygen to the cornea is from ambient air. Contact lenses decrease the amount of oxygen getting to the corneal surface. Below a critical oxygen level, debated to be between 40 and 75 mmHg, corneal hypoxia occurs and the cornea swells (Mandell and Farrell, 1980; Holden et al., 1984). The adverse military flying environment includes aircraft cabin pressure that is decreased from normal sea level and cabin humidity that is usually much lower than normal. This cabin environment is shown to result in calculated oxygen levels under contact lenses that may be substantially reduced from normal and thus needs consideration.

MILITARY AIRCRAFT CABIN PRESSURIZATION

At sea level the ambient air pressure is about 760 mmHg (14.7 psi); however, the ambient pressure rapidly decreases as altitude increases (Spells, 1965). U.S. Air Force aviation can be divided into two basic aircraft cabin pressurization schedules (Heimbach and Sheffield, 1985). Both are isobaric-differential pressurization systems in which cabin pressurization begins as the aircraft ascends through 5,000 –8,000 feet, and then the isobaric function maintains this pressure until a preset pressure differential (psid) is reached between the ambient and cabin pressures. This psid is then maintained with continued ascent, and the resulting cabin pressure can be written as: ambient psi + psid = cabin psi.

The typical cabin pressurization schedule for fighter-attack-reconnaissance (FAR) aircraft is shown in Figure 1 For FAR aircraft the cabin may be unpressurized to about 8,000 feet (10.9 psi); then cabin pressure is held



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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium Effect of Aircraft Cabin Altitude and Humidity on Oxygen Tension Under Soft and Hard Gas-Permeable Contact Lenses Melvin R. O'Neal The uninitiated reader on contact lenses may wish to start with an encyclopedia overview of contact lens types and lens wear modalities and complications (O'Neal, 1988). The primary source of oxygen to the cornea is from ambient air. Contact lenses decrease the amount of oxygen getting to the corneal surface. Below a critical oxygen level, debated to be between 40 and 75 mmHg, corneal hypoxia occurs and the cornea swells (Mandell and Farrell, 1980; Holden et al., 1984). The adverse military flying environment includes aircraft cabin pressure that is decreased from normal sea level and cabin humidity that is usually much lower than normal. This cabin environment is shown to result in calculated oxygen levels under contact lenses that may be substantially reduced from normal and thus needs consideration. MILITARY AIRCRAFT CABIN PRESSURIZATION At sea level the ambient air pressure is about 760 mmHg (14.7 psi); however, the ambient pressure rapidly decreases as altitude increases (Spells, 1965). U.S. Air Force aviation can be divided into two basic aircraft cabin pressurization schedules (Heimbach and Sheffield, 1985). Both are isobaric-differential pressurization systems in which cabin pressurization begins as the aircraft ascends through 5,000 –8,000 feet, and then the isobaric function maintains this pressure until a preset pressure differential (psid) is reached between the ambient and cabin pressures. This psid is then maintained with continued ascent, and the resulting cabin pressure can be written as: ambient psi + psid = cabin psi. The typical cabin pressurization schedule for fighter-attack-reconnaissance (FAR) aircraft is shown in Figure 1 For FAR aircraft the cabin may be unpressurized to about 8,000 feet (10.9 psi); then cabin pressure is held

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium FIGURE 1 Typical cabin pressurization schedule for fighter aircraft. at this altitude until a preset pressure differential of 5.0 psid is reached at 23,000 feet (5.9 psi). This 5.0 psid is then maintained as aircraft altitude increases. Thus, at an altitude of 30,000 feet (4.4 psi) the FAR cabin altitude is 12,000 feet (9.4 psi), and at 39,000 feet (3.0 psi) the FAR cabin altitude is about 16,000 feet (8.0 psi). For Tanker-Transport-Bomber (TTB) aircraft, the cabin is held near sea level until the preset pressure differential, usually 8.6 psid, is reached at 23,000 feet (14.7 − 8.6 = 6.1 psi). At an altitude of 43,000 feet (2.3 psi) the TTB cabin altitude is about 8,000 feet (10.9 psi). Military aircraft routinely fly in the 30,000 to 45,000-foot altitude range, thus, cabin altitudes will frequently be between 8,000 feet (565 mmHg pressure) to 16,000 feet (412 mmHg pressure). CABIN ENVIRONMENT AND CONTACT LENSES Oxygen makes up about 21 percent of air at any altitude, and thus oxygen pressure is also reduced at higher altitudes. At sea level the partial pressure of oxygen (PO2) is 159 mmHg (760 mmHg × 0.21), but it is 118 mmHg PO2 (565 mmHg × 0.21) at the 8,000- foot cabin altitude and only 86 mmHg PO2 (412 mmHg × 0.21) at the 16,000-foot cabin altitude (see Figure 2). The amount of oxygen passing through contact lenses is directly related to the PO2 or “driving force” of oxygen in the air (Fatt, 1978). At higher altitudes the oxygen under a contact lens must therefore be lower. In addition, lower humidity is known to result in partial dehydration of soft lenses (Andrasko and Schoessler, 1980). Since oxygen passes through the fluid

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium FIGURE 2 Comparison of oxygen tension in ambient air at three altitudes. phase of a soft lens (Fatt, 1978), any water loss will decrease the oxygen transmission of the lens. Also, virtually no tear exchange occurs under soft lenses (Polse, 1979), and thus the oxygen under these lenses is due only to diffusion through the lens. Conversely, hard gas-permeable (HGP) lenses do not dehydrate and have the added benefit of the “pumping” of oxygenated tears under the lens during blinking (Fatt and Lin, 1976). This tear exchange increases the level of oxygen under HGP lenses by about 7–15 mmHg PO2 above the amount from diffusion (Efron and Carney, 1983; Fatt and Liu, 1984). A number of studies in altitude chambers and aircraft have assessed the subjective, visual, and corneal responses to contact lenses at aviation altitudes and low humidity levels (Eng et al., 1978, 1982; Brennan and Girvin, 1985; Flynn et al., 1986; Dennis et al., 1988). In general, they report only minimal corneal surface abnormality, variable subjective irritation, and little or no effect on visual acuity. However, there have apparently been no studies to assess the corneal swelling response in these environments. Although marked corneal edema would be necessary to affect the parameters measured, moderate corneal swelling may still occur. This may be important to the military aviator, since repeated corneal edema has been implicated in the cause of a number of corneal complications, such as epithelial microcysts, during extended contact lens wear (Weissman and Mondino, 1985; Polse et al., 1987).

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium PROPOSED “ACCEPTABLE” CORNEAL SWELLING/ MINIMUM OXYGEN LEVEL Published findings allow an attempt to derive an “acceptable” oxygen level under a contact lens. Holden and Mertz (1984) suggested that a contact lens with an oxygen transmissibility (Dk/L) of about 35 × 10−9 worn during sleep causes an allowable amount of corneal swelling for most individuals to return to normal corneal thickness the following day. The results of Polse et al. (1987) show that this Dk/L level reduces corneal complications during extended wear. These studies found approximately 8 percent corneal swelling with this Dk/L, which is about twice the 4 percent swelling that occurs normally each night during sleep in individuals not wearing lenses (Mertz, 1980). Adopting such a 2× normal swelling criterion would seem prudent. Repeated high levels of overnight corneal swelling appear to not only adversely affect the corneal epithelium but also to induce morphological changes in the corneal endothelium (Schoessler, 1983). These morphological changes may have an effect on endothelial ability to maintain normal corneal hydration (O'Neal and Polse, 1986), of which the long-term effects on corneal health have not been determined. The minimum oxygen level under a lens occurs during the critical closed-eye period of extended wear when the ambient oxygen pressure from the palpebral conjunctiva is decreased to only 55 mmHg PO2 (Efron and Carney, 1979). A contact lens with a Dk/L of 35 × 10−9 has a calculated oxygen tension under the lens during eye closure of about 25 mmHg PO2—the proposed “acceptable” minimum oxygen level. APPROACH A calculational approach is used to assess the possibility of corneal hypoxia with contact lenses during military flight operations. The oxygen levels of both soft and HGP contact lenses are calculated for sea level, two cabin altitudes, and different humidities. Given the assumptions involved in the calculations, the oxygen levels determined, although probably close, may not be the actual values. However, these calculated oxygen levels allow relative comparisons between lenses under military cabin environments and can also be compared to the proposed “acceptable” oxygen level. The calculations suggest that corneal hypoxic conditions could occur, particularly with soft lenses, that may approach the hypoxia that occurs during night wear. The resulting corneal edema may be high in some cases; however, the actual amount of corneal edema needs to be measured. Regardless, the maximum corneal edema and frequency of swelling to be allowed remain debatable.

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium CALCULATION OF OXYGEN TENSION UNDER CONTACT LENSES General Equation Equations and values of the empirical constants for calculating oxygen tension under a gas-permeable contact lens without the presence of tear pumping have been given by Fatt and St. Helen (1971). The contact lens and cornea are considered to be tightly joined, and the oxygen flux through the lens, jcl, is taken to equal the oxygen flux, jc, into the cornea. From jcl = Dk/L(Pa − P) and jc = aP1/2, setting these equal and rearranging gives the useful equation: Dk/L = aP1/2/(Pa − P), where Dk/L is the lens oxygen transmissiblity, a the empirical constant 0.24 × 10 −6 ml O2/cm2 × sec × (mmHg)1/2, Pa the ambient oxygen tension at lens surface, and P the oxygen tension at the corneal surface. The condition of little or no tear pumping applies to all soft contact lens wear (Polse, 1979) and for hard lenses worn during eye closure (O'Neal et al., 1984; Benjamin and Rasmussen, 1985). Using this equation, the relationship between lens oxygen transmissibility and calculated oxygen tension under a contact lens is shown in Figure 3 for soft lens open-eye wear at sea level and at cabin altitudes of 8,000 and 16,000 feet. Also shown, for comparison, is the low oxygen tension under the lens calculated for the FIGURE 3 Calculated oxygen tension under soft and hard gas-permeable lenses over a range of lens oxygen transmissibility for the open eye at three altitudes and for the closed eye.

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium critical closed-eye overnight period of extended wear. The upper curve in Figure 3 shows the higher level of oxygen due to tear exchange under hard versus soft lenses during open-eye wear. Effect of Humidity on Soft Lens Oxygen Transmission Since oxygen passes through the fluid phase of a soft lens, the diffusion (D) and solubility (k) of oxygen in the lens are related to the amount of water in the lens (Fatt, 1978). Fatt and Chaston (1982a) derived the relationship between the oxygen permeability (Dk) and percent water content (% H2O) of a soft lens at eye temperature as, Dk = 2.00 × 10−11 exp (0.0411 × % H2O). Lower humidity results in partial dehydration of soft lenses; and Fatt and Chaston (1982b), using the data of Andrasko and Schoessler (1980), have listed the effect of lower humidity on various soft lens parameters, including percent water content and lens thickness. Using data in their Table 4, the oxygen transmissibility (Dk/L) of the soft lens in the vial, the Dk/L during normal wear, and the Dk/L at a low 18 percent relative humidity were calculated and are listed in Table 1 for two frequently used water content soft lenses, 55 percent and 71 percent H2O soft lenses having 0.09- and 0.21-millimeter average lens thicknesses, respectively. The ambient oxygen transmissibility during normal wear is lower than the vial Dk/L by 11.3 percent and 14.2 percent for the 55 percent and 71 percent H2O lenses, respectively. This decrease in oxygen transmissibility becomes substantial under low (18%) humidity, with the ambient Dk/L now TABLE 1 Effect of Humidity on Soft Lens Oxygen Transmission Humidity Level % H2O Lmm(ΔL/L) Cal Dk a Cal Dk/L b 55 % H2O Lens Vial 55 0.090 (1.00) 19.2 21.3 Normal 51 0.086 (0.96) 16.2 18.9 18% 47 0.085 (0.94) 13.8 16.3 71% H2O Lens Vial 71 0.210 (1.00) 37.0 17.6 Normal 66 0.200 (0.95) 30.1 15.1 18% 58 0.183 (0.87) 21.7 11.9 a × 10−11 (cm2/sec)(ml O2/ml × mmHg); Dk = 2.00 × 10−11 exp(0.441 × % H2O). b × 10−9 (cm/sec)(ml O2/ml × mmHg). SOURCE: Fatt and Chaston (1982,Table 4).

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium much lower than the vial Dk/L by 23.5 percent and 32.4 percent for these lenses, respectively. Effect of Humidity/Altitude on Oxygen Tension Under Soft Lenses The aircraft cabin environment includes both lower humidity and cabin pressure that is decreased from normal sea level. Aircraft cabin humidity is frequently between a very low 5 to 10 percent relative humidity. Although data on lens changes are not available for these very low humidities, the low 18 percent humidity data noted above shows a dramatic effect on lens oxygen transmissibility. Using the calculated Dk/L in Table 1 the calculated oxygen tension under 55 percent and 71 percent H2O soft lenses in the vial, during normal wear, and at 18 percent relative humidity is shown in Table 2 for sea level and for cabin altitudes of 8,000 and 16,000 feet. At sea level the calculated oxygen under the lens is higher for the 55 percent versus the 71 percent H2O lens by 11 mmHg PO2 (23.4%) during normal wear and is 17 mmHg PO2 (51.5%) higher in low 18 percent humidity. More significant for aircrew, the calculated PO2 is 52.4 percent and 66.7 percent higher for the 55 percent versus the 71 percent H2O lens at low 18 percent humidity at cabin altitudes of 8,000 feet and 16,000 feet, respectively. As a comparison, for the 16,000-foot altitude and low-humidity aircraft cabin, the oxygen tension under the 71 percent H2O lens approaches the very low oxygen level calculated for soft lenses during overnight closed-eye wear. TABLE 2 Effect of Humidity/Altitude on Oxygen Tension Under Soft Lens   Calculated O2 Tension (mmHg) at: Humidity Level % H2O Calculated Dk/L a Sea Level Cabin at 8,000 ft Cabin at 16,000 55% H2O Lens Vial 55 21.2 62 42 25 Normal 51 18.9 58 38 23 18% 47 16.3 50 32 20 71% H2O Lens Vial 71 17.6 58 38 23 Normal 66 15.1 47 30 18 18% 58 11.9 33 21 12 a × 10−9 (cm/sec)(ml O2/ml × mmHg).

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium Effect of Altitude on Oxygen Tension Under HGP Lenses During open-eye wear of hard gas-permeable (HGP) contact lenses, tear exchange with blinking increases the oxygen level under the lens. Efron and Carney (1983), using the equivalent oxygen percentage (EOP) polarographic sensor technique, reported an average increase of about 2 percent O2 (15 mmHg PO2) under hard lenses with blinking. The time-averaged oxygen tension under hard contact lenses has been computed by Fatt and Liu (1984). The upper curve in Figure 3 as adapted from their Figure 2 indicates approximately a 7- to 12-mmHg PO2 (1−1.5 percent O2) higher level of oxygen under hard versus soft lenses during open-eye wear. Also, their equations show that the additional oxygen due to blinking is related to the ambient oxygen tension in the air. To derive the additional level of oxygen under hard lenses at the lower ambient oxygen tensions found in the aircraft cockpit, the ambient PO2 was multiplied by 0.075 (7.5%). This factor seems appropriate, since it is equivalent to 1.5 percent O2 (11.5 mmHg PO2) at sea level (i.e., 1.5 percent of 760 mmHg), which is in the middle of the range between the calculated and EOP techniques noted above. The additional oxygen under hard lenses due to blinking was thus taken to be 9.0 mmHg PO2 (118 mmHg × 0.075) at 8,000-foot and 6.5 mmHg PO2 (86 mmHg × 0.075) at 16,000-foot cabin altitude. The calculated oxygen tension under HGP lenses having oxygen transmissibilities from 20 × 10−9 to 40 × 10−9 (cm/sec)(ml O2/ml × mmHg) is shown in Table 3 for sea level and for 8,000- and 16,000-foot cabin altitudes. The oxygen tension under a medium (30 × 10−9 Dk/L) oxygen transmissibility HGP lens is much higher than that for soft lenses (89 vs. 58 mmHg PO2) even at sea level alone. For the low 18 percent humidity condition, when the soft lens partially dehydrates and the hard lens does not, the calculated oxygen tension under the hard lens is two to three times that TABLE 3 Effect of Altitude on Oxygen Tension Under HGP Lens   Calculated Oxygen Tension (mmHg) at: Lens DK/L a Sea Level Cabin at 8,000 ft Cabin at 16,000 ft 20 70 49 31 25 80 59 37 30 89 67 43 35 97 74 48 40 105 80 52 NOTE: PO2 added for tear exchange = 0.075 × ambient PO2. a × 10-9 (cm/sec)(ml O2/ml × mmHg).

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium under soft lenses at all cabin altitudes. The oxygen tension under HGP lenses appears to be enough to prevent most corneal swelling, even for the low (20 × 10−9 Dk/L) oxygen transmissibility HGP lens at the 16,000-foot cabin altitude. DISCUSSION AND RECOMMENDATIONS The calculated oxygen tension under 55 percent H2O content soft lenses is about 1 1/2 times higher than that for 71 percent H2O lenses under all conditions and is particularly noteworthy for the low-humidity, high-altitude conditions found in the aircraft cockpit. For the low-humidity environment, the medium-water-content lens was calculated to be above the proposed minimum oxygen level of 25 mmHg PO2 at the 8,000-foot altitude but not at 16,000 feet; however, the high-water-content lens would not meet this minimum oxygen level at either altitude. Indeed, for the 16,000-foot cabin environment, the 71 percent H2O lens is calculated to have the very low oxygen level that is found during closed-eye overnight wear. These calculations suggest that high-water-content soft lenses may not be the best choice, at least from an oxygen standpoint, for wear in the military aircraft cabin environment. Lens dehydration in low humidity also affects soft lens parameters and fit and may complicate the use of soft lenses by aircrew. It should be stressed that the oxygen levels presented are for a low 18 percent humidity. The aircraft cabin frequently has a very low 5–10 percent humidity that would result in even lower oxygen levels and greater corneal hypoxia. The calculated oxygen levels under soft lenses further suggest that normal everyday extended wear of soft contact lenses may not be a viable choice for military aircrew. The low oxygen level during overnight soft lens wear results in a substantial amount of corneal swelling in most individuals. In normal soft lens extended wear the cornea deswells the following day, although usually not completely (Holden and Mertz, 1984). However, during flight the lower oxygen under the lens could cause corneal swelling and affect its recovery in both FAR and TTB aircrew. Thus, significant amounts of corneal swelling could be present not only at night but also during daytime flight and would be further compounded for those aircrew flying many hours in a day, particularly TTB aircrew. Corneal swelling may be related to a number of corneal complications seen during extended wear, including epithelial microcysts and nonreversible changes in endothelial morphology (i.e., polymegethism) and may predispose the cornea to some of the other complications seen in extended wear. The added burden of corneal edema during the daytime may result in a higher incidence of corneal complications during extended wear in military aircrew. Notwithstanding the oxygen question during flight, it is intuitive

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium that the probability of complications increases as a function of years of extended wear. If the military adopts everyday extended wear for aircrew, it can be expected that a number of pilots will be grounded due to complications from long-term extended wear, many of whom will be lost just when they become fully trained and reach their peak. It would seem wiser to maintain corneal health with daily wear (nighttime removal) and switch to extended wear when necessary. This would allow aircrew members to wear contact lenses over a greater number of years and help preserve this important resource. HGP contact lenses are calculated to have much greater levels of oxygen under the lens in the aircraft cabin environment than any soft lens. HGP lenses would generally have much more oxygen under the lens than the proposed minimum oxygen level at both cabin altitudes. This higher oxygen level occurs because HGP lenses do not dehydrate in low humidity, as soft lenses do, and they get additional oxygen under the lens from the tear exchange that occurs with blinking, which does not occur with soft lenses. Importantly, HGP lenses can be made with much higher oxygen transmissiblity and thus have much greater levels of oxygen under the lens at all times, particularly during overnight closed-eye wear. Corneal deswelling the day after overnight wear is much more rapid and much more complete, returning almost to normal, with HGP lens versus soft lens extended wear (see Figure 3 of O'Neal, 1988). This rapid corneal recovery may be a critical factor in the much lower incidence of some of the corneal complications that occur during extended wear (Polse et al., 1987). The advantages of HGP lenses for use by military aircrew would seem obvious considering oxygen alone. Also, vision with hard lenses is generally better than with soft lenses, even more so for those with astigmatism. Preliminary results of a centrifuge study indicate that the new HGP lenses remain centered on the eye even under high G forces. However, concern has been voiced about the possibility of foreign bodies under hard lenses and discomfort under dry conditions. Foreign bodies may not be the problem some imagine since anecdotal comments from a number of NASA astronauts indicate no problems during T-38 training flights and even during microgravity while on-orbit when there is substantial dust floating in the shuttle. Irritation with dry eye during low humidity may be a problem for some individuals, but this will also be the case when soft lens dehydration causes a tighter-fitting and less comfortable lens. The U.S. Navy has for years allowed nonpilots to wear contact lenses, and many of them must be wearing HGP lenses. If there had been any significant problem with this lens type, it would be known; however, no such problems have been documented. Some individuals need soft lenses and some need HGP lenses to obtain the best vision and fit, and successful contact lens fitting will not occur without trying both lens types. The

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium USAF should continue research into the use of HGP lenses in the FAR environment and could initially obtain data on in-flight wear by rear-seat aircrew using such lenses. In summary, (1) high-water-content soft lenses, at least from an oxygen standpoint, may not be the best choice for use in the cockpit environment; (2) long-term normal extended wear will most likely lead to grounding of some aircrew due to increased corneal compromise over length of wear and is advised against; (3) flexible wear in which daily wear is used to preserve corneal health over many years of wear and extended wear when necessary is recommended; (4) HGP lenses have already been used successfully by pilots in fighter aircraft and would improve vision and corneal oxygen supply; and (5) continued laboratory and field study of HGP lens wear by aircrew is necessary. REFERENCES Andrasko, G., and J.P. Schoessler 1980 The effect of humidity on the dehydration of soft contact lenses on the eye. International Contact Lens Clinic 7:210–238. Benjamin, W.J., and M.A. Rasmussen 1985 The closed-lid tear pump: oxygenation? International Eyecare 1:251–256. Brennan, D.H., and J.K. Girvin 1985 The flight acceptability of soft contact lenses: an environmental trial. Aviation, Space, and Environmental Medicine 56:43–48. Dennis, R.J., W.J. Flynn, C.J. Oakley, and T.J. Tredici 1988 A Field Study on Soft Contact Lens Wear in USAF Military Transport Aircraft . USAF School of Aerospace Medicine Technical Report No. USAFSAM-TR-88 –4. USAFSAM/NGOP, Brooks AFB, Texas Dennis, R.J., W.M. Woessner, R.E. Miller, and K.K. Gillingham 1989 Effect of Fluctuating +G sub z Exposure on Rigid Gas-Permeable Contact Lens Wear . USAF School of Aerospace Medicine Technical Report No. USAFSAM-TR-89-23. USAFSAM/NGOP, Brooks AFB, Texas Efron, N., and L.G. Carney 1979 Oxygen levels beneath the closed eyelid. Investigative Ophthalmology and Visual Science :93–95. 1983 Effect of blinking on the level of oxygen beneath hard and soft gas-permeable contact lenses. Journal of the American Optometric Association 54:229–234. Eng, W.G., J.L. Rasco, and J.A. Marano 1978 Low atmospheric pressure effects on wearing soft contact lenses. Aviation, Space, and Environmental Medicine 49:73–75. Eng, W.G., L.K. Harada, and L.S. Jagerman 1982 The wearing of hydrophilic contact lenses aboard a commercial jet aircraft. Aviation, Space, and Environmental Medicine 53:235–238.

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium Fatt, I. 1978 Oxygen transmission properties of soft contact lenses. Pp. 83–110 in M. Ruben, ed., Soft Contact Lenses: Clinical and Applied Technology . New York: John Wiley & Sons Fatt, I., and J. C. Chaston 1982a Measurement of oxygen transmissibility and permeability of hydrogel lenses and materials. International Contact Lens Clinic 9:76–88. 1982b Swelling factors of hydrogels and the effect of deswelling (drying) in the eye on power of a soft contact lens. International Contact Lens Clinic 9:146–153. Fatt, I., and D. Lin 1976 Oxygen tension under a soft or hard gas-permeable contact lens in the presence of tear pumping. Journal of Optometry and Physiological Optics 53:104–111. Fatt, I., and S.K. Liu 1984 Oxygen tension under a gas permeable hard contact lens. International Contact Lens Clinic 11:93–105. Fatt, I., and R. St. Helen 1971 Oxygen tension under an oxygen-permeable contact lens. American Journal of Optometry and Archives of American Academy of Optometry 48:545–555. Flynn, W.J., R.E. Miller II, T.J. Tredici, and M.G. Block 1986 Soft contact lens wear at altitude: effects of hypoxia. Aviation, Space, and Environmental Medicine 59:44–48. Heimbach, R.D., and P.J. Sheffield 1985 Protection in the pressure environment: cabin pressurization and oxygen equipment. Pp. 110–113 in R.L. DeHart, ed., Fundamentals of Aerospace Medicine. Philadelphia: Lea and Febiger Holden, B.A., and G.W. Mertz 1984 Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Investigative Ophthalmology and Visual Science 25:1161–1167. Holden, B.A., D. F. Sweeney, and G. Sanderson 1984 The minimum precorneal oxygen tension to avoid corneal edema. Investigative Ophthalmology and Visual Science 25:476–480. Mandell, R.B., and R. Farrell 1980 Corneal swelling at low atmospheric oxygen pressures. Investigative Ophthalmology and Visual Science 19:697–701. Mertz, G.W. 1980 Overnight swelling of the living human cornea. Journal of the American Optometric Association 51:211. O'Neal, M.R. 1988 Contact lenses. Pp. 867–877 in J. G. Webster, ed., Encyclopedia of Medical Devices and Instrumentation. New York: John Wiley & Sons.

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Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium O'Neal, M.R., and K.A. Polse 1986 Decreased endothelial pump function with aging. Investigative Ophthalmology and Visual Science 27:457–463. O'Neal, M.R., K.A. Polse, and M.D. Sarver 1984 Corneal response to rigid and hydrogel lenses during eye closure Investigative Ophthalmology and Visual Science 25:837–842. Polse, K.A. 1979 Tear flow under hydrogel contact lenses. Investigative Ophthalmology and Visual Science 18:409–413. Polse, K.A., M.D. Sarver, E. Kenyon, and J. Bonanno 1987 Gas-permeable hard contact lens extended wear: ocular and visual responses to a 6-month period of wear. Contact Lens Association of Ophthalmologists Journal 13:31–38. Schoessler, J.P. 1983 Corneal endothelial polymegethism associated with extended wear . International Contact Lens Clinic 10:148–155. Spells, K.E. 1965 The physics of the atmosphere. Pp. 28–29 in J.A. Gillies, ed., A Textbook of Aviation Physiology . Oxford: Pergamon Press. Weissman, B.A., and B.J. Mondino 1985 Complications of extended-wear contact lenses. International Eyecare 1:230–240.