Reduced Pressure at Altitude

Military aircraft frequently fly at altitudes, ranging from 30,000 to 40,000 ft, at which air pressures are greatly reduced from those at sea level. Air pressure at 30,000 ft is 4.36 psi, just 30 percent of the ambient pressure at sea level (14.7 psi); at 40,000 ft, air pressure is only 2.72 psi, just 18 percent of that at sea level. As the concentration of air thus decreases with altitude, so does the concentration of oxygen (measured as its partial pressure) available to crew members and their eyes.

To offset these high altitude effects, aircraft cabins are routinely pressurized to mimic lower altitudes. However, this ability to artificially pressurize the cabin is limited by aircraft engine size and at high altitudes can only lessen, not eliminate, the effects of the thinning atmosphere.

Generally, high-performance (fighter-attack-reconnaissance) aircraft remain unpressurized to about 8,000 ft (10.9 psi). The pressure is then held constant until the difference between cockpit pressure and outside pressure is 5 psi—at about 23,000 ft. This pressure differential is maintained as the aircraft flies higher. For example, at 30,000 ft, the cabin pressure is equivalent to an altitude of 12,000 ft; at 40,000 ft, the cabin pressure is equivalent to an altitude of about 17,000 ft (O'Neal, 1990).

In tanker-transport-bomber-type aircraft, a sea-level atmosphere can often be maintained in the cabin until the pressure difference between the cockpit and outside is about 8.6 psi—at roughly 23,000 ft. Again, this pressure differential is held as the craft ascends, resulting in a cabin pressure equivalent to an altitude of 3,500 ft at 30,000 ft and cabin pressure equivalent to an altitude of 8,000 ft at 43,000 ft.

All things considered, aircrews spend much of their flying time at pressure-equivalent altitudes of 8,000 to 16,000 ft At these altitudes, substantially less oxygen is available relative to sea level. Since the amount of oxygen passing through a contact lens is directly related to the partial pressure of oxygen in the surrounding air, the amount of oxygen available under a contact lens will also diminish at higher altitudes. Air Force calculations suggest that at 8,000 ft the oxygen available beneath the lens may be 35 percent less than that available at sea level; at 12,000 ft, oxygen availability may be 60 percent less (O'Neal, 1990).

In addition, low cabin humidities may dehydrate soft lenses, inducing a further decrease in oxygen available beneath these lenses by as much as 15 percent Such reductions in oxygen availability may result in hypoxia with consequent corneal edema and other stresses to ocular physiology.

Low Humidity and High Air Flow

Aircraft heating and air conditioning systems circulate cabin air constantly and remove considerable amounts of moisture from it. Relative humidity in the cabins of most military aircraft sometimes reaches 5 percent and is normally in the 5 to 15 percent range (O'Neal, 1990; Dennis, 1990). These very dry conditions can lead to rapid desiccation of ocular surfaces. There is much anecdotal evidence for humidity-induced discomfort in contact lens wearers on both commercial airliners and military aircraft (Josephson, 1990; Dennis et al., 1988).

In addition to being quite dry, cabin air may also be moving rapidly, adding to the desiccation effect. It is not uncommon for pilots to have a stream of air directed at or near their faces to relieve fogging or sweat-related problems. In helicopters, especially those with doors open, there is also a great deal of air movement, with the possibility for drying effects.

High Particulate Count in the Cabin

All military aviation settings considered here are subject to contamination with particulate matter—from dirt and sand to bits of insulation—borne in the cabin air. Such particulates may be blown out by the air conditioning unit, borne aloft when the aircraft experiences “negative Gs” during maneuvers, or blown in from outside the craft in the case of helicopters. Helicopter missions in dusty or arid locations are especially at risk from particulate contamination.

Depending on the nature of the object and the type of lens worn, the presence of a foreign body beneath a lens can cause irritation, distraction, or acute pain severe enough to warrant removal of the lens or to temporarily incapacitate the contact lens wearer. In this regard, soft lenses are generally much more forgiving of particulate contamination than hard lenses. Foreign body involvement can also lead to later complications if corneal abrasion results or pathogens are introduced beneath the lens (Josephson, 1990).

Noxious Gases, Fumes, and Smoke

Noxious gases from aircraft exhaust; outgassing of aircraft components, weaponry, or cargo; and mechanical discharge from heating and air conditioning systems may all contaminate the cabin environment Organic solvent vapors, carbon monoxide and other partially combusted organic compounds, hydrazine gas from missile propellant, oil mists, ozone gas, and even carbon dioxide are just a few of many possible contaminants. If these compounds are absorbed preferentially by the contact lens and released later to the eye, they can cause ocular distress or damage. While rigid lenses will not generally absorb chemical vapors or fumes, hydrogel lenses may absorb some compounds, especially those that are water-soluble.

The effect of airborne fumes or vapors on the contact lens wearer will depend on their concentration, the exposure time, the affinity of the lens for these compounds and thus the amount absorbed, the rate of release of the compounds, and their toxicity or ability to cause allergic response in the eye (Josephson, 1990).

Data on the types and concentrations of gaseous contaminants in the military cockpit are scant in the public domain (many remain classified information), but anecdotal sources and those few literature reports available indicate that a wide array of compounds is present in exhaust gas, which is commonly found in trace amounts in the cockpit. Army investigators measuring toxic gases in the cockpit of a standard observation helicopter found that exhaust fumes consisted of a complicated mixture of over 200 compounds, including paraffins, olefins, napthenes, aromatics, acids, aldehydes, alcohols, ketones, ethers, esters, chlorinated hydrocarbons, and sulfur- and nitrogen-containing compounds. Fortunately, none of these was present in high concentrations (Stroud et al., 1980; Pollard et al., 1979).

Even if toxic compounds are found to be present in fairly high concentrations, it is hard to predict how they will interact with the lens/eye system or if they will precipitate a toxic reaction, since few studies have been carried out in this area. There is some evidence to suggest that contact lenses can act as a short-term barrier to some organic solvent fumes, such as xylene and trichloroethylene, resulting in a lower exposure than if no lens is worn (Josephson, 1990; Nilsson and Andersson, 1982).

Other evidence suggests that hydrogel lenses known to have taken up certain organic compounds do not exacerbate the eye's response to the chemicals compared with direct exposure of the eye to the compounds (Josephson, 1990; Nilsson and Andersson, 1982). However, it should be emphasized that there is no comprehensive understanding of the short-term effects of exposure of the lens/eye system to chemical vapors, nor have any studies addressed the long-term effects of the uptake and slow release of low levels of toxic compounds by hydrogels.

At a practical level, the most serious contaminant of the cabin environment may be cigarette smoke. Smoking is prevalent in many cockpits, especially tanker-transport-bomber-type aircraft, but also in high-performance aircraft as well. Many contact lens wearers find cigarette smoke a potent irritant. It has also been reported that ozone, which is present in increased concentrations in commercial airliner cabins and probably in military aircraft as well, acts as an irritant for both those who wear contact lenses and those who do not (Josephson, 1990).

Unhygienic Conditions for Lens Care

Good hygiene practices are essential to the safe removal, disinfection, and reinsertion of contact lenses. Lenses handled in unhygienic conditions risk contamination leading to infection or other complications. This is especially true with hydrogel lenses, for which the risk of introducing and culturing infectious agents is much higher than with rigid lenses. The importance of good hygiene is further magnified with lenses worn in an extended-wear mode. While the chances for contaminating lenses are fewer, due to less frequent insertions and removals, the consequences of contamination and subsequent infection are heightened when those infrequent lens manipulations do occur.

The question of hygiene is probably not much of a problem for fighter-attack-reconnaissance and tanker-transport-bomber personnel who have regular access to airfield facilities. However, Army and Marine helicopter pilots routinely experience prolonged missions amidst field conditions in which good lens hygiene is impossible. These pilots may live in camps without running water, electricity, or the means to adequately clean their hands to ensure a reasonably sterile lens insertion or removal. In combat situations, conditions may be worse still, as cleanliness takes a back seat to more practical survival considerations.

High Acceleration

As explained earlier, high G-forces experienced during rapid aircraft acceleration can influence lens placement, with possible consequences on vision. Older PMMA lenses with small diameters showed unacceptable movement downward, decentering from the pupil and affecting visual acuity. However, more recent trials with larger-diameter hydrogel lenses showed minimal movement up to 8 G _z (see Chapter 1).

Rapid Decompression

Rapid elevation gain as experienced by the crews of high performance aircraft can give rise to decompression effects such as bubble formation under the contact lens. The quantity of gas that can remain dissolved in a solution relates directly to the atmospheric pressure it experiences. Thus, when the tear film under a contact lens experiences rapid decompression, the formation of bubbles can occur (Flynn et al., 1987) as gas (nitrogen) is forced out of solution.

Bubble formation has been observed beneath both rigid and hydrogel lenses. In the case of hydrogels, these bubbles formed at 6,000 ft. However, for both hydrogel and RGP lenses, bubbles dissipated without causing any apparent visual effects (see Chapter 1).

Inadequate Lens Care Systems or Regular Follow-up Care

Successful contact lens use relies on a system of competent fitting and regular follow-up care by qualified specialists (i.e., optometrists or ophthalmologists), and the timely provision of lens care solutions and lens replacements. Ensuring that this system is in place and can function smoothly in wartime will be a matter of considerable logistical complexity and expense. Even in peacetime, establishing such a system is no simple task.

Currently, there is no uniformity among the services in terms of lens types employed, wear mode, lens care solutions, fitting, or follow-up care. For those with access to established military medical facilities, such as most Air Force aviators, the chances of receiving professional ophthalmic care are quite high. But for aircraft carrier-based aviators and Army units in the field, high-quality care and resupply with lens care solutions may prove a greater problem. For example, a carrier is typically responsible for maintaining the health of up to 8,000 personnel, but no dedicated eye professionals are currently assigned to carrier duty (Markovits, 1990).

Temperature Extremes

Aviators—especially helicopter pilots—may experience extremes in temperature both on the ground and in the air. Arctic helicopter missions may encounter temperatures of –40ºF, while missions occurring in desert regions may confront temperatures of 120ºF. Civilian clinical experience to date seems to indicate that such extremes pose few special problems for contact lens wearers. In spite of the ambient temperature, the cornea and related structures seem to maintain themselves with little ill effect.

Overmotivation

Military aviators comprise a select and highly motivated group of individuals. The pilot's job is an esteemed one and he is generally loath to jeopardize his flying time in any way. He realizes that contact lens-related complications might well result in being grounded until the complication clears and lens wear can be resumed. Thus, he may be inclined to avoid or delay reporting minor symptoms and hope for the best. In addition, a considerable “macho” ethic surrounds the flying profession that may encourage tolerance of considerable ocular pain without complaint. For this reason, some military authorities fear that aviators may not heed early warning signs that could presage more severe complications later on.

F-A-R Missions

Although F-A-R missions generally involve the most hypoxic conditions, these conditions are not ordinarily experienced for prolonged time periods. Excellent night vision is critical. It should be noted that current Air Force regulations require that contact lens users carry spectacles as a backup whenever they fly. These aviators are trained to be able to remove their lenses within 45 seconds if a lens problem develops and to replace them with their backup spectacles.

A mission profile involving characteristic environmental factors, visual requirements, mission length, spectacle-incompatible equipment, and other factors in a F-A-R mission is provided below.

• Environmental factors:
  high altitude/low oxygen;
  particulates;
  low humidity;
  high air flow;
  high G-forces;
  noxious gases/fumes.
• Mission duration: usually short (2 to 4 hours) time in air, but occasionally much longer; total contact lens wear time might include considerable ground time in addition to actual flight time.
• Spectacle-incompatible equipment: laser protective eyewear.
• Visual requirements:
  perfect (20/20) visual acuity (for pilots, especially naval carrier-based pilots);
  excellent night vision (for pilots, especially naval carrier-based pilots);
  unimpaired peripheral vision (for “checking six”).
• Other factors:
  Air Force requires that backup spectacles be carried in the flight suit;
  contact wearers cannot readily access their lenses due to head gear, but in an emergency can remove them within 45 sec;
  access to qualified ophthalmic specialists for follow-up care not always available (Navy; carrier pilots).

T-T-B Missions

Much longer times aloft typify T-T-B missions. Although hypoxic conditions are less severe than for F-A-R missions, humidity is just as low and may become more of a factor due to increased exposure time. However, in contrast to F-A-R pilots, T-T-B aircrews can more easily adjust or remove their contact lenses or apply artificial tears if drying becomes a problem. Perfect visual acuity is not as critical among the T-T-B aircrew, except among pilots. As in the F-A-R setting, Air Force contact lens wearers must have backup spectacles in their flight suits at all times.

A mission profile for T-T-B activities might typically involve:

• Environmental factors:
  high altitude/low oxygen (less severe than F-A-R);
  low humidity/high air flow;
  noxious fumes; cigarette smoke;
  particulates.
• Mission duration: longer than F-A-R, usually 12 to 24 hours, but could extend to 36 hours; likely extended-wear mode required for contact lenses during mission.
• Visual requirements:
  among pilots, visual acuity important but not as many critical tasks requiring perfect acuity as in F-A-R;
  perfect visual acuity not essential for nonpilots.
• Other factors:
  backup spectacles required in flight suit;
  in-flight handling of lenses not a problem.

Helicopter Missions

Helicopter missions differ considerably from both F-A-R and T-T-B missions in terms of environmental stresses encountered and mission duration. Helicopter flight is all at low altitude, so hypoxia is of no concern. However, these missions take place in the most severe conditions with regard to particulate contamination and hygiene. Low altitude flight in arid and dusty climates, often with open cabins, leads to nearly inevitable involvement of debris in the eye. Moreover, the nature of helicopter missions dictates that they operate in the field, often for weeks at a time. During this period, the unhygienic conditions of the field prevail, marking perhaps the most critical problem with regard to contact lens wear. Finally, helicopter personnel currently face the greatest spectacle incompatibility problems of any aviators, even as they face the greatest possible stumbling blocks to the successful use of contact lenses. A typical mission profile for attack helicopters (Army and Marines) might include:

• Environmental factors:
  low-altitude, high-particulate settings (deserts, arid regions);
  turbulent airflow;
  unhygienic conditions for extended periods;
  temperature extremes.
• Mission duration: likely to encounter field conditions for 1 week or longer, possibly without opportunity to remove lenses.
• Spectacle-incompatible equipment (absolute incompatibility: no spectacle use possible):
  head gear with special optics;
  chemical protection masks.

In summary, it is possible to characterize aviation environments for purposes of identifying the risks posed by the use of contact lenses. The remainder of the report will address these risk factors and recommendations to minimize their consequences.