In our increasingly mobile society, flying has become a way of life. In 2016, U.S. airlines carried a record 719 million domestic and 104 million international passengers (U.S. Department of Transportation, 2017). Those numbers do not reflect the flight attendants and pilots who fly, often several times each week. As of 2015, there were more than 590,000 active, certified pilots in the United States; approximately one-fifth were commercial pilots. According to the U.S. Bureau of Labor Statistics (2016), more than 113,00 flight attendants are employed by various airlines.

Many of our patients travel by air occasionally or on a regular basis, either as passengers or as crew members. It is important that we understand the implications of flying on the ocular surface, especially in the case of contact lens wearers. The primary concern in designing airplane ventilation systems is ensuring proper cabin air pressure. Failure to do so may result in hypoxia, fatigue, nausea, headaches, or pulmonary edema. Air quality is a secondary consideration in maintaining the closed environment of the cabin.

Your Eyes at 35,000 Feet

As altitude increases, the atmosphere becomes thinner, i.e., lower in atmospheric pressure, and the amount of moisture that it can absorb declines proportionally. Gładyszewska-Fiedoruk (2012) evaluated the air quality in an aircraft through a commercial flight lasting three hours. During the flight, carbon dioxide levels increased from 1,100ppm to stabilize at around 1,700ppm. Throughout the flight, the initial humidity level of 55% rapidly declined to 33% and eventually stabilized at 17%.

McCarty and McCarty (2000) provided a one-page survey evaluating dry eye symptoms to 4,000 commercial Australian pilots. The survey included information about the specific aircraft that they piloted, i.e., jet aircraft versus propeller-driven planes, average cruising altitude, smoking status, use of distance spectacles and contact lenses, and use of eye drops. The participants were also queried about experiencing common signs and symptoms of dry eye while in flight or independent of flying. Those included dryness, burning, itching, gritty sensation, injection, tearing, discharge, or foreign body sensation.

A total of 1,246 (31%) of the pilots responded. Among those, 72.3% reported dry eye symptoms while flying, but only 5.4% reported dry eye symptoms independent of flying. This data suggest that flying significantly increases the risk of dry eye.

Tesón et al (2013) subjected individuals who had confirmed dry eye to a simulated in-flight cabin environment (SIC). They evaluated symptoms and markers, such as clinical signs and tear levels of 16 inflammatory mediators, prior to and after two hours in SIC.

After SIC, dry eye patients became more symptomatic and suffered a significant decrease in tear stability and tear volume. They also showed a significant increase in global and area-specific corneal staining. Following SIC, tear film levels of IL-6 and matrix metalloproteinase significantly increased, while epidermal growth factor significantly decreased.

The implications are significant. Flying, especially more than two hours in duration, may lead to inflammation and dryness, two contributors to ocular surface disease. We should ask patients how frequently they fly as part of our dry eye screening process. CLS

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