While Humid Conditions May Impact Aircraft Performance, Water Is Your Friend

Airline travel remains the safest mode of transportation in the world, but the aircraft cabin environment can pose health-related challenges, especially on long-haul flights. While concerns of developing deep vein thrombosis may prompt you to periodically get up and stretch your legs, the most common challenge that flight crews and passengers experience is a loss of fluids, or dehydration.

According to the Mayo Clinic, “Dehydration occurs when the body uses or loses more fluid than it takes in. Then the body doesn’t have enough water and other fluids to do its usual work. Not replacing lost fluids leads to dehydration.”

Airline crewmembers and passengers can experience dehydration due to their increased breathing rate caused by lower oxygen pressures in the cabin. Approximately 70 percent of the human body is made up of water, and, in the standard airliner environment, individuals lose approximately eight ounces of water per hour, mostly from normal breathing.

Airline industry regulators typically require that airliners maintain a cabin pressure equivalent to an altitude of 6,000 to 8,000 feet above sea level. Aircraft interior conditions are managed by environmental control systems that regulate inbound and outbound airflow. For most airliners, cabin pressurization is a result of engine compressor bleed air, which is pulled from outside the airplane by the engines, dried, and used to pressurize the cabin.

However, newer aircraft models, including the A350 and the B-787 Dreamliner, are designed to maintain higher cabin pressures with more oxygen available. In most airliners, the air-pressure level in the cabin during cruise is about 8,000 feet. However, thanks to the use of carbon-fiber-reinforced plastics and other composite materials, the B-787 is able to maintain air pressure closer to 6,000 feet while the A350 can maintain approximately 5,500 feet.

The B-787 and A350 also do a better job than comparable models controlling cabin humidity, an important factor when addressing dehydration. Both airplanes accomplish this by retaining the water vapor produced by human breathing and perspiration, rather than expelling it from the aircraft. Consequently, they maintain as much as 22 percent humidity levels, compared to the 2 to 7 percent found in other airliners.

Keep in mind that mild dehydration can occur with the loss of just 1.5 percent of your body’s water level. Low humidity levels in the airplane also dry out sinuses and mucous membranes, which play an important role in protecting your body from airborne diseases.

A paper from the Aviation Medicine Advisory Service’s medical article library—Dry and High by Linda Werfelman—provides suggested steps to take to keep properly hydrated:

• “Drink about 2.0 quarts (1.9 liters) of water every 24 hours, although the exact amount varies widely. Drink before you become thirsty, and drink from a container that allows you to measure daily water consumption;
• Limit consumption of alcohol and caffeine. Both are diuretics, which increase the excretion of urine;
• Monitor work and recreational activities and stop what you’re doing if you feel light-headed or dizzy. Exercise can result in water loss that’s difficult to overcome quickly;
• Be aware of your physical condition, especially if you’ve recently been ill; and
• Remember that your body’s adjustment to a major change in weather, such as the sudden onset of hot weather, can take one to two weeks.”

U.S. Transportation Security Administration regulations allow pilots to bring an empty water bottle to work. Pack it in your flight bag and fill it once you’ve passed through airport security. Canadian Air Transport Security Authority regulations permit pilots to bring 100-milliliter/100-gram or smaller containers of liquids in their flight bags.

Stay hydrated to fly right!

This article was originally published in the August 2025 issue of Air Line Pilot.