The next time you fly, soak a terrycloth hand towel so that it’s dripping wet and hang it up on the flight deck. Then fly a leg that’s at least an hour-and-a-half long. At the end of that time, the towel will be bone dry, the water absorbed by the ultra-low humidity of the cockpit and cabin environment.

Now just think of what that environment is doing to your body. And, in an even more profound and perverse way, to your mind and your ability to make the judgments needed to fly an airplane.

The human body is mostly water–a lot of water, more than 10 gal in a 170-lb human. You know where your water comes in and where it goes later. Depending on input, your general health and the efficiency of your kidneys, you excrete anywhere from a pint to several gallons of water a day. And while you can’t do a lot about how much comes out of you, you can make sure enough goes in so that you avoid the kind of sneaky, debilitating dehydration that can unexpectedly render you unfit for flight.

One of the dangers of dehydration is that what looks like the solution can really be part of the problem. As a society, we tend to drink a lot, but not enough of it is what really hydrates best–water. Many people think coffee or tea hydrate efficiently because of the readily apparent side effects that result from drinking a lot of either. But that quick and easy outflow is a dangerous illusion; coffee and tea are diuretics, and they actually wring more fluid out of your body than they bring in.

Alcohol is also a powerful diuretic, one of the reasons your mouth feels that way after what P.G. Wodehouse’s gentleman’s gentleman Jeeves delicately calls a “late night out.” Even if an aviator rigorously observes the “eight hours bottle-to-throttle” rule, moderate alcohol consumption before a flight exacerbated by the hyper-dry aircraft environment can have a debilitating effect.

Of course, the longer your flight, the greater the effects of cabin dehydration. Airline pilots who routinely fly long overwater routes, such as Los Angeles to Singapore, carry gallon-size jugs of spring water with them in the cockpit, jugs that they empty in the course of those 12- to 18-hr flights.

Moisture Management

“OK, so I get thirsty,” you ask. “What’s the big deal about dehydration?”

Dr. Don Hudson, director of the Air Line Pilots Association (ALPA) medical office, calls dehydration “an underrated factor in many cases of pilot fatigue.” And as dehydration worsens, its effects become more serious and more dangerous because they can easily be mistaken for other conditions. The first symptoms are fatigue, shakiness, nausea and emotional instability, followed by headache, elevated body temperature, pulse and respiratory rate and dizziness. As the body loses more water, the symptoms made familiar by dozens of movies about wanderers stranded in the desert become real: slurred speech, weakness, delirium, swollen tongue, circulatory problems, inability to swallow and kidney failure.

However, for most who fly in the corporate and airline environment, conditions will seldom get that bad. For those aviators, dehydration results from overconsumption of caffeinated beverages and/or alcohol, or simply being too busy to notice their own needs.

An incident submitted to NASA’s Aviation Safety and Reporting System (ASRS) points out a hidden danger of dehydration. Here, a captain on a transcontinental flight reports on how his copilot became ill en route:

“Before departure, the first officer mentioned that the seafood entrée he had consumed the previous evening was giving him an upset stomach and he reported some diarrhea. [Several hours later, in cruise flight at 41,000 ft] the first officer asked to be excused from the cockpit to use the lavatory. As he was leaving the cockpit, I heard a commotion behind me and a flight attendant seated on the jump seat started to scream. I turned around just in time to see the first officer falling backward over the flight attendant and a catering beverage insert that was routinely placed on the flight deck. He fell against the seat, where he struck the shoulder harness lock handle. As he slumped sideways across the cockpit, it was my opinion that he was unconscious.”

The pilot gave his first officer oxygen and he regained consciousness in about 45 sec. He was then helped into the jump seat, where he stayed on the oxygen and recovered somewhat. Through the rest of the flight, alternating bouts of nausea and diarrhea had the copilot shuttling between right seat and lavatory, leaving the captain to struggle through the unanticipated added workload as the only functional pilot in an airplane designed to be controlled by two. Just before landing, the newly revived first officer found himself capable of performing his duties and the landing was uneventful.

In a hospital soon after, doctors found the first officer to be in a state of extreme dehydration, so extreme that intravenous rehydration with two large bags of saline solution was needed.

Loss of Electrolyte or Why Gatorade Sells

This use of saline raises an important point. Proper hydration is necessary to maintain the appropriate balance of blood electrolytes (mineral salts dissolved in the blood, among them sodium, potassium, calcium, magnesium and phosphates). The amount of water in the body and the concentration of electrolytes in the blood are closely related, and both must be maintained at the proper levels for the human body to function properly. This is especially true in the case of the most important electrolyte–sodium, found in carbon-based life forms bonded with chlorine to become that white stuff in the shaker on the dinner table next to the pepper. A high concentration of sodium in the blood causes the body to retain water. It holds back water to dilute the sodium. Conversely, a low sodium concentration causes kidneys to excrete more urine to increase the sodium level. That’s why salt tablets actually stave off dehydration in extreme conditions.

When a person is dehydrated, the concentration of sodium in the blood usually rises. The person then becomes thirsty and drinks. Most people become thirsty with a 1.5-quart deficit, or a loss of 2 percent of total body weight. This level of dehydration triggers a thirst reaction. The problem is that the thirst reaction arrives too late and is turned off too easily. A few mouthfuls of water will turn this mechanism off, probably not enough for adequate hydration.

As any sports medical professional will tell you, the simple act of becoming thirsty means that something in the body is already out of balance. This is why athletes such as distance runners or even relatively sedentary baseball players are encouraged to drink fluids every chance they get.

If the amount of water taken in is not sufficient, your body tries to conserve what it has. The kidneys excrete less urine and perspiration decreases. The body borrows water from cells to replace water missing from the bloodstream. If this process is allowed to continue for long, the cells start to dry out and cease proper function.

When brain cells stop functioning properly, confusion results. The loss of electrolytes (principally from perspiration) usually goes hand-in-hand with loss of water, the start of a vicious cycle because low electrolyte levels slow movement of water from body cells into the blood, further worsening the effects of dehydration.

For mild cases of dehydration, simply drinking water will do. If the body has been moderately depleted of electrolytes (particularly sodium and potassium), commercial sports drinks are effective. More advanced cases of dehydration require victims to receive the fluids they need intravenously, since the cellular impact of dehydration has impaired the patient’s ability to rapidly absorb the fluids they need.

Something Else To Worry About

Failure to stay adequately hydrated over time is a suspected contributor to kidney stones, rocklike masses of calcium in combination with either oxalate or phosphate that can block the human urinary tract with painful, sometimes deadly, results. For as yet undetermined reasons the number of people in the U.S. with kidney stones has been increasing over the past 20 years.

The most widely accepted theory behind kidney stone formation is supersaturation crystallization, a process that starts when dehydration causes an imbalance in the liquids and dissolved solids in urine. The kidneys must maintain a proper amount of water in the body as they remove harmful waste materials. If dehydration occurs, the urine may become overloaded (supersaturated) with substances that cannot dissolve in water. These chemicals and trace elements combine to form crystals that slowly build up and form a stone. White Americans are more prone to develop kidney stones than African Americans, and stones occur more frequently in men. Kidney stones strike most typically between the ages of 20 and 40. Once a person gets more than one stone, others are likely to develop. A person with a family history of kidney stones may be more likely to develop stones.

Writing not long ago in American Airlines’ Flight Deck magazine, American captain George Shanks described the agony of his own experience with kidney stones, along with the revelation that came with its successful treatment. “I will explain the error of my ways,” he wrote. “I fly international. Long flights. I don’t drink enough water. I sit for long periods of time without getting out of the seat to stretch my legs and relieve myself…I brought this on myself.”