Dehydration in the Cockpit


Well-Known Member
Hey, here is a paper I just got done writing regarding dehydration. For all those who are interested.

Becoming a pilot can be the most exhilarating experience in a person’s life. Along with the training you begin to see that a day spend entirely on the ground is not nearly as rewarding as one where you fly. But flying has risks. Yet it is the understanding of these risks that the entire pilot training program is hinged upon. Understanding risks and anticipating risks is just what piloting is about. However, concentrating on keeping the plane flying, and not crashing into the ground, can often distract pilots from other elements of their health. Each year pilots are required to maintain proficiency with the flight controls and report to physicians for medical evaluations. Yet, pilots are very susceptible to commonly overlooked health risks, among the most common is dehydration.

Dehydration can be mentally debilitating yet many pilots welcome dehydration and actually practice it. Bizarre as this may seem, the pilot, whose workload in the cockpit is often frenetic, either cannot or will not be able to visit a restroom while in the air and will commonly chose not to drink fluids in order to minimize trips to the bathroom. However choosing what you drink can determine your long-term effectiveness while piloting.

The allure of the coffee buzz is an addiction that most do not see as such. Although sipping a cup of coffee in the morning can wake you up, you will also be closing off an important physiological pathway in your kidneys. Coffee is a diuretic; actually it is caffeine within coffee that is a diuretic. Therefore talking about coffee means we are talking about any fluid with caffeine in it, or any diuretic. Caffeine acts as a stimulant by affecting the adrenal function of the body. Once ingested, caffeine will stimulate the production of both epinephrine and norephinephrine (A Pilot Study of Some Physiological and Psychological Effects of Caffeine) The quick result is increased energy. It also constricts the blood vessels and elevates heart rate. A diuretic is any substance that impedes the process by which the body both absorbs and reabsorbs water within the blood stream. The diuretic works as a switch to turn off the pathway by which the body maintains a certain level of water; this process takes place in the kidney. Once the diuretic is in the blood stream the kidney will no longer absorb as much water; additionally the body will tend to shed water already within the body. The affects of a diuretic depend on the amount ingested and the type. Caffeine’s affects lasts somewhere between 1/2 of an hour and two hours. This adds an additional problem when caffeine is used as a stimulant; as the affects wear off, more caffeine is desired to maintain the same level of awareness. Ironically, the effectiveness of caffeine diminishes with use and increasing dosages is the only means to continue stimulation. As you ingest more caffeine, over the long term you build a tolerance and in the short-term you become edgy and stressful. Meanwhile fatigue sets in while the desire to drink more coffee continues (Stress and Fatigue in Flying Operations). And to top of the complications, the body begins to become chronically dehydrated. You will also become less aware of you surroundings, possibly nauseous, sleepy, dizzy, and suffer from muscle cramps.

Coffee is popular and common with the early morning pilot who wakes to prepare for a day of flying. The use of caffeine can lead to uncomfortable symptoms and possibly lend itself to difficulty sleeping. Often, after a stressful day in the cockpit pilots wish to relax and turn to alcohol to aid in relaxation. Alcohol is the second most common form of a diuretic in our diets. Not only is it a diuretic but it also can disturb the REM functions of sleep resulting in a less restful night sleep and hence a greater need for coffee in the morning.
Mechanisms of Dehydration

Dalton’s Law governs gases in our atmosphere. “It tells us that the total pressure of any mixture of gases (with constant temperature and volume) is the sum of the individual pressures (also called partial pressure) of each gas in the mixture. Also, partial pressure of each gas is proportional to that gas's percentage of the total mixture.” (Linda D. Pendelton, 1999) In this law we see that the proportions of gas in the upper atmosphere are relative to the altitude where they are found due to the diminishing pressure in the atmosphere as a function of altitude. One important element to remember is that this governs water vapor too. However, when pilots talk about Relative Humidity they need to see that the ability for a parcel of air at 15,000ft to carry water vapor diminishes directly proportionate to it pressure. When determining Relative Humidity for altitudes, always remember you measuring a fraction of the Relative Humidity at sea level. For example, using the chart below, when parcel of air at 15,000ft has a Relative Humidity of 30%, that parcel of air contains only 56.64 percent of the water vapor that the same parcel of air, at sea level, would have if measured at 30% Relative Humidity. Or, that parcel of air at high altitude has a Relative Humidity of 9% at Sea Level. The catch is that it would only measure 9% if it were the same temperature. An additional factor in water vapor carrying capacity is temperature changes quickly the higher we go in the atmosphere thus diminishing the amount of water vapor a parcel of air can hold.
ALT mmHg InHg %MSL
SL 760 29.92 100.00
1000 733 28.86 96.40
2000 706 27.82 92.98
3000 681 26.82 89.63
4000 656 25.84 86.36
5000 632 24.89 83.18
6000 609 23.98 80.14
7000 586 23.09 77.17
8000 565 22.22 74.26
9000 542 21.38 71.14
10000 523 20.57 68.75
15000 429 16.89 56.64

Standard Atmosphere, F.A.A. 1980

Translating this into water loss means we must take into account air temperature of cockpit, temperature of the skin and perspiration, and respiration rate. These are all factors in determining the amount of fluid loss a person will have over time. Studies have indicated that most humidity levels in aircraft while cruising is between 5-10%. Contrast this against the guidelines that are set for air quality in buildings where 20% is the minimum and 30% is the norm.

Why not humidify the cockpit and the plane environment? Unfortunately, there are many benefits to having a dry air inside the cabin. The condensation that would develop inside the cabin either at high altitude or upon decent could possibly create corrosion problems for the airframe. Additionally, this condensation, which would form between the outer skin of the aircraft and the insulation blankets within the fuselage, could lead to short circuited wires. In fact in testing, Boeing found that after replacing the old wet blankets with dry blankets within a 737-300, there was an 80-pound difference weight due to water. Within the cabin the dry air is also an environment inhospitable to bacteria, fungus, and potential viruses.

Symptoms of dehydration are headache, nausea, sleepiness, dizziness and cramping. The long term effects of dehydration are called mild chronic dehydration and can lead to the development of kidney stones in the bladder, increased risk of urinary tract cancers, increased risk of colon cancer and increased risk of breast cancer. Passing a kidney stone can be a painful ordeal. While taking an Emergency Medical Technician course, my instructor said that on the pain scale, 10 being the highest; giving birth is measured as a 7 and passing a kidney stone is a 10; she had experience with only one of these.

I interviewed Dr. Howard Donner MD, for this report. Dr. Donner is a prominent High Altitude physician; He has worked for NASA on developing medical protocols for the Space Shuttle as well as having been a doctor on numerous Mount Everest expeditions. He also holds a Commercial Pilot’s license while living in Telluride, Colorado. Dr. Donner’s general approach to dehydration is that is far more serious and prevalent than we commonly believe. Although dehydration is not a killer, it can and will affect a person’s performance and general feeling of wellbeing. Additionally, Donner believes that one of the greatest factors in jet lag is actually dehydration. “The air within the cabin is extremely dry. Combine this with the tendency for fluids to arrive in small plastic cups and this puts the pilots or passengers on the backside of the dehydration curve. One way of staying aware of how dry the air has become is to pretend the body as the only humidifier on the plane. Each exhaled breath is nearly 100% saturated with water; although it is an uphill battle, use your body to humidify the dry air inside the cockpit.”

And additional health concern with mild chronic dehydration is Deep Vein Thrombosis (DVT). DVT occurs when the body produces clots, often in the leg and “ part of the clot can break off (embolise) and travel to the lung circulation, where it lodges in and blocks one of the pulmonary arteries – ‘ pulmonary thromboembolism’ (PTE). This in turn can lead to low blood oxygen, circulatory failure, collapse and death.” (Department for Transport, Health in Aircraft Cabins) This is most likely to occur in a person who has been sitting still for long periods. Dehydration also dramatically increases the chances of emolism. When asked, Dr. Donner commented on the likelihood of DVT. “It is inactivity, dehydration, and some additional factor all combined that lends to clotting and therefore DVT. Whenever you are dehydrated you have increased the proportion of red-blood cells to plasma and you will clot easier. However, a person who is reasonably healthy will not exhibit these tendencies with out illness or injury as the additional factor.”

Some people measure hydration by both the color of their urine and it pungency. However, there are flaws in both those practices. Once a person has ingested a diuretic, the body will stop absorbing and begin shedding water that was once in the body. The end result is that urine quickly becomes clear giving the same appearance of being hydrated. A common misconception is that once you are drinking fluids you are hydrating. This leads many to believe that drinking any fluid will aid in replacing fluids.

Believing that more pungent your urine is a sign of dehydration is accurate. However, the body can have pungent urine while processing ingested compounds within the kidney. While the kidney is processing excessive amounts of vitamins, minerals, or drugs, urine will be pungent, giving the appearance of being dehydrated, yet you may not be. Donner believes using the color of your urine as the guideline is the easiest and simplest method of staying on top of dehydration. On a normal day, your urine should be not appear dark or bright in color. As a guideline you should need to pee at least once per two hours.

Hydration is an elusive state. In fact by the time the body triggers the mechanism for thirst, it is already to late. By the time a person becomes thirsty, the body has typically lost 1.5 quarts of fluid, or roughly 2% of your total body weight. Adding to this mild chronic dehydration, quenching a thirst can seem quite easy however, gulping down cold fluids typically will not hydrate you until you have drunk enough to replenish the loss. Drinking fluids with potassium and other electrolytes will also aid in replenishing the body of this loss. The best method for hydration is to have water bottle in cockpit. Stay alert of the frequency you urinate and avoid diuretics.
Summer heat tends to overcome the Jetstreams' AC so I usually drink a lot of water and juice from the galley. I'm not a huge coffee drinker (except when I'm cold) and I try to not go overboard on cokes as well.
I'm not a huge coffee drinker (except when I'm cold)

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I actually read someplace that coffee is the worst thing you can drink when you get cold as it actually makes you colder (the caffene).
Screw it! Coffee ROCKS!
Screw it! Coffee ROCKS!

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Yeah.... right up until you realize you blow 25 big ones per week on COFFEE alone!
602, I think you are mistaking alcohol for caffeine. Alcohol will actually bring your body temp down as well as decrease the profusion of blood in the extremities. Despite the image of the big dog coming to save you with a jug of Schnapps on his collar, having alcohol when you are cold only makes you colder.

Caffeine makes you edgy and elevates your heart rate. I suppose you could argue it speeds metabolism and runs you dry on energy quicker. But in studies they have not found a correlation between metabolism and caffeine, with the exception of it burning fat stores more rapidly, up to three hours after ingestion when taken in moderation before an endurance athletic event.

Yeah.... right up until you realize you blow 25 big ones per week on COFFEE alone!

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You are drinking the wrong coffee or else you have coffee IV hooked up to your arm. Stay away from Starbucks and places where it costs $2-3 per cup. Drink coffee from McDonalds, BK, or Waffle House for less than a buck including refills. Or {gasp} drink free coffee from the coffee pot at work!
Dave, come on, quality coffee is the only way to go. I would imagine McDonalds, etc's coffee would be the same as the coffee at work. There is no better money spend than on a good quality coffee buzz. It is like having great wine without the buzz; you get a great taste and great buzzzzzzzzz.

My solution: One shot of espresso in the afternoon, if I need it. ( notice the verb, need, HA) I wrote the dehydration paper because it is so near to my heart . . . I mean kidney.