Hydration on Flight. How much should you drink?

Disclaimer! This is NOT an opinion piece, but rather a collection of various readings and clippings which serve to spur further exploration in the topic. These are not full articles but simply excerpts from the bulk of reading material that is available.  As much citation and references were taken with regards to the topic. Legitimacy and accuracy of the clippings are read at your own discretion.
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Hydration on Flight
the importance of staying hydrated
Base Line of 8 Glasses of Water

As rule of thumb from various sources, to stay properly hydrated under normal conditions, you need to drink eight glasses, containing eight ounces of water or other non-caffeinated drinks per day. That equates to about 2 liters per day.

Your body loses about 2 liters or 66 ounces of water per day via your urine, sweat, breathing and bowel movements. Your most basic water requirements involve drinking to replace these water losses. This is where the eight glasses of water per day rule is based, which helps you replace lost water. However, some people need more or less water. Keep in mind that you also take in water through the foods you eat. Soups and fruits like watermelon have high water content that also can contribute to hydration.

The Aircraft Environment
High and as Dry as the Sahara

- Marie Claire
Dehydration is a major issue when traveling by airplane,’ Yasmin tells us. ‘The problem arises due to spending long periods of time in a climate-controlled environment where the relative humidity can be as low as 10-15%, which is three times drier than the Sahara desert!’

‘In an average 10 hour flight, men can lose approximately two litres of water and women around 1.6 litres. This means that on a London to Sydney flight a passenger could lose up to 4 litres and 8% of their bodily water.’
And since research shows that spending six hours in a climate simulator that mimics conditions on a plane, leads to a significant decrease in hydration status, with around 2% loss in total body water content over this time, despite them drinking 400ml of water, it’s safe to say, flying is bad for hydration levels.

‘Around 50% of this decrease is due to ‘insensible water loss’, mostly due to respiration. Insensible water loss is due to a combination of the mildly hypoxic environment which increases breathing rate and the dry cabin air and is difficult to prevent or reduce this type of water loss.’

‘A 1-2% decrease in hydration has significant impact on the health and mood of passengers and can affect cognition, attention, memory and critical thinking, as well as leading to feelings of tiredness, fatigue and irritability.’

Oh, and if that’s not enough, because your mucosal membranes become dehydrated while flying, their ability to trap bacteria or viruses becomes less efficient and effective so you’re more likely to catch an illness.

Pass us the big bottles of water, we say!

 According to The Airliner Cabin Environment and the Health of Passengers and Crew

Humidity Levels in an Aircraft Environment

- At cruise altitudes, the outside air contains very little moisture, and the main sources of humidity in the cabin air are respiration and evaporation from the skin of occupants. The steady supply of dry outside air is more than sufficient to flush the human-generated moisture from the cabin and maintain a low moisture content in the air, typically 10–20% relative humidity at cruise altitudes. Such values of relative humidity are below comfort guidelines (American Society of Heating, Refrigerating, and Air-Conditioning Engineers. 1992).
- Theoretically, at least, cabin air could be humidified to comfortable values. But a number of problems are associated with such humidification, including the weight penalty associated with the water that would need to be carried, the biological growth that is often associated with humidifiers, and the maintenance requirements of humidification systems.

- The humidity required for passenger comfort might exceed that which generates some of the safety concerns for the aircraft operations described previously. Whole-cabin humidification systems are therefore not normally included on aircraft. Air supplied to the cockpit is humidified on a small fraction of the current aircraft fleet, but on most aircraft the cockpit is normally drier than the passenger cabin air because of the higher ventilation rates in the cockpit.

- With that amount of moisture generation and the FAA minimal design flow rate of outside air of 0.042 kg/s, the water vapor concentration in the cabin air will be 0.0050, or 0.5%; this corresponds to a relative humidity of about 18% at typical cabin air temperatures.
That value might be a slight underestimate of the humidity because some occupants, particularly the cabin crew, will be more active than others.
From Google*The relative humidity in the Sahara Desert is 25 percentThe annual averages for relative humidity in Singapore range throughout the day from a maximum of 96 percent to a minimum of 64 percent. Humidity levels are fairly steady in Singapore, year round. Overall, December is the city's most humid month.*

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Hydration and Safety

According to FAA (Federal Aviation Administration) From: Pilotfriend.com/Plane and Pilot.com (Nina Anderson is a Hawker pilot, FAA Wings Program human factors seminar leader and ISSA Specialist in Performance Nutrition.)

There is scant attention given to it. Most pilots overlook it. Some shrug it off, while others simply don’t know about its effects in the cockpit. The problem?

Pilot dehydration. Most pilots are unaware of its devastating effects and symptoms, which can increase the risk of aircraft incidents and accidents, even during a mildly warm day.

In order to heighten general aviation’s awareness of this often overlooked condition, the Federal Aviation Administration (FAA) has recently added pilot dehydration—its symptoms, causes, effects and corrective actions—to its latest Practical Test Standards list of physiological conditions. The FAA believes that educating pilots about dehydration will not only decrease aircraft incidents, but also save pilots’ lives one day.

Most pilots associate dehydration with thirst and assume that they can drink any type of liquid. This isn’t always the case. A pilot’s dehydration condition is caused by a lack of water within the body cavity due to high body temperatures, a dry aircraft environment, excess caffeine, antihistamines and inappropriate fluid intake. Many soft drinks, teas and juice drinks don’t constitute adequate hydration substitutes, as they contain caffeine and sugar that may compromise absorption of the water content.

Hot cockpits and flight lines also cause dehydration.

What about the 72-degree/22degree C cockpit? Pilots should concern themselves in that environment, too, since average humidity in the cockpit is low, causing a dramatic increase in fluid loss. 

Everyone must be aware that un-replaced water losses equal two percent of body weight and will impact your body’s ability to regulate heat.

- 3% water loss of Body Weight There is a decrease in muscle cell contraction times.
- 4% water loos of Body Weight there is a five to 10 percent drop in overall performance, which can last up to four hours.

The symptoms of dehydration go beyond thirst. In an effort to respond to the brain’s need for fluid, the kidneys reabsorb water through the urine, creating fluid retention and frequent urges to visit the bathroom.

Dry skin is also an indicator of dehydration, as the skin gets most of its moisture subdermally. The brain is 75 percent water and, when it needs to replace lost fluid, it can manifest certain symptoms, such as headaches, light-headedness and fatigue. Dehydration also contributes to:
- fuzzy thinking
- poor decision-making
- dizziness
- muscle fatigue.
Long-term effects
- wrinkled skin
- impaired memory function
- dry hair
- brittle nails
- constipation
- susceptibility to colds
- because of extremely dry nasal passages, sinus infections. 

*Best to drink cold 4.4Degree C Water*

So how do you avoid dehydration in the cockpit? You’ll need to permanently attach yourself to a water bottle and drink from it regularly. The Federal Air Surgeon Bulletin suggests drinking cool, 40-degree Fahrenheit 4.4degree C water before feeling thirsty. This will help you stay ahead of the game, keeping you hydrated before the “thirst mechanism” sets in. 

An alternative to water is to simply drink mineralized (electrolyte) water. Electrolyte drinks, more commonly known as sports drinks, are generally designed to replace the fluids (water) and electrolytes (sodium, potassium, chromium, manganese, etc.) lost during stress, body temperature regulation and exercise. Most contain sugars which may lower a pilots systemic blood-sugar levels and precipitate fatigue. 

The FAA also suggests staying away from coffee, sodas and teas—otherwise called diuretic drinks. These beverages contain caffeine, alcohol and carbonation, which causes excess urine production or decreased voluntary fluid intake—a sure sign of dehydration.

Coffee Diuretic effects

Coffee is basically the life-saver of most college students. It gives us that energy we need to get through the day filled with classes and other activities. Coffee has tons of health benefits like helping to lower your risk of getting type 2 diabetes, cancer and can help burn calories but is also gets a bad rep for being dehydrating because they are a diuretic (they make you pee!)

But is coffee actually dehydrating? And if so, how much water do we need to drink to offset the coffee we intake? If the old rule of eight glasses a day of water is true, then we might have to really start upping our water intake.
Luckily, it turns out coffee isn’t doing as much damage as we think. Our hydration levels because of coffee can actually differ in the short term and long term. In the short term, yes you probably need to be drinking more water. If you drink a lot of coffee in a short amount of time then you will need to pee more, thus losing hydration. This means that for every cup of coffee or two cups of black tea, you should drink one cup of water to make up for the diuretic effect.

However, in the long term you may not need to make up for coffee’s effects at all. After a while of drinking coffee your body actually adjusts and can actually become less dehydrating. But if you stop drinking coffee for a period of time and then start drinking it again, you lose that tolerance to coffee and you should drink water, in the 1 to 1 ratio, until your body adjusts again.

It is not always easy to tell if you are dehydrated but the pee check usually is a good indicator. If your pee is too dark yellow, then you definitely need to be drinking more water. Also, if you are feeling thirsty, sweating excessively, exposed to heat, weakness or dizzy you may be dehydrated.

However if it’s hot or you’re flying at higher altitudes, you will need to drink more to prevent dehydration, as these conditions tend to increase the rate of water loss from the body.

You’re best off to sip a little at a time. So bring a couple of water bottles into the cockpit and drink regularly while you’re flying, and keep safe!

Sources:
http://www.marieclaire.co.uk/life/travel-dehydration-509326
https://www.ncbi.nlm.nih.gov/books/NBK207472/ The Airliner Cabin Environment and the Health of Passengers and Crew
http://www.flyingmag.com/technique/tip-week/pilot-hydration
http://www.planeandpilotmag.com/article/flying-high-and-dry/#.Wdq3eDBx1PY
https://spoonuniversity.com/lifestyle/this-is-how-much-water-to-have-to-drink-to-offset-your-coffee-addiction

Image Sources
https://esnoticia.co/noticia-13552-8-glasses-of-water-a-day-a-myth
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https://www.google.com.sg/imgres?imgurl=http%3A%2F%2Fichef.bbci.co.uk%2Fwwfeatures%2Fwm%2Flive%2F1280_640%2Fimages%2Flive%2Fp0%2F5h%2F41%2Fp05h41nr.jpg&imgrefurl=http%3A%2F%2Fwww.bbc.com%2Ffuture%2Fstory%2F20170919-how-flying-seriously-messes-with-your-mind&docid=oRZTdpSWD41oeM&tbnid=XxL_lgM1CLKqAM%3A&vet=10ahUKEwi00cGU2erWAhWFu48KHU1CBaYQMwhRKCIwIg..i&w=1280&h=640&client=firefox-b-ab&bih=701&biw=1536&q=dry%20airplane%20cabin%20environment&ved=0ahUKEwi00cGU2erWAhWFu48KHU1CBaYQMwhRKCIwIg&iact=mrc&uact=8
https://www.google.com.sg/imgres?imgurl=http%3A%2F%2Flostempireherbs.com%2Fwp-content%2Fuploads%2F2015%2F10%2Fdehydration.jpeg&imgrefurl=https%3A%2F%2Flostempireherbs.com%2Fcoffee-and-dehydration%2F&docid=zxc7GJb2E_xwBM&tbnid=Im67SvoNc2ARSM%3A&vet=10ahUKEwjP5ZGx2erWAhVC6Y8KHXTpBx0QMwhYKCkwKQ..i&w=750&h=545&bih=701&biw=1536&q=coffee%20diuretic%20effects&ved=0ahUKEwjP5ZGx2erWAhVC6Y8KHXTpBx0QMwhYKCkwKQ&iact=mrc&uact=8
http://www.snopes.com/medical/graphics/coldwater.jpg