The effects of flying on your body

Flying takes its toll

“A six hour flight has exactly the same effect on your body as sitting for six hours atop an 8,000 foot mountain in the Atacama Desert (the driest place on Earth)”

Below we will tell you why.

An aircraft cabin and Atacama desert 15% Relative humidity
Atacama and an aircraft cabin are equivalent in air dryness (Atacama credits). A .Whittall

The combination of low atmospheric pressure and therefore less oxygen, dry cabin air and inactivity coupled with lack of sleep, and can make you feel dizzy and tired during a flight. If you add to this alcohol or caffeinated beverages you may feel even worse.
You can’t avoid some of these factors but you can do things to reduce their side effects:

1. Low Air Pressure

Atmospheric pressure at sea level is equivalent to a column of water 33 feet high (10 meters) pressing down on you (14.7 psi – pounds per square inch). As you climb or fly upwards the pressure decreases simply because there is less air above you.

You don’t notice the difference in pressure (i.e. feel lighter or decompressed) but it has an important consequence: as pressure decreases so does the quantity of molecules in a given volume of air (Boyle’s Law) and this is the cause of altitude sickness: there is not enough oxygen in the air for your body to use.

At the normal cruising altitude of an airplane (35,000 ft., or 10.650 m) atmospheric pressure drops to 3.8 psi -or 26% of the sea level pressure, therefore the quantity of oxygen in the air is also 26% of that found at sea level. This reduced oxygen content will lead to death due to hypoxia (lack of oxygen).

Therefore aircraft cabins are pressurized but not to sea-level values because the structural strength of the airplane determines how much differential pressure the cabin can tolerate (Differential pressure is the difference between the external and internal pressure). On average planes can manage around 8 psi of differential pressure (they could be built to withstand higher pressures but they’d be much heavier and use more fuel, becoming far too costly to fly).

This means that aircraft are pressurized to an intermediate value: at 35,000 ft. with an external pressure of 3.8 psi and a maximum differential pressure of 8 psi, the inner pressure can’t exceed 11.8 psi.

This pressure of 11.8 psi is equivalent to flying at an altitude of 6,214 ft (1,894 m) which provides you with 81% of the oxygen content you’d have at sea level. This is a tolerable trade off.

Air flight regulations limit the maximum cabin altitudes to 8,000 ft. (2,440 m) however a study involving 204 flights found that the average value is 6,214 ft – with a maximum of 8,915 feet (2.717 m).

Hypoxia

So you should not suffer from hypoxia because in general people begin to notice the effects of lack of oxygen at altitudes in excess of 8,200 ft (2.500 m).

But some of its mild symptoms may be felt by sensitive people. These are sleep disruption, oliguria (reduced output of urine), tachycardia (racing, uncomfortable heartbeat) and dizziness.

Furthermore, sitting in a cramped seat for hours and not keeping properly hydrated may hamper blood flow and reduce oxygen levels in your cells, provoking hypoxia.

You may feel faint if you stand up all of a sudden during a flight due to these factors, so flex your arms and stand up slowly.

In-flight flatulence

Another side effect of low pressure is that gases expand: a volume of 100 ml at sea level becomes 130 ml at (6,000 feet – 1.830 m). That is a 30 percent increase in volume for all gases, including the gases trapped inside body cavities like your middle ear for instance. And that is why your ears pop with altitude. Actually the air expands and flows from your ear to your throat through the Eustachian tubes, equalizing pressure in a gradual manner bu, if this happens all of a sudden it causes the “popping” feeling.

Important tip. If you have an ear, nose or sinus infection the congestion will not allow the air to flow freely and may cause pain and even rupture your ear drum. So avoid flying with those conditions.

Expanded gases will also bloat your gut and you and your fellow passengers will pass more gas inflight. Farts are part of the airline cabin experience!

But don’t worry, cabin air is renewed roughly 25 times an hour so the gases will fade away fairly quickly -hopefully unnoticed.

Avoiding gas build up. You can eat foods that reduce flatulence before you fly, try rice, dairy products and fish. Avoid peanuts.

Another discomfort is a bloated stomach. Stay away carbonated drinks before flying: if you had a soda before takeoff, the gas trapped in your stomach will expand it by 30%.

2. Very Dry Air

The airplanes’ cabin air is pressurized by the aircraft’s turbofan (jet) engines, which compress it. Compression heats the air which has to be cooled, it is then mixed with air that is recirculating from the cabin and temperature is adjusted to keep it comfortable for the passengers.

But the plane intakes outside air which at an altitude of 35,000 feet has less than 1% humidity.

The only source for humidity in the aircraft is the original humidity of the cabin air when the plane took off and the moisture lost by the passengers and crew as they breathe and perspire.

This means that (see this paper and this article) that air humidity decreases inside the cabin to values of around 10 to 15%. And the longer the flight, the drier it gets.

This value is lower than the relative humidity of the Driest Place on Earth, Atacama Desert in Chile
 which averages 17.3% RH!

Humidifiers could be used but airlines argue that condensation causes corrosion and the water would add weight to their flights –meaning higher costs.

glass with fresh water
Drink plenty of water and keep hydrated

Nasty side effects of dry cabin air

Your body is used to an average of +60% relative humidity (the typical value in temperate countries) so this extremely dry in-flight atmosphere has many negative effects:

  • According to this paper flu viruses survive longer, and spread more easily in dry air
  • It dries out your nose’s sinus membranes irritating them; it does the same to your throat and bronchial system. If you suffer from asthma keep your medication handy.
  • It also causes dryness of the eyes because it increases the evaporation of tears. Loss of moisture irritates your eyes. Wear glasses instead of contact lenses and use moisturizing eye drops.
  • Your skin also loses moisture becoming dry and itchy. Moisturize with cream and pack some lip balm for your dry lips.
  • Your body loses more water by evaporation (as you breathe and sweat), and you dehydrate. Loss of water makes your blood more viscous and your heart has to pump harder to circulate it through your body. Flow becomes more inefficient and oxygen levels drop slightly inside your cells, and waste toxins build up in them too.
    This stagnation in blood flow is worsened by sitting for long periods of time which causes blood to collect in your feet and lower legs. Your feet swell and blood may produce blood clots.
    So do the on board exercises, flex your feet and walk to the back of the plane and back frequently. It will keep blood and oxygen flowing and remove wastes.

More on clotting and Deep Vein Thrombosis in a coming post.

Avoid dehydration

  • Stay away from alcohol because the it dries out your cells and is a diuretic it will make you urinate more frequently.
  • Caffeine found in coffee, tea and sodas and energy drinks is also a diuretic.
  • Skip the salty snacks, they dehydrate you.
  • Drink water to replace the loss: the Aerospace Medical Association recommends drinking eight ounces of water (240 cl) for every hour you’re in the air.
    But don’t overdo it because too much water can dilute the electrolytes in your blood and make you feel ill.

Summing it up

Drink one glass of water every hour you fly, avoid booze and caffeine. Get up and walk around the cabin regularly, flex your muscles, and enjoy your flight.

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