An Airplane Cabin at 35,000 feet
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). The higher you climb upwards, away from sea level, there is less air above you so the pressure drops.
Even though you don't notice it (you won't feel buouyant) the decreased pressure has a significant impact on your body's oxygen intake.
When pressure drops, the amount of molecules or air, contained in a given volume, falls following a law discovered by Robert Boyle in the 1600s.
And this means that there are less molecules of oxygen in every breath you take.
At the normal cruising altitude of an airplane (35,000 ft., or 10.650 m) the atmospheric pressure drops to 3.8 psi, which is almost one quarter of the sea level pressure. This means that the quantity of oxygen in the air is also 26% of that found at sea level.
Nobody can live for long with so little oxygen: death ensues quickly due to hypoxia (lack of oxygen).
For this reason aircraft cabins are pressurized (higher pressure means more oxygen), but it does not reach sea-level values because the airplane cabin would have to be built to withstand a higher differential pressure.
Planes are designed to have a certain structural strength which limits the differential pressure that the cabin can tolerate (Differential pressure is the difference between the external and internal pressure).
Planes can manage around 8 psi of differential pressure (they could be built to resist higher pressures but then they would be heavier and require more fuel to fly: costs would soar).
The trade-off is that aircraft are pressurized to an intermediate value: at 35,000 ft. with an external pressure of 3.8 psi and a cabin that can resist a maximum differential pressure of 8 psi, the inner pressure can’t exceed 11.8 psi.
So the plane is lighter but you get less oxygen!
A pressure of 11.8 psi is roughly equivalent to being at an altitude of 6,214 ft (1,894 m), which has aroung 81% of the oxygen content you’d get at sea level. Not too bad, it’s like living in Denver Colorado.
But it can get worse: even though air flight regulations limit the maximum cabin altitudes to 8,000 ft. (2,440 m), a study found that the average value is actually 6,214 ft – but the maximum was 8,915 feet (2.717 m).
2. Hypoxia and its consequences
So you should’nt suffer from hypoxia at 6,214 feet because people begin to notice the effects of lack of oxygen at altitudes higher than 8,200 ft (2.500 m).
But a small proportion of sensitive flyers will suffer from some of its mild symptoms, such as sleep disruption, dizziness, and "oliguria" (reduced output of urine) and uncomfortable heartbeat (tachycardia).
To make matters worse, sitting immobile, strapped into a cramped airplane seat for long periods of time and not keeping properly hydrated may restrict blood flow and decrease oxygen levels in your body’s cells, causing hypoxia.
Lack of oxygen will make you feel faint if you stand up all of a sudden during a flight. Tip: flex your arms and legs to improve circulation and then stand up slowly.
3. In-flight flatulence
Yes, passing gases or farting during a flight is very common problem that is rarely spoken about.
At low pressure, gases expand: at 6,000 feet (1.830 m), a volume of 100 ml at sea level becomes 130 ml (8 fl. oz. expands to 10.4 fl oz.). A 30% increase in the volume of all gases, including those inside your body: your gut and your middle ear.
Expanded gas inside your ear is the reason that 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 but, 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.
But let's get back to farting: Expanded gases will also bloat your gut and you and your fellow passengers will pass more gas. Farts are part of the airline cabin experience!
But don’t worry, cabin air is renewed about 25 times every hour, diluting the gases which will hopefully fade away quickly.
Tips for 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 from carbonated drinks before flying: if you had a soda before takeoff, the gas trapped in your stomach will expand it by 30%.
4. Very Dry Air
The aircraft’s cabin air is pressurized by the plane’s jets which compress the thin air and heat it; it is mixed with air reculating from the cabin and the temperature is adjusted to make 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 humidity inside the cabin is the original humidity of the cabin air at take off and all the moisture lost by the crew and passengers (evaporated pespiration and the water vapor exhaled as they breathe).
This value is lower than the relative humidity of the Driest Place on Earth, Atacama Desert in Chile which averages 17.3% Relative Humidity!
Humidifiers could be used to improve the situation but airlines say that it leads to condensation and corrosion; furthermore the extra water would add weight to their flights and higher fuel costs.
5. Nasty side effects of dry cabin air
Our bodies are accustomed to an average relative humidity that is over 60%; the typical value in temperate countries; so the very dry 15% has many negative effects:
- This paper says that flu viruses survive longer and spread far more easily in dry air
- The sinus membranes inside your nose become very dry and this irritates them. The same thing happens to your throat and bronchial system. Tip: If you suffer from asthma keep your medication handy.
- Dry air makes tears evaporate quicker so your eyes become dryer. This loss of moisture irritates your eyes. Tip: Wear glasses instead of contact lenses and use moisturizing eye drops.
- Your skin also loses moisture and becomes dry and itchy. Tip: Moisturize with cream and pack some lip balm for your dry lips.
- Your body loses even more water by evaporation (as you breathe and sweat), and you dehydrate. Loss of water "thickens" your blood more (its viscosity increases) so your heart has to pump harder to circulate it through your body. This makes blood flow more inefficient causing oxygen levels to drop slightly inside your cells, disrupting its processes and causing waste toxins to build up in them.
- Blood "stagnation" is made worse by the fact that you sit for long periods of time: the muscles are inactive so they can’t pump body fluids back up toward the heart. The position of your legs when you are seated also increases pressure in your leg veins. This causes blood to collect in your lower legs and feet. Your feet swell and the constricted blood vessels coupled to thickened blood may produce blood clots, this is a condition known as Deep Vein Thrombosis (DVT).
Avoiding clotting and swelling
So do the on board exercises, flex your feet and walk to the back of the plane and back frequently (once every hour). Elevate your feet and legs frequently. It will keep blood and oxygen flowing and remove wastes.
Deep Vein Thrombosis or DVT affects 1 out of every 6000 people who fly and can cause pain, serious trouble and even death.
Air travel compression socks may help prevent them.
You can also take an aspirin before flying (though it is not proven that its blood-thinning effect may help to prevent DVT).
The most serious complication of DVT is a pulmonary embolus, that is, a blood clot breaking away and heading to your lungs where it can cause death.
Also note that persistent calf symptoms may occur after a DVT.
- Stay away from alcohol because the it dries out your cells and is a diuretic it will make you urinate more frequently: causing even more water loss.
- Caffeine, which is found in coffee, tea and sodas and energy drinks is also a diuretic.
- Skip the salty snacks, they dehydrate you and raise your blood pressure too.
- Keep hydrated: 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.