Question: What causes increased flatulence on airplanes?
Short answer: Decreasing air pressure.
Long answer: Many people have the subjective impression that they fart more while on airplanes. In at least one case, excessive flatulence has led to a flight being grounded and the police being called in. Medical professionals have called for the introduction of fart-filtering materials into airline seat cushions and blankets. But is it true that people fart more often while in flight? As with so many things flatological, there is very little in the way of empirical data.
Of course, the idea is entirely plausible, for reasons of simple physics. As we have noted previously, the Ideal Gas Law dictates that as air pressure decreases, gas volume increases, so that a fixed amount of gas in someone’s rectum will take up more space when atmospheric pressure is low. The increased volume in an enclosed space might then increase the impetus to release the gas by farting. A similar explanation applies to High Altitude Flatus Expulsion, the increase in flatulence that occurs as hikers climb a mountain peak. This was actually studied as early as 1820 by the Russian physician Joseph Hamel, who reported “pneumatic flatulence” during an ill-fated ascent of Mont Blanc in Geneva (Simons & Oelz, 2001).
Commercial aircraft typically fly at very high altitudes, on the order of 36,000 ft. Breathing would be nearly impossible at this altitude without the intervention of systems for pumping pressurized air into the cabin. While these systems are very effective, they do not restore the air pressure to what would normally be found at sea level. Instead, the cabin pressure is usually maintained at a level that would be experienced by a hiker who climbed to an altitude of about 8,000 ft. Thus, cabin pressure during airline flight is quite low relative to most people’s daily experience, setting up the physical conditions for increased flatulence.
To study this phenomenon empirically, we recently undertook a series of airplane trips on standard commercial aircraft. During each trip, we performed detailed measurements of air pressure and ambient fart smell, using an air quality sensor placed strategically within the airplane. Below we show illustrative results for two of these trips:
The panel on the left shows a roughly 5-hour flight. Take-off occurred approximately 40 minutes into the recording, and it was followed by an abrupt drop in air pressure (red line), from just over 1000 hPa to about 760 hPa. This pressure drop was accompanied by a barrage of farts that started 30 minutes after takeoff and lasted for about an hour (blue line). These farts were unlikely to have come from any one person, as the sensor was placed near the aisle, amidst circulating air. Rather, the increased stink reflects the aggregate contributions of many passengers and crew members.
The fart smell peaked at a level 300% higher than pre-takeoff levels, before declining sharply over the ensuing 30 minutes. For the remainder of the flight, fart stink was actually quite low, presumably as a consequence of the very effective air circulation systems of modern airplanes. Indeed, air quality for most of the flight was actually better than what is found in the baseline miasma of a typical home.
Note that the HEPA filters used to remove contaminants from airplane air probably do nothing to reduce fart smell. But about 60% of the air at any time is drawn from outside the plane, and this is surely responsible for the elimination of fart smells.
The figure panel on the right shows a much shorter flight. As in the previous example, takeoff caused a decline in air pressure, which in turn led to a spike in fart smell. The increased fart output lasted for about 45 minutes, which is similar to the duration of the flight, so that the passengers were in this case inhaling farts for the entire trip.
There are several important consequences of these observations:
1) Because the initial release of farts lasts for about an hour, the smelliest flights will be the shortest ones, as the stink will be present for the entire duration on these flights.
2) The increased flatulence appears to be triggered by the decrease in air pressure, rather than the low air pressure per se. The physiological reason for this is unclear. Perhaps the passengers exhaust their supply of fart gas during the first hour after takeoff. Or perhaps some other process causes the body to adapt to reduced air pressure.
3) The Ideal Gas Law further states that increases in temperature will have the same effect as decreases in pressure (V = nRT/p). It would therefore be interesting to examine levels of fart smell for conditions in which temperature increases abruptly, such as when people step into a sauna.
These questions will be the topic of future investigations.