I’ve done a lot of long-haul travel recently… And somewhere over a large ocean, sandwiched between a window seat I can’t see out of and a stranger who’s made the bold choice to remove their shoes, I found myself wondering: why does everyone on this plane seem to be slowly releasing their farts into a sealed metal tube that I am also sitting in? Is this just me? Is it the food? Is it physiological? And… perhaps more pressingly… what exactly am I breathing?
It turns out this is a legitimate area of scientific inquiry, with a medical acronym and everything. Welcome to HAFE: High Altitude Flatus Expulsion.
HAFE was first formally described in a 1981 letter to the Western Journal of Medicine by two doctors: Paul Auerbach and York Miller. They had noticed the phenomenon during expeditions in the San Juan Mountains of Colorado at around 11,000 feet. They were not the first to observe it; the mountaineer Joseph Hamel had written about it as early as 1820, which tells you something about how long humans have been gassing their way up mountains. But Auerbach and Miller gave it a name and a mechanism, and the mechanism is elegantly simple: Boyle's Law.
Boyle's Law states that at a constant temperature, the volume of a gas is inversely proportional to the pressure surrounding it. As you ascend to higher altitude, atmospheric pressure drops. The gas already sitting in your intestines… a perfectly normal mixture of nitrogen, carbon dioxide, hydrogen, methane, and trace amounts of the sulphur compounds responsible for the more memorable olfactory experiences… has no choice but to expand to fill the lower-pressure environment around it. At 8,000 feet, intestinal gas can expand by up to 30%. At cruising altitude in a commercial aircraft (cabins are typically pressurised to the equivalent of 6,000 to 8,000 feet), the same physics applies. You are not imagining it. The gas gets bigger and occupies more space.
But here is where it gets more interesting. In 2013, a group of Australian researchers found that HAFE was occurring at altitudes as low as 5,900 feet, and that the peak in flatulence frequency happened eight to eleven hours after arrival at altitude… not immediately upon ascent. If it were purely Boyle's Law in action, you would expect an immediate effect as gas expanded. The lag time pointed to a second mechanism.
Their explanation involved carbon dioxide dissolved in the bloodstream. At altitude, atmospheric pressure drops, which means the partial pressure of gases, including CO2, in the environment also falls. CO2 naturally dissolved in blood and body fluids begins to diffuse out of solution and into the intestinal lumen, adding to the gas volume in the gut over several hours. The researchers recorded an average of 14 farts per person in the post-ascent period. This is roughly double the baseline rate, and they argued this was too large an increase to be explained by gas expansion alone. It is worth noting that this study involved participants ascending from 1,000 to 5,900 feet in 40 minutes, so for those of you ascending via gondola to a ski resort, you now have some advance notice of what to expect roughly a third of the way through your après ski session.
Add to this the confounding factors that cluster around high-altitude travel: the freeze-dried and high-fibre trail food on hiking expeditions, the airline meals engineered for nobody's digestive comfort, the carbonated drinks that introduce ready-to-expand gas before you’ve even taken off, the prolonged sitting that slows gut motility... and then you have a fairly comprehensive recipe for a gassy experience.
Now, the question I suspect more of you are actually wondering about: is any of this being blown at your face?
The visceral concern here is that you are sitting in a recycled air environment with 200 other people who are all experiencing some version of the above, and that the air being circulated around you contains meaningful quantities of whatever is being expelled… faeces. This concern is understandable but is, reassuringly, not really supported by the evidence.
Commercial aircraft cabin air is a 50/50 mix of fresh outside air drawn in through the engines and recirculated cabin air that is passed through HEPA filters before being returned to the cabin. Boeing's HEPA filters operate at 99.97% efficiency at particle sizes of 0.3 micrometres. This filter covers bacteria, most viruses, and the aerosol droplets that carry them. Studies of cabin microbial burden have found airborne bacterial and fungal levels considerably lower than typical ground-level public spaces.
Crucially… and this is the bit worth knowing.. air from the aircraft toilets is not recirculated into the passenger cabin. It is vented separately, and no studies have identified aerosol release from onboard waste systems into the passenger environment. So, thankfully, the air you are breathing has not been through the bathroom… and the main thing being recirculated is other people's low-grade anxiety and the ambient scent of reheated pasta.
👋 For the new joiners: I’m Suraj, also known as Dr Sooj - a primary care doctor & health content creator. I love navigating the complex world of health and wellness and breaking down complicated concepts.
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References:
1. Auerbach PS, Miller Y. High altitude flatus expulsion (HAFE). West J Med. 1981;134(2):173–4. PMID: 18748805
2. Greenwald AJ, Allen TH, Bancroft RW. Abdominal gas volume at altitude and at ground level. J Appl Physiol. 1969;26(2):177–81. PMID: 5765206
3. Slaney G, et al. High altitude syndromes at intermediate altitudes: a pilot study in the Australian Alps. Med Hypotheses. 2013 Oct;81(4):547-50. PMID: 23891043
4. National Academies of Sciences, Engineering, and Medicine. Research on the Transmission of Disease in Airports and on Aircraft. Washington DC: The National Academies Press; 2015.
5. Sheffer AL, et al. Microbial burden and diversity of commercial airline cabin air during short and long durations of travel. PLoS ONE. 2020; PMC7099242