Stuck in the middle seat next to someone with raging body odor? You could get kicked them off the flight.
Several US airlines reserve the right to kick passengers off of a plane if they are have a bad odor. It’s just a line in a pages-long document known as the conditions or contract of carriage that airlines include on their websites.
Delta says (pdf) it could deny transportation “when the passenger has a malodorous condition.” You can’t fly with American Airlines if you have an “an offensive odor not caused by a disability or illness.” United Airlines goes a step further in denying service to “passengers who have or cause a malodorous condition (other than individuals qualifying as disabled).”
The rules don’t only apply to the US. A passenger was reportedly kicked off an Air Canada Jazz flight in 2010 after passengers complained about an odor. Air Canada’s conditions of carriage state that the airline can deny service “when the passenger has an offensive odor (for example, such as from a draining wound).”
Body odor—or ozochrotia as scientists like to call it—is a result of bacteria living on your skin. When we sweat (and we all have to in order to keep our bodies from heating up) it’s a bacterial Thanksgiving. Families of bacteria called Corynebacteria, Propionibacteria, Micrococcus, and Staphylococcaceae go to town on the mix of salt and water, and churn out smelly acids.
What you’re wearing can exacerbate your body odor, too. Polyester, which is found in sweat-wicking athletic gear, is a low-tech petri dish for Micrococcus in particular. Although cotton shows sweat stains and stays wet for longer, bacteria are less able to grow on these surfaces after they’re gone.
Just like sweating, passing gas is a normal part of the human condition dictated by bacteria. Flatulence occurs as our gut bacteria break down various foods. Depending on the variety of microbes and foods, farts can take on all sorts of smells, and can even indicate certain gastrointestinal disorders.
Farting is benign, although awkward in close quarters. What’s worse is in airplanes, cabin pressure is lower than what we experience on the ground. Instead of being 760 mm Hg (one atmosphere), air cabin pressure mimics the pressure of being at 6,000 to 8,000 feet (1800–2400 meters) above sea level, or about 600 to 560 mm Hg. At lower pressures, gases—including those in your intestines—expand, Danish researchers explained in 2013. “This larger amount of intestinal gas will have to be released via the anus into the cabin air,” they write.
Fortunately, simple hygiene like showering and using deodorant or antiperspirants (which work by directly killing the bacteria or preventing you from sweating to starve them to death, respectively) should be enough to keep you smelling well enough to fly. Excessive sweating, a condition called hyperhidrosis, or any kind of digestive condition that causes more gas than usual could be argued to be medical conditions—and thus exempt from most airlines’ rules regarding odor.
It is up to airline crew or the complaints of passengers to them to detect an offender. Flight attendants may just try to move one of the passengers. But there isn’t a science to it. Much like airlines’ dress codes, what could be an “offensive” odor is highly subjective. One passenger’s body odor is another’s fragrance.