Is It True That You're Breathing in Fecal Particles When You Detect That Unpleasant Odor?

Midway through your morning jog, you slow down and come to a halt. Ahead lies a path lined with vibrant wildflowers in full bloom. Overnight, this once green and brown landscape has turned into a colorful spectacle. You anticipate a delightful fragrance, but instead, an unmistakable and unpleasant odor fills the air. The park hosted a marathon yesterday, and the rows of portable toilets are now releasing a foul stench.

The foul odor enters your nostrils and passes over structures called nasal turbinates or nasal concha. These bony features are found in the upper nasal cavity, the space behind your nose. Resembling rolled-up scrolls, the nasal turbinates increase the surface area inside the nasal cavity [sources: Encyclopedia Britannica, Netter Medical Illustrations].

When air passes the nasal turbinates, it arrives at the olfactory epithelium, a small area on the roof of the nasal cavity about the size of a postage stamp. This region houses millions of olfactory receptor neurons. These neurons are activated by odor molecules and send signals to the olfactory bulb, located beneath the brain's frontal lobe, via the olfactory nerve. The brain then deciphers these signals to identify smells. Humans can distinguish around 10,000 different scents, though each person may perceive the same odor differently, even under identical conditions [source: Binns].

But what happens when your nose picks up an unpleasant smell, like feces? Are you actually breathing in fecal particles when you detect such an odor? Scents are detected when tiny molecules released by surrounding objects enter our noses. These objects can range from freshly brewed coffee and blooming flowers to, unfortunately, a recently used public restroom.

Farts consist of gases like nitrogen, oxygen, methane, hydrogen, and carbon dioxide. However, the foul smell is primarily due to volatile methyl sulfides, with hydrogen sulfide gas and methanethiol contributing to a lesser extent. These compounds are produced by bacteria in the digestive system and are detected by the nose as volatile organic compounds [source: Cormier]. Importantly, you're not inhaling solid fecal matter, just gases. (As Popular Science notes, fecal bacteria are everywhere, but only a tiny fraction of them could potentially cause illness).

However, certain situations might result in inhaling more than just gases. A nurse's curiosity about whether her flatulence in the operating room could contaminate the area led a microbiologist to investigate. He had a colleague fart onto two petri dishes placed 5 centimeters away—once with pants on and once without. The next day, the dish exposed to the pants-free fart showed bacterial growth. Analysis revealed that these bacteria were typically found in the intestines and on the skin. This experiment demonstrated that if someone farts naked near your nose, you could inhale bacteria carried by airborne droplets of fecal matter [source: British Medical Journal].