The expulsion of intestinal gas, scientifically known as flatulence, is a normal biological process. Understanding what happens when gas is released in a liquid medium requires examining the gas’s chemical makeup, the physics of gas in water, and the potential release of other biological material. The consequences of passing gas in water are far different from doing so on dry land.
The Chemical Composition of Flatulence
The majority of the gas expelled during flatulence is composed of five non-smelling gases: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. These collectively make up over 99% of the volume released. Nitrogen is often the most abundant gas, primarily resulting from swallowed air. The other odorless gases, like hydrogen and carbon dioxide, are byproducts of bacterial fermentation in the large intestine as microbes break down undigested food particles.
The characteristic and often unpleasant odor is caused by trace compounds, typically making up less than 1% of the total volume. These compounds are volatile sulfur-containing molecules, most notably hydrogen sulfide, which imparts a rotten-egg smell. Other smelly compounds include methyl mercaptan, dimethyl sulfide, indole, and skatole, which contribute a fecal scent. The concentration of these potent trace compounds, heavily influenced by diet, determines the pungency of the flatulence.
How Water Alters Gas Physics and Sound
When gas is expelled underwater, it immediately forms bubbles due to the surrounding liquid medium. The gas has a much lower density than water, causing the bubbles to rise rapidly toward the surface due to buoyant force. As the bubbles ascend, the pressure exerted by the water column decreases, causing the gas within the bubble to slightly expand. The water momentarily contains and shapes the gas cloud, preventing the rapid, turbulent dispersion that occurs in air.
The most noticeable difference is the near-total absence of sound compared to passing gas in the open air. The sound of flatulence is caused by the vibration of the anal sphincter and surrounding tissues. When this occurs underwater, the sound energy must transfer from the gas bubbles into the surrounding water. Water has an acoustic impedance thousands of times higher than air, creating an extreme impedance mismatch at the gas-liquid boundary. This mismatch causes almost all the sound waves to be reflected back into the gas, effectively trapping and muting the sound energy.
The Hygiene Factor: What Else Is Released?
Flatulence is rarely just gas alone, which raises a hygiene consideration in a shared water environment. The force of the gas being expelled can aerosolize microscopic particles of fecal matter and bacteria from the rectum. While largely invisible, these tiny, bacteria-laden droplets are propelled outward.
This particulate matter is typically contained by clothing when out of the water. However, in a swimming costume or while unclothed, these particles are released directly into the water. These microscopic solids carry bacteria like Escherichia coli and other intestinal flora. Although the concentration of bacteria released is small compared to the contamination risk from a fecal accident, the release of these particles contributes to the overall microbial load in the water.