In mammals like humans, the gases come from two sources: swallowed air (for example, while eating, talking or drinking) and gases naturally created by bacteria in the body during digestion. Of the gases that come out during flatuence, most of it is nitrogen (which has the chemical formula N and makes up between 20% to 90% of all the gases), followed by hydrogen (H), carbon dioxide (CO), oxygen (O) and methane (CH).