When someone passes gas, odour molecules and bits of faecal matter from that person’s bowels are expelled into the air. How much is released depends on the filtering efficiency of their underwear and pants. Odour molecules are very tiny, so a lot of those escape into the air … they’re what causes your nose to register the bad smell. Particles of faecal matter that are very small, along with intestinal bacteria and other germs, may also make it into the air … and you breathe them in and swallow them.