The chemistry of halitosis may seem complicated, but in the end it all boils down to a few simple, smelly molecules. Volatile sulfur compounds (VSCs), which are substances excreted by oral bacteria, make up the bulk of oral odor. However, other compounds can tinge one's mouth with bad breath.
While organic compounds like skatole and putrescine can give the mouth a rotted odor, many experts agree that ammonia is one chemical that you don't want to smell on your breath. A study published in the journal Critical Reviews in Analytical Chemistry recently examined different methods of detecting ammonia on exhaled air. Its word for the physical causes of ammoniac breath is "hazards."
Amino acids in the body create ammonia all the time, since the compound is the body's primary source of nitrogen. Once it has been used, the liver converts ammonia to urea, which then passes through the kidneys and out in the urine. The scent of ammonia, interestingly enough, is similar to this human waste product, though it tends to have an alcoholic whiff to it.
Basically, if a person's breath reeks of ammonia, there is a very high chance that they are experiencing liver or kidney failure. Studies in the journals Kidney International and Digestive Diseases and Sciences point to high oral ammonia counts as indicators of end-stage renal disease and hepatic encephalopathy, necessitating immediate medical attention.
However, individuals whose breath smells of everyday VSCs can simple use oral care probiotics to crowd out odor-causing bacteria.