There is a curse that has plagued humanity since ancient times. The Greeks fought it by chewing aromatic resins, while the Chinese resorted to egg shells. In the ancient Jewish Talmud, it’s even considered legal grounds for divorce. This horrible scourge is halitosis, otherwise known as bad breath. But what causes it, and why is it so universally terrifying?

Well, think of some of the worst odors you can imagine, like garbage, feces or rotting meat. All of these smells come from the activity of microorganisms, particularly bacteria, and, as disgusting as it may sound, similar bacteria live in the moisture-rich environment of your mouth. Don’t panic. The presence of bacteria in your body is not only normal, it’s actually vital for all sorts of things, like digestion and disease prevention. But like all living things, bacteria need to eat. The bacteria in your mouth feed off of mucus, food remnants, and dead tissue cells. 

In order to absorb nutrients through their cell membranes, they must break down the organic matter into much smaller molecules. For example, they’ll break proteins into their component amino acids and then break those down even further into various compounds. Some of the foul-smelling byproducts of these reactions, such as hydrogen sulfide and cadaverine, escape into the air and waft their way towards unsuspecting noses. Our sensitivity to these odors and interpretation of them as bad smells may be an evolutionary mechanism warning us of rotten food and the presence of disease. Smell is one of our most intimate and primal senses, playing a huge role in our attraction to potential mates. 

In one poll, 59% of men and 70% of women said they wouldn’t go on a date with someone who has bad breath, which may be why Americans alone spend $1 billion a year on various breath products. Fortunately, most bad breath is easily treated. The worst smelling byproducts come from gram-negative bacteria that live in the spaces between gums and teeth and on the back of the tongue. By brushing and flossing our teeth, using antibacterial mouthwash at bedtime, gently cleaning the back of the tongue with a plastic scraper and even just eating a healthy breakfast, we can remove many of these bacteria and their food sources. 

In some cases, these measures may not be enough due to dental problems, nasal conditions, or rarer ailments, such as liver disease and uncontrolled diabetes. Behaviors like smoking and excessive alcohol consumption also have a very recognizable odor. Regardless of cause, the bad smell almost always originates in the mouth and not the stomach or elsewhere in the body. But one of the biggest challenges lies in actually determining how our breath smells in the first place, and it’s unclear why. It may be that we’re too acclimatized to the smell inside our own mouths to judge it. 

And methods like cupping your hands over your mouth, or licking and smelling your wrist don’t work perfectly either. One study showed that even when people do this, they tend to rate the smell subjectively according to how bad they thought it was going to be. But there’s one simple, if socially difficult, way of finding out how your breath smells: just take a deep breath and ask a friend.