According to the American Dental Association (ADA), halitosis has dozens of causes. It has so many, in fact, that even the organization does not list them all. Here are the agents of bad breath that the ADA mentions, along with a few that it does not, like tonsil stones and thrush.
Smoking - Smoker's breath comes from the tar, smoke particles and chemicals like ammonia and carbon monoxide that cling to the palate. Avoiding bad breath caused by smoking is as simple as avoiding smoking, which is the leading cause of preventable death in the U.S., according to the centers for Disease Control and Prevention.
Savory foods - Eating meals that include garlic, onions, oils, meat, leeks or other pungent foods can contribute to foul-smelling breath. The smell comes first from rotting food particles between the teeth and second from anaerobic bacteria, which feed on food bits and emit odorous sulfuric compounds. Avoiding those foods may not be feasible, but brushing and flossing is.
Poor dental hygiene - Oral diseases like gingivitis, tooth decay and periodontitis can cause halitosis separately or in concert. Caused by poor dental hygiene, these conditions can be prevented by simply brushing the teeth at least twice a day and flossing regularly.
Medical conditions - Occasionally, bad breath may be a sign of a medical disorder developing somewhere within the body. Conditions like diabetes, kidney failure and stomach illnesses like peptic ulcers can cause bad breath. However, these are rare agents of halitosis.
Postnasal drip - Sinusitis or a cold can cause postnasal drip, a condition in which mucus runs down that back of the throat. This mucus can give off a foul smell and may also give odor-causing oral bacteria sustenance.
Thrush - A yeast infection of the mouth, thrush presents itself as a whitish coating on the tongue, along with powerful bad breath.
Tonsil stones - These small white accumulations of collagen occur when food particles get stuck in folds on the tonsils. They cause an extremely foul odor, and may be coughed up or dislodged after gargling.
Dry mouth - While dry mouth may occasionally be a medical condition - called xerostomia - often it is caused by common actions, like sleeping with the mouth open, talking and exercising. It is also known to occur in those taking certain medications. To neutralize the odors created by anaerobic bacteria, which thrive in a dry oral environment, individuals may consider rinsing the mouth with a specialty breath freshening product.