Food debris becomes trapped, undergoes slow bacterial putrefaction and release of malodourous volatiles. Food packing can also cause a localized periodontal reaction, characterized by dental pain that is relieved by cleaning the area of food packing with interdental brush or floss. Acrylic dentures (plastic false teeth) – inadequate denture hygiene practises such as failing to clean and remove the prosthesis each night, may cause a malodour from the plastic itself or from the mouth as microbiota responds to the altered environment. The plastic is actually porous, and the fitting surface is usually irregular, sculpted to fit the edentulous oral anatomy. These factors predispose to bacterial and yeast retention, which is accompanied by a typical smell. Oral infections
Oral ulceration
Fasting
Stress/anxiety
Menstrual cycle – at mid cycle and during menstruation, increased breath VSC were reported in women. Smoking – Smoking is linked with periodontal disease, which is the second most common cause of oral maloduor. Smoking also has many other negative effects on the mouth, from increased rates of dental decay to premalignant lesions and even oral cancer. Alcohol
Volatile foods  – e.g. onion, garlic, durian, cabbage, cauliflower and radish. Volatile foodstuffs may leave malodourous residues in the mouth, which are the subject to bacterial putrefaction and VSC release. However, volatile foodstuffs may also cause halitoisis via the blood borne halitosis mechanism. Medication – often medications can cause xerostomia (dry mouth) which results in increased microbial growth in the mouth. Nose and sinuses
In this occurrence, the air exiting the nostrils has a pungent odour that differs from the oral odour.
The esophagus is a closed and collapsed tube, and continuous flow of gas or putrid substances from the stomach indicates a health problem—such as reflux serious enough to be bringing up stomach contents or a fistula between the stomach and the esophagus—which will demonstrate more serious manifestations than just foul odour.In the case of allyl methyl sulfide (the byproduct of garlics digestion), odour does not come from the stomach, since it does not get metabolized there. Systemic diseases
There are a few systemic (non-oral) medical conditions that may cause foul breath odour, but these are infrequent in the general population. Such conditions are:
Fetor hepaticus: an example of a rare type of bad breath caused by chronic liver failure. Lower respiratory tract infections (bronchial and lung infections). Kidney infections and kidney failure. Carcinoma. Trimethylaminuria ("fish odour syndrome"). Diabetes mellitus. Metabolic conditions, e.g. resulting in elevated blood dimethyl sulfide.Individuals affected by the above conditions often show additional, more diagnostically conclusive symptoms than bad breath alone. Delusional halitosis
One quarter of the people seeking professional advice on bad breath have an exaggerated concern of having bad breath, known as halitophobia, delusional halitosis, or as a manifestation of olfactory reference syndrome. They are sure that they have bad breath, although many have not asked anyone for an objective opinion. Bad breath may severely affect the lives of some 0.5–1.0% of the adult population.