The right temperature, humidity, substrate, nutrients, presence or absence of oxygen, and pH are necessary to cause bad breath. The normal salivary pH of 6.5 suppresses the growth of gram-negative bacteria that activate certain enzymes and cause malodor. Retentive surfaces such as periodontal pockets, the interproximal dental surfaces, and the tongue are ideal sites for harboring bacteria, accumulating plaque, and The intensity of breath odor tends to increase with age, probably because of regressive changes in salivary glands that affect the quantity and quality of salivary flow. Dentures are a common culprit, especially older ones made of vulcanite, a porous material that encourages the accumulation of food debris and Periodontal disease is responsible for one third of halitosis cases. The saliva of persons with periodontal disease tends to putrefy more rapidly and produce a more disagreeable odor.
Interdental pockets develop that can retain food.
When the bacteria in and around the pocket decompose food particles, the consequent inflammatory process leads to a rise in the substrates and growth factors needed to promote the growth of Inflammatory processes of the oral cavity such as stomatitis, glossitis, gingivitis, and cryptic tonsils can lead to the development of fissures and ulcerations of the oral mucosa that trap food particles, bacteria, desquamated cells, and tissues. Lower respiratory tract conditions such as bronchiectasis, necrotic tumors, and cavitary lesions tend to produce a putrid oral odor similar to that of rotten meat. The halitosis typically occurs late in the disease process after more obvious or urgent symptoms have already appeared. Halitosis of rapid onset that progressively intensifies may suggest an infective source such as one secondary to carcinoma or other localized airway disease. Perhaps the best known example of a systemic source is the fruity, sweet breath odor of diabetes and diabetic ketoacidosis, caused by pulmonary expiration of acetone and ketones.