This is caused by volatile chemicals in the circulation that transfer to the lungs during alveolar gas exchange and come out in the breath.  Most of these chemicals are produced by your body.  The concentration of these chemicals in your breath reflect their concentration in your blood.  Examples of these chemicals include alkanes, hydrocarbons, and aldehydes.  Whether any given chemical will give a mouth odor depends on its concentration, the character of the odour, and what other odorants are present in combination.  

Examples of conditions that can cause breath odor include liver disease, kidney disease, asthma and sleep apnea.  Starvation, dieting, and Ketoacidosis can give your breath an acetone odor. 

From the desk of Bassel Paul Gebrael DDS, Dip. Perio, FRCD(C), FICD

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https://dentaforte.com/type-3-airway-gastroesophageal-halitosis-based-on-aydin-2014-source-of-odor-is-gas-leakage-from-the-stomach/