| Significant VOCs in breath | Sources | References | 
      
        | Acetone | Glucose metabolism in the human body | [48] | 
      
        | Isoprene | Mevalonic acid pathway of cholesterol synthesis | [32] | 
      
        | Acrolein | Exogenous: cooked foods, formed from carbohydrates, vegetable    oils and animal fats, amino acids during heating of foods, occupational    exposure Endogenous: myeloperoxidase-mediated degradation of    threonine and amine oxidase-mediated degradation of spermine and spermidine    (markers for oxidative stress)
 | [49] | 
      
        | Ethanol | Exogenous : Alcohol consumption Endogenous: Several biochemical pathways like fatty acid    synthesis, microbial fermentation of the carbohydrates in the    gastro-intestinal tract
 | [50] | 
      
        | Acetonitrile | Exogenous: Smokers, drug addicts, passive smokers Cut-off for active smokers established at 20.3 ppb (study    based on PTRMS)
 | [51] | 
      
        | Acetaldehyde | Endogenous: Local production in upper airway or mouth;    metabolism of ethanol | [36] | 
      
        | Benzene | Exogenous : Passive smoking, occupational | [37] | 
      
        | Toluene | Exogenous: Occupational, indoor | [52] |