Composition of Human Breath: On the whole human breath is made up by a mixture of Nitrogen, Oxygen, Carbon Dioxide, Water and inert gases. The residual tiny fraction comprises of more than 1000 trace volatile organic compounds (VOCs) that have concentrations in the range of parts per million (ppm) to parts per trillion (ppt) by volume. Speaking of their origin, the generation of these volatile substances might take place in the body (endogenous) or they might be absorbed as contaminants from the environment (exogenous). The number of VOCs that have been found till now in human breath is more than 1000, in which just a few VOCs are common for every human being. These common VOCs, comprise of ethane, isoprene, methanol and acetone and are all products of key metabolic processes. These VOCs provide a lot of information in clinical diagnostics. The majority of the constituents as well as trace VOCs in breath get exchanged between the blood and alveolar air at the blood–gas interface in the lung. However NO is exceptional and is released into the airway when there is airway inflammation. The endogenous constituents present in human breath like inorganic gases (NO and CO), VOCs (pentane, isoprene, ethane, acetone), as well as other usually nonvolatile substances like isoprostanes, peroxynitrite, or cytokines, can be quantified by the help of breath condensate. Endogenous constituents can endow with important information regarding a potential disease state. While on the contrary, exogenous molecules predominantly halogenated organic compounds, help in indicating if a person is recently exposed to drugs or environmental pollutants.
Since breath tests are one of the least persistent means by which an individual’s disease state or exposure to a drug or an environmental pollutant can be monitored, curiosity in breath analysis for clinical diagnosis has grown over the years. On the basis of the origin of the substances that are found to be present in human breath, whether endogenous or exogenous, the archetypal applications of breath tests are categorized into two major groups namely: diagnosis of disease as well as evaluation of exposure to environmental pollutants.