CSI: Breathprint

We shall start by considering the composition of analytically useful breath. All samples of end-tidal breath (the air collected at the end of a normal breath) contain approximately 78 percent nitrogen, 13 percent oxygen, 5 percent carbon dioxide, 3 percent water vapor and aerosols, 0.9 percent inert gases and 0.01 percent that contains a mixture of as many as 1000 different volatile compounds. The molecular profiles of these latter compounds form the basis of the proposed breathprint and are determined by the concentrations and identities in inhaled air and blood. Volatiles in blood are made up of exogenous molecules and/or their metabolites (those that have been inhaled and crossed into the bloodstream; entered the bloodstream via dermal absorption or the gastrointestinal tract; or produced by foreign cells) or endogenous molecules and/or their metabolites that have been produced by cells and tissues throughout the body. (Semi-volatile and non-volatile molecules can also be exhaled as they are entrained within aerosols created in the airways.)

Now, let’s examine the inhalation route of exposure in detail. For the purposes of this discussion, we will assume that the breath sample is collected from a healthy young male (height 183 cm, weight 84 kg, body surface area 2.06 m2, body mass index 25.1 kg/m²) breathing tidally under autonomic control. This male, at rest, will have a minute ventilation (the amount of air breathed in 60 seconds) of 8.4 liters and will inhale approximately 0.50 m³ of inspiratory air in an hour. The first 185 ml of any exhaled breath for this male will reflect the concentrations of molecules present in the inhaled air. Only a fraction of the inspired concentrations of molecules that reach the alveolar surface will cross the membrane into the blood and the remainder of the molecules will be exhaled in the subsequent breath. The molecules that enter the blood can reach tissues, be stored, metabolized or excreted unchanged. These molecules provide a personal history of this male’s relatively recent exposure to exogenous molecules in inspiratory air. With the exception of acetone and isoprene, the concentrations of exogenous molecules generally dominate any breathprint. Obviously, exogenous contributions to the breathprint will be variable and time dependent – just like the external environment. It is possible to use this portion of the breathprint – what we call the exogenous breath exposome – for legal and security purposes; working in illegal drugs or explosives manufacture involves the use of characteristic molecules that can help identify those involved in such suspicious activities.

The ingestion of foods and beverages contributes significantly to the breathprint but in a highly variable and time-dependent fashion and is unlikely to be unique. Similarly, bacteria present in the oral cavity and upper respiratory tract contribute significantly but not uniquely. Infections by foreign organisms will change the breathprint but in an episodic, non-unique way. On the other hand, the species and strains of the bacteria found in our gastrointestinal tract – the ‘microbiome’ – could contribute uniquely to our breathprint but would vary with diet and the use of probiotics or antibiotics.

The remaining contributions to the human breathprint are endogenous. These processes occur in everyone and will not be unique, although their concentration profiles may vary by phenotype. Although disease states may appear to produce unique molecules in the breathprint, these results are likely a reflection of analytical method detection limits, because abnormal physiologies can only increase or decrease concentrations.

Evidently, there are many factors that contribute to the human breathprint and every such breathprint must be considered only as a ‘snapshot’ in time. And in fact, the method for sampling the breath is a major breathprint determinant so the same protocol must always be followed. Unlike methods in conventional biometrics, such as genetic codes, retina/iris pattern, or fingerprint whorls and ridges, the human exposome is not stable but rather a constantly moving target. There could be a fraction in the aerosol component of the human breath-borne exposome that is stable and reflective of the individual, but this would most likely be represented by large molecules derived from specific protein-coding sections of genetic material. Regrettably, such ultra-trace level breath protein analysis is not commonly available. In short, the breathprint is useful for insight into health status, recent exposures, and possibly threat assessment, but certainly not for distinguishing individuals from one another.