Respiration is the fundamental biological process that sustains life by facilitating the exchange of gases between an organism and its environment. While inhaled air is a mixture of atmospheric gases, the air we push out is chemically and physically distinct. This difference is a direct record of the body’s metabolic activity, reflecting the consumption of fuel and the presence of hundreds of trace compounds. The composition of exhaled air serves as a dynamic signature of internal bodily functions.
Primary Components and Concentration Changes
The most significant change in exhaled air involves the proportions of the three main atmospheric gases: nitrogen, oxygen, and carbon dioxide. Inhaled air is composed of approximately \(78\%\) nitrogen (\(\text{N}_2\)), \(21\%\) oxygen (\(\text{O}_2\)), and \(0.04\%\) carbon dioxide (\(\text{CO}_2\)). Nitrogen remains largely inert in the respiratory exchange process, holding steady at around \(78\%\) in exhaled air.
The concentration of oxygen decreases substantially to about \(16\%\) to \(17\%\). This \(4\%\) to \(5\%\) drop represents the oxygen absorbed by the blood and transported to the body’s cells for cellular respiration. Simultaneously, the carbon dioxide level rises dramatically from \(0.04\%\) to approximately \(4\%\) or \(5\%\) in the exhaled breath. This increase is the waste product of cellular metabolism, released back into the bloodstream to be expelled by the lungs.
The Physical State of Exhaled Air
Beyond the chemical gas exchange, exhaled air differs from inhaled air in two physical properties: temperature and humidity. The air breathed out is significantly warmer because it is heated as it travels through the respiratory tract. While the air reaches core body temperature (\(37^\circ\text{C}\)) deep in the lungs, it typically exits the mouth or nose between \(31^\circ\text{C}\) and \(35^\circ\text{C}\).
The air is also nearly saturated with water vapor as it passes over the moist surfaces of the airways. Relative humidity deep in the lungs is close to \(100\%\), though external measurements range from \(42\%\) to over \(90\%\) at the mouth. This high moisture content causes visible fog on a cold day, as the warm, saturated air rapidly cools and condenses into tiny droplets.
Trace Compounds as Health Markers
In addition to the primary gases and water vapor, exhaled air contains a complex mixture of hundreds of Volatile Organic Compounds (VOCs). These metabolic byproducts are present at trace concentrations, often in parts per million or even parts per billion. These molecules originate from various biological processes and diffuse from the bloodstream into the lungs. Analyzing these VOCs provides a non-invasive window into a person’s internal biochemical state.
One well-known metabolic marker found in breath is acetone, a type of ketone. Acetone is produced when the body increases the breakdown of fatty acids for fuel, a process known as ketosis. High levels of breath acetone can be diagnostic, often pointing toward uncontrolled diabetes where the body cannot use glucose efficiently and must burn fat instead.
Other trace gases reflect the activity of the gut microbiome. Gases like hydrogen and methane are produced by these microorganisms during the fermentation of food. Elevated concentrations of these gases can be used to diagnose conditions like small intestinal bacterial overgrowth (SIBO) or certain digestive issues. Specific compounds like ethanol or isopropanol can also be detected in the breath following consumption, reflecting both internal metabolism and external intake.