What Is the Chemical Composition of Exhaled Air?
Explore the chemical makeup of exhaled breath, a complex byproduct of metabolism whose composition changes with diet, activity, and physiological state.
Exhaled air is the gaseous mixture expelled from the lungs during breathing. Its composition is a direct result of gas exchange within the lungs, where the body takes in oxygen and releases metabolic waste products. This process transforms inhaled air before it is breathed out.
Dominant Gases in Exhaled Air
The air we exhale is composed of four main gases whose proportions are altered by bodily processes. Nitrogen is the most abundant, making up about 78% of both inhaled and exhaled air. Its percentage remains largely unchanged because the body does not use it in significant amounts, so what is breathed in is breathed out.
In contrast, the concentration of oxygen decreases significantly. While inhaled air contains approximately 21% oxygen, exhaled air holds only about 16% to 17%. This reduction occurs because oxygen diffuses from the alveoli in the lungs into the bloodstream. This oxygen is then transported throughout the body to be used by cells for cellular respiration.
The opposite is true for carbon dioxide, a waste product of cellular respiration. Its concentration increases from 0.04% in inhaled air to about 4% to 5% in exhaled air. The blood transports this waste gas from the cells back to the lungs to be expelled from the body.
Finally, exhaled air is saturated with water vapor. The warm, moist environment of the lungs causes water to evaporate from the surfaces of the airways. As a result, the amount of water vapor in exhaled air can range from 5% to 6%, which is considerably higher than in the air we inhale.
Trace Substances in Our Breath
Beyond the primary gases, human breath contains a complex mixture of hundreds of other compounds in trace amounts. These substances, often measured in parts per million (ppm), make up less than 1% of the total exhaled volume. This volatile fraction of breath provides a window into the body’s metabolic state.
The most revealing trace substances are volatile organic compounds (VOCs), which are byproducts of various metabolic processes. Some of the most common VOCs found in the breath of healthy individuals include isoprene, a hydrocarbon linked to cholesterol synthesis. Another is acetone, a product of fat metabolism.
Other VOCs frequently detected in breath include ethanol, methanol, and pentane. The presence and concentration of these compounds can originate from normal bodily functions, diet, or substances absorbed from the environment. Researchers have identified over 1,000 different trace compounds in human breath.
Influences on Exhaled Air Makeup
The chemical makeup of exhaled air is not static and changes in response to several factors. A person’s physiological state, diet, and health can alter the levels of both dominant gases and trace volatile organic compounds (VOCs). These factors all leave a signature on the breath.
Physical activity is a major influence on breath composition. During exercise, the body’s metabolic rate increases to meet higher energy demands. This leads to greater oxygen consumption and a corresponding increase in carbon dioxide production, raising the concentration of CO2 in exhaled air.
Diet directly impacts the substances found in our breath. For instance, individuals on a ketogenic diet exhale higher levels of acetone as their bodies metabolize fat for energy. Consuming foods like garlic can release sulfur-containing compounds, while drinking alcohol leads to the exhalation of ethanol.
A person’s health status can cause distinct changes in the breath’s VOC profile. For example, uncontrolled diabetes can lead to very high levels of exhaled acetone, giving the breath a sweet, fruity odor. Different patterns of VOCs have also been associated with various conditions, including lung diseases and metabolic disorders.