Humans primarily exhale carbon dioxide (\(\text{CO}_2\)), a natural waste product of the body’s energy production. Carbon monoxide (CO) is a toxic gas not normally produced in significant amounts by healthy metabolism. \(\text{CO}_2\) is a simple byproduct of living, while CO is a dangerous toxin that interferes with the body’s ability to use oxygen. The distinction between these two gases is fundamental to human biology.
The Breath of Life: The Role of Carbon Dioxide
Carbon dioxide is the byproduct of cellular respiration, the process by which the body converts food into usable energy. Inside the cells, glucose and other nutrients are broken down using inhaled oxygen. This metabolic process generates adenosine triphosphate (ATP), the cell’s energy currency, and \(\text{CO}_2\) as a waste product.
The \(\text{CO}_2\) created at the cellular level must be efficiently removed to prevent a toxic buildup that would lower the body’s pH. It travels through the bloodstream, primarily dissolved in the plasma or bound to hemoglobin, back toward the lungs.
Once it reaches the small air sacs, or alveoli, in the lungs, the \(\text{CO}_2\) diffuses out of the blood and into the air space. Exhalation then expels this gaseous waste product, completing the respiratory cycle. This constant exchange ensures the body maintains a stable internal environment.
What Makes Carbon Monoxide Dangerous?
Carbon monoxide is a colorless, odorless, and tasteless gas, making it difficult to detect without specialized equipment. It is produced by the incomplete combustion of carbon-containing fuels, from faulty furnaces, car exhaust, or fires. Unlike \(\text{CO}_2\), carbon monoxide is not a product of healthy human metabolism and is toxic when inhaled.
The danger of carbon monoxide lies in its ability to bind to hemoglobin in red blood cells with a much greater affinity than oxygen—about 200 to 250 times stronger. When CO binds, it forms carboxyhemoglobin, preventing that portion of the blood from carrying oxygen. This displaces oxygen, leading to cellular hypoxia.
This binding also causes remaining oxygen molecules to be held more tightly, making them harder to release to tissues. Organs with high oxygen demands, such as the brain and heart, are the most susceptible to damage. Even low concentrations of the gas can quickly lead to serious health complications.
Trace Amounts: What Exhaled Carbon Monoxide Indicates
Although \(\text{CO}_2\) is the main exhalation product, trace amounts of carbon monoxide can be found in the breath of healthy individuals. This normal amount of CO is produced endogenously as a byproduct of heme metabolism. The enzyme heme oxygenase breaks down the heme component of old or damaged red blood cells, releasing carbon monoxide, iron, and biliverdin.
The level of exhaled carbon monoxide (eCO) becomes a significant diagnostic marker when elevated above this normal trace amount. The most common reason for a marked increase in eCO is environmental exposure, such as from smoking or inhaling polluted air. Since environmental CO binds to hemoglobin, the lungs excrete it, making exhaled breath a direct measure of the body’s CO burden.
Elevated eCO can also indicate underlying medical conditions where heme breakdown is accelerated, such as hemolytic anemia. Increased levels are also proposed as a marker for systemic inflammation. Measuring exhaled CO is a practical clinical tool used to monitor smoking cessation and screen for carbon monoxide poisoning.