Humans exhale both carbon dioxide and carbon monoxide with every breath, though in vastly different quantities. Carbon dioxide (COâ‚‚) is the heavyweight: it makes up roughly 4% of exhaled air, tens of thousands of times more concentrated than the trace amounts of carbon monoxide (CO) measured in parts per million. COâ‚‚ is the familiar waste product of energy metabolism, while endogenous CO is a byproduct of a completely different biochemical process involving the breakdown of hemoglobin. The two gases serve different biological roles, and measuring each one in exhaled breath has led to surprisingly distinct clinical applications.
Why You Exhale So Much Carbon Dioxide
Every cell in your body that burns fuel for energy produces COâ‚‚ as a waste product. When glucose, fatty acids, or amino acids are broken down in the mitochondria to generate the energy molecule ATP, carbon dioxide is released during the reactions of the citric acid cycle.1Anesthesiology. Does Aerobic Respiration Produce Carbon Dioxide or Hydrogen Ion and Bicarbonate? This COâ‚‚ has to get out of your body, and the lungs are the exit route.
Getting COâ‚‚ from the tissues to the lungs is more complex than it might seem. Only a small fraction of COâ‚‚ travels dissolved in blood plasma. Most of it is converted into bicarbonate ions inside red blood cells and shuttled to the lungs in that form; a smaller portion binds directly to hemoglobin as a carbamate compound.2PubMed. Carbon dioxide transport and carbonic anhydrase in blood and muscle Once blood reaches the capillaries surrounding the air sacs in the lungs, the process reverses: bicarbonate is converted back into COâ‚‚ gas, which crosses into the airways and is breathed out. The efficiency of this system depends on pH, oxygen levels, and the enzyme carbonic anhydrase, which speeds the bicarbonate-to-COâ‚‚ conversion.3PubMed Central. Simultaneous blood-tissue exchange of oxygen, carbon dioxide, bicarbonate, and hydrogen ion
The amount of COâ‚‚ you exhale fluctuates with your metabolic rate. At rest, your body produces and exhales roughly 200 milliliters of COâ‚‚ per minute. During intense exercise, that number can increase fivefold or more as muscles demand more fuel. The ratio of COâ‚‚ produced to oxygen consumed, known as the respiratory exchange ratio, shifts depending on whether your body is burning mostly fat or mostly carbohydrate. Burning carbohydrate produces more COâ‚‚ per unit of oxygen than burning fat does, so during hard exercise, when carbohydrate use ramps up, your COâ‚‚ output rises disproportionately.4PubMed Central. The Respiratory Exchange Ratio is Associated with Fitness Indicators Both in Trained and Untrained Men: A Possible Application for People with Reduced Exercise Tolerance
Where Exhaled Carbon Monoxide Comes From
Carbon monoxide in your breath is not a sign that something has gone wrong. Your body produces CO on purpose, primarily through an enzyme called heme oxygenase. This enzyme breaks down heme, the iron-containing molecule at the core of hemoglobin and other proteins, into three products: biliverdin (a precursor to the yellow pigment bilirubin), free iron, and carbon monoxide.5PubMed. Carbon monoxide: endogenous production, physiological functions, and pharmacological applications Two active forms of this enzyme exist: HO-1, which ramps up in response to stress and inflammation, and HO-2, which runs at a steady baseline level in tissues like blood vessel walls and the brain.6PubMed Central. Carbon monoxide as an endogenous vascular modulator
Because CO is produced wherever heme is being recycled, the rate of production tracks the rate of red blood cell turnover. Old or damaged red blood cells are constantly being cleared by the spleen and liver, and each hemoglobin molecule that gets dismantled releases CO. The gas enters the bloodstream, binds to hemoglobin (forming carboxyhemoglobin), and eventually diffuses into the lungs, where it is exhaled in trace quantities. In a healthy nonsmoker, exhaled CO sits around 1 to 3 parts per million.7PubMed Central. Assessment of Carbon Monoxide in Exhaled Breath using the Smokerlyzer Handheld Machine: A Cross-Sectional Study
Carbon Monoxide as a Signaling Molecule
One of the more surprising discoveries of the past few decades is that CO is not just metabolic waste. It belongs to a small family of gases, alongside nitric oxide and hydrogen sulfide, that function as signaling molecules in blood vessels and other tissues.8PubMed Central. Redox Regulation of Endogenous Gasotransmitters in Vascular Health and Disease At the tiny concentrations the body produces, CO helps relax blood vessel walls by activating calcium-sensitive potassium channels in smooth muscle cells, contributing to the regulation of blood flow.6PubMed Central. Carbon monoxide as an endogenous vascular modulator It also appears to play a role in dampening inflammation and protecting cells from oxidative damage.
This is worth emphasizing because carbon monoxide has a well-earned reputation as a poison. At high concentrations from external sources like car exhaust or faulty furnaces, CO binds hemoglobin with roughly 180 times the affinity of oxygen, crowding oxygen off and potentially causing tissue suffocation.9PubMed Central. Carbon Monoxide as a Potential Therapeutic Agent: A Molecular Analysis of Its Safety Profiles But at the minuscule amounts the body produces internally, CO acts as a beneficial messenger. The difference between poison and signaling molecule comes down entirely to dose and delivery route. Research into using CO therapeutically, administered in carefully controlled amounts, is an active field precisely because the body already knows how to use this gas productively.
How Smoking Changes the Picture
Cigarette smoke contains large amounts of carbon monoxide, and this dramatically alters exhaled CO levels. Where a nonsmoker typically breathes out 1 to 3 ppm of CO, a regular smoker can exhale well above that range. Clinicians use handheld CO breath analyzers (often called Smokerlyzers) to verify smoking status. One study found that an exhaled CO cutoff of 12 ppm could identify whether someone had smoked within the past eight hours with about 90% sensitivity and 94% specificity.10PLoS ONE. Assessing Recent Smoking Status by Measuring Exhaled Carbon Monoxide Levels
Exhaled CO monitoring has also become a tool for evaluating tobacco harm reduction. When smokers switch to electronic cigarettes or heated tobacco products, their exhaled CO levels tend to drop into the nonsmoker range within months, reflecting the absence of combustion in these alternatives.11PubMed Central. Exhaled Carbon Monoxide Levels in Forty Resistant to Cessation Male Smokers after Six Months of Full Switch to Electronic Cigarettes (e-Cigs) or to A Tobacco Heating Systems (THS) This does not mean those alternatives are harmless, but the CO measurement offers an objective marker of one specific harm reduction dimension: the elimination of combustion byproducts from the airway.
Exhaled CO and Air Pollution
Smoking is not the only external factor that raises exhaled CO. Ambient air pollution, particularly from traffic, contributes measurably. A study comparing nonsmokers living in large cities versus small towns found that city-dwelling nonsmokers were far more likely to exceed an exhaled CO threshold of 4 ppm.12PubMed. Influence of air pollution on exhaled carbon monoxide levels in smokers and non-smokers. A prospective cross-sectional study Other research has shown that exhaled CO rises measurably after just eight hours of exposure to outdoor air pollution, making it a potential biomarker for tracking short-term pollution exposure.13PubMed Central. Exhaled carbon monoxide: a non-invasive biomarker of short-term exposure to outdoor air pollution
This has a practical implication that sometimes trips up clinicians: a nonsmoker living near a busy highway could produce an exhaled CO reading that overlaps with a light smoker’s range. Any clinical interpretation of breath CO has to account for the person’s environment, not just their tobacco use.
Medical Uses of Exhaled CO Beyond Smoking
Because exhaled CO directly reflects how much heme is being broken down in the body, measuring it offers a window into conditions involving accelerated red blood cell destruction. In newborns, this has become especially useful. Neonatal jaundice, the yellowing of a baby’s skin caused by bilirubin buildup, can result from excessive hemolysis, and catching that hemolysis early matters for preventing dangerous complications like kernicterus. Since the breakdown of heme produces CO and bilirubin in equal amounts, a simple breath test measuring end-tidal CO (corrected for ambient CO in the room) can flag which babies have hemolysis significant enough to require treatment.14PubMed. End-tidal carbon monoxide and hemolysis One study found that this noninvasive measurement identified infants who would have been sent home with undetected hemolysis had only standard blood typing been used.15Journal of Perinatology. End-tidal carbon monoxide for routine monitoring of significant hemolysis in the management of newborn hyperbilirubinemia
Exhaled CO also rises with airway inflammation. Asthmatic patients who are not on corticosteroids exhale roughly twice the CO of healthy subjects, around 5.8 ppm versus 2.9 ppm in one study. Corticosteroid treatment brought those levels back down near normal.16PubMed Central. Raised levels of exhaled carbon monoxide are associated with an increased expression of heme oxygenase-1 in airway macrophages in asthma: a new marker of oxidative stress The mechanism appears to involve the stress-inducible form of heme oxygenase, HO-1, which ramps up in inflamed airways and generates more CO as a byproduct. Meta-analyses confirm the pattern of elevated exhaled CO in asthmatics, and there has been interest in using it as an inexpensive monitoring tool, particularly in settings where more expensive biomarker tests are unavailable.17PubMed Central. Exhaled carbon monoxide in asthmatics: a meta-analysis 18PubMed. Association between exhaled carbon monoxide and asthma outcomes in Peruvian children
Medical Uses of Exhaled COâ‚‚
Exhaled carbon dioxide has its own set of clinical applications, and they look quite different from those of CO because they reflect whole-body metabolism and breathing function rather than heme turnover. The workhorse technology here is capnography, the continuous measurement of COâ‚‚ concentration in exhaled breath. By tracking end-tidal COâ‚‚ (the COâ‚‚ level at the very end of an exhalation, which closely approximates the COâ‚‚ level in the blood leaving the lungs), clinicians get real-time information about three things at once: whether the lungs are ventilating properly, whether blood is circulating to the lungs, and whether metabolism is producing COâ‚‚ at a normal rate.19PubMed Central. Applications of End-Tidal Carbon Dioxide (ETCO2) Monitoring in Emergency Department; a Narrative Review
Capnography is now considered essential whenever a patient is intubated or being ventilated, because a sudden drop in exhaled COâ‚‚ can signal that the breathing tube has slipped out of the airway or that the patient’s heart has stopped circulating blood. It has become standard in emergency departments and is increasingly used in prehospital settings by paramedics.20Journal of Paramedic Practice. Prehospital end-tidal carbon dioxide measurement During CPR, rising end-tidal COâ‚‚ is one of the earliest indicators that chest compressions are generating meaningful blood flow, and a sustained jump in the reading often heralds the return of a heartbeat before a pulse can be felt.
Your Body Clock and the COâ‚‚ You Exhale
The mix of fuels your body burns, and therefore the amount of COâ‚‚ you exhale relative to the oxygen you consume, is not constant throughout the day. Research on circadian rhythms has shown that the respiratory quotient (the ratio of COâ‚‚ produced to Oâ‚‚ consumed) varies with internal biological time even when food intake is controlled. In one study, fasting respiratory quotient was about 2.5% lower in the biological evening compared to the biological morning, reflecting a shift toward burning more fat and less carbohydrate as the day progresses. Carbohydrate oxidation peaked in the morning while fat oxidation peaked in the evening.21Cell Press (Current Biology). Universal circadian rhythmicity of resting energy expenditure uncoupled from circadian rhythmicity of body temperature and motor activity
The practical meaning: even at rest, the composition of your exhaled breath shifts subtly over 24 hours. Your body is not a machine running on the same fuel mix around the clock, and your exhalations reflect that. For metabolic testing, this means the time of day a test is conducted can influence results, which is why standardized protocols typically require morning fasting measurements.
Breath Testing With Labeled COâ‚‚
One clever clinical application exploits the fact that COâ‚‚ produced by specific biochemical reactions can be distinguished from background COâ‚‚ using isotope labeling. In the urea breath test, a patient swallows a capsule of urea labeled with the carbon-13 isotope. If the bacterium Helicobacter pylori is present in the stomach, its abundant urease enzyme splits the labeled urea into ammonia and carbon-13-labeled COâ‚‚. That labeled COâ‚‚ enters the bloodstream, reaches the lungs, and shows up in exhaled breath, where it can be detected with a simple analyzer.22PubMed Central. The 13C urea breath test in the diagnosis of Helicobacter pylori infection Because carbon-13 is a stable, non-radioactive isotope, the test is safe enough for children and pregnant women. It remains one of the most accurate noninvasive ways to diagnose H. pylori infection, and it works precisely because exhaled COâ‚‚ faithfully reflects what is happening metabolically inside the body.
What Else Is in Your Breath
COâ‚‚ and CO are far from the only gases you exhale. Exhaled breath contains more than 3,500 distinct compounds, the vast majority of which are volatile organic compounds present in tiny quantities.23PubMed Central. Human exhaled breath analysis Nitric oxide, for instance, is another exhaled gas that has found a well-established clinical role: fractional exhaled nitric oxide (FeNO) testing is used routinely to monitor eosinophilic airway inflammation in asthma. Acetone levels in breath rise when the body shifts heavily toward fat metabolism, as happens during prolonged fasting or poorly controlled diabetes. Hydrogen and methane in exhaled breath are produced not by human cells but by gut bacteria fermenting undigested carbohydrates, which is the basis of breath tests for lactose intolerance and small intestinal bacterial overgrowth.
The broader field of breath analysis, sometimes called “breathomics,” is exploring whether patterns of exhaled volatile compounds can serve as early detection tools for conditions ranging from lung cancer to liver disease. The technology is still mostly in the research stage, but the underlying principle is straightforward: your breath is a gas-phase sample of your blood chemistry, and any metabolic process that produces or alters volatile compounds has a chance of leaving a detectable signature in what you exhale. COâ‚‚ and CO just happen to be the two carbon-oxygen gases we understand the best so far.