What happened at the 2024 Tour de France
During the world's biggest cycling race, a report by the website Escape Collective revealed that at least three teams in the peloton were using a carbon monoxide rebreathing (CO) device. Among them were the race's two biggest forces: UAE Team Emirates, of Tadej Pogacar, and Visma Lease a Bike, of Jonas Vingegaard. Israel Premier Tech was also cited, and later Ineos Grenadiers admitted using the same technique.
The reaction was immediate. Since carbon monoxide is a toxic gas, linked to fatal household poisonings, the very idea of elite athletes inhaling the compound sounded alarming. The cyclists, however, downplayed everything.
The teams' version: it is just a test
Pogacar called the procedure a "very simple test". According to him, the athlete blows into a balloon for about a minute, measures hemoglobin mass and repeats two weeks later. Vingegaard took the same line and said there was "nothing suspicious", comparing the effect to "smoking a cigarette". For the teams, the goal is purely diagnostic: measuring how much hemoglobin the body produced before and after an altitude training camp, to know whether that training block worked.
Why the method is scientifically legitimate
On this point, the cyclists are right. Single-dose CO rebreathing is the gold standard of sports science for measuring total hemoglobin mass. The technique was refined by Nicole Prommer and Walter Schmidt and published in 2005.
The logic is elegant. Carbon monoxide binds to hemoglobin with much higher affinity than oxygen, forming carboxyhemoglobin. By inhaling a known amount and measuring how much CO was absorbed, one indirectly calculates how many hemoglobin molecules circulate in the blood. Since hemoglobin is oxygen's vehicle, measuring its mass is measuring the athlete's real O2 transport capacity. One measurement takes a few minutes and raises carboxyhemoglobin briefly and transiently.
One diagnostic measurement takes carboxyhemoglobin to about 10%, above the threshold considered safe of approximately 6.4%. Even so, a single short exposure is considered to have no effect on performance.
Gatterer et al., 2024 (Frontiers in Physiology)Where the tool becomes a problem
The controversy is not in the exam. It is in what happens when the same inhalation is repeated many times. By chronically exposing the body to low doses of CO, an artificial hypoxia is created: cells perceive less available oxygen and the body responds by producing more hemoglobin, exactly as it would on a high mountain. The difference is that this happens without leaving home.
In a controlled study, participants inhaled a bolus of CO five times a day for three weeks. The result was a 4.8% increase in hemoglobin mass, an effect comparable to an altitude camp, with a tendency toward a VO2max gain of about 4 mL/min per additional gram of hemoglobin.
Schmidt et al., 2020 (Medicine & Science in Sports & Exercise)Not every dose works. Ryan et al. (2016) tested a lighter protocol, with ten inhalations over twelve days, and found no relevant effect on hemoglobin or VO2max. Wang et al. (2019), with five inhalations per week, reported a 3.7% increase in hemoglobin mass. The pattern is clear: the doping effect depends on high, sustained dose and frequency. The line between diagnosis and cheating is, literally, how many times you inhale.
The UCI decision
The International Cycling Union entered the debate and, from February 10, 2025, banned repeated carbon monoxide inhalation. The justification was health, not just fair play: outside a medical setting, repeated exposure to the gas can cause headache, nausea, dizziness and, in severe cases, arrhythmias, seizures and loss of consciousness. The organization also admitted that the long-term effects of this practice are unknown.
The rule did not ban the exam. Under medical supervision, one measurement remains authorized, with a second inhalation allowed only after a mandatory two-week interval. In practice, the UCI turned a legitimate diagnostic tool into something controlled by exact dose, separating responsible use from abuse.
Carbon monoxide is a toxic gas. This text is informative and does not recommend any CO inhalation outside a supervised medical setting. It does not replace professional evaluation.
The verdict of the best evidence
The science is uncomfortably symmetric here. The single dose is a well-established, useful laboratory exam that helps the athlete understand whether altitude training was worth it. The same substance, inhaled several times a day for weeks, stops measuring the adaptation and starts manufacturing it, with real health risk and no proven benefit that justifies the danger. What separates medicine from doping, in this case, is not the molecule. It is the frequency.
