What is carbon monoxide rebreathing and why is the entire Tour de France talking about it? Tadej Pogačar and Jonas Vingegaard insist it's "nothing suspicious" and "just a simple test", but others are less convinced

Carbon monoxide has been the talk of the Tour de France this week, the race's biggest stars and teams fielding questions on a controversial 'rebreathing' technique used to test and optimise riders' altitude training. However, given the health questions that inevitably follow an admission about inhaling a deadly gas such as carbon monoxide, plus the potential for the practice to be abused for performance gain, it's not hard to see why so many have raised safety and ethical concerns.

The discussion was sparked by an at-length piece by Escape Collective who broke the news that top-level Tour de France pros, namely riders on UAE Team Emirates, Visma-Lease a Bike and Israel-Premier Tech, have access to a carbon monoxide rebreather — an expensive piece of kit that allows for precise dosing of the gas into the lungs.

Now, at this point the natural reaction is almost certainly to respond: why? For what possible reason would you wish to inhale a poisonous gas that can make you seriously ill if you breathe it in?

Well, carbon monoxide testing provides an accurate means of measuring haemoglobin (a protein that facilities the movement of oxygen in red blood cells), so for example during an altitude camp it can be used to track a rider's progress over a block of training.

The practice does not break anti-doping rules and is a technique that has been around for decades in the worlds of science and medicine. Where things get more murky, however, is with the potential for misuse.

Inhaling carbon monoxide — using the same device as is used for the tests carried out at UAE Team Emirates and Visma-Lease a Bike altitude camps — could assist riders' bodies to produce more red blood cells, an act that, while not banned by WADA, one would assume is in conflict with the anti-doping authority's stance against artificial manipulation of blood. 

At this point it's worth pointing out that there is no suggestion cycling teams are using the carbon monoxide rebreathing device for any purpose other than haemoglobin testing. However, equally, it's not difficult to see how the situation has raised concerns about the potential for athletes to abuse such equipment in search of performance gain, potentially risking their own health in the process.

This is an ethically questionable practice that has likely already arrived in sport, the US-based cycling website reporting that: "Several other sources claimed the technique was already in use in various sports. One sports scientist confirmed he had seen an athlete — in a sport he would not name — inhaling carbon monoxide for performance gains and conveyed his concern the practice could prove a blight on the Paris Olympics."

"There is nothing suspicious about it" and "just a simple test" — what have those riders and teams using carbon monoxide rebreathing had to say about it?

The communication from UAE Team Emirates, Visma-Lease a Bike and Israel-Premier Tech has been clear — it is only used for testing and monitoring altitude training. Never for direct performance gain.

There was some confusion this week when Tadej Pogačar initially said: "I don't know what it is", when responding to a question about the practice. UAE Team Emirates then promptly offered a clarifying statement to The Times' David Walsh admitting they do use the carbon monoxide device. The current race leader then told reporters 24 hours later that he "didn't quite understand the [previous day's] question".

"It's a test in altitude camp to see how you respond to altitude," he elaborated. "You need to do this test, it's like a two or three-minute-long test. You breathe into a balloon for one minute and then you see the haemoglobin mass, and then you need to repeat it two weeks after.

"But I did just the first part of the test. I never did the second part because the girl who was supposed to come after two weeks didn't come. It's not like we're breathing exhaust pipes every day in the cars. It's just a pretty simple test to see how you respond to altitude training."

The Slovenian's great rival Jonas Vingegaard too has been quizzed on his use, explaining: "It's to measure how much haemoglobin you have in your blood. There is nothing suspicious about it.

"We measure how many red blood cells you have in your blood and the effect of altitude training camp. One thing is what you have thought training at altitude did for you. Now we actually have something that can measure it. They [the doctors] say that they send something into the lungs that is similar to smoking a cigarette. We measure the day we get to our high altitude camp and then [on] the day we go back down. Then we see the difference in how much haemoglobin is built up. There is nothing more to it."

Israel-Premier Tech also said their "sole use of the Detalo Blood Volume Analyzer is for testing purposes to measure the impact of altitude training".

A UAE Team Emirates spokesperson added: "It is a well established, safe, professional method that is backed by a very large amount of research. It is used widely by athletes, climbers and other persons who are exposed to altitude for professional sports or other physical activities."

"It's not a therapy; it's a diagnostic tool that we use to essentially work out what our athletes' physiology is," the team's medical director Adriano Rotunno told Escape Collective, emphasising that there is no performance enhancement to the measurement method.

Others have been less relaxed about the use of carbon monoxide rebreathers, Uno-X Mobility telling Escape Collective it "is not something we are even considering", while EF Education-EasyPost boss Jonathan Vaughters also nailed his team's colours to the mast: "It's not something we have ever done or would do."

Both teams also didn't see the benefit in measuring haemoglobin mass, with another source unequivocal in their condemnation of the potential further use of a rebreather to inhale carbon monoxide for performance gain, telling Escape Collective: "That would be blood doping, make no mistake."

All credit to Escape Collective for breaking this story.