Laura Lewis explains what carbon monoxide rebreathing is and whether the method should be outlawed
Carbon monoxide rebreathing hit the headlines at the 2024 Tour de France as a method used, according to an initial report by Escape Collective, by UAE Team Emirates, Visma-Lease a Bike and Israel-Premier Tech.
A statement released by Tadej Pogačar’s UAE Team Emirates later clarified the method has been used to test haemoglobin levels and, in turn, the impact of altitude training. Crucially, the practice is not banned by the World Anti-Doping Agency (WADA).
“Carbon monoxide rebreathing is a method to assess total haemoglobin mass and has been used in altitude training and research for over 20 years. It is a well-established, safe, professional method that is backed by a very large amount of research,” read the statement.
“We measure the haemoglobin mass at the start of an altitude training camp and at the end of the camp. In this way we minimise our athlete’s exposure to carbon monoxide and not the other way around. Any links to our team using this type of technology for unethical purposes are being made without any merit and without any facts or evidence to support them.”
In this guest column, originally published in Cycling Plus magazine, the United States Anti-Doping Agency’s (USADA) director of science, Laura Lewis, explains what carbon monoxide rebreathing is, its relatively long history in sport from a scientific standpoint, and whether the method should be banned.
Carbon monoxide has a much higher affinity than oxygen to bind to haemoglobin, meaning that if you breathe it in, you have less oxygen attached to haemoglobin, so you enter a hypoxic state, as you would at altitude.
Picture the situation as a bus and the seats as haemoglobin. The larger customers are the carbon monoxide and take up all the seats, leaving little room for the thinner oxygen customers.
It does eventually unbind (it has a half-life of around 12 hours), but in very high doses all the seats are used up, so there’s no room for oxygen and the result is death.
But that’s at high doses. We’ve been using it in science for years to measure the total amount of haemoglobin someone has. You ask athletes to inhale a bolus [single large dose] of carbon monoxide – 75 to 125ml of gas, or around 1.25ml per kg of body weight.
The ‘bong’ test
In what cyclists used to affectionately call the ‘bong’ test, you have this glass-like spirometer, which looks a little like a flute with a big bag at the end. You’re rebreathing through this closed circuit for two minutes. That’s how long it takes for the carbon monoxide to mix into your system.
There’s also an automated machine that some of the teams have been talking about that’s come out of Norway, but I’ve never used it. The one we’ve used is really portable. It usually raises the carbon monoxide levels in your blood to around 5 per cent.
You’d normally have levels of around 0.5 per cent just from exhausts and pollution, while a smoker’s is always about 3 per cent. For a normal adult, those levels would be at 2 per cent by the next morning and back to normal within a day; quicker if you’re exercising in that time.
A long history
I did the test every stage during the 2010 Tour Down Under and showed that the haemoglobin levels of Cycling Australia’s national team remained stable.
I’ve used this test at altitude around the world to see how riders are reacting to the rarefied air, to find out if the training is working, as you want the haemoglobin levels to rise during an altitude camp thanks to their bodies making higher levels of natural EPO [erythropoietin].
I used it at the top of the Stelvio, then analysed the results at the Mapei lab, which has its headquarters in Milan. It’s a legitimate way to measure haemoglobin response to altitude.
We also use it to see how riders are reacting to training and if they need iron supplementation, for example. You can also measure haemoglobin mass really accurately with the Evans Blue Dye test, but that requires injecting a person with a tracer, so isn’t used.
Ethical boundaries
At one point, we looked into using the carbon monoxide rebreathing method as an anti-doping test because of its accuracy in measuring haemoglobin mass; it’s not influenced by factors such as dehydration or plasma volume.
The idea was a rider might finish a time trial and you wouldn’t have to wait for them to drink or warm down to see if they’d taken a blood bag. You’d just get them to inhale the gas. But being a toxic gas, it didn’t take off because clearly you can’t force athletes to do this.
So, it’s always been used as a measuring tool. But in a study by Professor Walter Schmidt in 2020, he showed that it could be an alternative to altitude training, though he did flag up the ethical argument.
In the study, 11 subjects inhaled a small amount of carbon monoxide five times a day for three weeks. This reduced the oxygen transport in their bloodstream by around 5 per cent, which corresponded to being at an altitude of about 2,500m. After three weeks, their total amount of haemoglobin had increased by 5 per cent.
But Schmidt’s was a very controlled experiment. The worry is that if you don’t have the proper machines and experts to measure and calculate everything, you could give someone too much and they could end up in a lot of trouble.
Should carbon monoxide rebreathing be banned?
Should it be banned? Would people be silly enough to take a noxious gas? It’d be hard to test for, but it could be something WADA [World Anti-Doping Agency] would look further into.
There’s a question of ethics, certainly with how these tests are approved. Every research study needs to go through an ethical review board to allow human subjects, and I question why it’s necessary to be doing these studies.
Yes, those researchers will control their studies well, but I just think the risks are dangerous if this information gets into the wrong hands.