Image Credit: ASO/Charley Lopez

“At Team Visma Lease a Bike we solely used it to test whether our altitude training was working or not. If the technique is being misused (to enhance performance), then I understand the decision to ban it. Inhaling carbon monoxide is comparable to smoking a cigarette. Many people smoke multiple cigarettes a day. I’m not afraid of one cigarette. Not that I’ve ever smoked a cigarette myself, but beyond that, I don’t want to take a stance on this issue.”

Jonas Vingegaard, speaking to Sporza in November 2024

As most people will be aware, carbon monoxide gas is the highly poisonous cousin of a gas we all all happily breath in and out all the time – carbon dioxide. A single carbon atom connected with a triple bond to a single carbon atom. Exposure at high levels can be life-threatening or even deadly. Even at lower levels, prolonged exposure can lead to serious health issues, including cardiovascular and neurological damage.

It seems strange, therefore, that the gas we nervously monitor with detectors in our homes could also be the key to unlocking the next level of performance gains for elite athletes. Nonetheless, through precise dosing using a device called a ‘rebreather’ it has the ability to be exactly that. Romain Bardet certainly thinks it may be the reason for the stratospheric trajectory of some of his colleagues in the pro peloton.

This article provides a brief analysis of the use of carbon monoxide rebreathers in professional cycling, including a brief explanation for their mechanism and use, and an examination of the current and potential future regulatory positions.

What is a rebreather and how are they used?

Carbon monoxide rebreathers have two main potential uses.

The first is simply as a measurement tool, where the rebreathing device is used to track and measure key blood values, and altitude training programs optimised using the results.

The test performed is relatively simple. The athlete breathes in a controlled mixture of oxygen and carbon monoxide for a period of time, and a small amount of blood is drawn pre- and post-session for testing.

Carbon monoxide bonds almost completely to haemoglobin in the blood, far more efficiently than oxygen does, to form a stable complex called carboxyhaemoglobin which can be measured in a blood gas analyser. Taking the amount of carbon monoxide dosed by the rebreather and comparing the amount of carboxyhaemoglobin before and after the test allows precise calculation of an individuals total haemoglobin mass. The higher an individual’s haemoglobin mass is, the higher their VO2 Max. Several World Tour teams including Visma- Lease a Bike, UAE Team Emirates, and Israel-Premier Tech have all confirmed they have used a rebreather device in this way to track and optimise the effects of altitude training.

However, there is a second, more controversial, use of rebreathers – carbon monoxide inhalation for performance-enhancing purposes (which, for clarity, no professional cycling team is openly doing).

In short, the rebreather can deliver a controlled amount of carbon monoxide to the athlete, creating a temporary state of hypoxia by reducing the blood’s oxygen-carrying capacity. The effect is similar to that of altitude training, spiking erythropoietin (EPO) hormone production. This in turn leads to a higher red blood cell count and improved oxygen-carrying capacity to an individual’s muscles when undertaking exercise.

When used in combination with traditional altitude training, this method of rebreather use can create an even further performance benefit – a so-called state “super altitude”, increasing the intensity and duration of its effects.

What is the current regulatory position?

As the global authority responsible for regulating doping in sports, WADA’s Prohibited List (which is updated annually) sets out what substances and methods are banned across all sports, including professional cycling.

As of the most recent update to the Prohibited List, WADA has not explicitly banned the use of carbon monoxide as a substance, nor has it explicitly prohibited the use of carbon monoxide rebreathers as a method.

The Prohibited List does, however, include a broad category for methods that manipulate blood and blood components, as follows:

M1.1. The Administration or reintroduction of any quantity of autologous, allogenic (homologous) or heterologous blood, or red blood cell products of any origin into the circulatory system except donation by Athletes of plasma or plasma components by plasmapheresis performed in a registered collection center.

M1.2. Artificially enhancing the uptake, transport or delivery of oxygen. Including, but not limited to: Perfluorochemicals; efaproxiral (RSR13); voxelotor and modified haemoglobin products, e.g. haemoglobin-based blood substitutes and microencapsulated haemoglobin products, excluding supplemental oxygen by inhalation.

M1.3. Any form of intravascular manipulation of the blood or blood components by physical or chemical means.

M1 of the 2024 Prohibited List

This category covers practices that are intended to enhance oxygen transport, such as blood doping and the use of erythropoiesis-stimulating agents (ESAs). Carbon monoxide rebreathers when used as a ‘performance-enhancing’ device (the second use outlined above) rather than a measurement device (the first use outlined above) would arguably contravene M1.2 – however their mechanism is distinct to the (albeit non-exhaustive) examples given.

Clarity incoming?

The UCI maintained a wall of silence on the issue for several months after Escape Collective broke the story of rebreather use in the summer. However, in a press release of 12 December 2024, it has now publicly discouraged their use and piled the pressure on WADA to provide clarity:

“The UCI clearly asks teams and riders not to use repeated CO inhalation. Only the medical use of a single inhalation of CO in a controlled medical environment could be acceptable. The UCI is also officially requesting the World Anti-Doping Agency (WADA) to take a position on the use of this method by athletes.”

UAE Team Emirates has already confirmed it has stopped using rebreathers, and has no intention to resume their use.

Although WADA’s approach to regulating new substances and methods is typically conservative, in a recent statement by WADA to Reuters, it made clear that although it was investigating the effects of repeated use of rebreathers to “Artificially enhanc[e] the uptake, transport or delivery of oxygen”:

“Exposure to carbon monoxide (CO) has been discussed by WADA’s prohibited list expert advisory group on several occasions… There is no general consensus on whether CO can have a performance enhancing effect and no sufficiently robust data currently supports that proposition… In any event, it is generally acknowledged that it can be dangerous for health so it would not be recommended.”

However, there is propensity for that position to change. If the use of rebreathers were to be considerd for inclusion on the Prohibited List as a prohibited substance or method, Article 4.3.1 to the WADA Code sets out that they must meet at least two of the following criteria: it enhances performance, it poses a health risk to athletes, or it violates the spirit of sport. One would have thought that rebreathers have the potential to satisfy all three.

4.3.1.1 Medical or other scientific evidence, pharmacological effect or experience that the substance or method, alone or in combination with other substances or methods, has the potential to enhance or enhances sport performance;

4.3.1.2 Medical or other scientific evidence, pharmacological effect or experience that the Use of the substance or method represents an actual or potential health risk to the Athlete;

4.3.1.3 WADA’s determination that the Use of the substance or method violates the spirit of sport described in the introduction to the Code.

Article 4.3.1.1 to 4.3.1.3 of the 2021 WADA Code

So, deadly gas or no more harmful than smoking a cigarette? Either way it appears the debate will rumble on a while longer.

Words: Nick Williams