Carbon Monoxide Rebreathing: A Legal Test, a Possible Edge and a Health Ban

Timeline

1. 2020A published study finds that repeated carbon monoxide inhalation raises hemoglobin mass by 5% over three weeks.
2. Jul 2024Escape Collective reports that three top teams use carbon monoxide rebreathing; the teams say it is for measurement only.
3. Aug 2024WADA says there is no consensus that carbon monoxide enhances performance and that it is not prohibited.
4. Jan 31–Feb 1, 2025The UCI Management Committee approves a ban on repeated carbon monoxide inhalation.
5. Feb 10, 2025The UCI ban takes effect.
6. Sep 2025WADA publishes its 2026 Prohibited List, banning non-diagnostic CO rebreathing (M1.4) from January 1, 2026.
7. Feb 10, 2026The UCI amends its medical rules to cross-reference WADA's M1.4 ban and set out disciplinary sanctions.

The ethical issues

• Rules vs. spirit: using legal methods in ways that may enhance performance.
• Athlete health and the duty of care of teams and governing bodies.
• The line between measurement and enhancement.
• Regulating before evidence of misuse exists.
• Inequality of access to advanced sports science.

Stakeholders

Stakeholder	Interests & obligations
Riders	Their health, fair competition and access to legitimate science.
Teams and performance staff	Effective training tools and competitive advantage.
The UCI	Rider health and the sport's credibility.
WADA	A consistent, evidence-based Prohibited List.
Sports scientists and device makers	Legitimate use of measurement tools.
Amateur athletes	Protection from dangerous imitation.

The case

At the 2024 Tour de France, a little-known sports-science tool became a talking point. Escape Collective reported that UAE Team Emirates, Visma-Lease a Bike and Israel-Premier Tech had used carbon monoxide (CO) rebreathing. Each team said it used the method only for testing.^[1][3] Escape Collective’s July 12 report said Visma and UAE confirmed they had access to the equipment for testing but denied using it to enhance performance.^[3] After one dominant climbing performance at the Tour, the French sports newspaper L’Équipe ran the headline “A domination that raises questions,” and its questions included CO rebreathing.^[1]

Background

CO rebreathing measures total hemoglobin mass and blood volume, which a standard blood test cannot do. A small, precise dose of CO is inhaled with oxygen; after a few minutes, a blood sample is analyzed.^[1] UCI’s own description notes that CO is “commonly used in sports medicine” for this purpose, especially to examine the effects of endurance training and altitude.^[2] One maker of rebreathing devices told the Observer that the method dated to 1960 and that “there is absolutely nothing new to it.”^[1] UAE Team Emirates described it as “a well-established, safe, professional method.”^[1]

The concern was a different use. CO binds to hemoglobin far more strongly than oxygen, temporarily lowering the blood’s oxygen-carrying capacity and triggering a response similar to altitude.^[1] In a 2020 experiment, 11 subjects who inhaled CO five times a day for three weeks increased their hemoglobin mass by 5%, with gains in endurance comparable to an altitude camp.^[1] The Observer reported that there was “no evidence of WorldTour teams using it for these purposes.”^[1]

Repeated inhalation is dangerous. The UCI listed symptoms such as headaches, nausea, dizziness and confusion, which can progress to heart-rhythm problems, seizures, paralysis and loss of consciousness.^[2] The device maker warned that people copying the research with “crude home-made equipment” could reach “doses that are outright lethal.”^[1]

The rules and governance

In August 2024, WADA said that its Prohibited List expert group had discussed CO but that “there is no consensus on whether it can have a performance-enhancing effect, and no sufficiently robust data currently supports that proposition.” It added that CO “can be dangerous to use, so it would not be recommended.”^[1] A sports physiologist predicted a ban, noting that gases such as xenon and argon, used to stimulate red blood cell production, had been banned a decade earlier after their use came to light around the 2014 Sochi Olympics.^[1]

The UCI acted on its own. At its January 31–February 1, 2025 meeting, the Management Committee approved a ban on repeated CO inhalation, effective February 10, 2025, “to protect the health of riders.”^[2] The rule:

• forbids possession, outside a medical facility, of commercial CO rebreathing systems connected to oxygen and CO cylinders, for all license-holders, teams and anyone acting for them;
• allows CO inhalation only in a medical facility under an experienced medical professional, limited to one inhalation to measure hemoglobin mass, with a second allowed only two weeks later; and
• requires top-tier teams to record each inhalation in the rider’s medical file.^[2]

The UCI said the ban was independent of the World Anti-Doping Code, and that it had asked WADA “to take position on the repeated use of CO in and out of competition.”^[2] Its president called the ban “a bold and necessary stance … on medical grounds.”^[2]

In December 2024, UAE Team Emirates’ performance coordinator said the team had stopped using CO rebreathing, describing it as “an exercise that we co-ordinated over 18 months” to assess riders’ improvements during altitude camps.^[4]

Aftermath

The UCI’s health-based rule took effect before the 2025 season’s major races.^[2] WADA then answered the UCI’s request. The 2026 Prohibited List, published in September 2025 and in force from January 1, 2026, added a new prohibited method, M1.4: “The use of re-breathing systems or equipment to deliver carbon monoxide, unless performed as a diagnostic procedure under the supervision of a medical or scientific professional.”^[5] WADA explained that “the non-diagnostic use of carbon monoxide (CO)” was now prohibited, while diagnostic uses “such as total hemoglobin mass measurements or the determination of pulmonary diffusion capacity” were not.^[6] A year after saying there was no consensus on a performance effect, WADA had banned the method across all sports, leaving the line between testing and training to be drawn by who supervises the procedure and why.^[1][6] WADA’s summary of the major changes gave its reasons in plain terms. The non-diagnostic use of CO “can increase erythropoiesis under certain conditions,” it said, and the wording was chosen “to differentiate between illicit use and the intake resulting from natural combustion processes (e.g. smoking), the environment (e. g. exhaust gases) or diagnostic procedures.”^[7] Cyclingnews noted that there was still no evidence that the teams named in 2024 had used CO to enhance performance. It quoted Visma’s Jonas Vingegaard from November 2024: “I understand if it’s misused, but I never knew that it could be misused… we only used it to test if the altitude camps are working or not.”^[7]

The two rules now overlap. In a memorandum dated February 10, 2026, the UCI amended its medical rules so that the section opens by noting that CO inhalation equipment is prohibited in and out of competition under “category M1.4 of the Prohibited Methods List.”^[8] The UCI’s own ban remains in place. It states that CO binds to hemoglobin “approximately 200-fold higher than that of oxygen” and that its rules apply “autonomously and independently of any other regulations.” It also tells courts and arbitral panels to “respect the strictly medical purpose of this rule, which is to protect the athletes’ health.”^[8] The amended text gives the UCI Disciplinary Commission power to sanction possession or use of CO rebreathing systems outside a medical facility. It treats “the proven use of repeated inhalations of CO, and/or any inhalation not intended for the determination of total Hb mass and blood volume” as an aggravating factor.^[8] The UCI rule therefore sits alongside the WADA prohibition. The UCI frames its rule as health protection, while WADA’s rule targets the method’s possible effect on red blood cell production.^[6][7][8] The sources reviewed for this case report no sanction under either rule as of October 2026.

The core dilemma

CO rebreathing is a legitimate measurement tool. Repeated use may enhance performance and is dangerous, but evidence of misuse in professional cycling was lacking, and WADA did not find enough evidence to prohibit it. Should governing bodies restrict a legal method before misuse is proven, and on what grounds?

Competing values

• Innovation vs. integrity: sports science against methods that blur into enhancement.
• Health vs. autonomy: protecting riders against their freedom to use legal methods.
• Evidence vs. precaution: waiting for proof against acting early.
• Equal access: advanced science for wealthy teams against a level field.

Strongest arguments

For the ban: repeated CO inhalation is dangerous, the research shows a potential edge, and cycling’s history makes precaution reasonable. Allowing only supervised, infrequent measurement protects legitimate use.

Against the ban: no evidence showed teams were misusing CO. WADA found no consensus on enhancement, and the ban burdens legitimate science on the basis of suspicion.

For a middle path: the UCI’s rule permits supervised measurement while banning repeated use and home kits, as it did.

Applying the frameworks

Utilitarianism

A utilitarian lens weighs the health risks and the potential for an unfair edge against the costs of restricting a useful measurement tool. Precautionary rules may prevent serious harm at modest cost.

Virtue ethics

A virtue lens asks what integrity means for teams pushing the limits of legal science, and whether a culture of seeking marginal gains can drift into gamesmanship.

Care ethics

A care lens focuses on riders’ health and on governing bodies’ and teams’ duty of care, including toward amateurs who might copy dangerous protocols.

Walk it through: the Simple Decision Model

1. Spot the issue. You sit on the UCI Management Committee. Stakeholders include riders, teams, scientists and WADA. Less visible are amateur riders. The codes are the UCI medical rules and the World Code.
2. Gather facts. How is CO used by teams? What does the research show about enhancement and risk? Beware the halo of cycling’s doping history and the availability of dramatic headlines.
3. Test options. (a) Take no action and wait for WADA. (b) Ban all CO use. (c) Ban repeated use and home kits while allowing supervised measurement.
4. Check codes and law. Check your authority under the medical rules and the relationship with the World Code.
5. Decide and document. Explain the health basis and the limits.
6. Communicate and follow up. Publish the rule, monitor compliance and ask WADA to review.

What’s settled and what’s contested

• Settled: teams’ acknowledged use for measurement; WADA’s 2024 position; the UCI’s 2025 rule.
• Contested: whether CO enhances performance in practice; whether any team misused it; whether WADA’s 2026 ban on non-diagnostic use draws the line in the right place.

Why this case matters

The CO rebreathing debate shows how legal sports science can raise integrity and health questions, and how governing bodies can act on health grounds when anti-doping rules lag.

Key terms

Rules vs. spirit of the game

Rules are the written minimum; the spirit is the fairness and respect the rules exist to protect. Rules set the floor; spirit sets the ceiling.

Equalizer vs. enhancer

Whether a technology or adaptation restores a level field (adaptation) or gives an advantage beyond it (augmentation).

Duty of care

The responsibility of coaches, organizations and institutions to protect the health, safety and well-being of those in their charge. It is not a license for total control.

Non-maleficence

The duty to avoid causing harm, often paired with care ethics when setting safety rules.

Gamesmanship

Pushing the rules to their limits, without technically breaking them, to gain an edge. It tests, and can erode, the game.

Paternalism (hard and soft)

Restricting someone’s choices for their own good. Hard paternalism overrides competent, informed adults; soft paternalism steps in when information or autonomy is compromised.

Definitions from the Ethics Cases glossary.

Discussion questions

1. Teams said they used carbon monoxide rebreathing only to measure hemoglobin. Should a legitimate measurement tool be restricted because it could be misused?
2. WADA found no consensus that carbon monoxide enhances performance. Was the UCI right to ban repeated inhalation on health grounds alone?
3. Is there an ethical difference between gaining an edge through altitude training and through repeated carbon monoxide inhalation?
4. Does advanced sports science create unfair advantages for wealthy teams?

Sources

1. Road cycling: how a legitimate blood test became the focus of performance abuse claims — The Guardian (The Observer)
2. The UCI bans repeated inhalation of carbon monoxide, and introduces measures to ensure participation of best teams in top-tier road events — Union Cycliste Internationale
3. Exclusive: Tour riders are inhaling carbon monoxide to optimise altitude training (July 12, 2024) — Escape Collective
4. UAE Team Emirates confirm squad have stopped using carbon monoxide rebreathing (Dec. 10, 2024) — Cyclingnews
5. World Anti-Doping Code International Standard: Prohibited List 2026 — World Anti-Doping Agency
6. WADA publishes 2026 Prohibited List (Sept. 2025) — World Anti-Doping Agency
7. WADA moves to ban carbon monoxide rebreathers as a performance-enhancing method (Sept. 24, 2025) — Cyclingnews
8. Memorandum: Part XIII – Medical Rules, amendments applying on 10.02.2026 (Ban of carbon monoxide inhalations outside medical facilities) — Union Cycliste Internationale