The British cycling team INEOS Grenadiers confirmed that during their 2025 training camp, the team used carbon monoxide (CO) rebreathing tests. This announcement comes in the wake of a report that riders from the team were required to sign consent forms before participating.
According to the team, the practice was medically supervised, entirely legal, and purely diagnostic in nature. But was it? The announcement ignited a debate across the cycling world about the line between ethical boundaries and legitimate performance science. Exactly how far should cycling go in the name of performance science?
The CO rebreathing method has been around for decades in exercise physiology. The technique measures an athlete’s hemoglobin mass and blood volume, giving medical staff key insight into each cyclist’s oxygen-carrying capacity. The test requires the athlete to inhale a very small, controlled amount of carbon monoxide, which binds to red blood cells before being safely expelled by the body.
Supporters of the method describe the test as a standard, low-risk diagnostic tool. According to INEOS, the team has never used the test to enhance the performance of its cyclists; rather, it’s a scientific measurement under strict medical oversight to monitor athlete health and adaptation.
Optics matter in a sport that’s seen its fair share of doping scandals over the years, and is currently still trying to rebuild public trust. The idea of athletes breathing carbon monoxide, even in trace amounts, has fans unsettled. Critics see the test as another example of cycling flirting with scientific limits.
Experts remain split over the test. Some sports physiologists cite that the test is a valuable tool for recovery monitoring and individualized training. Other experts say that the benefits of the test are minimal and the mere association with carbon monoxide could undermine the efforts to renew the credibility of the sport at a time when transparency is paramount.
The UCI (Union Cycliste Internationale) has yet to comment on these reports, but is reportedly looking into medical testing guidelines. Earlier this year, the UCI banned the use of repeated inhalation of carbon monoxide (CO) for performance enhancement due to health risks. WADA (World Anti-Doping Agency followed suit, banning carbon monoxide (CO) rebreathing beginning in 2026.
INEOS historically adhered to a ‘marginal gains’ approach. This move highlights the peril and power of scientific innovation. As the competitive edge of every cyclist continues to narrow, the sport faces another imbalance of science and ethics. Cycling has to find a way to balance ethics and science. Although the pursuit of perfection can be intoxicating, the sport has to maintain public trust.