Carbon monoxide (CO) rebreathing in Cycling

As of February 10th 2025, the latest substance ban by the UCI has come into place, pertaining to the use of carbon monoxide rebreathing, a practice that has increased in prominence over the past number of years. Similarly to other decisions of this nature, it has had its own share of surrounding controversy, with some questioning its harm, and others raising alarm bells – but just because it’s been banned, have we seen the last of it?

What is it, and why are they doing it?

Carbon monoxide (CO) rebreathing is quite literally all in the name. Before it’s administered, tests are conducted to establish the baseline levels of carbon monoxide in an athlete’s breath and blood. Then, using a rebreathing device, a precise, known amount of CO diluted in oxygen is inhaled for two minutes. Upon entering the blood, it combines with haemoglobin in the red blood cells to form carboxyhaemoglobin. Following this, the same tests are run to get a second set of data that can be compared to the initial baseline, enabling the measurement of critical blood metrics, such as haemoglobin (Hb). In this way, it has been used as a data collection exercise, particularly by teams to establish a more accurate impact of altitude training on their athletes.

When at altitude, where atmospheric pressure and oxygen levels are much lower than at sea level, the body is forced to adapt to these conditions. The body must work harder to oxygenate the blood, so fatigues faster. But the athletic benefit actually comes from adaptation to lower oxygen levels, as it increases red blood cell production (the body’s oxygen transporters). When returning to lower altitudes to compete, where oxygen is in greater abundance, athletes experience improved VO2 max and lactic acid tolerance.

Haemoglobin (Hb) is the specific protein in the red blood cells that enables oxygen transportation. There is a direct correlation between total Hb mass and VO2 max, in that the higher one’s Hb mass, the higher one’s VO2 max, because the more haemoglobin in the body, more oxygen can be transported. Haemoglobin mass is therefore the most important determinant when it comes to measuring the blood’s oxygen carrying capacity, and therefore evaluating athletic responses to training stimuli such as at altitude.

Prior to its introduction, it was solely a full blood count (not dissimilar to one you may have had at the GP) before and after an altitude camp that would be used to assess the outcome for each rider. However the flaw with this method is that if the blood volume increases, but haemoglobin levels remain the same, hematocrit levels will fall, meaning it’s actually a rather poor indicator of altitude success.

Whilst a red blood cell count could be used, this fails to take into account the impact that training at altitude can have on overall blood volume, which becomes a limiting factor in the measuring process, and where CO rebreathing succeeds.

Dr David De Klerk of UAE Team Emirates describes how CO rebreathing does this as: “If you could take all the hemoglobin in your body and put it on the scale and weigh it, that would be the best way of doing it, and that is essentially what carbon monoxide rebreathing does” Blood volume is no longer the limiting factor in measurement because you are merely measuring the total haemoglobin mass.

So you can see how performing this pre and post altitude training provides the most accurate insight to teams as to how their riders are performing as a result. Similarly to altitude, it has further potential uses in assessing the efficacy of heat training, which recent studies have shown to increase Hb mass in a similar way to altitude. As such, it isn’t contrastingly different to using a power meter, there isn’t an individualised advantage but merely provides data.

At first glance, it all seems above board, yet the controversy occurs over speculation that it could be abused as a performance enhancing tool – and a dangerous one at that. The Escape Collective published information outlining that CO inhalation protocols could not only measure the impact of altitude training, but also mimic them. The frequent inhalation of CO simulates a hypoxic response, stimulating an increase in haemoglobin levels, such as is achieved by physical exertion at altitude.

Two particularly prominent studies have demonstrated this; firstly one in 2019 where participants who inhaled CO prior to running on a treadmill five times a week for a month had an increase in Hb mass and VO2 max to a greater extent than participants who ran on the treadmill alone. The second study in 2021 particularly focussed on amateur cyclists, who inhaled a measured dose of carbon monoxide five times a day over a three week period – the result being a 4.8% haemoglobin mass and VO2 max.

We know all too well the extreme lengths both athletes and their teams will go to to gain the edge on competitors, and there has been fear that with the strength of this evidence, this would be no exception. The factor that has finally lead to its ban by the UCI isn’t the lack of clarity over the potentially damaging long term effects, but that CO causes a change to occur in the blood – an outcome, irrespective of cause, that is banned under the organisation.

As it stands, the UCI has authorised a single CO inhalation in a medical facility under expert supervision, for the purpose of measuring total Hb mass, and a second inhalation is only being permitted 2 weeks after the first. However all this being said, at the time of writing, the World Anti-Doping Agency (WADA) has still not taken a position on the issue despite repeated calls to do so.

The fact that there is not a blanket ban on the practice, and the limitations theoretically change nothing for the teams that do have it in their arsenal, leaves the question of a loophole that may be exploited, or a weakness in enforcement that may be overcome by those prepared to take the risk. At a surface level, it can be perceived as a positive move, however I don’t think we’ve seen the back of CO rebreathing and its controversies quite yet.