In the last few weeks the World Anti-doping Agency (WADA) have published their new list of prohibited substances and methods, as it does each year. This year they have added a new method onto the list – the non-diagnostic use of carbon monoxide (CO). Isn’t CO in the atmosphere all around us, I hear you ask? Yes, it is, in very small amounts. So why is this now banned?
Carbon monoxide is formed by the burning of hydrocarbon products (wood, coal, gas and so on) in an inefficient way. Complete combustion forms carbon dioxide and water, but if there is a little too much or too little oxygen available CO is formed. This often occurs in poorly ventilated spaces or with poorly maintained appliances like gas heaters or car engines. You can’t see or smell CO, which makes it very dangerous as it can build up and exposure to a high amount can be fatal. According to the CO Research Trust (CORT) “studies show that the brain is especially vulnerable to CO, which can lead to cognitive and neuropsychiatric problems, and potentially contribute to diseases including stroke and dementia”. There are several ongoing research projects investigating chronic low dose exposure in our School at Sheffield Hallam, funded by CORT and led by Dr Mari Herigstad.
I’ve previously written about carbon monoxide poisoning here.
Non-diagnostic use of carbon monoxide (CO) was added to the Prohibited Methods as a new section, M 1.4 (https://www.wada-ama.org/sites/default/files/2025-09/2026_list_explanatory_note_en_final_september_2025.pdf)
When deciding to ban a drug or method WADA run three tests. Is it:
- Potentially performance enhancing?
- A health risk for athletes?
- Against the ‘spirit of sport’?
If a drug or method fails two of these three tests it should be banned. In the case of CO they must have decided that it is potentially performance enhancing (see below) and also a potential health risk (as detailed above and here). I am not sure whether they also deem it contrary to the spirit of sport, but it can fail on the two others anyway.
So what is diagnostic or non-diagnostic use?
In endurance sports one of the most important aspects that helps predict performance is the ability of the body to carry oxygen. Oxygen is carried in the blood by haemoglobin within red blood cells, delivering it from lung to muscles. CO can be used in diagnostic testing to help measure the total amount of haemoglobin in the athletes body. One common use of this test is to measure the athletes total haemoglobin before and after a trip to altitude. The athlete and their team can then see what the effects of the training is, how the body gets used to altitude and what gains can be made. This helps planning and optimising performance prior to major competitions. The test involves inhaling small amounts of CO and the exhaled breath is measured. This diagnostic use will be allowed under future rules, whilst the ban targets non-medical, potentially performance-enhancing applications.
Non-diagnostic use is when an athlete repeatedly inhales a small amount of CO, so the level in the blood remains low but continuous. CO binds to haemoglobin forming carboxyhaemoglobin. CO has stronger binding to haemoglobin that oxygen, so it lowers the efficiency of oxygen delivery. Everyone has a small amount of carboxyhaemoglobin in their blood, you would expect to find a CO saturation level up to around 5 %, rising to 10 % or so in smokers. A fatal saturation of CO is usually above 35 %. So you can live with small increases. Repeated low dose use, over several weeks, simulates a low-oxygen (hypoxic) environment, similar to what happens during altitude training. The body will then produce more red blood cells to mitigate this.
Some papers suggest a corresponding increase in red blood cell mass and exercise performance, so it definitely fails that potential to enhance performance test.
How do we detect this use and stop it?
It is fairly straightforward to analyse CO exposure using a blood gas analyser to analyse carboxyhaemoglobin. The test most frequently used is unlikely to be able to differentiate between smoking, exposure through the natural world or ‘rebreathing’. So how will they monitor this? It is very unlikely they will catch people in the act, and CO will actually dissipate from the blood fairly swiftly. This is a warning really, unless they have developed a test for biomarkers? It is however unlikely to be available currently. The athlete biological passport will pick up significant variations in blood parameters and is the most likely mechanism currently to detect use. Although even then, can it differentiate between altitude training, an altitude tent or CO rebreathing? Unlikely. We’ll see if anyone fails a test for it in the next few years!
Thanks for reading!