Will cycling ban carbon monoxide rebreathing?

Cycling's world governing body the UCI is expected to vote for a ban on the use of carbon monoxide rebreathing equipment this week, after it was established last year some teams were using it to measure haemoglobin levels in their riders' blood before and after altitude training.

While there's nothing in the rules that prevents it, there are questions around whether it can be used to enhance performance if used repetitively - or if it is simply too dangerous to inhale a deadly gas as a scientific measurement.

So far, so bannable, it seems. But there is growing confusion in the sport about what a ban will really mean, given the UCI is expected to allow it in a "medical setting".

BBC Sport understands any ban would likely to come into effect straight away if sanctioned.

What is carbon monoxide rebreathing?

Carbon monoxide rebreathing is a process that has been around for a long time in endurance sports - it involves an athlete inhaling a small amount of carbon monoxide, a deadly gas given off during the process of burning any material, because it gives a reading of how much haemoglobin is present in the blood.

This is usually done at the beginning and end of altitude training.

Haemoglobin is a protein found in the red bloods cells that carry oxygen around the body. If there is more haemoglobin in the blood, more oxygen can be carried to the muscles, giving an athlete more resistance to lactic acid burn - helping them go faster for longer.

The device is a machine that is connected to an external supply of oxygen and carbon monoxide, and the athlete uses an integrated rebreathing circuit.

Cycling and blood will never have an easy association, thanks to Lance Armstrong's infamous use of transfusions - which produce more red blood cells - to cheat his way to Tour de France victories around 20 years ago.

Who is doing it?

The current rebreathing issue would perhaps not be so prevalent if some of the riders and teams involved weren't the biggest in the sport, with three-time Tour de France winner Tadej Pogacar and his UAE Team Emirates-XRG squad being one, along with his closest rival Jonas Vingegaard of Visma Lease A Bike.

Stories of its use emerged at last year's Tour, a season in which the winning margins of some of the sport's best riders were vastly increased.

Can it enhance performance?

The maker of the device being used by teams admits enhancing performance could be possible, albeit easily detectable.

"When you inhale carbon monoxide you reduce [the] oxygen content of blood, so all of a sudden the body [behaves] as if you are at high altitude," says Carsten Lundby co-founder of rebreathing equipment manufacturer Detalo Health.

"If you inhale it once, it's like being exposed to high altitude for a couple of hours – this will do nothing to your performance.

"If you inhale carbon monoxide four or five times a day over the ratio of a month, you will get more haemoglobin, but this would be a misuse of our device - this is not what it was intended for," adds Lundby, who is also professor at the University of Lillehammer in Norway.

"And due to real-time surveillance of each machine, we can easily identify whether our devices would be misused."

The UCI is not necessarily looking to snuff out cheating with a ban. It says it is seeking the ruling on grounds of safety, saying the practice still could be used in a "medical setting".

But Lundby and teams are confused.

"At first I was happy to see the UCI taking a stance but then a bit baffled on the wording that these be allowed to be performed in a medical setting. I'm not aware of any medical settings where this has been done," says Lundby.

"Colleagues and myself have been in touch with the UCI stating that 99% of measurements are not performed in a medical environment but by medical-trained personnel. I'm sure the wording will be changed."

What the teams think

One team who have bought a device and employ Lundby himself to use it are Chris Froome's Israel-Premier Tech.

"Israel-Premier Tech's sole use is for testing purposes to measure the impact of altitude training," they say.

And Visma Lease a Bike said it was "nothing new, a method used for decades to measure the effects of altitude training".

They added: "It's a completely normal practice in multiple sports, conducted with experienced scientists and their equipment. In this way, it is neither harmful to health nor performance-enhancing."

What does Wada think?

So, if you can enhance performance, can it be traced? In theory, any change on blood values would be identified by an athlete's biological passport.

But the World Anti-Doping Agency (Wada) is not taking any chances and is looking into it, telling BBC Sport there is uncertainty if misusing the process would register on the biological passport because of a lack of research.

"Exposure to carbon monoxide (CO) has been discussed by Wada on several occasions," a representative said. "There is no general consensus on whether CO can have a performance-enhancing effect and no sufficiently robust data currently supports that.

"But Wada is currently investigating the effects of frequently and repeatedly using this CO rebreathing method vis-a-vis artificially enhancing the uptake, transport or delivery of oxygen.

"In any event, it is generally acknowledged that it can be dangerous for health so it would not be recommended."

What might happen next?

Many feel the issue has been stirred up by the cycling media and the UCI is simply taking an approach to demonstrate proactivity.

The UCI is expected to announce any decision on 1 February, following president David Lappartient's formal request to the executive body, at a meeting in France.

It is also uncertain when Wada will know if any further action needs to be taken but teams say they are prepared to work with the UCI on whatever decision is made.

Until then, it's yet another practice which raises questions in a sport that could do without them.