The Tour de France’s two leading riders – Tadej Pogacar and Jonas Vingegaard – have admitted using a carbon monoxide rebreathing technique to monitor their blood following altitude training.

The technique, which does not contravene anti-doping rules, has emerged following a report on the website Escape Collective which says that multiple teams in this year’s race are using the practice.

Carbon monoxide rebreathing is a measurement tool that allows teams to track haemoglobin – a protein that facilities the movement of oxygen in red blood cells – and so monitor the physiological impact of training at an elevated height.

However, concern has also been raised about the potential for athletes to inhale carbon monoxide which, while also not against anti-doping rules, is believed to have some performance-enhancing impacts and would be more controversial.

Pogacar’s UAE Team Emirates have confirmed their use of carbon monoxide rebreathing to measure haemoglobin and said that it was widely employed by athletes, climbers and other people who are exposed to altitude for professional sports or other physical activities.

Vingegaard, who rides for Team Visma-Lease a Bike, also said that it was a diagnostic tool. “It’s to measure how much haemoglobin you have in your blood,” he told the Danish newspaper Politiken. “There is nothing suspicious about it. We measure how many red blood cells you have in your blood and the effect of altitude training camp. It is one thing [to presume] what training at altitude did for you. Now we actually have something that can measure it.

“They [the doctors] say that they send something into the lungs that is similar to smoking a cigarette. We measure the day we get to our high-altitude camp and then [on] the day we go back down. Then we see the difference in how much haemoglobin is built up. There is nothing more to it.”

Pogacar, the race leader, initially appeared confused when he was asked about the issue on Tuesday. “I heard this, I was thinking about car exhausts, I don’t know,” he said. “I have no comment on it. I don’t know what it is. Maybe I am just uneducated.”

After the stage finish on Wednesday, he clarified the position. “Yesterday, I didn’t quite understand the question. It was not a question posed like this. It’s a test in altitude camp to see how you respond to altitude. You need to do this test, it’s like a two- or three-minute-long test. You breathe into a balloon for one minute and then you see the haemoglobin mass, and then you need to repeat it two weeks after.

“But I did just the first part of the test. I never did the second part because the girl who was supposed to come after two weeks didn’t come. It’s not like we’re breathing exhaust pipes every day in the cars. It’s just a pretty simple test to see how you respond to altitude training.”

Pogacar and Vingegaard have produced a series of exceptional performances in again dominating the Tour de France this year. Indeed, Vingegaard admitted that he had surpassed his best physical performance even in losing more than a minute to Pogacar on the stage finish to Plateau de Beille. “Tadej must have improved on all parameters since he could ride so fast,” he said. “Sunday, I was better than I have ever been before. These are probably the two best performances on a mountain that have been delivered in the history of cycling.”

Pogacar also confirmed that, statistically, it had been the best performance of his career. “It was really crazy when I checked my numbers after the stage. This was especially the case at the moment where Matteo Jorgenson led and Jonas then attacked. There, I achieved the highest numbers in my career.”