The fatal incident happened at an On Air gym in eastern Paris on Saturday

A woman who was severely injured while having cryotherapy treatment at a fitness centre in Paris has been left brain-dead,local prosecutors say.

The 34-year-old was in an ultra-cold cryotherapy tank at the On Air gym on Boulevard Voltaire when the accident happened.

A 29-year-old employee supervising the treatment,named only as Alison,was found dead at the scene and tests later found she had suffocated.

This prompted speculation that nitrogen may have leaked from the machine,displacing oxygen in the chamber and the room where it’s located.

Cryotherapy uses short exposures of extreme cold to bring purported benefits such as reducing muscle soreness and stress.

Treatments see nitrous oxide or vaporised liquid nitrogen used to quickly cool air surrounding the patient below between -110C and -140C for up to three minutes.

A stock image of a cryotherapy chamber used in similar treatments (Picture: AFP)

Cryotherapy is popular with athletes in intense training but there is no scientific consensus on whether it actually benefits recovery or simply feels good.

The Mayo Clinic in London says cryotherapy research is ‘still in its infancy’ and that any benefits can probably be obtained just as easily with a bag of ice or a swim in cold water.

Three people who were in contact with the victims of the Paris gym incident were also treated.

Some 150 people were evacuated from the gym at the time of the incident,which is under investigation by authorities.

Previous cryotherapy incidentsinclude frostbite,severe burns,and other fatalities.

The treatment came under scrutiny in the US in 2015 after a woman froze to death at a Las Vegas spa.

The 24-year-old was believed to have entered one of the spa’s chambers after business hours to relieve some aches,and was discovered the next day by a co-worker.

According to one account,she was ‘frozen in solid ice’ after being trapped inside for more than 10 hours.