PARIS: A woman injured during a fatal cryotherapy session at a gym in France´s capital earlier this week is now brain-dead, the prosecutor´s office said on Friday.

The client, in her early thirties, was admitted to hospital in a critical condition after the accident late on Monday claimed the life of an employee in her late twenties. The client has been brain-dead since Thursday, the Paris prosecutor´s office said.

An autopsy on the first victim showed she suffocated due to a lack of oxygen, it added, which might confirm the theory of a nitrogen leak into the cryotherapy chamber.

Cryotherapy uses vaporised liquid nitrogen or nitrous oxide to lower the skin´s surface temperature to below minus 100 degrees Celsius (minus 148 Fahrenheit) for a recommended time of no more than three minutes.

Nitrogen is a colourless, odourless gas. It makes up around 80 percent of the air we breathe, while oxygen accounts for 20 percent.

But a nitrogen leak in a closed space could lead to oxygen depletion. Advocates say whole-body cryotherapy is effective in reducing muscle soreness, stress, rheumatism and various skin conditions — like ice baths.