A team of four British former special forces soldiers, including a UK government minister, has set a new record for the fastest ascent of Mount Everest without on-mountain acclimatization, summiting the 8,849-meter (29,032-foot) peak in under five days.

The high-speed expedition, completed early Wednesday, relied on xenon gas to pre-acclimatize to low-oxygen conditions, a method that has sparked debate in the mountaineering community, according to BBC News.

Typically, climbers spend six to eight weeks acclimatizing on Everest before attempting the summit.

The team’s use of xenon, a gas purported to simulate high-altitude conditions, enabled their rapid climb.

Organizers hailed the approach as a breakthrough, but critics in the mountaineering industry question its safety and efficacy, citing limited scientific consensus on the gas’s use.

Accompanied by five Sherpa guides and a cameraman, the team reached the summit and began their descent shortly after.

While their feat marks a record for an unacclimatized ascent, it does not surpass the fastest overall Everest climb, set by Lhakpa Gelu Sherpa in 2003, who summited in 10 hours and 56 minutes after acclimatizing on the mountain.

“They started on the afternoon of 16th May and summited on the morning of the 21st, taking four days and approximately 18 hours,” expedition organiser Lukas Furtenbach told the BBC.

The four former soldiers, including veterans minister Alastair Carns, slept in specialized tents for six weeks to acclimatize to lower oxygen levels before traveling to Nepal.

From Kathmandu, they flew to Everest base camp and began climbing immediately, using supplemental oxygen like other climbers, according to Mr. Furtenbach.

Typically, climbers spend weeks moving between base camp and higher camps to adjust to reduced oxygen at high altitudes.

Above 8,000m, in the “death zone,” oxygen levels are only a third of those at sea level.

However, the four British team members skipped this acclimatization process.

“The team made a three-month acclimatisation programme in simulated altitude before coming to Nepal,” Mr Furtenbach said.

The simulated altitude was achieved using hypoxic tents, where oxygen levels were reduced using a generator to mimic the low-oxygen environment found at high altitudes.

Two weeks before the expedition, the climbers also inhaled xenon gas at a clinic in Germany, according to Mr. Furtenbach.

This method is used to help pre-acclimatize the body to the stress of high-altitude climbing.

“It helps to protect the body from altitude sickness,” he said.

Some researchers suggest that xenon gas can boost the production of erythropoietin (EPO), a hormone that helps the body produce more red blood cells.