A sold-out basement concert venue in downtown Aspen was evacuated early Monday morning after a build-up of carbon dioxide led to breathing problems for patrons, sources said.

No one was seriously injured at Belly Up Aspen during the crowded show by Dutch DJ Tiësto, though the incident proved scary for at least one person in attendance.

“It was insane,” said a 38-year-old woman who attended the concert and asked not to be identified. “We all thought we were going to die. No one could breathe.”

The problem appeared to stem from a cryogenic special effect that suffered a burst pipe, said Michael Goldberg, Belly Up owner. Goldberg said he was still in the process of determining exactly what happened later Monday morning.

“What we think … is a line burst that feeds the cryo,” he said, adding that a video taken that night shows a pipe next to the stage rupturing. “But we’re still not to the bottom of it.”

Cryogenic special effects use carbon dioxide-fed cannons to create plumes of fog, according to online sources. The effect is a common part of dance music shows, and Belly Up has carbon dioxide monitors that functioned properly and notified local police and fire departments of the build-up, Goldberg said.

The system that governs the effect won’t let it be used when carbon dioxide reaches a certain level, and it wasn’t in use when the monitors detected the spike in carbon dioxide, he said. The spike was likely the result of the pipe rupture.

After the spike was detected, Belly Up staff began telling people to leave, Goldberg said.

Aspen police were called to Belly Up at 12:32 a.m. after reports that “people at the sold-out show were leaving because they were having trouble breathing,” said Aspen Assistant Police Chief Bill Linn.

“When officers arrived, they were talking to people as they were exiting who said they couldn’t breathe and something was not right,” Linn said. “It seemed like a very dangerous situation.”

He said bar staff “did the responsible thing” and evacuated the club as quickly as possible after officers arrived — patrons who checked coats and jackets had to leave without them — and the show did not continue, he said.

The 38-year-old Aspen resident who attended the Sunday show said she was drinking water all night and was initially with friends near the stage, where she remained for about the first hour of Tiesto’s performance. She said she left the front area because she felt short of breath and went and stood near the bathrooms for another hour or so.

At one point, she said she went into the bathroom and found one woman helping another who appeared to have passed out.

“I was really short of breath at that point, so I walked out and got some air,” she said.

A few minutes later, numerous people began coming up the stairs, some of them hyperventilating and others grabbing the handrails to steady themselves, she said.

“They were like, ‘What’s going on? I can’t breathe,’” she said. “People were freaking out everywhere.”

Jake Andersen, Aspen deputy fire chief, said Belly Up staff reported that the problem was with a malfunctioning fog machine, though Goldberg later said that wasn’t the cause. Firefighters reported seeing several carbon dioxide tanks in a closet near the stage, Andersen said.

Fire officials monitored the air quality inside the venue and brought oxygen levels back up to normal, Andersen said.

Paramedics checked out a couple of concertgoers at the scene and released them, but no one was taken to the hospital, Linn said.

“Obviously it was an alarming situation,” he said. “I’m very glad it turned out to be not as dangerous as it seemed.”

Belly Up holds a maximum capacity of 450 people, though the crowd was likely a bit smaller than that because of no-show ticketholders, Goldberg said.

jauslander@aspentimes.com