A flight takes off from Los Angeles International Airport in this July 2022 file photo.
CNN  — 

A terminal at Los Angeles International Airport has been cleared and deemed safe after an apparent gas leak that sickened four people in Terminal 8, according to a tweet from the airport.

The four people hurt in the incident Monday were all airport employees, according to an update provided by the Los Angeles Fire Department.

The condition of the most severely sickened victim was upgraded from grave to critical earlier Monday.

A woman and three men were working in or near a utility room “when a popping sound was heard, and the apparent release of Carbon Dioxide vapor took place,” an LAFD report said.

A deluge of carbon dioxide from a fire suppression system inside a subterranean utility room displaced all of the oxygen inside the area, LAFD Capt. Erik Scott said in a news conference.

The most seriously injured man, described as being in his 50s, was found not breathing and without a pulse inside the utility room. CPR was immediately performed and advanced life support given by emergency responders as he was taken to a nearby hospital, the Fire Department said in an incident report.

“The carbon dioxide displaced the oxygen in his bloodstream and he went into cardiac arrest,” Scott said of the man seriously injured in the incident.

This appears to be an isolated incident, Scott added, noting that there was no explosion or fire.

Where the gas leak happened

The incident occurred in Terminal 8 near the baggage area. The terminal was cleared of passengers after the incident. The area reopened around 1 p.m. ET (10 a.m. PT), according to a tweet from LAX airport.

A Federal Aviation Administration ground stop for arriving United Airlines was also lifted, the airport said.

All other terminals and flights were operating normally, according to a tweet from LAX airport.

“Check your flight status with the airline directly for the best information,” the airport tweeted earlier on Monday.

United told CNN it was” temporarily consolidating” its operations to Terminal 7 and was contacting impacted customers with new departure time and gate information.

Passengers passing through security bound for Terminal 8 were being held in Terminal 7 after screening, Transportation Security Administration spokesperson Lorie Dankers said earlier on Monday.

Dankers said all TSA employees are OK, and the incident did not involve a TSA checkpoint.

Top image: A flight takes off from Los Angeles International Airport in this July 2022 file photo.(AaronP/Bauer-Griffin/GC Images/Getty Images)

CNN’s Marnie Hunter contributed to this report.