Four workers at Los Angeles International Airport on Monday were sickened when a fire-suppression system inside an electrical room went off, releasing carbon dioxide — sending one to a hospital in grave condition and leading fire officials to evacuate Terminal 8, which serves United Airlines and United Express.

At around 7 a.m. the four were inside the electrical room “when a popping sound was heard, and the apparent release of carbon dioxide vapor took place,” Brian Humphrey, a spokesman for the Los Angeles Fire Department, said in a statement.

One worker, a man in his 50s, “was found pulseless and non-breathing inside the utility room,” Humphrey said.

Capt. Erik Scott, a spokesman for the LAFD, said the system was designed to snuff out an electrical fire without water. When triggered, the system spews enough carbon dioxide to push out any oxygen in the room, depriving a fire of any fuel and quickly putting it out. But with the four workers inside, they got a full blast of the odorless, colorless gas, Scott said.

“It was a complete deluge of that system — it doesn’t come out as a short sporadic amount of carbon dioxide, but rather a full deluge,” he said outside LAX on Monday afternoon.

“That displaced all of the oxygen inside — three people quickly got out, but one adult was still inside,” the captain said.

Scott said breathing in that amount of carbon dioxide would have “replaced the oxygen in (the man’s) blood. … He went into cardiac arrest.”

Paramedics attempted CPR on him at the airport, then took him to a hospital, where his condition improved. The other three workers were described by authorities as being in “mild distress” after the gas release, and were treated at the scene and not hospitalized.

Authorities didn’t reveal why the system was triggered. The utility room where the gas release occurred is in the baggage area of Terminal 8, at the end of a hallway far from areas for travelers.

Scott said all four workers are independent contractors.

During the evacuation, LAFD sent those in Terminal 8 to Terminal 7. The terminal re-opened before 10:30 a.m.

Some United Airlines flights were delayed.