Green Bay, WI – A cracked furnace in a downtown Green Bay hotel caused a carbon monoxide leak Thursday morning that hospitalized two people, fire officials said.

The leak could have been more dangerous if it had happened at night when people were sleeping, Green Bay Fire Department Lt. Nick Craig said.

“If this had happened at 11 p.m., it could have been deadly,” Craig said.

The Days Inn, 400 N. Washington St., was evacuated at about 9 a.m., and two guests were taken to St. Mary’s Hospital Medical Center for treatment of carbon monoxide poisoning. Hospital officials said Thursday afternoon the people were treated and released.

Firefighters found readings of more than 1,000 parts per million of carbon monoxide levels on the top floor. The normal levels are 3 to 4 parts per million.

“That’s absolutely deadly,” Craig said.

The two people hospitalized had been on the fourth floor, where readings were deemed very high at 800 parts per million, he said.

Firefighters worked into Thursday afternoon to ventilate the five-story hotel after evacuating people from 19 rooms.

North Washington Street between Pine and Main streets was closed for several hours. The hotel was closed Thursday and was expected to reopen sometime today, Craig said.

A crack in the furnace allowed carbon monoxide to escape and be distributed through the building. It was not known Thursday afternoon if the hotel had any working carbon monoxide detectors. Firefighters sounded a fire alarm to alert those inside.

A person who answered the phone at Days Inn at 1 p.m. would only say the hotel was not open Thursday afternoon and that staff was dealing with the situation.

The incident was discovered after two guests called an ambulance because they thought they had food poisoning and complained of headaches.

Green Bay Fire Department Lt. Ann Peggs, who responded on the call, questioned whether high levels of carbon monoxide caused their symptoms because headaches typically do not come with food poisoning.

Peggs did a good job recognizing a potential problem, Craig said. “She decided to take the extra step.”

Carbon monoxide is the leading cause of accidental poisoning deaths in America, according to the federal Centers for Disease Control and Prevention. The odorless, tasteless and colorless gas claims nearly 500 lives, and causes more than 15,000 visits to hospital emergency departments annually.

Early symptoms of carbon monoxide poisoning such as headaches, nausea and fatigue are often mistaken for the flu because the deadly gas goes undetected in a home. Prolonged exposure can lead to brain damage and even death.