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Fourteen people were hospitalized for carbon monoxide poisoning Wednesday after an incident involving construction equipment at a Yale University-owned building, officials said.

New Haven Mayor Justin Elicker told USA TODAY that first responders received a call shortly before 8 a.m. regarding an unconscious person found on a sidewalk. The individual, identified as one of the construction workers, was transported to a hospital and was treated for extremely high levels of carbon monoxide in his bloodstream.

Elicker said emergency crews were then sent to check for a gas leak at the building, where the New Haven Fire Department's carbon monoxide detectors measured "about 10 times the safe amount of carbon dioxide in the air."

Crews evacuated the construction site and an adjacent building also owned by Yale, said Karen Peart, a spokesperson for the university. A total of 14 people were hospitalized, including nine construction workers and five university employees.

It was later determined that the workers were using a propane-fueled saw to cut concrete, according to Elicker.

"When using that type of equipment, you are supposed to be in an area that's properly ventilated and using carbon monoxide monitors," Elicker said. "And they were not using carbon monoxide monitors. It appears they did not realize that there was so much carbon monoxide in the air."

Person remains in critical condition

According to Rick Fontana, New Haven’s emergency operations director, the worker found lying unconscious outside of the building was taken to a hospital’s hyperbaric chamber in Brooklyn, New York, where he was in critical condition.

Elicker said four people have been discharged while the remaining are still at the emergency room for medical monitoring.

When responding personnel arrived at the scene, they discovered 13 people at the building with elevated carbon monoxide levels and complaining of headaches, according to Fontana.

Fontana said a typical home carbon monoxide detector sounds an alarm when it detects 35 parts per million. In Wednesday's incident, there were 350 parts per million, or 10 times the permissible level.

“That carbon monoxide, it’s not like you could smell it or see it or feel it,” he said. “Everybody thought that it was being vented properly until we were notified of this group of people.”

At about 11:30 a.m., Peart said responders from Yale determined that carbon monoxide levels were within safe levels at the construction site and none was detected in adjacent areas.

New Haven's building inspector has issued a stop work order for the site, according to Elicker, and the Occupational Safety and Health Administration is investigating the incident.

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What is carbon monoxide poisoning?

According to the Centers for Disease Control and Prevention, carbon monoxide, or CO, is a deadly odorless and colorless gas. It can be found in fumes produced by burning fuel in vehicles, small engines, stoves, lanterns, grills, fireplaces, gas ranges, or furnaces.

The gas can build up in enclosed or partially enclosed spaces, which can poison people and animals who breathe it in, the center said. Inhaling carbon monoxide fumes prevents the body from using oxygen properly, which can harm organs, including the brain and heart.

The CDC describes symptoms of carbon monoxide poisoning as "flu-like." The most common symptoms are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion, according to the center.

"Every year, at least 420 people die in the U.S. from accidental CO poisoning," the center said. "More than 100,000 people in the U.S. visit the emergency department each year due to accidental CO poisoning."

Carbon monoxide poisoning sometimes happens when people are trying to protect themselves against cold weather. The CDC says the risk of carbon monoxide poisoning increases when heating systems, especially home heating systems, run for hours.

Earlier this month, nearly 50 people from a Utah church were hospitalized for carbon monoxide poisoning. The church said the incident was due to a malfunction with the heating system. A Massachusetts family was also discovered dead in their home this month, where authorities found elevated levels of carbon monoxide.

How to prevent carbon monoxide poisoning

The CDC recommends installing battery-powered carbon monoxide detectors in homes, especially in bedrooms so the alarms can awaken those sleeping. Anyone who suspects carbon monoxide poisoning is urged to call 911 immediately.

The center also recommends the following safety tips to prevent carbon monoxide poisoning:

• Never use a generator in enclosed spaces. Place generators outside and far from open windows, doors, and vents.
• Make sure your heating system, water heater, and other gas-, oil- or coal-powered appliances are serviced yearly by technicians.
• Also remember to service any appliances in your home that emit gas.
• Clean and check chimneys every year.
• Never use a gas oven to heat a home as it can cause a carbon monoxide build-up.
• Never burn any type of charcoal indoors.
• Never run a vehicle inside an attached garage. For detached garages, leave the door open to allow air to flow.

Contributing: Krystal Nurse and Minnah Arshad, USA TODAY; The Associated Press