Emergency services were forced to respond to a carbon monoxide leak at a daycare centre in Allentown, Pennsylvania, which left 27 people including children injured.
A total of 27 people including children and teaching assistants were transferred to four nearby hospitals. All were reported to be in a stable condition.
Parents were notified and some took their children to the hospital themselves, while others were treated outside the nursery premises. In total, 27 students and eight staff were evacuated.
Allentown spokesperson Genesis Ortega said: "The cause of the carbon monoxide leak is under investigation. As of late Tuesday morning, firefighters were still at the daycare trying to “mitigate the problem and ventilate the structure"."
A call for medical help was first received in relation to an unconscious child in the incident at Happy Smiles Learning Centre on Wabash Street early on Tuesday morning.
Several children were seen vomiting in the daycare car park.
The owner of the centre, Jesenia Gaudreaux, said she received a call at 7:30am that something was wrong with one of the children.
She said: "All of a sudden he just collapsed. They tried to bring him back, but he was just not responding. So my staff called 911 right away."
After assessing the situation, gas company UGI confirmed the incident was caused by a "malfunctioning heating unit as well as a blocked venting system".
Early reports say every Allentown ambulance had to respond to the scene and several children were taken away on stretchers.
Allentown said in a statement: "When [Allentown Fire Department] arrived on the scene to treat the unconscious child, a carbon monoxide monitor on the firefighter's equipment started beeipin and alerted the team to an issue.
"The firefighter then went and retrieved a gas metre which indicated dangerously high levels of carbon monoxide in the building."
Authorities declared a level 1 mass casualty, meaning there are warnings medical responders could become overwhelmed by the number of casualties.
Captain John Christopher of the Allentown Fire Department said their equipment alerted them to the presence of carbon dioxide when they attended the centre.
"We immediately exited the scene and reevaluated," he said.
Carbon monoxide is extremely dangerous as it displaces oxygen in the blood when it is breathed in, meaning the body becomes starved of the oxygen it needs to function.
The deadly gas is hard to detect as it is odourless and initial symptoms may include dizziness, feeling drowsy or sick and headaches.
Around 430 people in the US die every year as a result of accidental carbon monoxide poisoning, while a further 50,000 end up hospitalised.
