A Pennsylvania nursery school called 911 after a child fell unconscious. It ended up being carbon monoxide poisoning and 29 kids and employees were sent to the hospital.

• Twenty-nine adults and kids were hospitalized after a carbon monoxide leak at a Pennsylvania daycare center on Tuesday.
• The incident happened at the Happy Smiles Learning Center in Allentown.

More than two dozen people — including children — were hospitalized on Tuesday morning because of a carbon monoxide leak at a Pennsylvania daycare center, a fire official said.

The initial 911 call made from the Happy Smiles Learning Center in Allentown was about an unconscious child, Capt. John Christopher of the Allentown Fire Departmentm t0ld Insider.

But when firefighters and EMTs arrived on the scene at around 7:30 a.m., monitors on their medical bags went off, revealing a carbon monoxide leak.

According to Christopher, 29 children and workers at the daycare center were taken to four hospitals in the Lehigh Valley area. They were all in stable condition, the fire official said.

Christopher told WFMZ-TV that every ambulance in the area responded to the incident at the Happy Smiles Learning Center.

A total of 25 children and eight adult staffers were ultimately evacuated from the daycare center, Christopher told Insider.

The cause of the leak was not immediately clear. Happy Smiles Learning Center did not immediately respond to a request for comment by Insider.

Carbon monoxide is a colorless, odorless gas that can build up indoors, causing headaches, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion, according to the Centers for Disease Control and Prevention. Breathing in too much carbon monoxide can even kill, the CDC said.

The CDC said you can prevent carbon monoxide poisoning by making sure to put new batteries into your carbon monoxide detectors in the spring and fall, as well as having your heating and water systems checked by a technician each year.