More than two dozen children and employees at a daycare in Allentown, Pennsylvania, were hospitalized Tuesday after a carbon monoxide leak.

At around 7:30 a.m. ET, firefighters responded to a call about an unconscious child at the Happy Smiles Learning Center, according to Allentown spokesperson Genesis Ortega.

The firefighters were alerted to a heavy presence of the gas on their carbon monoxide monitors. A further investigation found "dangerously high levels" of the colorless, odorless gas, prompting a full evacuation of the building, Ortega said.

According to Allentown Fire Captain John Christopher, ambulances from several cities arrived at the scene.

At least 27 children and staff were taken to hospitals by ambulance, Christopher said. He added that another five people arrived at the hospital, possibly through private transport.

Ortega said all 27 patients who arrived by ambulance were in stable condition.

Authorities are still investigating what caused the leak. Ortega added they were also working to mitigate the problem and improve the building's ventilation.

More than 400 Americans die every year from carbon monoxide poisoning, according to the Centers for Disease Control and Prevention.

Poorly ventilated small spaces where carbon monoxide is trapped are hazardous, according to the Mayo Clinic. Carbon monoxide poisoning can also happen when people try to use generators indoors after a power outage.