On Tuesday, nearly three dozen adults and children were evacuated from an Allentown, Pa., day care center after emergency workers responding to a report of an unconscious child detected carbon monoxide, an invisible, odorless gas that kills hundreds of people in the United States each year.

At least 28 of the 27 children and eight adults who were evacuated from Happy Smiles Learning Center were taken to four area hospitals, according to officials. The leak was caused by a faulty heating unit and a clogged venting system, according to the local Morning Call newspaper.

“Thank you to our neighboring municipalities for also responding quickly and arriving on scene to offer mutual aid,” the city of Allentown said in a statement. All of the patients are “stable, and there is no ongoing threat to the neighborhood or community.” According to the statement, the center’s license has been suspended as a result of the incident.

According to local media, patients had three to ten times the normal amount of carbon monoxide in their blood, according to Andrew Miller, chief of pediatric emergency medicine for Lehigh Valley Health Network.

According to the Centers for Disease Control and Prevention, at least 400 people die each year in the United States from non-fire-related carbon monoxide poisoning. Approximately 4,000 people have been hospitalized.

In February, the Allentown City Council passed a safety ordinance requiring day-care facilities to install carbon monoxide detectors. The Happy Smiles Learning Center and the city’s other 160 day-care centers, on the other hand, had until October 27 to comply, and the center had passed an inspection in July. Happy Smiles owner Jesenia Gautreaux told the Morning Call that the center lacked detectors but that she planned to install them.

The Allentown Fire Department and the Happy Smiles Learning Center could not be reached for comment Tuesday night.

The incident prompted a state legislator to renew calls for colleagues to support a pending bill that would require carbon monoxide detectors in child-care centers across Pennsylvania.

State Sen. Wayne D. Fontana (D) said in an email to colleagues that he later posted on his website that tragedy “was narrowly avoided.”

“I urge you to pass Senate Bill 129, which is currently in the House Health Committee, that would require inexpensive battery-operated devices” be placed in child-care centers, he said.

According to Fontana, the bill passed the Pennsylvania Senate unanimously last month and at least twice in previous years. It is currently awaiting approval from the lower legislative chamber. In a brief statement on Twitter, state Rep. Jeanne McNeill (D) urged the House to bring the safety bill to a floor vote, citing Tuesday’s incident.

One of the children who was evacuated from the facility appeared shaken, telling a local television station that she became concerned after seeing a classmate on the floor with his eyes closed. She remembered feeling dizzy and having pain in her head, both of which were symptoms of carbon monoxide poisoning. Signs include vomiting, chest pain, and nausea.

According to CDC data, many unintentional carbon monoxide poisonings occur between December and February. “When winter temperatures drop and home heating systems run for hours,” the CDC says, “the risk of carbon monoxide poisoning rises.”

The gas is typically produced by fumes from furnaces, kerosene heaters, vehicles with running engines in closed garages, stoves, lanterns, gas ranges, portable generators, and charcoal or wood burning.

The CDC recommends that people check or replace carbon monoxide detector batteries every six months, inspect heating systems and burning appliances once a year, and never run a gasoline-powered engine less than 20 feet from an open window. According to the CDC, carbon monoxide poisoning is “completely preventable.”