CHICAGO (CBS) — More than 100 students and several faculty members were taken to hospitals for precautionary reasons Thursday morning, after a carbon monoxide leak at a South Chicago neighborhood elementary school.

Horace Mann Elementary School, 8050 S. Chappel Av., was evacuated around 8:30 a.m. after carbon monoxide detectors in the building went off, according to the Chicago Fire Department.

Fire officials said the leak appeared to come from the school’s heating system, which has been shut off.

Chicago Public Schools officials said 104 students and 7 staff were taken to various hospitals for precautionary reasons. Fire Department officials said all of the patients were in “very good” condition.

“The safety of our students and staff is of the utmost importance to the district. We are in contact with parents of affected students, and we will work with the Fire Department to get to the bottom of what happened today,” CPS spokeswoman Emily Bittner said in an email.

It was at least the third time this school year that a Chicago public school has had a carbon monoxide incident.

In late October, more than 70 students and several adults at Prussing Elementary School in the Jefferson Park neighborhood were hospitalized after the school’s boiler malfunctioned, causing carbon monoxide fumes to begin filling the building.

Two weeks ago, two teachers at James Shields Elementary School in Brighton Park fell ill from carbon monoxide. No students were in the building at the time, and were kept out until the building was cleared of the toxic fumes. While initially blamed on a faulty boiler, Chicago Public Schools officials later said there was no carbon monoxide leak, and that fumes that had been properly vented from the boiler room were able to re-enter the building through an “improperly opened door” that created a downdraft.

Last month, the district began distributing 5,000 additional carbon monoxide detectors in its schools, in response to the incident at Prussing.

Fire Department spokesman Larry Langford said Mann was one of the schools to receive new carbon monoxide detectors after the incident at Prussing. As a result, school officials knew immediately when carbon monoxide levels became elevated, and were able to evacuate the school before anyone became seriously ill.

“It looks as though they’re in good condition, due in part to the fact that these alarms were installed, and went off early,” he said.

Langford said Mann had carbon monoxide detectors in the boiler room, tunnels connecting the boiler room to the building, and in the hallways.

“They started progressively going off, and the principal called 911, and we started rolling, and we got here early enough to make a difference,” he said.