The first ambulance rushed to Ernst Prussing Elementary School on Friday morning after a student passed out.
When these first responders arrived, their carbon monoxide alarms went off -- indicating dangerously high levels in the air -- according to the Chicago fire spokesman.
Just over 30 minutes after that first call, 10 ambulances had been dispatched to the scene and everyone was ordered out of the school, the spokesman said.
The 78 were taken to Swedish Covenant Hospital, Community First Medical Center, Presence Resurrection Medical Center, St. Mary hospital, Illinois Masonic Medical Center in the northern Illinois city and to Lutheran General Hospital and West Suburban Medical Center outside of it, according to the fire spokesman.
It was not immediately clear what caused the high carbon monoxide levels or how high they were.
A message on the school website said that "an environmental condition" discovered around 10:15 a.m. (11:15 a.m. ET) "required us to evacuate the school building."
Overhead images from CNN affiliate WLS showed students sitting outside the building on Friday afternoon, some of them lined up on the pavement and several wearing Halloween costumes.
Those not taken to hospitals were moved to another elementary school that Prussing Elementary identified as "our nearest evacuation site."
Larry Langford, another fire department spokesman, said that a large mobile ventilation unit was set up at the school to get fresh air into the building.
Carbon monoxide is an odorless, colorless and potentially deadly gas produced when cars, small engines, stoves, furnaces and other devices burn fuel.
According to the Centers for Disease Control and Prevention
, those with carbon monoxide poisoning often experience headaches, dizziness, weakness, upset stomachs, vomiting, chest pain and confusion. Some 20,000 Americans each year go to emergency rooms, 4,000 are admitted to hospitals and more than 400 die as a result, the agency states.