It happened around 6:30 Saturday morning on the 1000 block of Chestnut Avenue.
Fire and EMS crews were called after reports that one person was unconscious and another person was dizzy and lightheaded.
Firefighters quickly determined that there were elevated levels of carbon monoxide in several locations.
Residents of the apartment building were evacuated.
At least five people were taken to St. Frances Hospital for observation.
Additional crews including PSE&G were called to the scene to assist.
Officials say a faulty heater appears to be the culprit.
