After some investigation by the Baltimore City Fire Department, the culprit was discovered to be a plug-in pumpkin spice-scented air freshener.
As a precaution, the school was evacuated and hazmat crews were called in to investigate the source of the odour that was causing some people to have difficulty breathing.
Several tests showed negative results for any toxic materials, said fire department spokesperson Roman Clark.
According to a statement released by the Cristo Rey Jesuit High School, several students and staff were evaluated by emergency medical staff who were on the scene.
Two students and three staff members were sent to a hospital to be treated for nausea and stomach problems, said Clark. One of the adults was reportedly ill due to an unrelated cause.
Once firefighters opened all the windows to air out the building, the situation was deemed safe again and classes resumed on Friday.
According to Heiser, the school has 350 students and 50 staff on campus.