A long time ago, myself and some of my constituents were asked to investigate the cause of "Sick Building Syndrome" at a 1950's elementary school. The office personnel complained of headaches and went home sick way more than the norm. Turns out there was an incomplete portion of concrete block firewall separating an air return chase from the boiler room. Negative pressure sucked the flue gas from the draft hoods and into the Ventilated Airstream. This went on for thirty-some years. So thankfully this building has been replaced.