Evanston firefighters were called to the Church Street Station condominiums Sunday afternoon after a carbon monoxide detector registered an unacceptably high amount of the potentially dangerous substance.
No one was injured in the mid-afternoon incident, and there was no evacuation, as fire crews traced the origin to the 17th floor mechanical room.
Battalion Chief Dan Lynch told Evanston Now that a CO detector went off in a vacant unit, and after that, the source was determined to be the mechanical area on the top floor.
Representatives of the gas company were called in to pinpoint the exact point of origin, which Lynch said may have been a hot water heater.
That work was ongoing as of about 3:45 p.m.
Lynch noted that the CO reading was approximately 250 parts per million in the mechanical area. The ideal amount is zero, although the danger was not deemed to be enough to order residents out of the building.
An ambulance was standing by just in case there turned out to be any trouble.