Superior, WI – Authorities in Superior blame a malfunctioning furnace for sickening two people in an apartment with carbon monoxide.

WDIO-TV reports a carbon monoxide alarm came in after 3 a.m. Sunday. Firefighters found low levels of the odorless gas in the hallway, but the levels jumped in an apartment on the first floor.

Battalion Chief Erik Sutton says two adults in the apartment were taken to the hospital and found to be suffering from toxic levels of carbon monoxide. Their condition was stable as of Sunday morning.

Sutton said the apartment had a CO detector, but the battery had been removed. A CO alarm in a neighboring apartment alerted residents to the problem.