A heating unit on the roof of a Madison business was causing elevated readings of carbon monoxide in an entryway of the building, according to the Madison Fire Department. Firefighters were finishing up an EMS response at an east side restaurant in the 4300 block Lien Road near the East Towne Mall complex when they noticed a smell of natural gas in the entryway. Using their air monitor, the crew identified elevated readings. The crew found the source to be a heating unit on the roof that wasn’t operating properly. The fire department says carbon monoxide readings in the bathrooms serviced by this heating unit reached over 60 parts per million. People exposed to this level of carbon monoxide typically show symptoms after 4-5 hours of continued exposure.