Residents of a Madison duplex were left without a home for a night after carbon monoxide levels in their basement reached almost fatal levels. The Madison Fire Department says people living on the 600 block of North St. on the city’s near east side called in describing symptoms typical with carbon monoxide exposure. Firefighters evacuated the building after confirming high carbon monoxide levels, and followed the trail to the basement, where the levels were highest. MFD says spending three hours in the basement would have made for life-threatening carbon monoxide exposure. Symptoms like headaches would begin after 1-2 hours. The gas seemed to be coming from a furnace and water heater. MFD turned the gas off, and worked on venting the basement. The residents were asked to find somewhere else to stay for the night.