Madison, WI – Three people became ill from carbon monoxide fumes Thursday afternoon in their Near West Side apartment, the fumes apparently coming in through the furnace vent system.

The incident happened at about 12:30 p.m. in the Oak Tree Apartments in the 2100 block of University Avenue, the Madison Fire Department said.

“The three women living in the apartment reported feeling sick and having fainted at one point,” said MFD spokeswoman Cynthia Schuster.

The apartment dwellers went to a neighbor’s apartment so firefighters and property managers could figure out where the carbon monoxide was coming from.

“Engine 4 detected carbon monoxide readings in the apartment at 88 parts per million,” Schuster said.

Health officials say anything over 70 ppm can cause medical problems.

The furnace in the apartment, the only natural gas appliance in the unit, was shut off.

“Firefighters retrieved an electric fan to ventilate the unit as they explored adjacent apartments, to see whether carbon monoxide was affecting others,” Schuster said.

When elevated levels were found in other apartments, firefighters discovered the exhaust and intake vents for the furnace were located just 12 inches apart from one another.

“Under normal circumstances, this may not be an issue, but with plenty of frost and ice built up around the pipes in the record-breaking low temperatures, and a slight breeze coming in at just the right direction, crews surmised that might be why carbon monoxide was circulating among multiple units in the building,” Schuster said.

Property managers worked with Madison Gas and Electric to remedy the problem, and the occupants of the apartment who had been sick were feeling better and didn’t seek additional medical care.