Madison, WI – “Dangerous and potentially lethal” levels of carbon monoxide were discovered at a downtown Madison apartment complex Monday, the City of Madison Fire Department stated, which were likely caused by a van left running outside the building.

Crews were called just after 5:30 p.m. to evacuate the 128-unit complex on the 1000 block of University Avenue after Madison Gas & Electric noted high readings of carbon monoxide. An upper floor reported a reading of 500 parts per million (ppm), while other levels were at 250 ppm.

Firefighters installed fans to begin ventilating the building and went door-to-door to get residents out of the building.

MFD noted that several apartments had high levels of carbon monoxide. Officials say some residents had taken down their carbon monoxide detectors, removed the batteries and put them in the hallway.

Two people met with paramedics on scene for mild symptoms of carbon monoxide poisoning, but neither needed to be taken to a hospital.

Firefighters aired all apartments out until a reading of zero ppm was reported. Residents were evacuated for about two hours while crews worked in the building.

The department stated that the carbon monoxide build-up was due to a carpet cleaning service van that had been running for several hours outside an exterior stairwell door.

Symptoms of carbon monoxide exposure over time can be similar to the flu and cause confusion, disorientation and death.