Chicago, IL- A hazardous materials situation has prompted the evacuation of a Near North Side office building early Thursday afternoon.

The Chicago Fire Department responded to a Level One HazMat situation at 770 N. LaSalle, Fire Media Affairs tweeted at 12:14 p.m.

The carbon monoxide was measured at 200 ppm but the levels are “falling fast,” according to Fire Media.

The building was evacuated, according to Fire Media. All heating and cooling units are being checked, fans have been put in place, and windows are open where possible.

The source of the carbon monoxide was a gasoline-operated air-conditioning generation being used on the first floor of the building by a construction crew, according to Fire Media.