Chicago, IL- Six women were taken to hospitals after an illegal hot-water heater hookup in a beauty salon apparently led to elevated carbon monoxide levels in a South Loop building Wednesday night, officials said.

The women were all at the beauty salon at 1020 S. Wabash Ave. when the leak was discovered about 9 p.m., according to the Chicago Fire Department’s media office. The building, which also includes apartments, was evacuated as firefighters called for a hazardous materials response.

The women were taken to hospitals in fair to serious condition, officials said.

An Emergency Medical Services Plan 1, which sends at least six ambulances to a scene, was called as precautionary measure for the incident, said Commander Frank Velez, a fire spokesman. When firefighters arrived, they found elevated carbon monoxide levels of 300 parts per million but ventilated and decreased the levels significantly in the eight-story residential building, he said.

The hot water heater was poorly vented and firefighters shut it off, officials said. An investigation is ongoing.