A Sioux City apartment complex was evacuated Monday afternoon for carbon monoxide.

Sioux City fire crews arrived just after 12:00 p.m, at the Stellart Apartments on 21st and Nebraska for an alarm.

Firefighters found unsafe levels of carbon monoxide and evacuated the building. The residents were kept warm on a city bus.

Crews say that the cause was a worker using a cement cutter in the basement without proper ventilation.

One person was treated on the scene.