Madison, WI – A buildup of carbon monoxide in a north side apartment building was caused by a gas-powered pressure washer running for six hours, according to a release from the Madison Fire Department.

Firefighters were called at 10:02 p.m. Tuesday to the 2500 block of Calypso Road for a report of carbon monoxide detectors sounding in an apartment building and smoke visible from the basement.

Investigators said they found the pressure washer running in the basement.

The apartment building was evacuated and ventilated. A Madison Metro bus was brought to the scene to shelter apartment residents.

A 10-month-old was evaluated and released at the scene.

The residents were allowed back in after the building was ventilated.