HOUSTON (AP) — A dozen people in southwest Houston are hospitalized after carbon monoxide leaked into their apartments.

Houston firefighters were called Saturday night to the Vista on Gessner apartments. A fire district chief told KHOU that the department evacuated the building after residents complained of nausea, vomiting and severe headaches.

Their conditions were not immediately known.

Firefighters believe a problem with a vent serving dryers in the complex caused carbon monoxide to flow into the apartments nearby.