Eight people were treated for complications due to carbon monoxide exposure early Sunday, after rescue workers were called to a boardinghouse in Northwest D.C.

D.C. Fire and EMS officials said they found high levels of carbon monoxide in the three-story home in the 500 block of U Street when they arrived about 8 a.m. Officials say the deadly gas probably emanated from a malfunctioning furnace.

Eight occupants were taken to the hospital with a range of mild to moderate symptoms, but all were non-life-threatening. A ninth person arrived at the hospital shortly before emergency personnel had reached the home.

Rescue officials took to Twitter to remind residents to make sure they had working alarms that can detect the odorless, colorless toxic gas.