CHICAGO (CBS) — Three people went to the hospital Thursday morning after residents of an apartment building used an oven for heat.

Firefighters were called to the two-story building on the 4100 block of West Adams Street at about 10 a.m.

Crews measured dangerous levels of carbon monoxide in the building and took one person to Mount Sinai Hospital in fair condition.

Two others who took themselves to a hospital were also being treated, police said.

A fire department spokesman said CO levels were at 500; anything over 10 is dangerous enough to trigger a CO detector.

Residents had been using an oven to heat the apartment, police said. The building was deemed safe about 10:30 a.m.

(The Sun-Times Media Wire contributed to this report.)

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