Eight people were taken to hospitals Saturday after being exposed to carbon monoxide at a spa in Back Bay, a Fire Department spokesman said. Firefighters responded to Exhale Spa on Arlington Street at 2:21 p.m. because some people in the building complained about lightheadedness, said the spokesman, Steve MacDonald. About 30 spa workers and patrons were evaluated for carbon monoxide exposure, MacDonald said, and no injuries appeared to be life-threatening. The eight people were taken to Massachusetts General Hospital and Tufts Medical Center. Firefighters vented the building and shut off the hot water-heating system and clothes dryers, MacDonald said. A spokeswoman for the company said the spa would be closed until further notice.