Fifteen tenants were hospitalized and others left in the cold this morning after high carbon monoxide levels forced them out of their Crotona Park East apartment building. Firefighters responded to 1531 Vyse Ave. at around 2:30 a.m. after they received reports that three people had been exposed to the gas. Within hours, however, officials were knocking on every door, urging residents to leave the building. The fifteen hospitalized tenants were taken to Jacobi Medical Center. Officials say they believe the leak started in the boiler room. Con Edison, the Department of Buildings and the Department of Housing Preservation were called in to investigate. Tenants were allowed back in the building after waiting for almost five hours.