Nearly 600 workers at Norwood (MA) Hospital will join the Service Employees International Union as the result of an election. Norwood became the third hospital in the Boston-based Caritas Christi Health Care system to unionize this year. The bargaining unit at Norwood Hospital will represent 594 respiratory therapists, radiology technicians, licensed practical nurses, nurse assistants, pharmacy technicians, paramedics, secretaries, housekeepers, dietary workers, and others.