Boston: Effective January 10, 2010 only photoelectric smoke detectors will be allowed within 20 feet of a kitchen or a bathroom containing a bathtub or shower. According to fire officials this is because photoelectric detectors are less prone to false alarms when steam or smoke is produced. Outside of this area you must have both a photoelectric and ionization smoke detector.
This change mirrors an existing provision of the state building code which prohibits the installation of ionization smoke detectors in new construction. By placing a similar requirement in the fire code older homes built prior to 1975 are captured.