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Smoke alarms in hallways of dwellings are specified in many jurisdictions, sometimes as the only required alarms and sometimes in conjunction with smoke alarms in other rooms, usually bedrooms. How effective are they compared with the same alarms in rooms but interconnected so that when one is activated they all sound? There are two major functions of smoke alarms â€” reliable and rapid detection of smoke and reliable and effective warning of building occupants. Both of these aspects are investigated using the data from a recent project intended to help determine the most appropriate location(s) of smoke alarms in dwellings in Australia. A sound level of at least 75 dBA but preferably higher in rooms is required to make it very likely that building occupants similar to those recently killed in dwelling fires in Australia would respond when asleep. The sound level in rooms due to hallway alarms was found to vary considerably and was generally well below this level even with the room doors open and was lower again with the door closed. The time taken for a particular type of smoke alarm to activate and the probability of it activating at all when in hallways was found to be significantly greater than the time taken for a similar smoke alarm to activate when in the room of fire origin (RFO). The RFO door being closed was also found to be very likely to prevent smoke alarms in hallways or rooms other than the RFO from activating, and at the very least to very substantially delay activation. Overall, hallway smoke alarms were found to take substantially longer and to be significantly less likely to activate than similar alarms in the RFO. Taking into consideration both the received sound level in a room and the time and likelihood of alarm activation it is concluded that hallway smoke alarms are much less effective than interconnected smoke alarms in every room. It appears likely that if there are interconnected smoke alarms in every room in a dwelling the probability of an occupant being killed in the event of a fire is much less than if there are unconnected hallway smoke alarms instead.