An alarm for detecting radiation and/or pollutants such as smoke, carbon monoxide or the like has a housing, a mounting plate for mounting the housing to a flat surface, cooperating means on the housing and the mounting plate engageable to secure the housing to the mounting plate, and an alarm circuit including a detector for detecting the radiation and/or pollutants. The alarm also has a switch in the alarm circuit in the housing and a magnet on the mounting plate, the arrangement being such that the magnet actuates the switch in response to engagement of the housing with the mounting plate thereby to arm the circuit. A battery compartment in the housing receives a battery for the alarm and stop means prevent proper engagement of the cooperating means in the absence of the battery. The mounting plate is generally planar having an upper surface and a through-opening for receiving a fixing such as a screw for fixing the mounting plate to a generally flat surface. The through-opening is elongate having an enlarged portion for passage of the head of the fixing through the through-opening, a narrowed portion for retention of the head and a neck portion therebetween. The alarm circuit also preferably includes actuation means configured to scroll through the various functions of the alarm when the actuation means is activated in a particular fashion.