Methods and apparatus, including computer program products, for processing, at a sensor node within a sensor network, a set of sensor readings to determine whether an event has occurred, the set of sensor readings including one or more sensor readings for corresponding environmental conditions. The processing includes determining whether a sensor reading exceeds its corresponding threshold value for an environmental condition, and if so, comparing the set of sensor readings with a history log file to determine whether a false alarm event has occurred. The history log file includes one or more sets of historic sensor readings for corresponding environmental conditions, each set being associated with an environmental situation.