A system for detecting hazardous agents is based on cellular telephone networks. The cellular telephones in public circulation have at least one detector for detecting at least one hazardous agent and broadcast alarm packets upon detecting the hazardous agent. A computer or computer network interconnected with telephones by the cellular telephone network processes the alarm packets and sets an alarm condition for the hazardous agent in a vicinity of the cellular telephone by calculations based upon alarm packets from other telephones with detectors for the hazardous agent in the vicinity. A map of the contaminated area is also created.