A chemical, biological, radiological, and nuclear weapon detection system is disclosed that incorporates a mechanism to reduce the probability that a false alarm will be issued. In particular, the mechanism causes an alarm to be triggered when the amount of a hazardous material reaches a threshold, but changes the threshold based, at least in part, on environmental (e.g., meteorological, etc.) characteristics (e.g., whether is it precipitating or not, whether it is sunny or not, etc) that effect the efficacy of a chemical, biological, radiological, or nuclear weapon. Given that there are environmental factors that make an attack less effective, and given that terrorists are aware of this, the illustrative embodiment is less likely to issue an alarm when the environmental factors suggest that an attack is less effective, and, therefore, less likely. The illustrative embodiment accomplishes this by changing the threshold needed to issue an alarm based on one or more the environmental factors.