1. An apparatus comprising: a surveillance system that monitors subjects within an environment to generate monitoring data; a detector that analyzes the monitoring data to identify a number of indicator instances exhibited by at least a portion of the subjects, wherein each of the number of indicator instances is an instance of a corresponding event indicator in a set of pre-selected event indicators; and an analyzer that evaluates the number of indicator instances identified with respect to a set of criteria to determine whether at least a portion of the number of indicator instances meets an event detection threshold, thereby indicating a detection of an occurrence of an event of interest; wherein the set of criteria include that a certain number of the indicator instances detected occurred within a selected period of time, that a certain number of the indicator instances detected occurred within a certain distance relative to each other, and that a certain number of indicator instances were detected for each of multiple pre-selected event indicators; wherein each of the multiple pre-selected event indicators comprise a reaction or a response that is physical, physiological, psychological, or a combination pf physical, physiological, and psychological; and wherein the event of interest is a presence of a hazardous material in a location relative to the environment and wherein the at least a portion of the subjects exhibit the number of indicator instances in response to exposure to the hazardous material.