Transit security detection system

A method and system for processing exposure to an item of interest in a transit system is disclosed. Detectors are sensitive to the item of interest such as chemicals, biologic compound, radiation, nuclear, and/or explosives to produce detector or other information. A threat assessment value is determined periodically. The fare media is read for card information, which can be used in addition to detector or other information. The assessment value along with the card information, detector information and/or other information is used in determining if an action is taken, for example, if a user can pass a portal to get access to the transit system.