The Department of Homeland Security Science and Technology Directorate (real name, no kidding) is so worried that it will be conducting a study on the behavior of "airborne contaminants" in the subways on May 9-13.
The document, "Environmental Assessment of Proposed NYC Subway Tracer Particle and Gas Releases for the Underground Transport Restoration (UTR) Project" Version 8, dated Jan. 28, 2016, states it will release a "non-toxic, inert, odorless gas and particle tracers" into the subway at three stations and take samples at approximately 55 subway stations in Manhattan and parts of Brooklyn and Queens.
The substances will be released at what every New Yorker knows are the biggest targets: Penn Station, Grand Central Station and Times Square.
MORE:
http://nypost.com/2016/03/23/youre-not-the-only-one-worried-terrorists-could -attack-subways/