The Department of Homeland security today released non-hazardous gas into the New York city subway as part of a study designed to help authorities know how to respond in case a biological weapon, like anthrax or ricin, were released in the nation's largest public transit system.The harmless particulate tracers -- colorless, odorless, and "biologically inert," according to DHS -- will be deployed over the next five days from Grand Central, Times Square and Penn Station subway platforms. Meanwhile, equipment at more than 55 stations throughout Manhattan will measure airflow, officials said."We know that subways are targets in general, so we are just trying to make sure that we are prepared for something bad that might happen," DHS Science and Technology Program Manager Dr. Donald Bansleben said.Knowing "how quickly and how far something might spread through the subway system will help us in developing better capabilities to protect the citizens," FDNY Battalion Chief Bob Ingram added.The NYPD conducted similar tests in July 2013, when researchers released invisible, odorless gas into three above-ground and four underground locations within the system.