Researchers with the U.S. Department of Homeland Sescurity (DHS) are continuing their study of airflow throughout the Massachusetts Bay Transportation Authority (MBTA) underground subway system and surrounding urban areas this week by releasing non-toxic, inert, odorless gas and particle tracers.

Commuters will notice scientific equipment and researchers with electronic monitoring devices throughout the MBTA subway system and nearby urban areas of Boston and Cambridge from July 29 through August 5, while the DHS continues the scientific study.

Led by the DHS Science and Technology Directorate (S&T), this continuing study will gather data on the behavior of airborne contaminants if they were to be released into the subway system.

“Previous phases of this study focused on the behavior of airborne contaminants within the subway system. This follow-on study will help us better understand how airborne contaminants released into the subway impact surrounding urban areas,” said S&T Program Manager Teresa Lustig. “With a clear understanding of how contaminants from the subway may spread to above-ground city centers we can use that information to enhance emergency planning and coordination across multiple jurisdictions in the response to emergency events.”

“Our customers and employees should know that the MBTA is working closely with our federal partners in order to make the transit system as safe as possible,” MBTA Transit Police Chief Paul MacMillan added.

The study involves releasing non-toxic, inert, odorless gas and particle tracers into the subway system and nearby locations. Particle and gas concentrations will be sampled in more than 20 stations covering the entirety of the underground portion of the MBTA subway system as well as numerous locations in Boston and Cambridge.

While the deliberate release of chemical or biological agents is of primary concern, the study will also help researchers understand airflow characteristics for smoke or unintentional spills of chemicals or fuels —providing information for MBTA to develop evacuation, ventilation and other incident response strategies. The information will also aid in the design of next generation chemical and biological agent detection systems.

Although the equipment will be visible, the study will not disrupt normal activities or present any inconvenience to the public. Commuters may notice a variety of electronic equipment positioned in subway stations, aboveground in Boston and Cambridge, or being carried by researchers.

The study will be conducted by researchers from Argonne National Laboratory, Lawrence Berkeley National Laboratory, FLIR Systems Inc., and an international team from Defence Science and Technology Laboratory of the United Kingdom.

The tests will use sulfur hexafluoride and perfluorocarbon gas tracers, both innocuous, non-toxic, inert gases that have been used in dispersion experiments since the 1960s. Sulfur hexafluoride is a harmless, common tracer gas used for indoor and outdoor air testing. Perfluorocarbons are used in eye surgery and other medical applications. Inert particle tracers, tagged with rare earth elements and a common optical brightener, will also be used. The optical brightener is used in many laundry detergents and in paper manufacturing. These tests will not use any biological materials, only inert particle tracers.