Boston Globe 
July 31, 2012
Scientists are fanning out throughout the MBTA this week to release and trace the movement of nontoxic, odorless gases and particles, in a federal experiment to simulate and understand how contaminants might move through the subway in a chemical or biological terrorist attack.
The research expands on previous studies of subway airflow in Boston in 2009 and 2010, with the tests this time examining not just how gases and particles move underground but also how they might flow out of subway tunnels and stations to city streets, said Teresa Lustig, program manager for the US Department of Homeland Security’s Science and Technology Directorate.
Authorities said subway riders should not be alarmed at the sight of tracer samplers — clearly marked boxes that draw in air for measurement — or of scientists in fluorescent vests accompanied by MBTA Transit Police officers.
Near the site of their release, the plumes of gas and particles may appear as a fine mist, but otherwise they will be invisible and harmless to riders as they flow through the subway system, officials said.