Boston Globe 
Aug 19, 2010
Researchers working for the US Department of Homeland Security will release odorless, nontoxic gases in more than 20 MBTA subway stations for a week, beginning tomorrow, to study the possible effects if airborne contaminants were to be released there, officials said.
Electronic devices will track the innocuous agents, according to federal officials, and subway travel will not be affected.
Results of the study will be used by designers working on new chemical agent detection systems, the officials said.