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.

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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.

The testing, which began Monday and will continue through the end of the week, is being done at off-peak hours and should not disrupt commuters, they said.

“The public should know that this is a very safe test, that we’ve done this in the past very successfully, and that they’ve actually done similar comparison tests in the Washington system,” said Deputy Chief Lewis Best, the homeland security and emergency preparedness commander for the Transit Police. “Boston’s the oldest subway, so it’s good to get a comparison study done, and we’re very fortunate to have been selected.”

The T’s age and comparatively poor ventilation led to its selection as a test site for the federally funded research, following testing in the newer Washington subway system in 2007 and 2008. According to Homeland Security, the tests use innocuous, inert, nontoxic tracer gases that have been staples of airflow experiments since the 1960s—sulfur hexafluoride, commonly used in indoor and outdoor air testing, and perfluorocarbons, also used in eye surgery and other medical applications.

The particulates released with the gases are meant to simulate biological materials but are not biological themselves. They include materials tagged with a brightening agent found in laundry detergent or with “rare earth” elements such as gold, silver, and iridium.

“To help protect a transit system against a chemical or biological attack ... we need to understand and characterize the movement of contaminants,” Lustig said. “If we understand how that movement occurs once it’s released underground and we understand how it moves above ground, we can then assess the impact on the city center.”

That understanding will inform federal, state, and local emergency-response preparation, officials said. The MBTA research—conducted this week inside and above 20 subway stations in Boston and Cambridge—builds off the earlier work in Boston and Washington, Lustig said. Scientists plan to return this fall or next spring for a fourth Boston phase, she said.

The multiyear study and the recommendations that will follow carry a budget of $1.6 million, all of it federally funded, according to the T.

The feedback from riders has been favorable, Best said, citing familiarity with earlier rounds of study and an ongoing awareness campaign.

This research is distinct from federal testing of new biological sensors planned for three Red Line stations in Cambridge and Somerville next month. Some residents expressed concern at a hearing in May, but officials have stressed that the bacterium used, known as Bacillus subtilis, will be released in killed form. Even in its activated form, the bacterium is not considered harmful to people and is used in farming and as a supplement in human and pet food.