BOSTON — Scientists are releasing gases and fluorescent particles into Boston’s subway tunnels on Friday to study how toxic chemicals and lethal biological agents could spread through the nation’s oldest subway system in a terrorist attack.

It’s part of a weeklong study commissioned by the U.S. Department of Homeland Security to figure out ways to quickly minimize the impact of an airborne assault on the nation’s 15 subway systems and protect the nation’s infrastructure. U.S. subway systems include 810 miles of track in tunnels and accounted for about 3.45 billion trips taken last year, according to the American Public Transportation Association.

The scientists are monitoring concentration of the gases — which are invisible to the naked eye and nontoxic — and particles as they move throughout the system and then up into the streets above, pushed by turbulence created by trains thundering through the tunnels. Researchers use electronic devices to take air samples at more than 20 Massachusetts Bay Transportation Authority stations and in subway cars.

Test results will be used to craft ways to quickly detect an attack so authorities can shut down subways to limit the spread of contaminants.

Federal officials say similar tests were conducted in 2008 in the Washington, D.C., area, serving as an excellent contrast to the Boston study. The Massachusetts subway system, which opened its first tunnels in 1897, is poorly ventilated, while Washington’s is relatively modern and well-ventilated, DHS officials said. Read more at www.google.com