New York, New York – US federal researchers will release nontoxic particles and gases into the New York City subway this month as part of a study on airborne terrorism threats, officials announced Sunday.

Researchers from the Homeland Security Department and the Massachusetts Institute of Technology aim to find "actionable data for emergency preparedness authorities," according to an advisory from the Metropolitan Transportation Authority (MTA).

The particles and gases that will be used in the study are designed to imitate biological and chemical agents, authorities said. The scientists will set up air testing devices at dozens of locations across the subway from October 18-29 to conduct the study.

The Homeland Security Department did an environmental assessment of the gases and particles to be released to ensure they're safe, officials said.

It's not the first time the feds have used the city's subway system as a testing ground for potential airborne attacks.

This month's research is part of the Urban Threat Dispersion project, which in 2016 conducted similar experiments in the subway.

But US officials haven't always notified straphangers they're involved in the test.

The Army in 1966 sprayed a bacteria officials said was harmless directly onto city subway riders as part of an experiment made public in 1980. The subway riders – accustomed to wild experiences underground – did not notice the test, which Army officials said made the system a prime target for a covert attack.

MTA officials promise the latest airborne tests will be much more transparent.

"Customers may see signage identifying specific locations involved in the study as they travel around New York City," the agency wrote in an advisory. "This study poses no health risks to the public, using materials that have repeatedly been used in prior outdoor and indoor airflow tests."