On Monday, the Department of Homeland Security and the Metro Transit Authority began a drill in which government agents will release a gas at over 100 locations to simulate a biological attack on the city. Authorities say the gas is "non-toxic" but will be released in closed spaces underground in the city's subways system as well as outdoors.

The event, which is being referred to as an "airflow and dispersion study," runs from October 18 through the 29th. It will simulate the aerosol release of a biological agent in a densely populated urban environment.

The project includes the release of "low concentrations of safe particle and gas tracer materials" as part of two programs, the Urban Threat Dispersion (UTD) program and the Chemical and Bio-defense Testbed (CBT) program, according to the DHS.