New York, NY —

Some New Yorkers may notice strange boxes on the subways and in Midtown over the next few weeks. They're sensors used by the Department of Homeland security to test air patterns -- and to find out what would happen if poisonous gases were released in the city.

REPORTER: Scientists from DHS and other federal agenies will release harmless tracer gases in office buildings, on the street and in the subway system. They say that will give them a picture of how dangerous chemicals and radiological materials might move.

Dan Valero works for the environmental protection agency. He says knowing how the gases move can be critical in an emergency.

VALERO: If a first responder knows that, then he or she is gonna be able to protect themselvse, and they're gonna be able to do a better job of making sure people evecuate and that their response is more effficacious.

REPORTER: This is the second test that DHS has conducted in the city. It could start as early as tomorrow, depending on the weather.