(WNBC-TV) The NYPD is releasing harmless gases into the subway system during the morning rush beginning Tuesday to study how chemical weapons could be dispersed through the air.

Police, working with Long Island’s Brookhaven National Laboratory, were spotted placing air-sampling devices in specific areas on the street and within the subway system across the five boroughs. Several researchers spent Tuesday morning outfitting the Columbus Circle subway station with the devices.

Researchers will track the movement of harmless tracer gases called perfluorocarbons.

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