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. 

The gases mimic how a chemical or biological weapon may react if released. They’ll be dispersed in low concentrations for 30 minutes only during the morning, city officials said. 

The project was announced in April. It will be funded through a $3.4 million federal grant.
 
Police Commissioner Ray Kelly says results from the airflow study will help the NYPD learn how airborne toxins travel underground within the subway systems and above ground near the entrances and exits. The aim of the study is to better safeguard the city against a potential chemical attack, Kelly said.