Harmless gas will be released in the NYC subway system during the morning rush beginning Tuesday to study how chemical weapons could be dispersed through the air. 

NYPD were spotted placing devices at some NYC subway stops in Manhattan and the 5 boroughs that would disperse the harmless gas. Several researchers spent Tuesday morning outfitting the Columbus Circle subway station with the devices. 

The NYPD are working with Long Island's Brookhaven National Laboratory to track the movement of harmless tracer gases called perfluorocarbons. 

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, Police Commissioner Ray Kelly said. 

The project was announced in April. It will be funded through a $3.4 million federal grant.

NYPD swear that the gas is harmless.  Although most New Yorkers are glad for the project, some New Yorkers are concerned that the 'harmless' gas could have long-term effects that scientists aren't aware of.

Are you nervous about gas in our subway system?