To study the movement of air through the city's subway system, the New York Police Department began releasing harmless gases — perfluorocarbons — in underground train stations Tuesday morning.

Working with Brookhaven National Laboratory, which is located in Long Island, police and technicians installed the air-sampling devices on streets and within the subway system across the five boroughs of the city, including Manhattan, Brooklyn, Queens, Long Island, and the Bronx. Several researchers were spotted by media reporters Tuesday morning installing a device at the Columbus Circle subway station near the epicenter of New York City.

Police Commissioner Ray Kelly says the project, funded by a $3.4 million federal grant announced in April, will help authorities better understand how air moves throughout the underground system and around entrances and exits, with the intent to better protect the public from possible biological or chemical attack.

The Brookhaven National Laboratory released the following statement: "If an industrial accident or a terrorist act resulted in the release of dangerous contaminants into the atmosphere in New York City or in its subway system, the city's first responders would have to decide quickly whether people should shelter in place or be evacuated, and what evacuation routes should be considered."

Although some people might find the tests alarming, Deputy Commissioner Paul Brown told reporters that the study was a necessary exercise, albeit one that explores a worst-case-scenario. "It's planning for the worst and hoping we don't ever have to use it," Brown said. "It's in the earnest interest of preparation," he said.

Funding for the study came from the U.S. Department of Homeland Security.