NEW YORK - The NYPD is teaming up with some national researchers to test the air flow in the city's subway system.

Brookhaven National Laboratory researchers are releasing a harmless, odorless gas called perfluorocarbon at a number of stations citywide as part of the Subway-Surface Air Flow Exchange program.

Air sampling equipment will be used to better understand the risks of contaminants in the air. The police department will then use the findings to improve its emergency response in case of an attack or an accidental release of hazardous materials.

The study costs $3.4 million and is funded by the U.S. Department of Homeland Security.

Two more days of testing are expected to take place sometime this month.