The New York Police Department, in concert with the Department of Energy's Brookhaven National Lab, will release colorless, odorless, benign gas throughout the New York City subway system this July to test how a chemical or biological weapons attack would spread, according to a release.

About 200 sampling devices will be deployed to examine how low concentrations of harmless gases known as perfluorocarbons get dispersed in the country's busiest public transport system.

Here's the description of the gas:

Perfluorocarbon tracer gases (PFTs) present no health or environmental hazard. They are non-toxic, inert, odorless, and invisible, and have been used in airflow studies since the 1980s, including a 2005 Urban Dispersion Program (UDP) conducted in Manhattan. PFTs also are used in medical applications including eye surgeries and artificial breathing systems.

And here's NYPD Commissioner Ray Kelly's statement on the event:

The NYPD works for the best but plans for the worst when it comes to potentially catastrophic attacks such as ones employing radiological contaminants or weaponized anthrax," said Police Commissioner Raymond W. Kelly, adding that, "This field study with [DOE'S Brookhaven Laboratory]'s outstanding expertise will help prepare and safeguard the city's population in the event of an actual attack."

The release wil occur on three non-consecutive days. The final dates have not been determined.