Testing began Monday to help the city better understand and emergency plan for airborne threats from hazardous or biological materials--whether accidentally or intentionally released into the air. 

The first release of non-toxic materials from the air-sampling study happened at Times Square. 

The non-toxic gas vapors and particles were released into the air as part of a study for the Urban Threat Dispersion Program. 

It has been four years in the making for the Department of Homeland Security, the Metropolitan Transportation Authority and scientists from across the country. 

The demo simulated an aerosol release of a biological or chemical agent into one of the city’s most densely populated areas. It will be tracked to see how far it travels amid skyscrapers and its ability to travel above and below ground in the subways. 

Data from the study will be given to New York City authorities to aid emergency planning and help keep people safe in the event of an airborne threat. 

Over 2,000 samples will be analyzed over two to three months to see where and how far materials traveled. 

After Monday, there will be four more testing and sampling days through Oct. 30, both above ground and subway locations throughout the city.