The study of the subway's airflow began Friday morning and was set to conclude about 3 p.m., cops said. View Full Caption flickr/Stephen Rees

NEW YORK CITY — The NYPD released a gas into the subway system Friday morning to simulate a potential airborne terrorist attack in the second day of a three-day airflow study, officials said.

Police, along with researchers from the Brookhaven National Laboratory, released gases at street level in Manhattan about 8 a.m. to trace their trajectory through the subway system, according to the NYPD.

The gas, called perfluorocarbon tracer gas, was deemed harmless by cops.

Researchers will be in the subway system with equipment until about 3 p.m., when the second day of the study concludes, police said.

The final day of the study will be announced in the future one day in advance, the NYPD said.