The New York City Police Department and federal officials are once again conducting air sampling today, as part of the largest urban airflow study ever conducted.
Researchers are trying to determine how a chemical or biological attack would affect the transit system and the city.
In addition to air sampling at multiple locations citywide, researchers will release harmless tracer gas in some Manhattan subway stations.
Police are also trying to decide the best locations for detection equipment.
It started earlier this month, and will be repeated one more day in July.