NEW YORK - The NYPD and a team of scientists are conducting the second of three subway air flow tests.

Researchers are dispersing a non-toxic, odorless and invisible gas inside Manhattan stations as part of the Subway-Surface Air Flow Exchange program. Samples will then be collected in all of the five boroughs to see how far the gas spreads.

The police department will use the results to improve its emergency response in the case of an attack or accidental release of hazardous materials.

The first test was conducted last Tuesday. The date of the third and final test has not yet been released.