NEW YORK - Homeland security officials are releasing an invisible, odorless, harmless gas into part of New York City's subway system as part of a test of how air moves through the tunnels and platforms.

They say the data would be useful in understanding how biological contaminants might move through the tunnels during a terrorist attack.

The test started Monday and will last for five days.

The gas released into the system contains tracing particles. Detectors have been installed in several stations to see where the vapors go.

There is no health risk to the public. The New York subway system was chosen because it is the largest in the country.