Homeland security officials will release an invisible, harmless gas Monday into part of New York City’s subway system as part of a test of how air moves through the tunnels and platforms.

The test begins at 11 a.m. and will last for five days. Officials said the data would be useful in understanding how toxic vapors or biological contaminants might move through the tunnels during a terrorist attack.

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

The MTA warned riders there may be some delays during the testing, WCBS 880’s Marla Diamond reported.

There is no health risk to the public.

(Source: WCBSTV)