The linked article says this;

Non-Toxic Gas to Be Deployed in 100+ NYC Locations, Including Transit, In Bio-Attack Readiness Test
The test gas is non-toxic and poses no health risk to the public
Published October 11, 2021 • Updated on October 12, 2021 at 10:10 pm

New York City's response plans for a chemical or biological attack will be put to the test in the coming weeks as part of a federal preparedness study.

The MTA says the Department of Homeland Security working alongside a team of researchers and city agencies will deploy a non-toxic gas this month at about 120 locations across the city, including transit.

Most of the locations will be above ground, including some parks. A number of below-ground subway stations will also be included, though details on which ones weren't known.

The tests will be conducted on five separate days between Oct. 18 and 29.