The MTA and the Department of Homeland Security are conducting a somewhat ghoulish experiment this month.
On five days between October 18 and 29, they'll be releasing non-toxic gas at about 120 subway stations to test NYC's response plans for a chemical or biological attack, according to the New York Daily News.
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The "airflow and dispersion study" will simulate the aerosol release of a biological agent in a densely populated urban environment, according to 311. Researchers will take air samples and materials that settle on the ground and other surfaces afterward.
According to the DHS, it'll take place at Times Square, World Trade Center Complex, Union Square Park, the Union Square subway stations, as well as the Oculus transit hub.
As mentioned earlier, the particle gas tracers are non-toxic and pose no health risks, according to the Daily News. The DHS says the non-toxic substances include salt, glycerol, maltodextrin (sweetener), a fluorescent brightener, non-coding DNA oligos, amorphous silica among others.
The gas tracers include sulfur hexafluoride as well as perfluorocarbon tracer. Sulfur hexafluoride is a safe gas commonly used in leak testing and is already present in urban backgrounds due to its use in the electric power industry. Perfluorocarbon tracer gases are safe, inert, odorless, colorless gases that have been used in many similar airflow studies in the past.
Between these dates, it's likely you'll see researchers and police officers outside of and inside subway stations.
This isn't the first time this test has been done in NYC—the last time it took place was in 2016. So don't worry if you see machines pumping gas into your station, it's just a precaution and it isn't harmful.