© Dan Nosowitz2nd Avenue Subway Station, Sans Gas.
No need to fear, though: it's an experiment to see exactly how gases are dispersed through the hundreds of miles of New York City subway tunnels.
Starting today, New York City authorities will be releasing perfluorocarbon gas into several subway stops, some above and some below ground. Sounds scary, but isn't: perfluorocarbon is a harmless gas, odorless and colorless, and it's being used in the largest airflow experiment ever undertaken.
Perfluorocarbon tracers, or PFTs, are used because they're artificial and do not occur in nature, so a very small amount can be detected fairly easily. The work of detection will be done by a large team from three national labs: the Brookhaven, Argonne, and Los Alamos National Laboratories. About 100 interns in addition to professionals from the labs will be constructing and monitoring small black-and-grey boxes in dozens of locations all around the city, all dedicated to checking the air for these tracers.
Those locations aren't merely in the subway; the thing about New York's extensive, massively complex subway system is that it's the quickest way for airborne contaminants to race through (nearly) all portions of the five boroughs. So these testing boxes will be installed on subway platforms, sure, but also on telephone poles above ground.