The New York City Police Department announced Wednesday that it will deploy, then track, what it calls “harmless” gases into the city’s subway system over three non-consecutive days this summer.

The plan, to be enacted in July, will investigate New York’s readiness to handle a chemical terrorist attack by dispersing the colorless gas and tracing it as it flows through the city, according to Scientific American. The test is expected to cost $3.4 million and is scheduled to be carried out in all five boroughs and dozens of stations on 21 of the city's 34 subway lines.

“The NYPD works for the best but plans for the worst when it comes to potentially catastrophic attacks such as ones employing radiological contaminants or weaponized anthrax,” police commissioner Ray Kelly said in a statement.

The police will use roughly 200 detectors to monitor the gas. Dubbed the Subway-Surface Air Flow Exchange, the test will be the largest of its kind and organized in cooperation with the energy department’s Brookhaven National Laboratory. They’ll use perfluorocarbon tracer gases (PFTs), which are frequently used to measure potential sites for underground construction.

Despite the science fiction catastrophe a plan of this magnitude naturally conjures, Fernando Ferrer, the acting chairman of the Metropolitan Transportation Authority maintained that it will not impact commuters.

“The NYPD, in partnership with the MTA, is responsible for keeping more than 5 million daily subway customers safe and secure,” he said in a statement Wednesday. “This study will bolster the NYPD’s understanding of contaminant dispersion within the subway system as well as between the subway system and street, thereby improving its ability to better protect both customers and the city population at large.”

The scheduled gas deployment comes years after investigators foiled an Al-Qaeda terrorist plot to bomb the New York subways, the largest public transportation system in the world. Three men had planned to detonate suicide vests just days before the eighth anniversary of the September 11, 2001 terrorist attacks in what US Attorney General Eric Holder at the time called “one of the most serious threats” to the United States since 2001.