This summer, New York City will witness what might be called an airborne non-toxic event, to corrupt a term coined in Don DeLillo’s 1985 novel White Noise.

Over three days in July, the New York Police Department and scientists from Brookhaven National Laboratory in Upton, N.Y., will release small amounts of a harmless, colorless gas in the subways and on the streets to trace its flow through the city, both above and below the surface. The aim of the $3.4-million airflow experiment is to investigate how a harmful agent would disperse in the event of an accidental release or a terrorist attack, such as the 1995 sarin gas release in the Tokyo subway by the Aum Shinrikyo cult.

“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 prepared statement.

A similar project, carried out in 2005, tracked the spread of aboveground gases in midtown Manhattan. This summer’s Subway-Surface Air Flow Exchange experiment will cover all five boroughs, including dozens of stations on 21 subway lines, and will employ some 200 detectors to monitor the dispersal of gas.

As in the 2005 test, the researchers will release gases known as perfluorocarbon tracers, which exist in such small quantities in the atmosphere—just a few parts per quadrillion, for some molecules—that their spread following a controlled release can be clearly tracked using sensitive detectors. Perfluorocarbon tracer gas systems were developed at Brookhaven in the 1980s and have been used to identify leaks in hazardous waste–containment systems as well as to trace the potential migration of airborne pollutants across distances of hundreds or even thousands of kilometers.

The gases are nontoxic, nonflammable, and chemically and biologically inert, according to the National Oceanic and Atmospheric Administration. But considering that 5 percent of Americans believe that airplane condensation trails are actually “chemtrails” deployed by the U.S. government for mind control and other nefarious aims, according to a recent poll, expect more than a few straphangers to steer clear of the subway system during the tracer tests.