A subway-borne chemical attack is one of those theoreticals that require the willful ignorance of regular passengers—for most of us, it's just better not to think about it. Not so for the NYPD, which yesterday announced a plan to test how a chemical or radiological attack would spread through the city's 200-odd miles of subway, by pumping an invisible gas through the system this summer.
The Subway-Surface Air Flow Exchange, or S-SAFE, has actually been in the making for over a year—that's how long it takes to plan a fake airborne toxic event in a city of more than eight million. Led by Paul Kalb, the principal investigator on the $3.4 million Department of Homeland Security-funded grant, the group will track the movement of small amounts of Perfluorocarbon tracers (or PFTs) through the five boroughs on three days in July. "The study will show us the worst case scenario," he explained over the phone today. "It'll be a close representation of how particles from a bioweapon or dirty bomb could move through the air."
What, exactly, are PFTs? They're a completely odorless, invisible, and non-toxic type of gas that happens to have incredible staying power, making it perfect for tracking purposes (it's even been used to trace counterfeit money). It also has a high vapor pressure, so it can pass through fabric and objects-and it's easy to detect, because it isn't found in nature.
PFTs are used to test all kinds of air-based chemical events, from gas leaks to pollution. Brookhaven National Laboratory, where Kalb works as an engineer, started using them to test disaster scenarios in cities back in 2005, when an EPA-led study released them near Madison Square Garden to test how a chemical attack would affect the canyon-like streets of midtown Manhattan. "We realized there was huge potential for taking a comprehensive look at the possibilities," says Kalb.
Unlike the 2005 study, S-SAFE will test the subways, which could play a major role in an attack since they carry air from all over the city. "Anything on the surface can get sucked down into the subway through air grates," Kalb explains. "And likewise, anything on the subway eventually makes its way to the street." The point of the study isn't to simulate the behavior of specific weapons. Kalb is doing something much simpler: mapping how air travels through one of the oldest and longest subways systems in the world.
Here's how it'll all go down. Like most studies, S-SAFE will depend on the help of many interns. In July, Kalb and his team will choose three days with particularly normal weather conditions (too windy or too still could skew the data) and mobilize 100 student interns to install breadbox-sized air sensors in subways stations and on lamp posts across the five boroughs. They'll focus the sensors in Manhattan, though every subway line and every borough will be represented. These 200 boxes will measure the amount of PFTs in the air every 15 minutes or so until the mid-afternoon, when New York's data-distorting sea breeze kicks in, which is the cue for the intern army to unlock the sensors and take them back to headquarters.
After the data is culled, the first step will be to generate a dynamic 3D "plume model" of the data points. In the age of Big Data, this will be one of the few pieces of information the city won't share—it's simply too sensitive. Instead, Kalb and his team will pass their models along to the NYPD, who'll fine tune their emergency response plans accordingly. That could mean anything from letting the MTA know which lines are the most vulnerable, to coming up with an evacuation plan that avoids "hot" zones on the map. Eventually, city officials could use a real-time animated model to predict how an attack will spread.
The big, outstanding unknown in this carefully choreographed dance? How the public will react. According to Kalb, there'll be a large-scale effort to let people know about the testing on the day it occurs, as well as signs that point people to an information hotline and a website explaining the project. "We've had to grapple with it," he says. "The best formula is to get the word out as soon as possible. You don't want to attempt this without letting people know."