station in New York City, it sends a blast of air rolling through the 100-year-old system’s enormous network of tunnels. That blast kicks up and carries a whole slew of particles. They swirl and float and fly. And then they deposit themselves all over everything. Over five days this past May, the US Department of Homeland Security (DHS) conducted a massive test to find out what would happen if those particles weren’t just harmless pieces of dust or lint: What if they were something more dangerous, like anthrax? How would they move around and where, exactly, would they land?

“New York is the largest subway system in the US and one of the largest in the world. They have over 5.5 million people on average on weekdays. It’s an enormous system, very complex, and it’s high on the target list of our adversaries,” says the project’s lead scientist Don Bansleben, a programme manager at the DHS Science and Technology Directorate.

The trains, he says, act like pistons when they move through the system. “They push material in front of themselves and pull material from outdoors behind them. The material basically goes everywhere. And during rush hours there are 4,000 subway cars in the system.”

For the test, 115 people from the DHS and several partners (including the US Environmental Protection Agency and seven American laboratories) set themselves up at 55 train stations (including two in Queens, one in Brooklyn, and one in New Jersey) and inside 10 different trains. Using air compressors, they shot one gram of a harmless, aerosolized, analogue of anthrax into the air every minute for 20 minutes. The subway system was running as normal the whole time.

With subways’ low security threshold and high passenger count, they’re especially vulnerable spots for a bioweapon attack. Similar tests that studied airflow within the NYC subways have been done before—those focused on gases, and how air flows from outdoors to indoors via station entrances. But this study is the first large-scale one that focuses on particles, which behave differently than gases, and which also include different kinds of bioweapons that aren’t gaseous, like anthrax.

It’s an enormous system, very complex, and it’s high on the target list of our adversaries.

The anthrax analogue, called DNATrax, was developed by one of those DHS partners, Lawrence Livermore National Laboratory, located about 70km from San Francisco. DNATrax is a material that comes from wheat and corn starch and has been used in the past to help the US Centers for Disease Control and Prevention (CDC) track food poisoning outbreaks. For this test they made two sets of particles—one at 2 microns and one at 5 microns (anthrax particles are about 10 microns). They then attached a series of very short chains of DNA onto each particle. The DNA, which is derived from a microscopic deep sea creature that’s clearly not often found in the subway, acts as a tiny barcode.

On each of the five days the team released different barcodes into the system (nine in total). In the end, that will help the researchers understand how the air moves on different days. “It also tells you about resuspension of the material,” says Bansleben. “A big concern is that the particles might deposit, but something may cause the material to come back up into the air.”

After the DNATrax was airborne, an array of sampling instruments placed throughout the system went to work. Some of them included systems the subway already has installed: Called BioWatch, it’s essentially a system of air filters that captures particles for later testing, as well as filtration devices borrowed from the Pentagon’s Force Protection Agency. They laid out one-by-one aluminium plates around the stations that they later swabbed. And finally they had team members walking around with portable testing devices and swatches of different types of cloth attached to their bodies like wool, nylon, and cotton.

“We wanted to get a sense of how much people are being exposed,” Bansleben says. How do particles attach to bodies “where people who may have walked into a cloud of something don’t know it, and then go out of the subway, go home, and potentially expose their family.”

We wanted to get a sense of how much people are being exposed.

On top of all that, the team wanted to get an idea of how previous airflow studies would compare to their own. There haven’t been many of these types of tests in the system.

So to improve this from a few previous years’ airflow tests, the team also released harmless perfluorocarbon gases into the system, which also acted as an experimental control: If the gas behaved the same way it did in the past studies, the team would know nothing funny was going on with the airflow in their tests.

In the end, the DHS and its partners collected 12,000 samples in total—7,000 surface samples from the clothing, air filters, and aluminium plate swabs and 5,000 samples from the gases. Now they’re off to the national labs to be analyzed. It will likely take many months to identify all the DNA present in each of those samples and create a detailed map of how and where all the DNATrax went.

So, what happens with the findings? They will be handed over to NYC’s emergency response and management agencies. None of the data will actually be released to the public, at least at first. Only government officials are getting a look, and the city itself will be able to decide how, and if, the results are more widely disseminated. That way, officials can get use out of the data, but also keep it out of the hands of anyone looking to execute the very type of attack the experiment tried to mimic.

Once in city agencies’ hands, the findings will help them respond to attacks by making more accurate contamination maps of the system. It will also help them determine the best locations to put future filters and sensors. Finally, because the data will contribute a lot of new information to subway air flow models used by transportation systems around country, it will assist the EPA and the federal government with developing post-attack decontamination plans.

“What we did was not an easy thing to do,” Bansleben says. But the success of the test means that New Yorkers—and subway travellers around the world—can breathe a little bit easier the next time a train blows into their station.

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