At 8AM ET this morning commuters were guinea pigs for a mock chemical attack on the New York City Subway. If you were on the subway you likely didn't notice anything — the test was silently set off and the gas dispersed through the system was harmless, odorless, and invisible. The planned attack was carried out by the New York Police Department and Brookhaven National Laboratory, and the gas that found its way though New York's tubes (and New Yorkers' lungs) is known as perfluorocarbon tracer. Researchers set off the gas at "several dozen" subway stations around the five boroughs for a sustained 30-minute attack, and around 200 air sampling boxes placed around the subway and on street level were set to track the gas as it moved around the city.

The purpose of this harmless attack? You may have guessed already, but it's said that the data on how gases move through the subway system and the city will help inform officials when making emergency response plans. As NYPD deputy commissioner Paul Browne told reporters, "it's planning for the worst and hoping we don't ever have to use it." Brookhaven also notes that the data could also assist if an industrial accident introduced harmful gases to the subway system. Typically, computer models can provide enough information to help when coming up with such emergency plans, but officials say that the complexity of New York's subway system and the aboveground structures made it difficult to accurately model such an attack.

This isn't the first time officials have launched a fake chemical attack on a public transit system — though today's test is the largest of its kind in the US. Similar studies have been carried out on a smaller scale in Washington, DC and Boston, and there are plans for an air flow test in London's tube this summer. Today's study will be replicated two more times through July 28th, and the project is funded by a $3.4 million grant from the US Department of Homeland Security.

Update: This article has been modified with additional information on the study and similar tests.