NEW YORK (CBSNewYork/AP) — Homeland security officials have released an invisible, harmless gas Monday into part of New York City’s subway system as part of a test of how air moves through the tunnels and platforms.

The gas will be released for five days at two of three major transit hubs: Grand Central, Times Square or Penn Station. The gas contains tracing particles that look like drops of perfume from an atomizer before quickly dissipating.

Detectors have been installed in more than 55 stations and some outdoor spots to see where the vapors go. The collection boxes pull air through filters and capture particles into bags for analysis. About 12,000 samples will be taken this week, federal officials said.

Donald Bansleben, program manager of the Chemical and Biological Defense Division of the U.S. Department of Homeland Security, told reporters on the subway platform that the exercise is meant to simulate the release of a biological agent.

“To understand where hazardous materials would travel in the subway if they were released, especially the particles,” Bansleben said. “It’s very valuable, especially for the emergency planners because you need to understand the situation that you might be facing.”

Chief Bob Ingram of the fire department’s counterterrorism unit said they also want to see how they can keep people safe, 1010 WINS’ Juliet Papa reported.

“Not only with evacuations, sheltering in place, but also the recovery of these types of facilities so that we can move people back into them and know that it’s safe,” he said.

While riders will be able to see the smoke, Bansleben said they should not be alarmed.

“Everything we’re releasing is harmless,” he said. “Nothing is hazardous to the public.”

Researcher Benjamin Ervin said the tests were very important.

“If there were to be a chemical or biological attack we would have to know how to test for it and sample for it once it’s happened,” Ervin said.

Most subway riders were on board with the test.

“We need to be prepared,” one man told 1010 WINS’ John Montone.

“It sounds OK to me, as long as it helps out God forbid something happens,” another man said.

“It’s something that I think we should be prepared for, because the country’s always under attack,” another woman said.

Although some were a little nervous about the gas.

“It’s gotta be done, but I’m not too happy about it,” one woman told CBS2’s Scott Rapoport.

The data will take months to analyze, Rapoport reported.

The MTA warned riders there may be some delays during the testing.

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