The largest-ever study of urban airflow began on Tuesday morning, as the New York City Police Department and federal government are trying to find out how a gas attack would affect the city.
For three mornings starting Tuesday, scientists are releasing some harmless gases known as perfluorocarbons for 30 minutes at the height of the morning rush at several subway and street-level locations in Manhattan.
The gasses are odorless, tasteless, and scientists say, harmless.
The measurements will be recorded inside special dark boxes on station platforms and other spots located throughout the five boroughs.
Officials hope to learn more about the risks of airborne contaminants including what could happen in a chemical, biological or radiological attack.
"It is an obvious target. We have to be vigilant, we have to be aware of that, and this is one of the many things that we do to protect the system," said Police Commissioner Ray Kelly.
The subway study is tied to a larger urban airflow research project that is being conducted over three days this month at above-ground locations in the five boroughs.
The NYPD plans to use data collected from the study to shape its response to a potential gas attack on the city and the subway, such as the 1995 sarin gas attack by a doomsday cult against the Tokyo subway system that killed 13 commuters and injured more than 1,000.
Even with a police officer posted nearby, the testing equipment can be a little hard to spot. Some commuters at the test sites told NY1 they were not even aware of the ongoing tests being conducted.
"We can never be too cautious today, especially with biochemical ways to poison people," said one New Yorker.
"If it's harmless I wouldn't have any reason to think they'd be releasing something into the air that could potentially affecting us in a dangerous way," said another New Yorker.
Police officials stressed the gases were dispersed in low doses and that the study's findings could be central to the city's safety. The results of the study could also help the NYPD respond in the event of an industrial chemical spill.
A similar air-flow study was conducted in Manhattan eight years ago. As for the next test, NYPD officials pledged to give the public 24 hours' advance notice.