(Newser) – Don't worry: The gas the NYPD will be releasing into the New York City subway system in July is totally harmless. The "per fluorocarbons" will be released as part of a $3.4 million-dollar study looking at airflow in the subway system, with the aim of helping authorities understand the risks of an airborne terrorist attack, the Huffington Post reports. The NYPD, along with experts from a number of leading laboratories, will track the "tracer gases" through the city, both in the subway system and on the street.

An NYPD statement on the study describes per fluorocarbons as harmless and non-toxic (though HuffPo notes that the EPA categorizes them as greenhouse gases). They're also odorless and invisible, so New Yorkers won't realize they're wafting by during the three non-consecutive days in July the study is conducted; that recognition will be left to some 200 air-sampling tracking devices installed throughout the city and subways. The study is "the first of its scale to study airflow in a dense, complex urban environment both below and above-ground," according to the statement, and it will help the NYPD map out an optimized response to any purposeful or accidental hazmat release. (Read more hazardous material stories.)

