Since it's actually cooler in some subway stations than it is above ground, it's the perfect day for the NYPD to continue its air flow study involving "harmless gases" being injected in various subway stations across the city!

Day one of the three-say study was on July 9, involved researchers dispersing "low concentrations of harmless gases known as perfluorocarbons at several subway and street-level locations in Manhattan for 30 minutes only during the morning of each study day." While the NYPD, which is conducting the study with Brookhaven National Laboratory, has insisted the gas is harmless in such low concentrations, a guerrilla brochure pointed out a 2011 study found that women exposed to perfluorocarbons had decreased levels of the female sex hormone estradiol, and, in some cases, early menopause. (PFCs, though, are very common.)

The sampling will be done at 3 p.m. The MTA said, "The MTA welcomes the urban airflow study conducted by the New York City Police Department and Brookhaven National Laboratory, which will generate valuable information on how to safeguard the New York City subway system against any airborne contaminants. The NYPD and Brookhaven are working in partnership with the MTA to introduce harmless tracer gases into the system and measure how they disperse.

"These gases are safe for our customers and employees, and the entire test will be performed with no impact on them and no interruption to service. The NYPD does an excellent job of keeping more than 5 million daily subway customers safe and secure, and this test will bolster its ability to protect them and the city at large."