The test is part of the military's national security preparation for the capital area.

Over the past few years, the defense agency has worked with Arlington County to set up chemical sensors throughout the county, where thousands of defense employees work in leased office space.

The Pentagon has also supplied the sensors and accompanying monitoring equipment to Arlington for the county's own use.

"Within minutes, if someone attacks the Pentagon, it becomes a problem for Arlington," Pengtagon Force Protection Agency Director Paul Benda said.

The sensors scan broad areas, Benda said.

If weather cooperates, the Pentagon will release perfluorocarbon tracers, which are commonly used commercially to detect leaks, and sulfur hexafluoride, a common window insulator filling, near the Jefferson Plaza building at 10am on Thursday and Friday.

Officials in yellow vests will set up 80 battery-operated samplers - toolbox-looking cases with 12 air tubes inside of them - throughout Crystal City and will check the air samples in the tubes afterwards to evaluate how quickly and how high the gases spread.

The data will help the Pentagon and Arlington shape their lockdown policies for chemical and biological attacks or accidents, and will also help them determine the most effective locations for sensors.

"We want to place our sensors so we can detect this stuff as quickly as possible," Benda said.

The test, dubbed "Urban Shield: Crystal City Urban Transport Study," is similar to one conducted in Manhattan a few years ago, officials said.

 http://www.examiner.com/a-1251975~Pentagon_to_test_invisible_gases_in_Crystal_City.html