WASHINGTON -- A bright blue sky and a chilly, gentle breeze – perfect weather for simulating a chemical attack on a quiet neighborhood near the Pentagon.

Defense Department staff and Arlington County, Va., workers hit the streets of Crystal City in yellow vests this week to see how toxins would move through an urban environment in a chemical attack or accident. They released a series of safe, clear, odorless gases and picked up data from sensors placed around buildings, in ventilation systems and on rooftops.

Should people evacuate downstairs or go upstairs? Should someone flip the air conditioning on or off? Would running the elevators pump gases up to higher floors? Could buildings be constructed better?

Atmospheric scientists and defense officials say chemical dispersion tests are invaluable for planning emergency response strategies, but a price tag of hundreds of thousands of dollars per test means they’re reserved for locations where the government has a high national security interest.

As for northern Virginia, it would be hard to find a part of the U.S. near more potential targets with symbolic and security value. There’s the Pentagon, the White House, Capitol Hill, the National War College, Reagan National Airport, a subway and highway system crammed with the nation’s leaders, and a slew of monuments and military bases.

The Pentagon is paying the bills and providing some of the manpower for Arlington’s study.

“We recognize that the investment that the department made in protecting the Pentagon could easily be expanded to protect the surrounding jurisdiction,” said Paul Benda of the Pentagon Force Protection Agency. “We have several leased facilities within the national capitol region that house Department of Defense employees. We have responsibility at PFPA to protect these folks.”

The Defense Department also plans to give  Arlington County more chemical detectors and a control system that would connect the sensors to a computer constantly running atmospheric models. A similar system focusing on the Pentagon already exists, Benda said.

Chemical dispersion tests date back to tracking pollution in the 1960s. Agencies didn’t  start using them for defense strategy until 9/11 brought home the threat of terrorism, Benda said.

“Once people realized that a terrorist might release a toxic gas, they started to ask, ‘What does that do?’” said Paul Kalb of the Department of Energy’s Brookhaven National Lab, which helped run the test.

The answers are never simple. Within a few yards of one test site, the wind was blowing two neon pink ribbons in opposite directions.

The study used harmless gases to see how toxins with similar properties might move. Some of the test chemicals were released by unscrewing a cap. Another system used a timed syringe to drip liquid that then evaporated.

To experiment with the atmosphere, one day the scientists shut off all the ventilation and elevators in a building and the next they turned them on.

Within a few minutes, rooftop devices had picked up the chemicals even with everything shut off. Detectors stored samples of the air in bags to show how much of the chemical was in the air during different time intervals. Complete analysis should take a few months, Kalb said.

“There are caveats,” said Christina Murata, chief science officer for the directorate of the Pentagon Force Protection Agency responsible for the test. “Our data is accurate for this day and these meteorological conditions.”

The test in Arlington was delayed four days because the wind was too strong, she said. However, there’s nothing to say a truck full of poisons gas can’t tip on a windy day.

The variability means it can be difficult to extrapolate data from one city to another. Salt Lake City, Oklahoma City and Manhattan have done studies, but defense officials decided a separate test needed to be done for Arlington.

Just don’t expect a test on every corner in America. The City of New York and the Environmental Protection Agency have expressed interest in more tests, but they need to track down funding first, Kalb said.

“We’re learning in each successive test,” he said. “But I don’t think it will ever get to the point where it will be cheap and easy to do.”