Police to Disperse Gas to See How It Would Flow in Terror Attack On three separate days this July, invisible and odorless gases will be released in subway stations and at street level in all five boroughs of New York City. But officials in the New York Police Department will not be alarmed - it was their idea. The gases, known as perfluorocarbons, will be dispersed to study how airborne toxins would flow through the city after a terrorist attack or an accidental spill of hazardous chemicals, the department said on Wednesday. Researchers supervised by the Brookhaven National Laboratory will use about 200 monitors to trace the paths of the gases they release. The police intend to use the information gathered in the test, which they said would be the biggest such urban airflow study, to hone their plans for emergency responses. One answer they seek is how the subway system affects the flow of air above and below ground. Knowing that will help them decide which subway lines may have to be shut down to limit the spread of hazardous material, said Paul Kalb, division head for environmental research and technology in the environmental sciences department at Brookhaven National Laboratory. "The subways play a major role in how air moves through Manhattan and the five boroughs," Mr. Kalb said. If you're in the subways and there's something released on the surface, you could be vulnerable. In the same way, he said, a gas released in the subway could affect people in a different part of the city. "It can spread further and in a way that you might not anticipate," he said. Researchers got a glimpse of that complication from a smaller study conducted in Manhattan eight years ago, Mr. Kalb said. In the summer of 2005, a different group of researchers released similar harmless gases in several locations in Midtown, including one subway station. Wanting to know more, the Police Department commissioned the laboratory, which is on Long Island, to conduct the more extensive two-year study at a cost of $3.4 million, Mr. Kalb said. The money came from a federal grant the police received from the Department of Homeland Security. "The N.Y.P.D. works for the best but plans for the worst when it comes to potentially catastrophic attacks, such as ones employing radiological contaminants or weaponized anthrax," Police Commissioner Raymond W. Kelly said in a statement. This field study with Brookhaven's outstanding expertise will help prepare and safeguard the city's population in the event of an actual attack. Mr. Kalb said his colleagues planned to enlist about 100 college students as interns to help set up the test and gather air samples to be analyzed. He said they would install small black-and-gray boxes containing monitoring equipment on subway platforms and lamp posts poles around the city. Then, the traceable gases will be released in seven different locations - three above ground and four below - on three nonconsecutive days in July. The results will be tracked by researchers from Brookhaven, the Argonne National Laboratory and the Los Alamos National Laboratory. To minimize fear, each release will be announced to the public a day in advance and the boxes will show a phone number and a Web site people can contact for information, the police said. "We're a little bit concerned that people are going to be nervous, especially after what happened in Boston," Mr. Kalb said. Clearly, people are trained to say something if they see something. The authorities" approach to this study is a world apart from the way the Army examined the spread of biological poison in the subways in the mid-1960s. According to a 1975 article in The New York Times, about 20 employees of the Army laboratory at Fort Detrick, Md., threw "bulbs" of a "simulated biological poison" on the tracks of two subway lines in Manhattan. "The bulbs burst and the wind of the passing subway trains" quickly spread the fake poison from 15th Street to 58th Street, the article said. It said the project's engineer had concluded that the subway system could not be safeguarded against that type of attack. If the attack were carried out during rush hours, the engineer said, it would "put New York out of commission."