In recent years a lot of smart people in and out of government have been working on devices to pick up on anthrax, tularemia, and a host of other potential bio-weapons. Other devices include portable scanners to home in on evidence of a dirty bomb while screening out background radiation from things like granite and marble, and chemical antennas to sound the alarm if nerve agents such as sarin or VX or choking agents like chlorine and phosgene are released into the air. One approach to chemical detection—used in Grand Central Terminal—relies on surface acoustic wave technology to scan for nefarious compounds. The technology features fast-vibrating crystals, usually made of quartz, coated to absorb specific molecules; if such molecules are present and stick on, the vibrations slow down, triggering a warning.

Still, the performance of current devices is underwhelming in real-world environments. And even the best tools can't do everything we wish they could. In the past, chemical sensors were designed mainly to flag a few compounds or members of one chemical family. Law-enforcement agents would now like to scan for 50 different hazards at once. But as Dr. Laura Skubal, a research scientist at Argonne National Lab who has been working on detection technology for 20 years, told me, there's nothing out there that can do that. Many sensor systems also require constant tweaking and calibration. Some cannot detect chemicals in concentrations lower than what's lethal. And in dirty or dusty environments, many will sound false alarms. During the first Gulf War, for instance, sensors on the lookout for chemical weapons were set off frequently by diesel exhaust and insecticides. To be fair, there has been at least some improvement since then.