So How's The Air?

Following 9/11, there was a renewed interest in placing chemical and biological (CB) sensors in subways to detect and alert authorities to the threat of terrorist CB incidents. We can thank Aum Shinrikyo and the 1995 Tokyo sarin incident for that. The funny thing is, while there are certainly those agencies still interested in using CB sensors in subways, you don't hear much about the actual success or failure of these concepts.
You might see one of these air samplers in the DC or New York subways, in the corner quietly chugging away. The challenge has been air particles - as this Science News article demonstrates, there is about 5-10 times the amount of particulates in subway air as compared to the air above ground. Automatic point detectors can false alarm to particulates or will jam up. Air samplers, on the other hand, don't alarm automatically but require the "man-in-the-loop" to extract samples and to analyse them. So the best we can do today is tell people what they were exposed to, not prevent them from being exposed in the first place.
This isn't great news, so we don't hear about the actual concepts of operation or the challenges these emergency responder agencies are going through. Every now and then, someone like the Los Alamos National Lab will show their wares and concepts. But it's important to anyone involved in homeland security to be able to understand the pros and cons of relying too much on detectors and air samplers that were designed more for battlefield use than for homeland security.
Does this mean we shouldn't trust detectors and monitors? Well, that depends. For military environments where the threat is high, the CB warfare agents are known, and the time duration of use is relatively short, detectors and monitors make sense. For antiterrorism situations where the threat is low or unknown (as compared to conventional threats), the weapons are unknown and many, and the time duration of use is year-round, detectors and monitors do not make sense. There are two exceptions - response teams need detectors and monitors for post-incident analysis and remediation, and it makes sense to place monitors and detectors at national special security events. Both are short-duration, focused efforts. Other than those exceptions, this issue is a money pit.
-- Armchair Generalist