Sniffer dogs don't do crowds

From Slate, via Bruce Schneier. According to the head of a large urban bomb squad:

It is, quite simply, "bullshit," he says, to think that dogs can walk through subway cars, or sniff people entering turnstiles, and detect whether they've brought explosives along for the ride.

For one thing, dogs work best in quiet places that have been cleared of people other than their handlers. In airports, they are best at sniffing luggage in secluded baggage areas. Canine performance has also been shown to "fall off exponentially," the bomb expert said, because of distractions like gusts of air, noise, food, and people—all realities, of course, of mass transit. Bomb-sniffing is also exhausting work—a kind of sensory sprint—that dogs can't sustain for more than 20 or 30 minutes out of every couple of hours. And as they move through an area, dogs need constant reassurance and reward; if they aren't talked to, given an explosive to find now and then, and allowed to run back and forth, they may lose interest in the game. The explosives and the scampering would be hard to offer in the subway.

All that means sniffing dogs probably aren't that reliable at entry points on the border, either. Many are trained at Lackland Air Force Base in San Antonio.