The Limits of Detection: The New Yorker In the wake of the Boston Marathon bombing, dogs seemed to pop up everywhere. Not just any canines, of course: highly trained detection dogs, at the side of police officers or federal agents. While it's too early to say exactly what kind of permanent changes we might see as a result of the bombing, one good bet is that the presence of these kinds of dogs at almost any event at which a large number of people gathers in a relatively small amount of space will become a regular occurrence. Many people will feel reassured just seeing them. But the public has only a tenuous grasp on just what the dogs are capable of. Walking through a busy international airport may lead you not only by explosive-detection dogs but also by dogs trained to detect bed bugs, drug-detection dogs, and dogs sniffing for illegal guava and other agricultural products being brought back into the U.S. by foreign travellers. But each is trained specifically on particular molecules or compounds, and pays other odors no mind at all. The guava dog will pass right over T.N.T. (For more on the training that dogs go through for work like this, see Burkhard Bilger's February, 2012, article from the magazine on New York City's canine units.) To be sure, detection dogs, and explosive-detection dog teams in particular, are extremely good at finding the odors for which they search. There are very specific training standards for these dogs, and the rate of success at discovery of explosives at the end of training hovers around ninety-five per cent, much higher than artificial-nose machines" performance. But that's training. What happens in the field? Well, there the handlers themselves gauge performance, and there are fewer objective reports about the success - or failure - of each outing. Furthermore, the performance of the dogs can be affected by a triad of contextual elements: situation, endurance, and conditions. Dogs must be trained in roughly the same circumstances as they will be performing. An animal who can detect as little as a picogram - a trillionth of a gram - of explosive in a spare basement room is dealing with an entirely different backdrop of scents when in a crowd or even in a windy environment, and will likely not perform at the same levels. Similarly, dogs trained to find a stationary explosive planted at a site may have trouble detecting a moving explosive - such as one being carried in a backpack. We untrained masses see the dogs" abilities, far exceeding our own, in a hazy supernatural fog: surely a dog, with his powerful nose, is noticing every scent - however slight - around him. Surely! Noses, alas, do not work that way. First of all, detection dogs need to "search," to actively explore their environments, pressing their noses into cardboard, into the leg of your pants, into the canvas of a tote bag. They burrow their noses deep into a smell. Ever notice that your dog would rather make full-nose contact with another dog's rump rather than smell it at a safe one-metre distance? The same principle is at work here. One can get more and better information up close. The dog cannot just smell any explosive from across a crowded room. This is the hamartia of bomb-sniffing dogs as the solution to all our explosives problems: a bomb-toting bad guy who sees dogs need only stay some distance behind them. Then there's endurance: performance changes over time. Dogs get tired, and less reliable, as the day goes on, just as we might. As for conditions, odors change in the weather, and dogs do, too. When they're panting, dogs don't sniff as readily as they normally might; an overly warm dog, forced to pant to cool himself, has a less reliable nose. And while we don't think of odors as seasonal (until, upon reflection, we remember: there is a smell of spring, of summer; the odors of winter are fewer), smells are more volatile in warm weather, and travel and disperse differently than in the cold. This too affects detection rates. Combine all of these factors, and a scenario like the marathon bombing presents ample challenges for a detection team: a densely peopled environment full of movement and scents, dogs that have been working for hours, and explosives that weren't stationary until almost the last possible moment. Compounding the potential problems that could be encountered with an increase in the use of detection dogs is that the supply of dogs suited to this kind of work is not infinite. While all domestic dogs are very good sniffers - the dog's primary sense is olfaction - not every dog is or can be a very good detection dog. Those selected for training are often not walk-ons: they might come from a breed line known to produce sensitive dogs, or, better, they are raised with the intention of eventually going into a training program. That's not to say that mutts from the local shelter aren't perfectly able trainees; no one breed has the monopoly on scent detection. Still, from within the pool of candidates a select few enter training, and not all dogs that do wind up successfully completing it. Training centers don't tend to publish a list of dogs that go through their program and then fail a certification test. To begin, the dog has to be motivated - interested in sniffing, and keen to do so for a reward (usually play with a handler, or food) - which not all dogs are. Does your dog's attention wander when you try to engage her in a hide-and-seek game? Maybe she's not a born detection dog. We think of the detection dog as an instrument, a kind of device - indeed, in the Supreme Court's recent decision about the constitutionality of a canine-assisted search on the front porch of a Florida resident, Justice Kagan called the detection dog on site "a specialized device." But no dog has yet been trained to sniff out malicious intent. Photograph by Josh Raab/Redux.