EDCs do not smell bombs. But they can be trained to be highly alert to the ingredients of explosive materials. At the MSA Security Training Academy, for example, odors of chemical vapors are imprinted on the olfactory cortex of the dog's brains—Pavlov-style by task repetition and reward :
MSA's dogs begin building their vocabulary of suspicious odors working with rows of more than 100 identical cans laid out in a grid. Ingredients from the basic chemical families of explosives—such as powders, commercial dynamite, TNT, water gel and RDX, a component of the plastic explosives C4 and Semtex—are placed in random cans. In addition, urea nitrate and hydrogen peroxide—primary components of improvised explosive devices—have joined the training regime.
EDCs learn not to scratch a potentially threatening material or device (avoiding a possible blow-up), but to respond by just sitting down when they sense one. Today, EDCs at airports, train stations, stadiums, fairs and banks are not even noticed by travelers and visitors; and if, they are greated with a friendly look or smile. The simple presents of EDCs hopefully keeps people safe and threats away.
Keywords: threat protection, chemistry, sensory system, dog's nose, Canis lupus.
References and more to explore
 MSA Security: Bomb Dogs [www.msasecurity.net/bomb-dogs/].
 Joshua Levine: The education of a bomb dog. Smithsonian July-August 2013, 44 (4), pp. 72-78 [http://www.smithsonianmag.com/ideas-innovations/The-Education-of-a-Bomb-Dog-213881241.html#Bomb-Dogs-explosive-detection-canine-training-1.jpg].