Washington ( CNN ) -- A man walks through Washington 's Union Station with a backpack and puts it down by a potted plant . No one notices him . He looks like any traveler . But inside his backpack are explosives . Five minutes later , a yellow Labrador retriever named Zeta enters the same crowded room , straining at her handler 's leash . She has picked up the scent of black powder in the air and tracks it to the foot of the ficus tree , where she sits and waits eagerly for a reward . This is not a thwarted terrorist strike but a demonstration of what a `` vapor wake '' dog can do . For years , specially trained dogs have run their noses over objects to screen for explosives . But vapor wake dogs can detect explosives in the air despite crowds , cross-currents and other odors . Their trainers say some of the dogs have been able to enter a room and pick up the scent as much as 15 minutes after explosives have passed through . Proponents say the dogs can detect a few grams of explosives . Most are trained to recognize a dozen , but some dogs have learned as many as 20 types , including TATP , the explosive used in the London transit bombings , and ammonium nitrate , which Timothy McVeigh used to bring down the Alfred R. Murrah Building in Oklahoma City , Oklahoma . If a dog needs to learn an explosive , it can do so in one or two days . `` Technology sometimes goes bad after two , three years , and you have to get new stuff , '' said Capt . William Parker of the Amtrak Police Canine Program . `` A dog gets better as the years go on . '' Parker says he is amazed by the dogs ' capabilities and compares them to top athletes . `` Michael Jordan was one of the best basketball players of all time , and these vapor wake dogs I put in the same category . '' At an Auburn University kennel in Anniston , Alabama , yellow Labrador retriever puppies romp and roll in a grassy pen . They have been bred for vapor wake traits , but only 1 or 2 percent of them will make the cut . Jeanne Brock , who manages the puppy program , says successful vapor wake dogs must have a strong instinct to hunt . `` They 'll hunt and hunt and hunt and wo n't come back without it , '' Brock said . John Pearce , who developed the program , says his best dogs are friendly but not overly social . `` They could care less about being petted , '' Pearce explained . `` We are looking for a dog that is just very strong and very independent in searching , pick up air scent very easily without the direction of a handler . '' At a few weeks old , the dogs are introduced to a variety of walking surfaces , like the slippery tiles they will find in a transit station . They are exposed to noises and large groups of people to make them more adaptable . Older puppies are then placed in prisons in Mississippi , Georgia , and Alabama , where inmates begin teaching them to use their noses . Eventually , they come back to Auburn and undergo a training regimen that is largely secret and is in the process of being patented . The dogs get 25 weeks of training , 14 of them with their permanent handlers . `` The dogs , when you get them , have more training than you do , and you have to play catch up , '' said Amtrak police officer Ed Ross , who works with Zeta . The U.S. Capitol Police have just started using the $ 20,000 vapor wake dogs . The Federal Protective Service has a few . But Amtrak Police have made them a key part of their explosive detection strategy . Parker says they provide a non-intrusive way of screening large numbers of passengers . `` You can take the dog to the situation ; you do n't need to take the situation to the dog , '' he said . `` They can go anywhere and adjust to the situation . They can be out in the cold . They can be out in the wind or the rain or the storm . What equipment can do that ? '' Although Amtrak 's dogs have hit on fertilizer , nitroglycerin pills and photo paper , which all contain the same chemicals as explosives , Parker said all the situations were quickly resolved . Rob Gillette , a veterinarian who is director of Auburn 's Animal Health and Performance Program , says dogs ' noses are remarkably sensitive instruments that sample the air several times a second . He compares a vapor wake dog to a chef who can discern the scent of a particular herb in a complex stew . For the dogs , Gillette says , finding explosives is simply a game : . `` At some point , this one scent comes up , and he 's like 'Oh , that 's the one I like ! That 's the one I get rewarded for ! I love this , so let 's go find this ! ' ``
Vapor wake dogs can detect small traces of explosives in air . Comment: Prisoners in Mississippi , Georgia , Alabama help train dogs .