Sniffer Dogs, the World of K9 Scent Detection

ogs possess a sense of smell many times more sensitive than the most sophisticated electronic instruments. The limits of their olfactory sensitivity are more limited by the capacity of measuring these limits than by a dog's nose. 

Comparing dog and human olfaction does not help us grasp the reality of a dog's amazing olfactory capabilities. We're not in the same league. Humans are primarily visual creatures, we see the world, dogs smell it. We smell emotively, dogs smell to investigate the world around them.

A trained dog can detect odors in a gram per trillion. What does a gram per trillion smell like? Imagine that the cinnamon roll you just bought at the bakery has about a gram of cinnamon in it. Now imagine the smell of one trillion (1 million million) cinnamon rolls.

For example, a drug-sniffing dog can detect a plastic container filled with marijuana that is submerged in the gasoline tank of a car. Examples defy our imagination.

What can dogs detect?

We are used to seeing sniffer dogs used by police, customs, and military units, although most people have little awareness of what canines can actually accomplish. In recent years, dogs have also been used extensively by the scientific community in the fields of medicine, archeology, and forensics.

Working dogs are smelling experts. “Vapor Wake” canines are elite bombs dogs that can track the scent of explosives in the air from a moving person carrying a bomb in a backpack. Dogs can be trained to detect concealed drugs, money, hazardous chemicals in very small dilution quantity, oil leaks pipeline buried seven feet below the ground, or tiny bed bugs hidden deep within drywalls, or mattresses. They can track humans and other pets long after they have disappeared, and find buried human bodies following natural disasters.

Dogs can detect with up to 90% accuracy early forms of breast, prostate, or skin cancer, the onset of a human epileptic fit, or of a diabetic hypoglycemic attack. In double-blind lab tests, canines proved able to correctly pick out the scent of children infected with malaria parasites 70% of the time. Recently, dogs have been used to detect cadavers in archeological tombs dating several thousand years back.

Dogs can be trained to smell early onset of disease - Dog Learner
Dogs can be trained to smell early onset of disease

What makes a dog’s nose so special?

A comparison of olfactory capabilities between humans and dogs is pointless because the dog has significant anatomical and physiological advantages.

A dog’s nose extends from the nostrils to the back of its throat, giving dogs an olfactory area 40 times greater than humans. Canines have between 200 million and 1 billion olfactory receptor cells depending on the breed, while humans have a mere 6 million. More importantly, 35% of a dog’s brain functions are assigned to smell-related operations. In comparison, a human brain assigns only 5% of its cellular resources to smell, and given the actual use we make of it, it seems like an over-investment.

Dogs also have significant physiological advantages over us when it comes to smell. They have separate sniffing channels for each nostril and can breathe in and out at the same time. They use their nostrils separately and differentially, depending on the side of the brain involved. Inhaled air can either take the breathing route or the sniffing route. Dogs also exhale out the side slits of their noses to minimize the odors displaced.

A dog can detect the tiny reductions in the concentrations of odor molecules that occur over short periods. This allows tracking dogs to determine which direction a person or animal has moved by sniffing the ground.

Canines and other mammals are also gifted with a kind of “second nose,” right under the bone separating the nostrils and above the roof of the mouth. That appendix goes by the suspicious-sounding name of vomeronasal organ, and it is as much a part of smelling as the nose is. The vomeronasal organ enables dogs to detect a kind of molecule that the ordinary olfactory route often cannot, such as pheromones. A pheromone is an olfactory signal sent between two members of the same species that can hold information about an animal’s social and sexual identity.

Are all dogs alike?

The short answer is no. Dog breeds vary in their smelling abilities, perhaps because genetically, their anatomy  differs. Of particular importance is the difference in face morphology—think of the differences between a Boxer and a Beagle. Other characteristics come into play, like the funny-looking Basset Hound’s floppy ears used to vent odors, and its short legs to be able to track closer to the ground.

The most anatomically endowed breeds when it comes to smell are hounds (Bloodhounds, Beagles), but other sporting or herding breeds such as Labradors and Dutch shepherds or Belgian Malinois are usually preferred for detection work because with a good nose comes a higher drive, and better trainability.

Drug detection dogs for police work - Dog Learner

How are detection dogs trained?

Working dogs are everywhere. You see them working in airports, at customs, in police departments, and an ever-increasing number of dogs working in conservation settings or pest control. Soon you will see dogs in public spaces detecting the COVID-19 virus.

Being a former police K9 instructor specialized in narcotics and explosives detection, I will describe the scent detection training methodology that I have used in the U.S. for law enforcement. Some fields of application may require a slightly different training approach, but mostly, the same principles apply.

Most scent-detection training facilities source dogs from breeders who are dedicated to produce working lines. In the U.S, most facilities import future police dogs at around 9 or 10 months of age from European breeders, mostly from Holland, Germany and Slovakia.

A small number of facilities have in-house breeding programs. The advantage of raising puppies in-house is that the dogs can be tested, selected, observed, and shaped from the a very young age. Selective breeding also allows to genetically obtain the best temperament and behaviors for the job, as in the case of guide dogs. 

Selection of scent-detection dogs

Police detection canines are selected at around 9 to 12 months of age with the help of an assessment test that allows trainers to score each dog based on several key criteria:

  • Social temperament
  • Fluid adaptability
  • High prey/hunting drive, strong possession 
  • Ability to use his nose more than his eyes
  • A strong focus and high level of persistency 
  • Stamina

It is worth noting that criteria used for selecting either explosives or narcotics detection dogs are different. Explosive dogs need be be calmer and more balanced in temperament, while narcotics dogs are required to be more social as they have to work in public places.

Scent detection training

Detection training can usually last 4 to 5 months, including the final 4-week training with the dog’s future handler. During that period, each dog is assigned to a specific trainer who has the responsibility to teach the dog the required search skills, and to build the high level of drive and motivation that are essential to make the dog successful.

Of all working canines that I have worked with, detection dogs are the ones that show the highest levels of drive and motivation. The level of physical —and often vocal—excitement demonstrated by a Belgian Malinois of a Labrador retriever when he is about to start working is nothing short of spectacular.

Unlike humans, trained detection dogs can separate mixed odors. This explains why trying to mask an odor does not work. If a drug smuggler wraps a pound of marijuana in sheets of plastic, a wrap of foil, and places it in a sealed can of coffee, a dog will smell each component separately and identify the drug within seconds. 

Phase 1: Hunting skills and drive building

What most people don’t realize, is that although dogs are naturally endowed to smell, they need to be taught to develop the skills to hunt with their nose. If you throw a toy in the middle of a patch of high grass, a dog will initially struggle to find it because he will instinctively rely on following the direction in which the toy was thrown. If you disorient or distract the dog before throwing the toy, he will be forced to use his nose. 

Hunting skills are progressively taught outdoors —in high grass and dense vegetation—or indoors among a number of haphazardly scattered cardboard boxes. The apprentice detection dog is leashed to a 30ft nylon line for control. Balls are used exclusively as bait to develop the dog’s drive to hunt for his favorite toy as a reward. Food rewards are never used, and each dog develops an attachment—and even an obsession—for his favorite toy. Trainers use verbal markers and praise to encourage the dog and to build his confidence. 

Phase 2: Odor imprint 

A scent detection K9 is a highly specialized expert. Dogs are trained to detect certain groups of odors exclusively. An explosives-detection dog will never work with narcotics, and a bed bug detection dog has no other odor imprint than these blood thirsty insects.

  • For explosives, most professional trainers have a special license to use real substances such as Nitrates, Chlorates, RDX, TNT, and PETN. Pseudo explosives exist, but studies have shown that they are not reliable for imprinting odors.
  • For narcotics, trainers use real marijuana and LSD, meth, pseudo heroin, and pseudo cocaine. Pseudo narcotics are synthetic substitutes for controlled substances that have similar odor characteristics to the real substance.

Once a dog has developed the necessary drive and hunting skills, the next phase is called odor imprint and consists in hiding small samples of the target odors (narcotic drugs, explosive compounds) in a small container together with a ball. The substances and the toy are placed together in a hidden metal can or PVC tube, small enough to prevent the dog from reaching for it.

The handler has the dog leashed on a long line, while an assistant taps randomly and quickly on various parts of the hiding area (boxes or scattered empty furniture) to attract the dog’s attention. Once the dog finds the hidden target, he gets praised and rewarded with a ball thrown by his trainer. The dog searches primarily for his toy, but his brain gets imprinted on the target odor in the process.

Handlers are instructed not to cue their dog. Dogs are keen observers and can easily watch our body language or gaze direction to guess where the hidden toy is.

During training, a dog will typically perform 20 odor imprint sessions (multiple substances hidden together, and individual ones) before moving on to the next phase.

Phase 3: Sit and alert

Once odor imprint is already well established, for the next phase trainers use a special device called a BSD box (Behavior Shaping Device) for fast timing of reinforcement and alert. Several empty boxes are used as decoys, and one BSD box contains the target odor secured at the bottom, and a hidden ball that sits on a spring-loaded mechanism, allowing the trainer to timely eject the ball out of the box with a remote control when the dog successfully alerts for the odor. 

The dog must sniff several BSD boxes aligned in a row, and alert the trainer by staring and sitting in front of the box that contains the odor (and most importantly the ball). Once the dog alerts in front of the correct box, the trainer ejects the ball as a reward.

A BSD box for scent detection training - Dog Learner

Phase 4: Loose hides

In this last phase, the dog is already reliably primed on his target odors and knows how to alert upon finding it. The trainer then gradually proceeds to hide small amounts of sealed substances (without the toy) in places that best reflect real-life situations, indoors and outdoors (walls, furniture, various decorative objects, plants, vehicles).

Detection training is complete once the trainer judges that the dog performs successfully and reliably in various environments, even with surrounding distractions.


Dogs are highly reliable and efficient scent-detectors. Numerous studies have established their proficiency and accuracy at locating a wide range of scents in varied circumstances. 

Trained scent-detection dogs can significantly reduce the amount of time spent searching for a target object, chemical, or species. Often more sensitive, reliable, and practical than electronic scent-detection devices, dogs are also relatively easy to train and put into action. 

In the future, we can expect to see dogs involved more widely in the fields of chemical detection, conservation, virus detection, and medical diagnosis.

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