Part 3: A Human’s Smell

Wednesday, November 01, 2017 12:30 AM | Front & Finish (Administrator)

Written by Michael Pumilia

The goal of Tracking under American Kennel Club rules is to find a human’s dropped object such as a glove.  A starting point is shown to the dog and handler. The dog is brought up to that point and given a reasonable time to identify the human’s scent to be followed. The tracklayer has already gone along a predetermined track at least half an hour earlier but no longer than two hours. If the dog is able to follow this track, find the object and indicate where it is with a desired response such as sitting, lying down, etc.; then the requirements for the Tracking Dog (T.D.) title have been accomplished. That’s it in a nutshell.

Now all we have to do is understand how to train the dog to learn what smell to follow and how to persevere long enough to get to the end of the track and find that dang object. So let’s try to understand what the human scent is that the dog actually smells and all the factors that influence how the human scent gets to the ground so the dog can follow it. For this purpose I created the “Fairly Reliable Emission Device” otherwise known as FRED. But I’m going to use “Fred” in these articles so I don’t have to type all those capitals each time I talk about FRED.

Fred exists in a world of scent molecules that he generates and all the lotions and potions he applies to his skin and clothing. When we first start training, “You are Fred.” Your dog knows your scent intimately. Later we switch to someone the dog may know such as a member of the family, a training buddy, neighbor, etc. In a tracking certification and a tracking test, the tracklayer is an unknown. Let’s explore all the aspects of Fred so you can understand what makes up your scent and everyone else. Here is a simple explanation from someone’s article written in 1992 [1]:

Note: Brackets [  ] indicate references found at the end of this article.

“The primary premise is that each human emits an individualized smell (scent). This scent is generated by bacteria attached to regularly-shed small, epidermal flakes; by perspiration; and by skin oils which are unique to each individual. There is also a gaseous component, which includes air exhaled from the lungs and/or expelled from the digestive tract. The gaseous scent is dispersed more rapidly than the other components but remains distinctly individual. The cumulative debris, also known as 'rafts', envelops each person in an invisible cloud that constantly drifts or disperses. Some of this debris settled on the ground or on the surfaces beneath the individual; some may be carried on air currents for long distances. The variables determining the distance the debris is carried and duration of scent emission are moisture, terrain, temperature and wind speed.”

Since 1992 a lot more has been learned about human scent and how it is dispersed. On the whimsy side is this from : [2]

2. To a dog, you stink

As clean as you are, and as much soap and perfume and deodorant you wear, you are still splendidly stinky to your dog. Every human has a unique scent fingerprint, and that’s pretty much everything a dog needs to tell one person from another. “To our dogs, we are our scent,” says canine cognition expert Alexandra Horowitz, author of the enlightening book, Inside of a Dog: What Dogs See, Smell, and Know. In that book, she writes this wonderful description:

“Humans stink. The human armpit is one of the most profound sources of odor produced by any animal; our breath is a confusing melody of smells; our genitals reek. The organ that covers our body –- our skin — is itself covered in sweat and sebaceous glands, which are regularly churning out fluid and oils holding our particular brand of scent. When we touch objects, we leave a bit of ourselves on them; a slough of skin, with its clutch of bacteria steadily munching and excreting away. This is our smell, our signature odor.”

Here is a more complete explanation of what composes the human scent. It comes from U.S. Patent 6,073,499 which describes a human sniff scanner [3]. The following is interesting highlights from the document.

  • “The entire outer layer of skin is shed every one or two days.
  • Some millions of skin flakes are shed by the average person every minute.
  • It is primarily composed of human skin.

The skin flakes released by the epidermis are immediately caught up in the upward motion of the human boundary layer. Their average size is much smaller than the interweave pores of almost all clothing fabrics; they move freely through the clothing and away from the skin where they were released. (This is proved by the fact that counts of bacteria shed from the body are about the same whether the subjects are clothed or nude.)

The thermal plume of  typical person while walking conveys some 7 million skin flakes away from the body each minute.

The entire human boundary layer is thus a heavily particle-laden flow containing an extremely large number of microscopic skin flakes.

All regions of the body generate such flakes and the human thermal plume likewise contains myriad skin flakes that have originated from all regions of the body.”

Interestingly, I found an answer to one of my questions from long ago in a recently found article from a couple of years ago. My question was about the smell of identical twins; they are genetically the same. Here’s the answer:

 “It was discovered that the canines were able to differentiate between fraternal twins, but experienced difficulty in differentiating between identical twins unless their environments differed. The high similarity of the type and abundance of Volatile Organic Compounds (VOC) identified for identical twins, as well as the differences in the VOCs that were identified for the fraternal twins, corroborate the previous canine differentiation studies demonstrating that identical twins can be differentiated based on human scent.” [4]

We are the accumulation of everything we are as a functioning person plus everything we eat, wear, emotions we are currently feeling, and what is blown onto our body. Our skin provides the envelope for all of this. The figure below provides an image of this.

Fred represents the typical person and is surrounded by his molecules of scent flakes. The shape of the distribution is a bell curve. You might remember from your schooling “being graded by the curve”. This is that curve. The closer you are to the center of gravity of Fred the stronger the intensity his smell is. The further from him you get, the faster the smell decreases. So the question can be raised as to how many molecules of scent are needed for a dog to identify Fred from all other smells. The chart [5] below is used as an example to show the probability of a dog correctly identifying several substances as percentage (%) of success versus parts per billion. It was one I  found back in 2005. Using the chart for example, the odds of finding Dimethyl Dinitro Butane (DDB) is 1 out of two tries (50 %) when the concentration is ½ part per billion. It’s shown as the green line. At 1 part per billion, the odds improve to 75 %. Now that gets a little hard to wrap your mind around because the numbers are so big. Instead, let’s imagine a red ping pong ball as a molecule of DDB. Now we will fill a football field with more white balls until we have a billion balls in all. That means the stack of balls for the entire gridiron would be over 40 feet high. For the dog to correctly identfy this Butane each time, the concentration needs to improve to about 10 red balls per 1 billion balls. So just add 9 red balls in lieu of the same number of white ones. That is a minute amount – 10 red amongst 1,000,000,000 balls.

By the way, Wikipedia says “Dimethyl Dinitro Butane is a volatile organic compound used as a detection taggant for explosives, mostly in the United States where it is virtually the only such taggant in use.”

As you can see though, it would take nearly 700 parts of Cyclohexanone per one billion parts of air for a dog to achieve 95% success.   

Human Tracking is much easier to train than explosive detection for instance. Dogs are used to human scents since near birth. We are living organisms that the critters can easily relate to, especially if we have treats.         


  • 1.     The use of dogs in search, rescue and recovery,  Vikki Fenton, BSN, Journal of Wilderness Medicine 3, 292-300 (1992) 0953-9859 © 1992 Chapman & Hall
  • 2.     7 Amazing Facts About Your Dog’s Sense of Smell, Maria Goodavage,
  • 3.     US6073499 Human Sniff Scanner, Gary S. Settles, Jun. 13, 2000
  • 4.     Furton KG, Caraballo NI, Cerreta MM, Holness HK. 2015 Advances in the use of odour as forensic evidence through optimizing and standardizing instruments and canines. Phil. Trans. R. Soc. B 370: 20140262.  © 2015 Royal Society
  • 5.     Hand-Held Explosives Sensor System, Charles D. Bosco, University Transportation Center for Alabama, UTCA Final Report 03306, Sep 16, 2003

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