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The Scent of a Crime

dog nose

Dogs can be used to detect a range of scents, including drugs, explosives, accelerants, currency, and living and deceased people. Credit: yellowsarah/iStock

By LaTara Rust & Rebecca Buis

Cadaver-detection dogs can’t be trained using human remains. How accurately can the complex scents emitted by decomposing bodies be mimicked when these dogs are trained?

Death can be a confronting topic for most people. Recent catastrophes such as the Malaysia Airlines plane crashes last year have made the fragility of human life abundantly clear, but the personal impacts of death can have lasting effects on the living.

Take a moment to imagine that someone close to you, a loved one, has gone missing. The fear of the unknown can be unbearable and many questions can cross your mind – have they become a victim of foul play? Are they trapped somewhere? Will they ever be seen again? And what were their last moments like?

Returning a loved one to their family brings enormous relief and peace of mind, but recovery of a deceased victim can also help families by assisting the mourning process so that healing can begin.

Answering these questions is also crucial for police and forensic investigators. Recreating a crime scene and the events leading up to it can be particularly ambiguous when the victim is missing or the evidence is not apparent. Locating a victim is a challenging task, especially when searching for victims of crime or disaster whose bodies may be concealed and not easily located.

In order to search for victims, an investigator can call upon a range of tools to assist the search process. One of these is man’s best friend.

Dogs have been used for scent-detection work since the 14th century. They assist by locating a range of scents, including drugs, explosives, accelerants, currency, and living and deceased people.

Their sensitivity to specific odours stems from a highly developed olfactory system – they possess approximately 20–60 times more odour receptors in their nasal cavities than humans. When a canine is scenting for an odour, small gaseous molecules called volatile organic compounds are inhaled into the nasal passage where they bind to receptors, triggering an electrical signal that is sent to the brain. Their brain functions like a database and, depending on the molecules present, generates a pattern from these signals to produce a unique identifier for that particular scent.

While the functional aspects of this scenting capability are well understood, there is still mystery surrounding the actual compounds that dogs detect in specific odours. Since we have not been able to train dogs to voice these explanations, we must use science to unravel this mystery.

Cadaver-detection dogs are highly valued as a search tool for victim recovery. Due to ethical and legal restrictions, handlers of these specialised dogs cannot use cadavers to train the dogs, even though this is the scent they are often asked to locate. Instead, handlers use training aids that include bone, teeth, blood, decomposition fluid and commercially-available artificial scents. However, each aid only provides a small picture of a much larger and more complex scent profile.

Researchers have yet to identify whether cadaver-detection dogs are detecting a single compound, a class of compounds or the entire odour profile when scenting decomposition odour. Our research is illuminating the invisible world of odours to better understand what molecules the dogs are detecting by using highly sensitive instrumentation that detects and identifies trace levels of volatile organic compounds. By analysing the chemical spectrum of decomposition odour and comparing these results with those obtained during cadaver-detection dog training, we aim to identify the key compounds that elicit a positive alert by the dogs. Such information is imperative for understanding the way in which dogs track a scent to the source and thus enhancing their success when searching for victims.

Our research focuses on two common scent-sources used by handlers: decomposition fluid and blood. Decomposition fluid is the liquid produced by the degradation of the body’s soft tissues and organs. Since the decomposition fluid is a liquefied version of the body, its scent profile should be similar to decomposed remains. However, this has never been proven and is a common assumption when training cadaver-detection dogs. Could it be that the scent produced by decomposition fluid actually differs to human remains? If so, do we risk training the dogs on an incorrect scent profile?

To answer these questions, we have chemically examined the scent profile of decomposition fluid and compared it to the scent profile of human remains. This comparison allows us to determine whether the training aid is an accurate representation of the scent that the dogs would be locating at a scene.

The same training aids are also presented to cadaver-detection dogs during training to examine their capability of detecting very small amounts of the odour. This scenario might mimic severely degraded remains or remains that are concealed, such as a burial, and therefore have a reduced odour profile.

To date we have shown that decomposition fluid produces a similar odour profile to human remains. Our results indicate that approximately 70% of the compounds produced by decomposed remains are also detected in the decomposition fluid training aids.

We have also tested the sensitivity of cadaver-detection dogs by diluting decomposition fluid to one-part-per-trillion, which is equivalent to one drop of fluid in an Olympic sized pool. The dogs have been able to detect these dilutions with very little difficulty, confirming their superior olfactory sensitivity.

For now, decomposition fluid appears to be a suitable training aid that is readily detectable by cadaver-detection dogs. The next challenge is to test this training aid in an environment that mimics a crime scene or mass disaster and produces many interfering background odours.

Although the dogs are typically searching for human remains, their ability to locate trace or latent blood is also important at crime scenes and following mass disasters where these types of samples can be found. Blood has a very distinct scent profile that does not closely resemble the odour of decomposed remains.

However, blood is easier to collect than decomposition fluid, so it’s more commonly used as a training aid. Our research assesses the validity of blood as a training tool, and the circumstances where it may or may not be appropriate.

Once blood is collected from a consenting donor, it may not be used for training until hours, days or even months after collection. Does the scent profile of blood change as the samples ages? And does this reduce training efficiency?

Our comparison of fresh and aged blood has shown very different scent profiles depending on the collection age of the sample. This confirms anecdotal evidence that dogs trained on fresh blood will have difficulty finding aged blood, and vice versa.

Storage of blood training aids also varies among handlers, and may compromise their quality as a training aid. Blood samples that are stored in a freezer or refrigerator produce a more complex scent profile than blood stored at room temperature. These variations must be considered when determining the optimal age and storage conditions for blood training aids.

The chemical make-up of blood is also influenced by a person’s diet, lifestyle and medication history, which adds an extra layer of complexity. Analysis of the scent profile of blood collected from a range of different people has allowed us to demonstrate a clear distinction from person to person.

However, cadaver-detection dogs have no difficulty in locating all samples, regardless of the variations observed in the scent profile of an individual’s blood. This finding suggests that the cadaver-detection dogs are recognising several key compounds that are common to all blood samples. By continuing research into blood training aids we hope to identify the significant chemicals responsible for the dogs’ success.

Australia is leading the way with the training and deployment of cadaver-detection dogs. They are an invaluable tool for search and recovery, most notably lending assistance in the aftermath of devastating bushfires and in missing person cases.

Internationally, cadaver-detection dogs are regularly deployed to mass disasters following earthquakes, tsunamis and airplane crashes. Their ability to track scents to their source over large distances is incomparable to other search tools currently available.

For this reason, it is important that they are given the best chance of success when searching for victims. To ensure this, we must chemically confirm that the training aids used for their training are sufficiently accurate to guarantee their success.

LaTara Rust and Rebecca Buis are doctoral students in the Centre for Forensic Science at the University of Technology Sydney.