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Research and Technology - Forensic Science Communications - October 2004

Research and Technology - Forensic Science Communications - October 2004

October 2004 - Volume 6 - Number 4

Research and Technology

Survivability of Human Scent

Rex A. Stockham
Explosives and Hazardous Devices Examiner
Explosives Unit
Federal Bureau of Investigation
Quantico, Virginia

Dennis L. Slavin
Bloodhound Handler
South Pasadena Police Department
South Pasadena, California

William Kift
Bloodhound Handler
Police Service Dog Unit
Long Beach Police Department
Long Beach, California

Abstract | Introduction | Background | Methods | Considerations | Conclusion | References


A new and innovative approach that uses human-scent evidence to identify bomb makers and arsonists is presented. The process of identifying and locating a suspect after an explosion or fire is often complicated by the fact that improvised explosive and incendiary devices generally employ a time-delay mechanism to allow evasion well before their functioning. One approach that uses specially trained bloodhound-handler teams as an investigative tool to identify people who had contact with the devices has been developed. Pipe bombs containing explosives with varying explosion velocities were functioned, and metal and plastic gas cans were burned with gasoline. The purpose of this feasibility study was to demonstrate the survivability of human scent after being exposed to extreme mechanical and thermal effects from the explosion and burning of various energetic materials and the potential for use in criminal investigations.


The ability of bloodhounds to effectively match collected scent to the correct person and follow that person through and across numerous environments to an effective conclusion is accepted in most courts and has been validated in a recent scientific study (Harvey and Harvey 2003). The authors identified no published studies that explore the durability of human scent.

In traditional bloodhound circles, the anecdotal information passed from trainer to student is that human scent is fragile and easily destroyed. Many dog handlers in the United States are taught that identifiable human scent disappears after 24 hours. European studies using properly trained scent-identification dogs showed acceptable performance levels with collected scent that was aged two weeks to six months (Schoon and Haak 2002).

This paper will demonstrate that human scent is durable and will remain identifiable after being exposed to extreme mechanical and thermal effects associated with the reactions of various energetic materials. This feasibility study does not address the durability of human scent as a function of physical parameters, such as surface temperature of materials. The effects encountered in the detonation of improvised explosive devices and the deflagration of improvised incendiary devices are not consistently reproducible due to the myriad of variables that can affect energetic material performance in an improvised device. In addition, conditions encountered by bloodhound handler teams during crime scene responses are never identical. Therefore, the test design discussed below reflects conditions encountered in an urban setting and examines the potential to use this technique in a criminal investigation.


Generally, there are two methods of bloodhound-handler training in the United States—traditional and specialized. The traditional method teaches a bloodhound-handler team to have the dog search for matching scent at the beginning of the trail by casting about. The handler then determines the presence or absence of matching scent by “reading” the dog’s behavior. Some groups have trained their bloodhounds to return to the handler and provide an alert upon determining that no matching scent is present in the area.

Scent collection techniques used to acquire human scent vary widely in the traditional bloodhound community. The most routinely used methods are direct scenting, swiping, and absorption. In the direct-scenting method, the bloodhound handler allows the dog to sniff the actual item of evidence. With the swiping and absorption methods, scent is transferred onto a gauze pad instead of using the actual item of evidence. Swiping, a direct transfer of scent onto a gauze pad, is achieved by wiping the pad across the surface of the evidence. Absorption, or placing the pad next to the scent article for an extended period, relies on the gauze pad’s ability to gather scent while being in direct or indirect contact with the evidence. Although these methods have been used for decades, the potential for negatively affecting trace evidence is clear.

The specialized bloodhound-handler teams use a different response system to indicate the presence or absence of matching scent at the start of a trail. Although traditional handlers must rely on their ability to “read” the dog’s behavior, the specially trained teams use a simplified yes or no response technique. At the start of the trail, if matching scent is present at the location being checked, the bloodhound trails. If no matching scent is present at the location being checked, the bloodhound refuses to trail. Once the bloodhound has started trailing, thus indicating the presence of matching scent, much of the handling techniques used in the traditional bloodhound community are relied upon.

The specially trained bloodhound-handler teams employ the Scent Transfer Unit (STU) to collect scent pads. The STU-100 is a portable vacuum collection unit that uses the flow of air to transport the components of human scent onto 11.25 by 22.86cm sterile surgical pads. (The pads are considered sterile for medical usage, not to denote an absence of any chemical compounds.) The vacuum’s intake funnel supports the sterile pad to allow the evidence to be placed on or near the pad. At full charge, the STU-100’s 12-volt fan pulls approximately 400 liters of air per minute across the surface of the evidence and through the pad, thus trapping the scent-causing materials. Where traditional scent evidence recovery techniques require direct scenting from the article of evidence or touching the evidence with a gauze pad, the airflow across the scent pad allows the evidence recovery personnel to immediately capture scent, thus minimizing the loss of other forensic evidence. It also provides a consistent type of scent article for presentation to the bloodhound.

When the investigators develop a suspect, the specially trained bloodhound-handler team is brought to a location recently visited by that person to conduct a suspect-location check. Typical locations for scent checks include the suspect’s residence or work because these locations provide large areas of deposited scent due to the frequent travels in and out of the buildings. Generally, case law in the United States requires that the dog-handler team be placed on a trail where the suspect was known or believed to have passed. In order to fulfill these requirements, the handler is placed on this fresh trail location and asked to introduce the previously collected scent pad to the hound. The handler knows that he has been placed on a known trail but is not told details of potential outcomes, thus keeping him blind. In addition, the handler does not know if the scent pad for presentation is a negative control or a scent pad collected from an article of the crime.

Because these specially trained bloodhounds provide a yes or no response, a positive response indicates to the investigator that additional investigative efforts should be exerted to determine the reason that the dogs matched scent from the evidence to the location. This type of positive-scent match is most often associated to a resident or frequent visitor to that location. Assuming that the scent article being used contains a viable amount of scent, a negative response during a location check provides strong evidence to eliminate the suspect from the investigation.

Bombers and arsonists typically employ some time-delay method in improvised devices to remove themselves safely from the scene of the crime, and many of these events go unsolved. Using scent collected from the devices, qualified bloodhound-handler teams can use scent pads to conduct suspect-location checks, thus providing a new tool to assist in the identification and capture of these individuals.


Test Material

Four pipe bombs and two gas containers were used for scent articles. Four 2.7 x 20.3cm schedule-40 steel pipes and eight end caps were purchased wrapped in plastic. The gas containers, one metal and one plastic, were purchased new and immediately placed into large plastic bags.

Twelve test subjects were selected from a local search-and-rescue organization that had not been used in any previous training or testing. Most target and decoy pairs chosen were the same sex and age. In order to deposit scent, the targets handled their respective items for approximately one to two minutes, placed the items into resealable bags, closed, and labeled the bags. During the explosion and collection process, the bomb technicians and scent-pad collectors were monitored by the test planners to minimize any scent cross contamination. To accomplish this, the technicians and collectors were required to wear a new pair of latex gloves each time a new device was handled. Because assembly and collection personnel could have contributed scent even while wearing gloves, they were not permitted to be present during the testing.

Pipe Bomb Preparation

Four pipe bombs were constructed using two low-explosive powders and two high-explosive materials. Goex (Doyline, Louisiana) black powder, a 6:1.2:08 mixture of potassium nitrate, charcoal, and sulfur, and Bullseye (Alliant Powder, Radford, Virginia) double-base smokeless powder, a combination of nitrocellulose and nitroglycerin, were chosen for the two low-explosive filled pipe bombs because of their availability and common use in domestic bombing incidents. In its legitimate form, smokeless powder is used for reloading ammunition, and black powder is typically used for a type of sport shooting. Kinepak, (Slurry Explosive Corporation, Oklahoma City, Oklahoma) a binary explosive consisting of a mixture of ammonium nitrate and nitromethane, and Composition C4, a military’s cyclotrimethylenetrinitramine (RDX)-based explosive were the selected high-explosive fillers.

Figure 1 is a photograph of a black powder explosive.

Figure 1. Photograph of a Black Powder Explosive

Figure 2 is a photograph of a smokeless powder explosive.

Figure 2. Photograph of a Smokeless Powder Explosive

Figure 3 is a photograph of a binary explosive.

Figure 3. Photograph of a Binary Explosive

Figure 4 is a photograph of a C4 explosive.

Figure 4. Photograph of a C4 Explosive

To ensure the safe initiation of each buried pipe bomb, a detonating cord booster was placed into the energetic material. Holes were drilled in one of the two end caps to allow for the insertion of a length of Dupont (Dupont-ETI, North Bay, Ontario, Canada) 70-grain per-foot detonating cord with a pentaerythritol tetranitrate (PETN) core. After the pipes were half filled with the explosive material, approximately 10cm of a 61cm length of detonating cord was inserted into each pipe with the remaining length protruding from the containment vessel. A U.S. military nonelectric blasting cap and black powder core time fuse were used as a time-delay system to initiate the detonating cord boosters.

Each assembled device was placed inside a 20-liter plastic bucket that was packed with dirt. The bucket was then suspended inside a 189-liter steel drum and detonated. This technique was used to recover as much postblast fragmentation as possible. Some devices required the steel drum to be partially buried to further restrict scattering of fragments. Fragment recovery was completed for each device immediately after detonation, and before the next device was exploded. A screen sifter and magnet were used to locate and collect the smaller pieces. The recovered fragments were placed inside polyethylene resealable bags.

The maximum approximate reaction product temperature of black powder and double-base smokeless powders are 2380K and 2200 to 3800K, respectively (Picatinny Arsenal 1962). The reaction product temperature in detonating explosives can exceed 5000K (Persson et al. 1993). The ignition source temperature of gasoline is 1083K (Henderson and Lightsey 1984), with a much higher flame temperature that is dependent on oxygen content. These temperatures are not the surface temperatures of the containers but are provided to demonstrate the range of temperatures that can occur in the reaction zone of various energetic materials.

Arson Device Preparation

Two gasoline containers, one plastic and one metal, were placed on the ground and covered with one half liter of gasoline. The gasoline was ignited and allowed to burn for two minutes. The fire was then extinguished with water. After cooling, the remains were placed in separate paper bags.

Dog-Handler Teams

Twenty professional and novice bloodhound-handler teams were used for this study including 16 handlers and 20 dogs. The bloodhound-handler teams using two dogs in this test were designated with the same numeric identifier but with a different alpha identifier (i.e., 4 and 4a).

Thirteen of the handlers were specially trained. The remaining three (10, 11, 14) were traditionally trained. Five handlers were full-time law enforcement dog handlers, three were reserve officers, and eight were civilians. The handlers’ experience levels ranged from 700 cases worked to no field-case experience. The dogs ranged in age from under one year to seven years old. Twelve teams had previously trained on arson debris. Three teams had previously trained on bomb debris.

Follow this link to Table 1.

Scent Collection

Scent pads were collected from the pipe-bomb debris by placing the fragments onto the STU-100 and running the machine for approximately 30 seconds. Scent pads were collected from the arson debris by placing the STU-100 intake funnel inside each bag and running the machine for approximately 30 to 90 seconds. The decision to place the evidence article directly on the scent pad or to place the intake funnel inside the evidence container was made specifically for ease of processing. No empirical data has been gathered to determine if either collection method is superior to the other.

In order to eliminate cross contamination, the STU-100 intake was cleaned according to manufacturer’s recommendation by using isopropanol swabs. The STU-100 was allowed to dry prior to collecting scent pads from each of the six devices. To minimize preparation time, most of the scent pads were split evenly with scissors into two sections after scent collection, thus creating two pads from one. Before cutting the pads collected from a new device, the scissors were cleaned with isopropanol swabs and allowed to dry. Half of the pads were collected with the STU-100 the same day the devices were functioned. The remaining halves were collected two weeks later. All of the pads were packaged in polyethylene, resealable bags and maintained at room temperature. The scent pads were randomly aged two days and 16 days before being presented to the dogs.

Trail Information

The trails were run in an urban public park frequented by joggers and people walking pets. Because it was previously demonstrated that bloodhounds are capable of identifying human scent that was vacuumed onto a scent pad (Harvey and Harvey 2003) and capable of matching that scent to a suspect on aged and contaminated trails through an urban environment (Harvey and Harvey 2003), no attempts were made to age or contaminate the test trails. For the test trails, 12 people walked a split trail; six scent targets and six decoys. On each trail, two people (target and decoy) were started at the same point and walked the first section of the trail side by side. After approximately 14 meters, they split at a 45-degree angle and continued another 18-27 meters to their respective hiding locations.

The bloodhound-handler teams were not permitted to see the trails being laid, and the target and decoy were hidden at the end of their trail.

Six stations were set throughout the park, one for each device. At each station, the target and decoy laid a new trail in a different location for each bloodhound-handler team so that no team ran the same trail. Each bloodhound-handler team completed one trail at each station. For each starting location, the teams were placed directly on the target and decoy trail. After placing the dogs in harnesses, the handlers were given an arbitrarily selected scent pad for presentation to their dog. The parameters recorded were as follows:

    • Did the dogs begin to trail?
    • Did the dogs identify the target person?

The following outcomes were recorded. For beginning to trail, a YES was recorded if the dog indicated the presence of matching scent at the start of the trail and began to follow. A NO was recorded if the dog gave no response to the presence of matching scent at the start of the trail. (Table 2)

For the identification of the target person, a YES was recorded if the dog trailed to and alerted on the person at the end of the trail. If the dog trailed to the decoy person and gave a positive identification, a false positive (FALSE) was recorded. If the dog trailed but did not identify either the target person or the decoy, a negative identification was noted. Because the teams were given two minutes to complete their test, the positive or negative identification results only reflect the immediate response given by the dog at the end of the trail. The trails were monitored and recorded by people without knowledge of the correct outcome.

The test design replicated common crime scene practice; therefore, no negative-control pads were introduced. In actual casework, the bloodhound-handler team is placed on a known trail and given scent collected from an instrument of the crime. If an identifiable amount of scent is present on the scent pad and the bloodhound finds matching scent at the start of the trail, the bloodhound-handler team follows the trail to its logical conclusion.

Follow this link to Table 2.

The overall percentage for a positive beginning to trail indication was 78.3 percent. Beginning to trail indications were calculated by using the scores of the bloodhound-handler teams that indicated positive at the start of the trail. A no-response indication at the start of the trail did not necessarily signify that there was no matching scent present on the pad or at the trail beginning. This negative alert may also indicate that the dog was not able to detect such low levels of material. (Table 2)

The overall combined score for positive identifications was 70 percent. The score for dogs that indicated matching scent by beginning to trail and correctly identifying the target person was 88.6 percent with no false-positive indications. (Table 2)

Eight dog-handler teams with a casework experience level under five conducted 48 trails with 34 positive begin-to-trail indications (70.8 percent) and 31 positive identifications (64.5 percent). (Table 3)

Five dog-handler teams with a casework experience level more than five and fewer than 100 conducted 30 trails with 24 positive begin-to-trail indications (80 percent) and 19 positive identifications (63.3 percent). (Table 3)

Seven dog-handler teams with a casework experience level over 100 conducted 42 trails with 36 positive begin-to-trail indications (85.7 percent) and 34 positive identifications (80.9 percent).

Follow this link to Table 3.


Several aspects of this study must be considered when factoring the significance of the findings. Overall, the dogs correctly identified the target person in 53 of the 80 bomb-debris experiments and 31 of the 40 arson-debris experiments with no false-positive identifications. The combined results and the absence of false-positive identifications supports the general reliability of this procedure and indicates that dogs can detect and identify human scent on bomb and arson debris.

Some positive identification could have occurred because a dog alerted on the first visual cue that it received. In the training of various teams, trails are set up so there is only one choice at the end of the trail—to identify the scent target. Teams that have trained with multiple decoys on the scent trail typically have dogs conditioned to check each person for a scent match. The specially trained teams in this study use multiple decoys in training. It is unknown whether the three traditionally trained teams use similar techniques.

The number of target identifications may have increased if there had been no time limit for the completion of each trail. The two-minute limit did not provide enough time for some of the bloodhounds to make a choice. This time limit may have also had a beneficial effect because the handlers did not necessarily have the time to entice their dogs to choose one target over another in order to complete the trail with a find, thus causing false-positive identifications.

No readily identifiable differences were observed that indicated the scent pads collected on the day of device functioning produced better results than the pads collected 14 days after the event. The following tables specify the scent-pad collection information and the test results for each pad.

Follow this link to Table 4.

Follow this link to Table 5.

Follow this link to Table 6.

Follow this link to Table 7.

Follow this link to Table 8.

Follow this link to Table 9.

Follow this link to Table 10.

The explosion and burning of the test materials in this series was conducted to maximize the chances of recovery. There are too many uncontrollable environmental parameters associated with the explosion and burning of materials to reliably replicate events associated with an actual crime scene. In each of these events the materials would have been handled differently and subjected to scattering, weather, and the influence of the actions of emergency personnel. Likewise, it is impossible in an experimental test scenario to control all of the environmental variables to accurately replicate trailing conditions experienced in casework. Consequently, the results derived from this type of feasibility test series only demonstrate the survivability of identifiable human scent and the potential to use it in an investigation. It does not indicate the ability of a particular breed, nor will it provide sufficient data to predict a scent dog’s reliability in casework or testing.

Caution must be applied when dealing with human-scent evidence. Because scent is easily transferred, a positive trail or identification resulting from any scent article only shows a scent relationship to the scent article and must be verified and corroborated through other investigative means (Stockham et al. 2004). This scent relationship generally establishes a direct or indirect link between a person and an article of the crime; it does not prove complicity.


This feasibility study demonstrated the ability of human scent to survive the extreme mechanical and thermal affects associated with the explosion and burning of various energetic materials. Furthermore, the ability of specially trained bloodhound-handler teams to match the collected scent to the correct person after these violent energetic events was demonstrated. By conducting suspect elimination checks with scent pads collected by the STU-100, a portable vacuum collection unit, this specialized approach has shown that it can assist in providing valuable lead information for investigators, focus valuable and often limited resources, and aid in the solution of crimes.


Harvey, L. and Harvey, J. Reliability of bloodhounds in criminal investigations, Journal of Forensic Sciences (2003) 48(4):811-816.

Henderson, R. W. and Lightsey, G. W. Effective flame temperatures of flammable liquids, Fire and Arson Investigator (1984) 35(12):8.

Persson, P., Holmberg R., and Lee, J. Equation of state of the explosion products. In: Rock Blasting and Explosives Engineering, CRC, Boca Raton, Florida, 1993, p. 102.

Picatinny Arsenal, Encyclopedia of Explosives and Related Items, PATR 2700, Volume 2, Dover, New Jersey, 1962, B170, C34-35.

Schoon, A. and Haak, R. Stability of the odor left on an object. In: K9 Suspect Discrimination, Training and Practicing Scent Identification Line-ups, Detselig Enterprices, Calgary, Alberta, Canada, 2002, pp. 47-48.

Stockham, R. A., Slavin, D. L., and Kift, W. Specialized use of human scent in criminal investigations, Forensic Science Communications [Online]. (July 2004). Available: www.fbi.gov/hq/lab/fsc/backissu/july2004/research/2004_03_research03.htm.