Europeans have used scent-discriminating dogs to aid criminal investigations
for over a century. Reputed to have a sense of smell 1,000 to 10,000 times
more superior than that of humans, a dog’s nose does offer a sensitive
forensic instrument.
Traditionally, dogs contributed to police work by stalking a suspect’s
track. During the latter part of the twentieth century, investigators found
a new way to employ the canine’s olfactory skills: perform a scent lineup
to connect a person with a crime via scent evidence. Dogs trained for scent
identification are “specialist, biological devices,” according
to Tomasz Bednarek, the Head of the Warsaw Metropolitan Forensic Laboratory
and an expert on oso-mology – a system of human scent identification.
Figure 1. Chromatograms of armpit samples produced from different people and
the same person at different times. Samples were taken from Male 1 ten hours
apart (Sampling 1 vs. Sampling 2) and several weeks apart.2
The Specialist, Biological Device’s Olfactory Abilities Scent
identification rests upon an assumption that humans have unique odor profiles
that remain constant over time, and that dogs can be trained to recognize
that unique scent in a mixture of other odors. What determines the uniqueness
of human scent? “As far as the bloodhound is concerned,” says Dr.
Lisa M. Harvey, “it appears that their scent discriminatory abilities
are based mainly on genetics.” Harvey, a member of the Biology Department
at Victor Valley College (Victorville, CA), points out that studies with rats,
mice, and humans indicate that scent may be based upon the expression of genes
in the Major Histo-compatibility Complex (MHC). “This is the
most polymorphic genetic code in the human genome,” she explains, and
since everyone – with the exception of identical twins – has a
different genetic code, variations in the MHC may provide the clues to the
bloodhound’s sensitive nose.
Other factors can contribute to the distinctiveness
of a person’s scent. Harvey’s group plans to publish their findings “that
nutrition and hygiene may play a minor contributory role if the individuals
are related or identical twins.” A person’s state of health may
also contribute to a characteristic scent. A recent study showed that dogs
can detect changes in physiology associated with human bladder cancer.
G.A.A. Schoon (Netherlands National Police Agency and Leiden University)
investigated how the age of odor trace evidence affects the performance
of Dutch and German
scent identification dogs.1Although the dogs performed faultlessly in matching
odors collected on the same day, their performance level dropped when presented
with scent evidence stored for two weeks. After this initial drop, aging did
not significantly diminish the dogs’ performance
even with scent evidence aged up to six months. Schoon suggests that the initial
drop is due to the volatile nature of scent.
Allison Curran, Scott Rabin, and
Kenneth Furton (International Forensic Research Institute, Florida International
University) have used gas chromatography-mass spectroscopy to analyze the volatile
components of human odor signatures (Figure 1). Dr. Curran explains that their
work “has developed discriminating terminology for the elements of an
odor profile to separate out the compounds which are present due to external
factors such as perfumes (tertiary odor), internal environmental factors such
as smoking or eating onions (secondary odor), and constituents that are stable
regardless of diet and environmental factors (primary odor). Human scent can
be described as a combination of volatile to semi-volatile compounds which
differ in ratio from person to person, along with additional compounds which
vary between individuals.” The group also examined the effects of aging
on scent samples and found measurable amounts of human scent compounds months
after transfer to sterile gauze. These studies, Curran says, reveal “a
relatively long persistence of the human scent compounds in a controlled environment.” What
happens to human scent in uncontrolled environments?
According to anecdotal information, identifiable human scent usually disappears
from an environment after 24 hours. But human scent can persist for much
longer than one day. In 2003, the FBI hosted a bloodhound research workshop
that included
a test to evaluate the feasibility of detecting aged human scent in a heavily
populated residential area. Researchers selected a test subject who had lived
in a Stafford, Virginia house for seven years before moving to Albuquerque,
New Mexico. Six months after the subject had moved to New Mexico, researchers
placed a bloodhound team at a Stafford intersection several houses away from
the old residence. After sniffing a letter that the subject had mailed from
New Mexico, the dog trailed to and
identified the correct house. In this study, the distinctive scent traces withstood
the deteriorating effects of weather. But scent can survive conditions much
more extreme than weather.
Bombers and arsonists often use a time-delay device,
which allows them to depart safely from the scene of the crime. In 2001, the
FBI and Southern California Bloodhound Handlers Coalition performed a study
to investigate whether human scent can connect a bomber or arsonist with the
debris left behind. The researchers prepared four pipe bombs – each with
a different type of explosive – and two containers of gasoline. Six test
subjects then handled a bomb or gas can for one to two minutes. After detonating
or burning the devices, researchers collected debris – such as that shown
in Figure 2 – and transferred scents from the debris to gauze pads. They
stored the pads for two to sixteen days.
On the day of the test, the bomb handlers and six decoys walked along trails
in a public park. After placing 20 bloodhound teams at the start of a trail,
the handlers gave their dogs sniff of a scent pad. The dogs trailed and identified
the target person in 53 of 80 bomb debris experiments and in 31 of 40 arson
debris experiments with no false identifications. The results showed that human
scent can survive extreme mechanical and thermal stress.
Figure 2. Debris from a binary explosive used in the scent study.3
Figure 3. The STU-100®.
Courtesy of Larry Harris, Big “T” Enterprises, Inc.
Scent Detection Applications
The canine nose can aid police by tracking or trailing an individual, or by connecting
a suspect with evidence in a scent lineup. Dogs can track a person without a
scent sample of their quarry. They achieve this by following the odor of crushed
vegetation
and disturbed ground, indicators of a fresh track. Tracking dogs may also use
traces of fresh generic human scent to aid their pursuit.
Trailing and scent
lineups require a scent sample. Traditional scent collection techniques
include direct scenting, swiping, and absorption. In the direct scenting method,
a
handler allows a dog to smell an article of evidence. In the swiping method,
a sterile gauze pad is wiped across the surface of evidence, and the pad
is then used as the scent source. The absorption approach relies upon the ability
of a sterile gauze pad to absorb scent if placed upon an item of evidence
for
an extended period of time. These traditional methods have a drawback:
collection of scent can remove or contaminate fingerprints, fibers, DNA, and
other trace
evidence.
Figure 4. Presenting scent evidence to a scent identification dog.
Courtesy of the Netherlands National Police Agency.
Figure 5. Scent identification dog investigating carrier tubes on a platform.
Courtesy of the Netherlands National Police Agency.
The latest collection method requires the Scent Transfer Unit (STU) to create
scent pads (Figure 3). The STU-100®– a portable vacuum – uses
airflow to convey the components of human scent onto sterile gauze pads.
The scent pads can then be stored in zip-lock or heat-sealable bags until
needed.
Police departments and the FBI use this device.
Supporters of the STU-100®say that the use of airflow to capture scents
minimizes the loss of other forensic evidence and produces a consistent scent
pad. When the FBI and the Southern California Bloodhound Handlers Coalition
tested the STU-100®, they concluded that either no scent cross-contamination
occurs from one pad to the next or, if contamination does occur, the amount
falls below the detectable threshold of the trained canine.
Lisa M. Harvey
and Jeffrey W. Harvey (San Bernardino Police Department) tested the STU-100®in
a study designed to investigate whether anecdotal evidence about a trailing
dog’s ability could be justified.4 Unlike tracking dogs, trailing dogs
have been trained to follow an odor presented on an article that contains the
scent of a particular individual. The dogs can identify this scent even when
mixed with scents of other humans. At least, this is their reputation. The
Harvey study presented bloodhounds with a challenge: identify and trail the
scent of individuals remaining on two day-old paths in parks, college campuses,
and urban environments. Veteran bloodhounds lived up to their reputation by
trailing and correctly identifying individuals under various weather conditions
and despite cross-trail contamination.
In the latest canine contribution to police investigations, a dog performs
a scent identification lineup to link a suspect with crime scene evidence. The
Dutch National Police devised a protocol to withstand the scrutiny of their
courts. Their procedure requires a scent
evidence object, a control object preferably made of the same type of material
as the scent evidence object, and scent carriers, typically ten centimeter
stainless steel tubes. To prepare scented test materials, a suspect and six
foils - adults not associated with the suspect – hold two scent carriers
for five to ten minutes. One of the foils is designated as the control person,
who stores the control object in a pocket while handling two scent carriers.
An assistant prepares the test room for a scent lineup by clamping both sets
of scent carrier tubes to a platform. The assistant arranges each set of
tubes as a row with a random sequence. The dog and its handler enter the room,
the
handler lets the dog smell the control object, and the dog searches the first
row for
a matching
scent. If the dog identifies a scent carrier handled by the control person,
then the handler repeats the procedure with the second row. If the dog identifies
the odor of the control person once again and has not shown particular interest
in the carrier tube held by the suspect, then the dog may proceed to the
suspect identification stage. Since the dog has retrieved both tubes held by
the control
person, six tubes remain on each platform: one handled by the suspect and
five by the foils.
To start the suspect identification stage, the handler returns to the first
row and allows the dog to smell the scent evidence. If the dog responds to the
odor of the suspect in both rows, then police conclude that the scent evidence
object and the suspect share an “odor
similarity.”
In this procedure, the dog’s handler does not know
who touched the scent carriers. The handler’s ignorance avoids the so-called
Rosenthal effect – a nonverbal, or even unconscious, suggestion to a
dog working in a lineup. Polish forensic laboratories that perform human scent
lineups also eliminate the Rosenthal effect by separating the roles of dog
handler and scent identification expert.
Making Legal Sense of Scent Evidence
Human scent can be transferred from one person or object to another. Consequently,
a scent relationship can establish a direct or indirect link between a person
and an article of a crime, but cannot prove that an individual participated
in a crime. While a scent lineup can aid an investigation, the results usually
cannot justify an arrest. “A canine identification of a person is one
reasonable suspicion indicator,” says Terry Fleck, a Deputy Sheriff
and Canine Handler in South Lake Tahoe, California. He explains that “the
officer or investigator needs to develop other reasonable suspicion indicators
in order to develop probable cause that the person committed a crime.” That
is, investigators will seek corroboration for scent identification.
U.S. courts have diverse views about the admissibility of scent lineup evidence.
In Winston v. State (Tex. App. 2002), for example, an appellate court noted
that 37 states and the District of Columbia admit scent trailing evidence to
prove the identity of the accused. As a relatively new use of the canine’s
olfactory skill, the scent lineup will require scientific proof of reliability
before identification evidence can withstand judicial scrutiny in all U.S.
courts. Until then, the scent lineup remains a valuable tool for law enforcement
during an investigation.
References
Forensic Science International 147:43 (2005).
Curran, Allison M.,
Rabin, Scott I., and Furton, Kenneth G. “Analysis
of the Uniqueness and Persistence of Human Scent.” Forensic Science
Communications 7(2) (April 2005).
Stockham, Rex A., Slavin,
Dennis L., and Kift, William, “Survivability
of Human Scent.” Forensic Science Communications 6(4) (October 2004).
J. Forensic Sci. 48:811 (2003)
Phillip Jones is a freelance writer and member of the National Science Writers
Association and the American Society of Journalists and Authors. He can be
reached at philljones@biotechwriter.com.
