The Benefits of Scent Evidence

On average, humans lose approximately 1 to 2 1/2 quarts of water through perspiration each day, with that amount greatly increasing as activity and temperature intensify.(3) Becoming saturated with perspiration and skin oils, raft serves as a "breeding ground" for the bacteria normally found on the skin.(4) This action of bacteria on cellular debris may, in fact, form the basis of the human scent trail. Although culture, diet, environment, heredity, and race influence it to some degree, the combination of bacteria, vapor, and cellular debris is believed to be unique to the individual, accounting for the singularity of human scent.

Using Scent Evidence

The uniqueness of human scent permits law enforcement to use scent evidence to

1) follow a suspect directly from a crime scene;

2) determine a suspect's direction of travel from a crime scene, possibly locating additional evidence that the suspect drops, abandons, or unwittingly leaves behind (e.g., footprints or tire tracks);

3) ascertain the whereabouts of a suspect;

4) identify a suspect in a lineup;

5) place a suspect at a particular crime scene or location;

6) establish probable cause; and

7) locate and recover missing persons, whether dead or alive.

Law enforcement agencies cannot rely on scent evidence alone to accomplish these tasks; they must use specially trained dogs to link the evidence to the individual and the crime.

Working with Scent-Discriminating Dogs

Dogs have an extraordinary number of olfactory sensory cells (220 million in a sheep dog, compared to 6-10 million in humans), enabling them to smell 44-100 times better than human beings/Although law enforcement agencies have used a number of breeds in police work, the bloodhound, with its keen sense of smell and innate determination, remains the best suited for locating individuals and evidence.(6)

A properly trained dog can successfully follow trails of up to 10 days old. Still, because human scent is affected by such environmental elements as wind, temperature, humidity, and other factors and also diminishes with time, a dog should begin working within 24 hours. Law enforcement agencies that bring dogs into an investigation right away increase the odds of preserving vital evidence.

Preserving the Crime Scene

Law enforcement first responders should make every, effort to preserve the crime scene or the last known location of a missing person ("point last seen"). To do this, they first should limit access to the area. A perimeter of at least 200 feet around the scene can prevent contamination and give dogs and their handlers an adequate area in which to obtain a scent trail. Because automotive exhaust masks scent, individuals arriving at the scene should turn off their vehicles' engines.

Pets also should be confined. Although keeping individuals from contaminating the crime scene may prove particularly difficult in some cases - for example, in missing-child investigations, where anxious family members and friends gather at the scene - doing so gives law enforcement the best chance to collect critical evidence.

Collecting Scent Evidence

Anything a suspect has touched, worn, or eliminated (e.g., bodily fluids, including blood and urine) can serve as scent evidence, but articles of clothing worn close to the skin work best. Investigators should avoid collecting clothing from hampers because it likely contains the scent of other family members or roommates.

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