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Contamination of Evidence

Contamination is the unwanted alteration of evidence that may affect the integrity of the original exhibit or the crime scene. This unwanted alteration of evidence can wipe away the original evidence, dilute a sample, or deposit misleading new materials onto an exhibit. Contamination can compromise the analysis of the original evidence to the extent that the court may not accept the analysis and the inference.

Causes of contamination

Contamination can take place in a number of ways including:

  1. Crime scene contamination: Crime scene contamination usually results through the actions of the personnel at the scene. The greater, the number of personnel at the scene, the more likely it is that the scene/evidence will be contaminated. Scene personnel can deposit hairs, fibers, trace material from their clothing, or destroy latent footwear or fingerprints. Crime scene personnel or anyone entering the scene can also deposit footwear patterns.
  2. Degree of contamination: The level of contamination risk to be expected is related to the type of crime scene and corresponding number of individuals who have access to the scene. At a burglary scene, the victim and the officer taking the report may be the only individuals present. In contrast, the investigators, crime scene examiners, coroner or medical examiners, prosecuting agency, etc. would usually visit a typical death scene. Family, friends and neighbors of the victim may be present as well. Obviously, due to the higher number of individuals in contact with the scene, the potential for contamination would be significantly higher.
  3. Environmental factors: Environmental conditions may also play a major role in the contamination of crime scene evidence. Wind, sun, rain, snow and temperature can play key roles in the destruction of the evidence at a crime scene. For instance, if there is blood at an outdoor crime scene and it rains, the blood may become so diluted that testing of the blood becomes impossible. The same would apply, if the blood was exposed to the sun on an extremely hot and humid day. The fluid would be decomposed or contaminated by bacteria to a point where further analysis would be impossible.

Steps to prevent contamination

The risk of contamination in all crime scenes is reduced by thoroughly protecting the scene. Consequently, determining the dimensionality of the scene should be the first priority. We cannot protect something, we do not recognize as part of the scene. Indoor scenes, by virtue of being enclosed structures, seem easier to secure. Outdoor scenes, on the other hand, are more difficult to secure because of the potential contamination by agents such as weather conditions and crowd. As a result, these types of scenes require more personnel to properly protect.

  1. Barrier tape, usually yellow in color and marked by the words “Crime Scene” or “Police Line” with the additional words of “Do Not Cross” is used to identify the outer border of the scene.
  2. Physical barriers are always needed to define the areas restricted to the public as well as other law enforcement personnel. The barrier used can be as simple as a rope or added markings with attached signs. Providing visual boundaries to the scene assists in restricting access and reducing contamination risks.

About the Author

author Sunil Sharma is an advocate; editor and compiler of legal commentaries, having authored more than 40 books.

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