The issue of contamination of physical evidence has painfully brought notoriety to several criminal cases. Webster's Dictionary defines contamination as; "to make impure, corrupt, by contact; pollute, taint." Potential contamination of physical evidence can occur at the crime scene, during the packaging, collection and transportation of the evidence to a secured facility or laboratory, and during evidence analysis and storage.

While forensic scientists in the laboratory are sensitive to the issue of contamination and have developed protocols to identify and reduce the risk of contamination, law enforcement has been slower to incorporate precautions in contamination prevention. Recent advances in forensic DNA technology are making it even more important that crime scene personnel become more sensitive to the issues of contamination.

The Crime Scene
Crime scene contamination usually results through the actions of the personnel at the scene. In general, the grater number of personnel at the scene, the more likely it is that the scene/evidence will be contaminated. Scene personnel can deposit hairs, fibers or trace material from their clothing or destroy latent footwear or fingerprints. Footwear patterns can also be deposited by crime scene personnel or anyone entering the scene. As Professor Locard has taught us, when two objects come in contact with each other they exchange trace evidence. Each time we enter a crime scene, we not only potentially leave trace evidence behind, but also take evidence away from the scene.

Forensic DNA analysis has become an increasingly powerful investigative tool. Analysis of biological fluids and now cells found at crime scenes can, with relatively high confidence, exclude/include a possible suspect and provide a numerical estimate of the similarity of the crime scene and suspect's DNA. The DNA technology being used in crime laboratories around the country can take samples that are very small or degraded and xerox the DNA present to provide a large enough sample to be analyzed. Because of the analyst's ability to xerox very small amounts of DNA from biological evidence, reducing the potential for contamination at crime scenes becomes ever more significant. Single hairs, perspiration and/or saliva inadvertently deposited by an investigator while at a crime scene can now cost valuable time and create the potential for excluding a viable suspect as well as cloud or confuse the interpretations of the physical evidence.

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About the Authors

Hayden B. Baldwin, B.S. is the Director of Forensic Enterprises, Inc. 15774 South LaGrange Road, PMB 385, Orland Park, IL 60462-4766, and is the Executive Director of the International Crime Scene Investigators Association
Cheryl Puskarich May, Ph.D. is the Assistant Director of the Criminal Justice Institute, Forensic Sciences Education Center, 7723 Asher Avenue, Suite B, Little Rock, AR 72204