Contamination Issues

Because extremely small amounts of DNA can be used as evidence, greater attention to contamination issues is necessary when identifying, collecting, and preserving DNA evidence. DNA evidence can become contaminated when DNA from another source gets mixed with DNA relevant to the case. This can happen if someone sneezes or coughs over the evidence, or if the person collecting the evidence touches his/her mouth, nose, hair, or any other part of his/her body, and then touches the area that may contain the DNA to be tested. In addition, environmental factors, such as heat and humidity, can accelerate the degradation of DNA. Degradation refers to the breaking down of DNA into smaller fragments by chemical or physical means. For example, wet or moist evidence that is packaged in plastic will provide a growth environment for bacteria, which can destroy DNA evidence. Biological evidence should always be thoroughly airdried, packaged in paper, and properly labeled, ensuring that the chain of custody-a process used to document the chronological history of the evidence is maintained. DNA evidence that is properly identified, collected, and preserved can be stored for years without risk of extensive degradation, even at room temperature.

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Understanding DNA Evidence: A Guide for Victim Service Providers
April 2001