Contamination is the accidental, deliberate, or neglectful introduction of material into a location which is likely to undermine the integrity of inferences that may be drawn of any examination or investigation made of it.
Contamination, or the suggestion of it, will undermine any examination and the evidence of material recovered within an investigation. So, it is important that risk of contamination needs to be identified at the earliest opportunity and steps are taken to eliminate it and demonstrate so.
Practical steps can be taken to avoid contamination from control of the crime scene to dealing with victims and suspected persons, establishing boundaries and protecting them. Developing areas of separation in scene investigation practice through the use of personnel and equipment can be shown to remove risk of contamination.
Contamination continues to be a consideration long after the initial scene examinations have been completed. So the packaging, storage, and transfer of any material collected must be thought through in advance.
Separation from any outside source of the material under investigation and the ability to demonstrate it is the only way to maintain the integrity of the investigation and any interpretation or conclusions gained from it.