The two major concerns in handling potential DNA evidence are degradation and contamination. Degradation can be brought about by overexposure to heat and humidity. Contamination can either be sample to sample or person to sample. Degradation is usually caused when bacteria, mold and mildew grow on a sample. This can lead to inconclusive results.
Contamination can lead to false or un-interpretable results.
This happens because the PCR reaction will preferentially work on fresh DNA over older samples. So, even if a small amount of fresh DNA comes into contact with a crime scene sample that has been exposed to the environment , it is possible for that fresh DNA to give results along with or instead of the crime scene sample. Contamination can happen both in the field or in the laboratory. Degradation that occurs at the crime scene , before collection , cannot be helped. The DNA testing methods utilized now can more often then not obtain reliable results from those.