Understanding and mitigating the risks that environmental DNA contamination poses to the recovery of forensic evidence from victims and suspects of rape and sexual assault.

The high sensitivity of current DNA analysis technologies poses significant anti-contamination challenges when recovering evidence from individuals in sexual assault referral centres (SARC)s and police custody forensic medical examination rooms where background DNA levels cannot be controlled as effectively as within a DNA laboratory setting. In response to reported instances of DNA evidence becoming compromised during recovery within SARCs, the UK Forensic Science Regulator (FSR) established anti-contamination guidelines for these facilities including requirements for environmental monitoring (EM) and target levels for air replacement with a view to managing the risk of air-borne contamination. Forensic samples were recovered from different volunteers during 24 forensic medical examinations across four SARC and four police custody suites, all utilising different cleaning and air replacement regimes. Of the 144 EM samples taken from high contamination risk areas of the forensic medical room, DNA was present in 84 % of these swabs. Significantly less DNA was found to be present in the SARC when compared to custody suites. Despite the high environmental DNA levels observed, none resulted in contamination of the forensic evidence recovered from the volunteer patients. This study assessed the real-life risk to evidential samples and the results demonstrated that provided appropriate anti-contamination measures were used in recovering and handling evidential samples, the risk of DNA contamination was effectively managed, even within facilities displaying high background levels of DNA. The results from this assessment of risk have enabled target levels of cleanliness to be defined in FSR guidelines1 together with revised requirements for airflow rates.