In routine forensic genetics cases, samples from crime scenes involving vehicles or house fires after use of fire extinguishers are frequently analyzed. In some cases, stolen vehicles or those used for criminal activities are set on fire to destroy evidence. In other cases, vehicles involved in car accidents caught fire, but certain circumstances need to be investigated and clarified. Additionally, fire extinguishers are sometimes used to deliberately destroy evidence at the scene of a crime, even when there is no fire. Either way, the use of fire extinguishers may impede short tandem repeat (STR) analysis and thus prevent the identification of individuals involved. 

To investigate the hypothesis that fire extinguishers or their contents could cause STR analysis failure, three commonly in Germany used extinguishers were selected for testing:  a powder extinguisher from Minimax, a foam extinguisher from TOTAL GmbH, and a CO₂ extinguisher from Smartwares. 588 mock samples of blood, saliva and epithelial cells were created and applied in different ways and volumes on various surfaces, including laboratory table and typical car surfaces, such as a steering wheel, glove compartment and a car door trim. The surfaces were sprayed with one type of fire extinguisher at a time, and then left for one and 25 hours before taking samples. For comparison, mock samples were not sprayed with a fire extinguisher and used as a reference.

No significant effect was found on DNA quantity in blood and saliva samples after fire extinguisher treatment. Only samples of 25 µl blood exposed to fire extinguisher foam and left for 25 hours demonstrated a decrease in concentration of 40 %. In epithelial cell samples, a very low DNA concentration was observed in from both the treated samples and the untreated control. With regard to STR analysis, no deterioration in completeness of STR profiles was observed for blood and saliva samples after contact with fire extinguishing agents. However, STR analysis of epithelial cell samples revealed a significantly lower number of complete profiles. 

In summary, a major effect of fire extinguishers on DNA analysis could not be found in our study.

Authors

• Sophie Fertyk (Institute of Legal Medicine, University Hospital Essen, Germany)
• Helen Konrad (Institute of Legal Medicine, University Hospital Essen, Germany)
• Katharina Firl (Institute of Legal Medicine, University Hospital Essen, Germany)
• Benno Hartung (Institute of Legal Medicine, University Hospital Essen, Germany)
• Micaela Poetsch (Institute of Legal Medicine, University Hospital Essen, Germany)