Impact of fire extinguishing agents on DNA quantity and integrity

In routine forensic genetics, samples from crime scenes after use of a fire extinguisher are often examined. These are usually vehicle or house fires in which the fire extinguisher was used to fight the fire. The vehicles include cars that have been stolen or used for criminal activities, or both, and then set on fire to destroy evidence. In addition, there are vehicles that have been involved in hit-and-run accidents and caught fire. Furthermore, fire extinguishers are sometimes used without a fire in order to deliberately destroy evidence at the scene of the crime. Either way, it can lead to an unsuccessful STR analysis and prevent the identification of the individuals involved. To investigate the hypothesis that the use of fire extinguisher or the chemicals they contain could be responsible for the failure, three different fire extinguishers were selected (powder, foam, and CO2 extinguishers), which are commonly used to extinguish small fires. Various surfaces were selected, including a laboratory table and typical vehicle surfaces. We created 620 artificial samples of blood, saliva, and epithelial cells on these surfaces, allowed them to dry, and then sprayed them with one type of extinguisher each. As a control, one cohort was not sprayed and used as reference. In summary, the use of the various extinguishers after a short incubation period (one hour) had no effect on DNA concentration and in STR analysis. However, with increasing incubation time (25 h) prior to sampling, the foam extinguisher cohort showed a reduction in DNA concentration, but there was no reduction of complete STR profiles for blood and saliva in any of the cohorts. Only the quality/completeness of the STR profiles based on epithelial cells decreased in the powder and foam extinguisher cohorts. The proportion of full profiles fell by 6% and 15% respectively, and the proportion of no evidence profiles rose by 17% and 13% respectively. However, the study did not reflect the impact of a real-world fire scenario. It could be assumed that completely charred surfaces or areas exposed to intense heat would yield poorer results in subsequent DNA analysis. Similarly, longer incubation times of fire extinguisher chemicals can have a greater impact and thus lead to a more reduction in the number of successfully typed samples.