Activity level evaluation of DNA evidence requires understanding of the transfer, persistence, prevalence and recovery of biological traces under case-specific circumstances. While studies on the probability of secondary transfer in defined scenarios are valuable for forensic practitioners, their applicability may be limited by the specific conditions tested or by inter-laboratory differences such as variations in DNA yield. Thus, fundamental knowledge regarding the mechanisms and processes that control the transfer of cells and DNA is essential to enable reliable activity level assessments in casework. We have investigated how different temperatures and relative humidities affect the drying of blood, semen and saliva stains on various fabrics. Dynamic vapour sorption was used to gravimetrically study the drying process with high resolution under controlled environmental conditions. Temperatures of 5°C, 20°C and 30°C combined with relative humidities of 25%, 75% and 95% were tested, representing different seasons and indoor/outdoor climates. Controlled transfer experiments were conducted to learn how the drying process, moisture levels and fabric characteristics impact the amount of transferred DNA. A primary fabric containing the stain was placed in direct contact with a secondary clean fabric, using glass plates and weights to apply uniform pressure. Various natural and synthetic fabrics were evaluated, representing materials commonly found in clothing, furniture and bed sheets. All three body fluids showed similar drying patterns with an initial linear phase, reflecting the evaporation of free water molecules. In the final stage of drying, the blood dried slightly slower than semen and saliva, likely due to a higher content of solid particles. Relative humidity had a substantial influence on drying, with 75% humidity leading to around three times longer drying times compared to 25% humidity for saliva on cotton fabric for all tested temperatures. Lowering the temperature from 20°C to 5°C increased the drying time with a factor of around two for all tested relative humidity levels. Transfer experiments showed that a high degree of moisture was necessary to enable the transfer of DNA from the absorbing fabrics. The findings indicate that the type of body fluid has limited impact on the drying time. Given their large effects on the drying process, the ambient temperature and relative humidity should be considered when evaluating the possibility of body fluid transfer in activity level assessments.

Authors

• Johannes Hedman (National Forensic Centre, Swedish Police Authority and Division of Biotechnology and Applied Microbiology, Department of Process and Life Science Engineering, Lund University, Sweden)
• Sofie Bergstrand (National Forensic Centre, Swedish Police Authority and Division of Biotechnology and Applied Microbiology, Department of Process and Life Science Engineering, Lund University, Sweden)
• Tess Nordenberg (Division of Biotechnology and Applied Microbiology, Department of Process and Life Science Engineering, Lund University, Sweden)
• Julia Bengtsson (National Forensic Centre, Swedish Police Authority and Division of Biotechnology and Applied Microbiology, Department of Process and Life Science Engineering, Lund University, Sweden)
• Lars Wadsö (Division of Building Materials, Department of Building and Environmental Technology, Lund University, Sweden)