Since the first use of DNA evidence in a criminal case in 19861, forensic scientists have considered biological material (such as hair, skin, and bodily fluids) to be relatively reliable physical evidence.
While early technology required a substantial amount of biological material to extract enough DNA to build an individual profile for analysis, researchers have since discovered that they can obtain reliable DNA from more than just bloodstains or visible fluids; they can also obtain it from “touch DNA” that is left behind on surfaces or objects such as doorknobs, window latches, or steering wheels. Although touch DNA can be essential for forensic casework, it also comes with its share of issues, including those related to:
The results from rigorous analysis of these complicated factors have important implications for how touch DNA is collected, analyzed, and interpreted.
In 2018, the Forensic Technology Working Group at NIJ called for “comprehensive, systematic, well controlled studies that provide foundational knowledge and practical data about ‘touch evidence’ persistence in the real world.” That same year, Dr. Meghan Ramsey’s group at the Massachusetts Institute of Technology (MIT) Lincoln Laboratory began quantifying how long touch DNA would persist on certain surfaces under specific conditions. Building on that knowledge, and in collaboration with Dr. Ramsey, scientists at South Dakota State University created predictive models of how DNA degrades on different surfaces under a range of environmental conditions.
The researchers addressed two central questions:
To address these questions, scientists deposited control DNA and touch DNA samples2 onto steel bolts and cotton fabric swatches. Then, they examined the DNA residue over time, across varying temperature and humidity combinations, and under UV light exposure (Figure 1).3, 4
The ability to obtain a DNA profile using short tandem repeats (or STRs), commonly used in forensic genetic analysis.Figure 1: Overview of the research model, including DNA deposit, environmental exposure, DNA recovery and persistence, and predictive modelling of DNA degradation.
To predict the amount of DNA degradation over time, Dr. Ramsey worked with her collaborators to fit the DNA degradation data (based on temperature and humidity exposure) to a linear, mixed effects model.5 In doing so, they found:
To further examine DNA degradation, Dr. Ramsey and colleagues compared the completeness — whether the DNA profiles could be submitted to a database for a potential match — of two DNA profiles: environmentally exposed touch DNA recovered from steel bolts and unexposed reference sample DNA from cheek cells (Figure 3).
Throughout the course of this research, low and variable quantities of touch DNA collected remained a challenge; the low quantities of the initial touch DNA that scientists could recover made it difficult for researchers to evaluate the level of DNA degradation properly. Future work aims to increase the initial amount of touch DNA collected to record its degradation more accurately over time.
Still, those in forensics and law enforcement can glean valuable information from this ongoing research regarding the persistence of DNA in certain environmental conditions. For instance, investigators are more likely to recover useable DNA in cool and dry indoor environments than hot and humid outside conditions. Moreover, they may have better success obtaining DNA from stainless steel objects than fabric.
“The results from the study have generated a number of recommendations for best practices that the forensic science community can use to interpret and evaluate touch DNA evidence in a laboratory setting,” notes Physical Scientist Dr. Tracey Johnson of the National Institute of Justice.
Collectively, these studies provide the most comprehensive information to date on the persistence of touch DNA evidence.
The work described in this article was supported by NIJ grant number 2018-DU-BX-0192 awarded to MIT Lincoln Laboratory.
This article is based on the grantee report, “Persistence of Touch DNA for Forensic Analysis” (pdf, 24 pages), by Meghan Ramsey.
National Institute of Justice, "Persistence of Touch DNA for Analysis," June 5, 2023, nij.ojp.gov: https://nij.ojp.gov/topics/articles/persistence-touch-dna-analysis
Read the report, “Persistence of Touch DNA for Forensic Analysis” (pdf, 24 pages), by Meghan Ramsey.
This article was origanially published by the National Institute of Justice at https://nij.ojp.gov/topics/articles/persistence-touch-dna-analysis. The Crime Scene Investigator Network gratefully acknowledges the National Institute of Justice for allowing us to reproduce the article Persistence of Touch DNA for Analysis.
Article posted June 10, 2023