Deoxyribonucleic acid (DNA) is the genetic blueprint for all living organisms, an invaluable tool for identification in forensic investigations. “Touch DNA” is a significant form of trace evidence, offering insights into who may have come into contact with an object.
Understanding Touch DNA
Touch DNA refers to genetic material, primarily from skin cells, left behind when a person makes physical contact with an object or surface. Everyday activities involve the constant shedding of skin cells, which are transferred upon touch, leaving microscopic traces of DNA.
While skin cells are the most common source, touch DNA can also originate from cells found in sweat or saliva, transferred from the hands to a surface. The minute amounts of this transferred material require specialized forensic techniques for collection and analysis.
Factors Influencing DNA Persistence
The duration for which touch DNA remains detectable and suitable for analysis varies considerably, influenced by several environmental and contextual factors. Environmental conditions play a significant role in DNA degradation. Higher temperatures accelerate the breakdown of DNA molecules, while exposure to ultraviolet (UV) light, particularly from direct sunlight, can rapidly degrade DNA, rendering it unusable. Humidity also impacts persistence; while low humidity can lead to desiccation, high humidity may promote microbial growth, further breaking down DNA.
The type of surface on which DNA is deposited also influences its longevity. Non-porous surfaces like steel or glass might hold DNA on their surface, making it vulnerable to removal, but can also protect it from absorption. Porous materials such as fabrics can absorb DNA, shielding it from immediate environmental damage or physical removal. The initial quantity of DNA deposited also affects how long it persists, as a larger starting amount offers more material for analysis. Exposure to elements like rain or wind, or repeated handling or cleaning, directly impacts DNA survival.
Typical Lifespan of Touch DNA
The lifespan of touch DNA is highly dependent on the factors discussed previously. Under ideal, protected conditions, such as indoors with stable temperatures and minimal handling, DNA can persist for months or even years. Studies indicate that DNA on fabric and steel can produce informative profiles for up to nine months when kept indoors.
In less favorable or harsh conditions, touch DNA can degrade rapidly, sometimes within days or hours. Outdoors, with environmental stressors like UV light and fluctuating temperatures, DNA on steel might remain usable for about six months, and on rubber surfaces for approximately three months. While DNA molecules might endure, the quality and quantity suitable for generating a complete forensic profile decrease significantly over time.
Forensic Utility of Touch DNA Over Time
The practical utility of touch DNA in forensic investigations depends on whether it remains suitable for generating a usable genetic profile. Even if DNA fragments are detectable after degradation, obtaining a comprehensive profile for identification can be challenging. This means that while some genetic material may remain, it might not provide enough information to link it definitively to an individual.
The difficulty of collecting and analyzing degraded samples poses a significant challenge for forensic scientists. Advancements in forensic techniques, such as highly sensitive Polymerase Chain Reaction (PCR) methods, have improved the ability to analyze smaller and more degraded samples. These technological improvements extend the practical “lifespan” of touch DNA for forensic purposes.