The team began with a computational question: does polystyrene physically interact with the molecular machinery of DNA analysis? Using in-silico docking, they modeled how polystyrene binds to Taq DNA polymerase, the heat-stable enzyme that copies DNA during the polymerase chain reaction, or PCR. The predicted binding energy was a favorable −4.57 kilocalories per mole, suggesting the polymer does not merely float passively beside the enzyme but engages with it in ways that could disrupt its catalytic work. The researchers then turned to the DNA itself, docking polystyrene against the mitochondrial DNA HVI region, a workhorse target in forensic sequencing, and against four short tandem repeat markers that anchor human identification: D8S1179, D13S317, D21S11, and D2S1338. The interaction energies ranged from −1.95 to −1.53 kilocalories per mole, with D8S1179 showing the strongest association. In plain terms, the plastic showed a measurable chemical affinity for the exact genetic regions forensic laboratories amplify every day.

Computational predictions, however, are only as good as their experimental confirmation. To probe what happens to DNA at the molecular level when polystyrene is present, the team used attenuated total reflectance Fourier-transform infrared spectroscopy, or ATR-FTIR, on calf thymus DNA mixed with the polymer. The spectra told a precise story. The deoxyribose C–O stretching peak shifted from 1066 to 1047 wavenumbers per centimeter, the asymmetric phosphate stretch moved from 1242 to 1260, and the purine ring C–N stretching band drifted from 671 to 737. Each of these shifts indicates that polystyrene is not a bystander: it perturbs the sugar-phosphate backbone and the nitrogenous bases of DNA, the very structures that primers must recognize and polymerases must read. A forensic assay depends on those molecular contacts happening cleanly and repeatedly; a plastic particle wedging itself into the chemistry undermines the entire chain of events.

The consequences showed up immediately in DNA quantification, the step that tells a laboratory how much human DNA a sample contains and therefore how to process it. When the researchers spiked known quantities of control DNA with polystyrene at concentrations between 25 and 100 micrograms per milliliter, the readings collapsed. A sample containing 0.5 nanograms of DNA was estimated at just 0.027 nanograms, and a 1.0-nanogram sample registered as 0.046 nanograms—underestimations by factors of roughly eighteen and twenty-two respectively. Part of the explanation lies in fluorescence interference. Real-time PCR instruments quantify DNA by reading fluorescent signals, and polystyrene is known to fluoresce and to absorb or scatter light in ways that scramble those readings. Notably, the internal positive control’s cycle threshold value barely moved, shifting from 27.83 in controls to 27.72 in treated samples, which suggests the problem was not classic PCR inhibition at the quantification stage but optical distortion—the instrument was effectively being blinded by the plastic.

Quantification errors alone would be serious, because an analyst who believes a sample contains almost no DNA may choose an aggressive low-template strategy or may even decline to test it at all. But the study found that polystyrene also attacks the amplification stage directly. When DNA spiked with 25 micrograms per milliliter of polystyrene was run through STR profiling, the markers D10S1248, TH01, and D12S391—all part of the expanded European and international standard sets—amplified poorly or dropped out. At 100 micrograms per milliliter, the quality-sensing markers within the quantification kit showed a heterozygote peak height balance of just 0.44, far below the balanced ratios expected in a clean reaction, which the authors interpret as significant PCR inhibition. The mechanism is likely multifaceted: polystyrene can sequester DNA strands and primers through the same binding interactions predicted in silico, compete with the polymerase, and interfere with the fluorescent chemistry that reports amplification progress.

The downstream effect on DNA profiles was consistent across every concentration the team tested. Short tandem repeat profiling, the backbone of human identification, depends on every locus amplifying reliably and symmetrically. When some markers fail, drop out, or produce imbalanced peaks, the resulting profile becomes harder to interpret, harder to match against a suspect or a database, and more vulnerable to challenge in court. The authors emphasize that such compromised profiles are especially dangerous for low-template DNA samples—the faint traces from a single touched surface, a fingerprint residue, or a hair—and for mixture samples containing DNA from multiple contributors, where every lost allele complicates the already delicate task of deconvoluting who contributed what. In other words, microplastic contamination hits hardest precisely where forensic science is already operating at its limits.

What makes the study timely is the growing recognition that microplastics are inside us. Recent reviews have documented polystyrene and other polymers in human blood, sputum, skeletal tissues, and a widening list of organs, and research on laboratory animals has linked polystyrene exposure to mitochondrial disruption and genotoxic effects. For forensic practitioners, this reframes contamination risk. It is no longer sufficient to think of sample contamination purely in terms of handling errors, environmental bacteria, or chemical inhibitors like humic acids and indigo dye. A victim’s or perpetrator’s own body may carry polymer particles into a bloodstain or tissue sample, and clothing, packaging, and plastic evidence bags may shed additional fragments onto exhibits during storage. The study’s authors, who have previously examined how metal contaminants interfere with STR analysis, position microplastics as an emerging contaminant class that forensic workflows have not yet been designed to detect or counter.

The researchers argue that the problem warrants immediate action and call for suitable mitigation strategies to strengthen the routine forensic DNA workflow. Practical responses could take several forms. Laboratories might incorporate purification steps that separate plastic particles from DNA before quantification, adopt quantification chemistries less susceptible to fluorescence interference, or add validation studies that characterize how common polymers—polystyrene, polyethylene, polypropylene, polyethylene terephthalate—affect each step from extraction to interpretation. Evidence-handling protocols could also be revisited, since plastic packaging is ubiquitous in forensic storage. The study stops short of prescribing specific remedies, but its message is clear: until mitigation exists, microplastic contamination is an unmeasured variable in casework, capable of silently shrinking DNA estimates by an order of magnitude and erasing genetic markers without any obvious warning sign to the analyst.

There is also a broader scientific payoff in the paper’s mechanistic approach. By combining docking predictions, infrared spectroscopy, quantitative PCR, and full STR profiling, the team built a coherent causal chain from molecular binding to profile degradation—a model that can now be applied to other polymers and other contaminants. The same framework previously illuminated how metals sabotage PCR, and it could guide the design of inhibitors-resistant enzyme formulations or buffer additives that shield DNA from polymer surfaces. For a field whose credibility rests on reproducibility, understanding why and how a contaminant distorts results is the first step toward neutralizing it. As microplastics continue to accumulate in bodies, waterways, and dust, the intersection of environmental pollution and forensic genetics is no longer hypothetical. This study provides the first detailed mechanistic account of how one of the world’s most common plastics collides with DNA evidence, and it suggests that crime laboratories worldwide may need to add a new item to their contamination checklist—one measured in microns.

Subject of Research: Mechanistic interaction between polystyrene microplastics and DNA and its impact on forensic DNA profiling

Article Title: Understanding the mechanistic interaction between DNA and polystyrene microplastic and the effect of microplastic on forensic DNA analysis

Article References: Yadav, N., Tehsin, S., Tanpure, D., Sahoo, S., Dash, A. A., Priyadarshini, K., Priyadarshini, P., & Dash, H. R. (2026). Understanding the mechanistic interaction between DNA and polystyrene microplastic and the effect of microplastic on forensic DNA analysis. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-04017-3

Image Credits: AI Generated

DOI: 10.1007/s00414-026-04017-3

Keywords: microplastics, polystyrene, forensic DNA analysis, DNA profiling, STR markers, PCR inhibition, Taq DNA polymerase, ATR-FTIR, DNA quantification, low-template DNA, contamination, International Journal of Legal Medicine

The research team, led by Maryam Sharafi Farzad and colleagues at the University of Copenhagen, set out to validate the Precision ID mtDNA Whole Genome Panel from Thermo Fisher Scientific specifically for telogen hair analysis and to implement it in routine forensic casework. Mitochondrial DNA offers a crucial advantage in this context: cells contain many more copies of mitochondrial DNA than nuclear DNA, and the molecule is far more resistant to degradation. Although mtDNA is maternally inherited and therefore lacks the discriminatory power to identify a single individual, it remains highly valuable for maternal lineage analysis, missing-person investigations, disaster victim identification, and cases where only degraded biological material is available. Traditionally, mtDNA typing relied on Sanger sequencing of the control region alone, a labor-intensive and costly approach with limited discriminatory power. Massively parallel sequencing technologies such as the Precision ID panel now allow entire mitochondrial genomes to be sequenced quickly and affordably, dramatically increasing both sensitivity and information content.

To establish the sensitivity of the workflow, the researchers prepared dilution series from NIST Standard Reference Material DNA and six reference samples, spanning an extraordinary range from 3 to 3,600 mitochondrial DNA copies per microliter. The results were striking. Complete mitochondrial genome sequences were consistently recovered in 99 percent of samples across the dilution series at DNA input levels as low as 47 mtDNA copies, demonstrating remarkable robustness under low-template conditions. Full profiles were obtained in half of the samples containing only 23 copies, and even some samples at 12 copies produced partial results. Statistical modeling using log-log linear regression revealed that library yield scaled nearly proportionally with mtDNA input in the NIST series, with a doubling of input DNA producing an approximately 2.16-fold increase in library output. Read depth increased linearly up to roughly 560 to 1,235 copies per reaction before plateauing due to saturation, underscoring the importance of accurate library quantification and normalization.

The heart of the study involved 100 volunteers who each provided 20 shed hairs collected from their own clothing, with 98 of them also contributing buccal swab samples for comparison. Microscopic examination identified the club-shaped roots characteristic of telogen hairs, and one hair per donor was selected for analysis. Quantification revealed that 98 of the 100 hair extracts met or exceeded the manufacturer-recommended input of 3,000 mtDNA copies, and both small and large mtDNA quantification targets were detected in every single sample. In contrast, 25 hairs contained no detectable nuclear DNA at all, and most of the remainder showed concentrations below 6.8 picograms per microliter with extensive degradation. The researchers also found that reducing the post-extraction volume from 50 to 30 microliters significantly increased both mtDNA copy number and sequencing read depth, a simple procedural change that boosted sensitivity without introducing contamination. Notably, donor variables such as age, gender, hair dye use, and washing habits showed no statistically significant association with mtDNA quantity or quality.

Comparing the mitochondrial genomes sequenced from 91 paired hair and buccal samples revealed fascinating subtleties of mitochondrial biology. Initial sequence concordance between hairs and their corresponding buccal references was 93.4 percent. However, when the team applied the interpretation guidelines of the International Society for Forensic Genetics and excluded low-confidence discrepancies associated with high strand bias, heteroplasmic variants, and sequencing artifacts, concordance rose to a perfect 100 percent. This finding is critically important for forensic practice: apparent mismatches between a hair and a reference sample frequently reflect technical limitations and tissue-specific heteroplasmy rather than true genetic differences. Heteroplasmy, the coexistence of multiple mitochondrial variants within an individual, arises from bottleneck effects and drift during tissue development, and previous research has shown that it can drift notably in hair. According to ISFG guidelines, a single mismatch or heteroplasmic difference should never be interpreted as evidence of exclusion, and this study provides empirical weight for that principle in the context of whole mitochondrial genome sequencing.

Reproducibility testing further strengthened the case for implementation. The researchers amplified and sequenced duplicate libraries from 43 telogen hair samples and 46 buccal samples, finding over 99 percent concordance of the software-classified true variants between replicates. The handful of discrepancies involved shifts between point and length heteroplasmy classifications, insertion-length differences at homopolymeric C-tracts, and low-frequency artifacts at known hotspots, none of which affected haplotype interpretation. Haplogroup assignments remained identical across all replicates and dilution levels, confirming that reduced template input did not compromise phylogenetic classification. The data analysis itself was also highly repeatable, with two independent runs of the Converge analysis software producing identical results across every measured parameter, from variant frequencies and read depths to strand bias metrics and variant state classifications.

Contamination control, a perennial concern in low-copy-number DNA work, was rigorously assessed. Of 16 negative controls processed during the study, none produced a complete mitochondrial DNA sequence. Six controls showed low-level mtDNA signal consisting of only two to eight variants, predominantly common polymorphisms, and none of these partial profiles corresponded to any of the analyzed samples, the NIST control, or laboratory personnel. Several variants in the controls were traced to primer binding regions, indicating primer variants rather than true biological signal. All work was conducted in a dedicated laboratory following ISFG and SWGDAM guidelines under the ISO 17025 validation framework, and every extraction, quantification, and sequencing batch included negative controls, providing a template for laboratories considering adoption of the method.

Perhaps most compelling for investigators is the performance of the validated workflow on real evidence. The team sequenced 11 archived DNA extracts from hairs collected in 2010 from concluded criminal cases, samples that had previously been deemed unsuitable for STR profiling because they contained no detectable or less than 7 picograms per microliter of nuclear DNA. Remarkably, these aged forensic samples yielded complete mitochondrial genome sequences with an average read depth of 1,037-fold, an exceptionally high coverage figure. Across the three fully reported casework samples, 102 variants were identified, 98 of which were confirmed as true variants with high confidence scores, and no nuclear mitochondrial DNA segments, or NUMTs, were detected to confound the analysis. The success demonstrates that even DNA extracts stored for over a decade can return informative whole mitochondrial genomes, offering hope for cold case reviews.

The study is not without caveats that laboratories must respect. A few amplicons, particularly those covering mitochondrial regions 8622 to 8796, 1317 to 1491, and 1948 to 2111, consistently underperformed across sample types, mirroring poor performance reported in earlier evaluations of the panel. Length heteroplasmy artifacts at homopolymeric stretches such as positions 309 and 11038 appeared with variant strand bias exceeding 0.7, requiring careful filtering. At very low template levels, below roughly 47 copies, stochastic effects increased the frequency of unlikely and unclear variant calls. The authors emphasize that reliable results from low-copy samples demand increased read allocation, replicate sequencing, manufacturer-recommended run thresholds, and stringent filtering of negative controls and NUMTs. The laboratory will continue using an input of 3,000 mtDNA copies for casework despite evidence that lower inputs can suffice, retaining a conservative margin for evidentiary work.

Taken together, the findings position whole mitochondrial genome sequencing of telogen hairs as a validated, sensitive, and contamination-controlled method ready for routine forensic implementation. With more than 85 percent of hair evidence in some laboratories currently unanalyzable by STR profiling, the ability to recover complete mitochondrial genomes from shed hairs with high read depth and near-perfect concordance when properly interpreted represents a meaningful expansion of the forensic toolkit. As massively parallel sequencing continues its migration from research laboratories into accredited casework units, hairs that once sat silently in evidence storage may soon speak, narrowing gaps in investigations, strengthening maternal lineage identifications, and giving cold cases a second chance at resolution.

Subject of Research: Validation of whole mitochondrial genome sequencing of shed telogen hairs for forensic casework

Article Title: Precision ID mtDNA Whole Genome Panel and sequencing of telogen hairs – perspectives for validation and implementation in casework

Article References: Sharafi Farzad, M., Claessens, F., Petersen, C. B., Pereira, V., Morling, N., & Mogensen, H. S. (2026). Precision ID mtDNA Whole Genome Panel and sequencing of telogen hairs – perspectives for validation and implementation in casework. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-03992-x

Image Credits: AI Generated

DOI: 10.1007/s00414-026-03992-x

Keywords: mitochondrial DNA, telogen hairs, massively parallel sequencing, Precision ID mtDNA Whole Genome Panel, forensic casework, mtDNA heteroplasmy, low-template DNA, STR profiling, profile concordance, contamination control, ISFG guidelines, hair shaft evidence