Packaging, Storage and the Cold Chain for Biological Evidence

DNA degradation in biological evidence follows two principal pathways: biological and chemical. Understanding both helps explain every packaging and storage rule that follows.

The biological pathway is driven by microorganisms. Bacteria are present on all environmental surfaces and on skin. When biological material is deposited at a scene, microbial colonisation begins immediately. At room temperature and in the presence of moisture, bacterial populations can double in 20 to 30 minutes. These organisms release extracellular DNases that fragment nuclear and mitochondrial DNA as a byproduct of feeding on cellular contents. Mould follows within hours to days, particularly in warm and humid conditions. Once microbial activity is established, it continues inside sealed packaging unless the item is sufficiently dry to deny the water activity microorganisms require, or unless temperature is low enough to halt their metabolism.

The chemical pathway operates even without microorganisms. Hydrolytic depurination is the most important abiotic process: water molecules attack the glycosidic bonds between deoxyribose sugars and purine bases (adenine and guanine), releasing the base and leaving an abasic site that makes the strand susceptible to strand breakage. Oxidative damage from reactive oxygen species, which are generated by UV exposure, heat, and metal ion catalysis, produces strand breaks and base modifications. Both processes are accelerated by moisture and heat, and both are slowed by drying and low temperature.

The net result of both pathways is DNA fragmentation: originally intact high-molecular-weight DNA (chromosomal DNA runs to hundreds of kilobases) is progressively broken into shorter and shorter fragments. Standard STR profiling amplifies target regions of 100 to 400 base pairs. As degradation progresses, the larger STR amplicons drop out first, producing partial profiles in which fewer and fewer loci return results. When fragments are predominantly below 100 base pairs, conventional STR profiling fails entirely, though mini-STR kits and next-generation sequencing methods can still recover information from shorter fragments.

Drying is the single most important step in preserving biological evidence collected at a scene. Most biological material encountered in casework, bloodstains, semen on fabric, saliva on cigarette ends, touch DNA on surfaces, is either already dry or can be dried before packaging. The target is to reduce the water activity of the biological material below the threshold that supports microbial growth, approximately 0.6 water activity, which corresponds roughly to a moisture content below 15 percent by mass.

At the scene or at the exhibit processing room, items are spread individually on clean paper or hung on a drying rail in a ventilated area at room temperature (15 to 22 degrees Celsius). Forced-air drying with a fan is acceptable provided the air is clean and uncontaminated. Direct heat, including hairdryers on a hot setting, is not used: temperatures above 37 degrees Celsius accelerate the very chemical degradation reactions the process is meant to prevent. UV exposure from sunlight should also be avoided for the same reason. Drying times vary: a thin bloodstain on cotton dries in 30 to 60 minutes; a heavily soaked garment may need several hours.

Items that cannot be dried before packaging present a particular challenge. Weapons with wet blood, items too large or fragile to dry safely, or items collected in conditions where drying facilities are not available must be packaged wet with explicit documentation and transferred to a processing facility as rapidly as possible, under refrigeration. The packaging note must state that the item is wet, the time of collection, and the ambient conditions. At the receiving facility, the item is dried before being repackaged for storage.

Swabs are among the most common biological evidence collection devices, and swab drying deserves specific attention. A swab used to collect a moist biological sample should be allowed to air-dry in its swab box (open, not sealed) for at least 30 minutes before the box is closed. Many collection kits include a dedicated drying tube or card for this purpose. Sealing a wet swab in a closed container, even temporarily, is the single most common packaging error in scene examination and the most frequent cause of avoidable DNA degradation in casework.

The choice between paper and plastic is not arbitrary. It follows directly from the physics of moisture and gas permeability.

Paper bags, paper envelopes, and cardboard boxes are the standard packaging for dried biological evidence. Exhibit labels are affixed to the outside, and the seal is signed and dated. Tamper-evident tape is used across the opening seam. The bag itself must not be stapled: staples puncture the bag and reduce structural integrity, and may introduce metal contamination relevant to trace evidence examinations.

Plastic bags have a legitimate but limited role. For wet items that must be transported before drying can occur, an inner paper layer can absorb surface moisture while an outer plastic bag contains any liquid, provided the transit time is short (hours, not days) and the items are kept refrigerated. For liquid samples such as whole blood in tubes, or urine samples, sealed airtight containers are correct because the liquid is the sample itself. For items that have been thoroughly dried and are being moved as an outer protective layer only, plastic is acceptable, but the inner packaging must still be paper.

Temperature requirements differ by evidence type and custody stage. The following applies to the three main stages: at scene, in transit, and in laboratory storage.

At the scene, the priority is drying and protection from environmental contamination. Exhibits should not be left in direct sunlight or in vehicles on hot days (vehicle interiors can reach 60 to 70 degrees Celsius in summer). Scene examination must proceed under shaded conditions where possible, and exhibits collected early in a long scene examination should be taken to a shaded, cool collection point rather than left in the sun while the examination continues.

In transit, dried exhibits can be transported at controlled room temperature provided the journey is short (same day). For overnight or multi-day transit, refrigeration at 2 to 8 degrees Celsius is required. Frozen samples must travel in insulated containers with dry ice (carbon dioxide solid, sublimation temperature minus 78.5 degrees Celsius) or in a dedicated freezer transport unit. The temperature of the transport environment should be continuously monitored with a data logger, not just checked at origin and destination, because temperature excursions during transit may not be apparent from end-point readings alone.

In the laboratory, three storage tiers apply to biological evidence. Dried exhibits in paper packaging are held at controlled room temperature (15 to 22 degrees Celsius) in a secure, climate-controlled storeroom. Exhibits with body fluids, touch DNA, or incomplete drying are refrigerated at 2 to 8 degrees Celsius. Liquid samples, extracted DNA, and long-term archival storage of high-value items use freezer storage at minus 20 degrees Celsius (standard freezer) or minus 80 degrees Celsius (ultra-low freezer for reference samples and extracted DNA). Laboratories must calibrate and continuously monitor storage temperature, with alarms set to alert staff when temperature moves outside the acceptable range.

A cold-chain break is any period during which refrigerated or frozen evidence warms beyond its required temperature range. The biological consequences depend on three variables: the temperature reached during the break, the duration of the break, and the state of the evidence (wet or dry, sealed or open).

For dried exhibits, a short temperature excursion (a few hours at room temperature) during transit typically has minimal biological effect if the evidence was thoroughly dried before packaging. The greater hazard is condensation: when a cold package warms, moisture condenses on the item inside if the package is opened or if the packaging is permeable. This condensation reintroduces water and restores the conditions for microbial activity. For this reason, refrigerated or frozen items must never be opened until they have fully equilibrated to the ambient temperature in a dry environment, and that equilibration must be documented.

For liquid samples or incompletely dried exhibits, even short cold-chain breaks can cause significant degradation. A frozen blood tube that thaws to room temperature for 24 hours before being re-frozen will show measurable DNA fragmentation on analysis. Each freeze-thaw cycle stresses cell membranes and creates ice crystals that physically shear DNA strands. The number of freeze-thaw cycles an item has undergone is therefore an important variable in interpreting analytical results from frozen samples.

The legal consequences of cold-chain failures extend beyond the scientific. In adversarial legal systems in the US, UK, and India, the defence is entitled to examine chain-of-custody documentation, including temperature logs, transfer records, and any documented excursions. A cold-chain break that is undocumented is more damaging to the prosecution than one that is documented and explained, because it suggests either a gap in record-keeping or active concealment. Courts in the UK have excluded DNA evidence where the chain of custody was inadequately documented, even where the analytical result was plausible. The principle that chain of custody must be continuous and documented is common to the Federal Rules of Evidence in the US, the Bharatiya Sakshya Adhiniyam 2023 in India, and the PACE Act 1984 and Criminal Procedure Rules in England and Wales.

Every exhibit of biological evidence requires a label that is affixed at the point of collection and accompanies the item throughout the chain of custody. The label must include: a unique exhibit reference number, a brief description of the item and the biological material present or suspected, the date, time, and location of collection, the name and signature of the collecting officer or examiner, and the condition of the item at collection (dry, wet, frozen).

At each transfer point, the receiving party signs the chain-of-custody form, records the time and date of receipt, and documents the condition of the item on arrival. Any deviation from expected condition, a seal that appears breached, packaging that is damp, or a temperature log showing an excursion, is recorded explicitly. Omitting these records does not make the problem disappear; it makes it impossible to explain at trial.

Laboratory accreditation under ISO/IEC 17025:2017, required of forensic DNA laboratories in the UK (by the Forensic Science Regulator), the US (under the DNA Identification Act requirements enforced via NDIS/CODIS participation), and most EU member states, mandates written procedures for evidence intake, storage conditions, temperature monitoring calibration, and handling of non-conformances including cold-chain breaks. Laboratories that are accredited must demonstrate compliance in audits: a temperature log gap, a missing intake signature, or an undocumented freeze-thaw event is a non-conformance that must be reported and corrected.

Digital temperature monitoring systems, which log temperature at defined intervals (typically every 15 to 30 minutes) and transmit alerts to designated staff when thresholds are exceeded, have become standard in accredited laboratories. These systems produce an auditable record that is far more reliable than manual temperature logs. A data logger accompanying a transit shipment produces the same function for the transport stage. The output of these systems is part of the evidence record for each stored item.