Plastic bags have become a ubiquitous part of modern life. We use them for everything from carrying home groceries to storing items around the house. One question that sometimes comes up about plastic bags is whether they can retain fingerprints from those who have touched them. In this article, we’ll take an in-depth look at whether plastic bags can hold onto fingerprints and for how long.
Can plastic bags retain fingerprints?
The short answer is yes, plastic bags can retain fingerprints from those who have handled them. How well and for how long depends on several factors.
Plastic grocery and shopping bags are made from materials like polyethylene and polypropylene. These smooth surfaces can capture fingerprint ridge detail when a finger comes into contact with them. However, plastic bags are not ideal for recording usable fingerprints compared to surfaces like glass.
The plastic material itself is flexible and lacks the hard, stable surface needed to cleanly capture all the ridges and details of a fingerprint. So fingerprints left on plastic bags will likely be smudged or distorted to some degree.
Factors that affect fingerprint retention
There are a few key factors that come into play in determining how well a plastic bag will retain fingerprints:
- Surface texture – Smoother plastics like polyethylene bags hold fingerprints better than textured bags.
- Pressure applied – More pressure when touching creates a better fingerprint imprint.
- Residue/moisture – Sweat, oils or water on fingers leaves better prints.
- Time – Plastic bags hold prints best right after touching but deteriorate over time.
Can fingerprints on plastic bags be used as forensic evidence?
fingerprints on plastic bags are generally not a preferred source of fingerprints for forensic analysis. The overall quality is often too low for identification purposes.
That said, fingerprints on plastic bags have been used as supporting forensic evidence in some cases. If the print is of a clear enough quality, forensic analysts may be able to find enough level 2 and 3 detail (like ridges and minutiae points) to provide association with a suspect. But this is dependent on the print quality.
How long do fingerprints last on plastic bags?
Fingerprints left on plastic bags will persist for some amount of time but will eventually deteriorate to the point where they can no longer be visualized or used for identification.
The length of time a discernible fingerprint imprint will remain on a plastic bag depends on factors like:
- Type of plastic – Some polymers hold prints better.
- Weather conditions – Heat, humidity, sunlight fade prints.
- Storage conditions – Prints last longer when bags are stored in cool, dark places.
- Handling – Prints deteriorate faster with more handling.
- Cleaning – Washing/wiping bags destroys existing prints.
Typical plastic bag fingerprint duration
On average, visible fingerprints tend to last on plastic shopping and grocery bags for:
- 1-2 days if left out in the open air
- 3-4 days if kept in a drawer or other dark place
- 1-2 weeks if kept in a cool, dark place like a refrigerator
Plastic bags meant for long-term storage may retain fingerprints a bit longer than disposable grocery bags. But even on these types of bags, fingerprints generally start to fade and become difficult to visualize under a forensic light after a month or two.
Can old fingerprints on plastic be revealed?
Even if a fingerprint is no longer visible on a plastic bag, it may still be possible to optically enhance it using forensic techniques.
Chemical developers like cyanoacrylate or ninhydrin can be applied to react with fingerprint residue and recreate a visible print. But fingerprints already degraded due to environmental exposure may be unrecoverable.
Case study examples
Here are some real world examples of plastic bags being checked for fingerprints as forensic evidence in criminal cases:
Grocery bag in murder case
In 2015, Canadian man Dustin Paxton was convicted of murdering his roommate. A key piece of evidence was a fingerprint matching Paxton’s found on a grocery bag used to dispose of the victim’s body. The print was believed to have been left 1-2 days before discovery.
Freezer bag prints lead to conviction
In another murder case from 2010, a clear fingerprint was recovered from a freezer bag containing the dismembered remains of the victim. The print matched the defendant, Keith A. Hunter, leading to his conviction.
Lunch bag fingerprint in jail sodomization
In 2017, an inmate in Clark County, Washington was charged with assault after his fingerprint was found on a lunch bag left at the scene of a rape and assault of another prisoner. The lunch bag print was a key linking the suspect to the crime.
In each of these cases, the plastic bags retained identifiable fingerprints long enough for forensic teams to recover, analyze and match them to suspects. But for bags left out in the open, prints likely would have degraded within days.
Techniques for recovering plastic bag fingerprints
Police forensics teams have a few techniques they can use to try to raise fingerprints on plastic bags as usable evidence:
Visual examination
The bag is carefully examined under bright light to look for any visible fingerprint impressions. Angling the bag and light can reveal prints.
Fluorescence
Under ultraviolet light, fingerprint residue may fluoresce and reveal prints not visible under white light.
Chemical development
Chemicals like cyanoacrylate, ninhydrin or DFO can be applied to react with fingerprint material and develop a visible print.
Vacuum metal deposition
In VMD, a thin layer of metal like zinc or gold is deposited onto the bag. This can visualize the ridge detail in fingerprints.
Forensic image scanning
Advanced systems like GelSight can digitally scan the surface to create a 3D image, revealing the shape of any ridge detail present.
Factors that affect plastic bag fingerprint recovery
Whether usable fingerprints can be recovered from a plastic bag depends on several factors:
- Fingerprint material quality – More sweat, oil gives better prints.
- Time elapsed – Longer times mean more degradation.
- Storage conditions – Heat and sunlight are bad for prints.
- Handling and use – The more the bag is disturbed, the worse for prints.
- Cleaning – Wiping or washing removes prints.
- Bag texture – Smoother plastics capture better prints.
Ideally, a bag should be set aside as evidence as soon as possible, minimizing handling and exposure to environmental conditions. The sooner forensics teams can attempt print recovery, the better.
Fingerprint persistence comparison by material
While plastic bags can retain fingerprints, they are far from ideal surfaces for preserving prints compared to other common materials.
Here is how plastic bags compare to some other common surfaces in terms of fingerprint persistence and recoverability:
| Surface | Persistence | Recoverability |
|---|---|---|
| Glass | High | High |
| Metal | High | High |
| Wood | Medium | Medium |
| Paper | Low-Medium | Low |
| Plastic bags | Low | Low |
As you can see, plastic bags are towards the bottom when it comes to persistently retaining identifiable fingerprint impressions compared to glass, metals and other common surfaces.
Why plastic bags are poor for fingerprints
There are a few reasons why plastics like polyethylene perform poorly for retaining fingerprint evidence:
- Smooth, non-porous surface makes it hard for prints to adhere
- Pliable and flexible material distorts ridge impressions
- Lacks hard backing needed to maintain print integrity
- Thin plastic degrades and fades prints faster
- Plastic additives over time degrade latent print residue
Together, these traits of plastic bags limit both the initial quality and lifespan of fingerprints left on them compared to more ideal forensic evidence surfaces.
Improving fingerprint capture on plastic bags
While plastic bags are not a prime surface for capturing identifiable fingerprints, there are a few things investigators can do to try to improve quality:
- Use smooth plastic – Textured plastic distorts prints more.
- Apply pressure when handling – Light touches leave less residue.
- Ensure fingertips are oily – Washed/wiped hands give weak prints.
- Store cool and dark – Slows degradation of latent prints.
- Avoid handling excessively – Each disturbance weakens prints.
- Process quickly – Chemical and optical methods work best on fresh prints.
Optimizing these conditions gives forensic teams the best chance of pulling usable prints from even less than ideal surfaces like plastic bags.
The future of plastic fingerprint evidence
While plastic bags today remain a lower-quality surface for fingerprints, future advances may improve their evidence potential in a few ways:
- New plastic polymers better capture fingerprint residues
- Stabilizer additives to slow fingerprint degradation
- Nanoparticle coatings to enhance prints on plastics
- Better visualization techniques to recover aged prints
- Machine learning for matching distorted plastic prints
With continued research into fingerprint persistence, capture and recovery methods, plastics could someday become a more reliable source of identity evidence, even if prints are distorted or degraded.
Conclusion
Plastic bags can retain fingerprints to some degree. But their smooth, flexible surface makes prints likely to be distorted and smudged compared to more ideal fingerprint evidence surfaces like glass. A variety of factors from material to environmental conditions determine if a print on a plastic bag will last long enough and maintain enough ridge detail for forensic identification. While not the strongest fingerprint medium, plastic bags have provided supporting evidence in some cases when handled and processed soon after touching. Continued advances in forensic techniques and materials science may someday improve plastic bags for capturing usable fingerprint evidence.
