Contaminated From The Start: Why Porous Materials Have No Place in Detection Dog Training

Blog 8, Contaminated From The Start: Why Porous Materials Have No Place in Detection Dog Training 

When it comes to detection dog training, the devil is in the detail — and sometimes those details are literally in the walls, boxes, and blocks you train with.

Too often, I see trainers using porous materials such as wood or concrete in their training environments. These materials absorb and retain odours, creating misleading scent pictures and contaminating the training environment from the start. This isn’t just a technicality — it fundamentally undermines the dog’s understanding of the target odour and can introduce serious long-term problems.

Why Porosity Matters

Porous materials trap odour molecules. Unlike non-porous surfaces (such as stainless steel or glass), porous materials don’t just carry the odour you put on them today — they hold on to yesterday’s, last week’s, and last year’s odours as well.

For the dog, this means they are no longer learning to discriminate the true target odour. Instead, they are responding to a cocktail of residual scents embedded in the material, which creates confusion, weakens indication reliability, and can even lead to false alerts in the field.

The Problem with ‘Cheap and Convenient’

One of the most common examples I encounter are wooden ammunition boxes used for training hides. Yes, they’re sturdy and easy to get hold of — but they’re also a nightmare for contamination. Wood absorbs oils, vapours, and even moisture from the environment, locking in odour and becoming virtually impossible to clean completely.

I often see some countries using wooden ammunition boxes during detection dog competitions. If I were taking part in a competition, I would not want wooden ammunition boxes to feature at all — they can create unfair conditions, with residual odour trapped in the wood confusing the dog and undermining the fairness of the test.

Even more concerning, we’re missing something fundamental about these boxes: many of these redundant ammunition boxes have actually contained ammunition, sometimes for years. They’ve been stored in warehouses, stacked with other ammunition-filled boxes, absorbing that odour day after day, month after month, even year after year. And yet, here we are — using those same boxes to train dogs to detect drugs or cash, dogs we specifically don’t want to detect ammunition.

Why are we doing this?

If these boxes are saturated with ammunition odour from years of use and storage, then we are introducing exactly the kind of contamination and conflicting scent picture we’re supposed to avoid. It’s illogical and undermines the very goal of reliable detection.

I’ve also seen concrete blocks being used as hide locations or focus walls. Concrete, with its porous surface and micro-cracks, traps odour over time. It might seem durable and practical, but it becomes a permanent source of confusing, residual scent.

Worryingly, I have even seen some UK police services using concrete blocks during the imprinting phase for new detection dogs, which I regard as a major flaw. Imprinting is the foundation of everything a detection dog will go on to do — and embedding residual odour into porous concrete at this critical stage risks introducing confusion and unreliability from day one.

Once, a police instructor told me it was fine because “the target odour was within a glass jar inside the concrete block.” Well — no, it isn’t fine. How do you stop the dog from sniffing the concrete block itself? How do you make sure the dog is focusing only on the contents of the jar, not the residual odour that’s already embedded in the concrete? You can’t. The dog is sniffing the whole picture — jar and block — and you’re teaching it a confused, unreliable association.

What about Luggage?

This same issue arises when training detection dogs on luggage for airport or border work. I always make sure I have plenty of luggage available — because in most cases, I will only use a bag or case once, to avoid contamination problems.

Like wood and concrete, luggage is porous and absorbs odour. The number of times I’ve arrived to train detection dogs for customs and seen the same suitcase or piece of luggage being used — over and over — is staggering. Sometimes it’s the same bag being used for weeks, even months.

That suitcase you’ve packed cigarettes into for months? It has absorbed all of that odour. So what is the dog actually detecting now — the contraband or the suitcase?

If the dog learns to associate that particular suitcase, with its embedded scent, as the target, you’re not training them to detect contraband at all — you’re training them to find that bag.

The Real-World Consequences

When dogs are exposed to contaminated training aids or environments, they start to generalise incorrectly. This can lead to them alerting in operational settings on surfaces or materials that have nothing to do with the target odour — simply because that’s what they learned in training.

If your dog is hitting on the scent of last month’s hide because it’s still embedded in the wood, concrete, or luggage, you’re not training them to find contraband — you’re training them to fail.

The Right Way

The solution is simple but non-negotiable: use non-porous materials.

Stainless steel, glass, ceramic, and certain high-grade plastics are ideal. These materials can be fully cleaned and sanitised, ensuring that the only odour present during training is the one you’ve deliberately placed there.

And when it comes to luggage, rotate and refresh your bags regularly — don’t let a single piece of luggage become the target.

Final Thoughts

Detection dog training is about precision. You can’t expect precise results if you’re building that training on contaminated, inconsistent materials. If you respect the work your dog is doing, respect the environment you’re asking them to work in.

It’s easy to blame the dog when indications go wrong, but often the fault lies in how — and where — we’ve trained them.

People often make fun of me because of how seriously I treat contamination and cross- contamination in detection dog work. But I stand by it — a few years ago, I even invested in a medical-grade autoclave specifically to sterilise all the stainless steel and glass items I use for detection dog training and imprinting. I stopped using porous materials many years ago because I’ve seen the confusion and failure they can cause.

A lot of what I learned about contamination came during the whisky dog project. We were training dogs to find odour — and trace odour — as low as parts per trillion, and one of the odours we were working with would contaminate anything it touched, even for just a second or two. Those lessons were hard-earned and incredibly valuable. I’ve carried them into all of my detection dog work since — with drugs, cash, tobacco, and beyond — and they’ve made me a better trainer and handler.

And whilst we’re on this subject, here’s a question — perhaps for another blog:

Cannabis. Why are people still storing their cannabis training samples in the same Peli case as their other drug samples, like MDMA and cocaine?

I know they’re all in glass jars — but cannabis is one of the worst odours for contaminating everything. Now you’ve got a cocaine jar that smells of cannabis. What are you really training your dog to find?

Start clean. Stay clean. And leave porous materials — and sloppy storage habits — where they belong: out of your training environment.