Concerns have been raised about devices in shops, banks and offices, designed to deter armed robbers by emitting a cloud of blinding fog. How do they work?
A fogging device used during a bank raid in 2007: courtesy Smoke Cloak
From watching a string of action movies, many of us are familiar with what protects banks - cameras, silent alarms, automatic shutters slamming down and exploding capsules of ink in wads of money.
But the use of fogging devices is now an established tactic to protect not just banks, but a host of other businesses.
"It might be an ordinary house where you have somebody with a lot of valuables," says Carl Gibbard, managing director of Concept Smoke Screen, "convenience stores, filling stations, jewellers, large factories, warehouses with items that stand a chance of being stolen where the thieves are relying on a smash and grab."
These devices, typically linked to an alarm or a panic button, consist of a well of either glycol or glycerine mixed with water and a heater. In a split second, the liquid is pumped over the heater and out where it condenses on contact with cold air.
The machines are linked to an alarm or panic button
They use a glycol and water mix and a heater to make fog
They can pump out 1,000 cubic metres in 30 seconds
Sold in UK since early 1990s
The intention is to disorientate the robber or burglar and cause them to abandon their efforts and flee.
"It's the same principle as a kettle and steam," says Mr Gibbard. A device might pump anything between 300 and 1,000 cubic metres of smoke into a room within 30 seconds.
Jewellers Goldsmiths and Watches of Switzerland have stopped seven attempted raids using the devices, Mr Gibbard says, and Boots and Tesco also have them installed.
Rival fogging device firm Smoke Cloak lists Barclays, Argos, Carphone Warehouse, PC World, and 150 schools among its clients, says managing director Paul Dards.
"Originally, they were mostly used in empty buildings for overnight protection, but as crime has become more violent, more places want to use them to protect their staff during working hours."
But there are those who are slightly concerned about the idea of an already edgy armed robber being suddenly wreathed in smoke.
"Armed robbery is a high intensity situation," says Roy Ramm, former head of the Metropolitan Police's Specialist Operations, as well as the Flying Squad. "What you really want is calm and for people not to be panicked. Most robbers are very susceptible [to panic]."
While fatal shootings during armed robberies are rare in the UK, many robbers who have shot people tend to reveal afterwards that they had not intended to fire, says Mr Ramm.
"In a situation where a room suddenly fills with fog, it gives somebody another reason to panic. You could quite easily end up with someone being shot."
Criminologist Prof Roger Matthews, of London Southbank University, author of Armed Robbery, concurs.
"It sounds a little bit like a double-edged strategy. The last thing you want to do is panic and confuse an armed robber. The danger is that people start doing strange and undesirable things like shooting people."
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But the smoke screen manufacturers dismiss criticism.
"When we first introduced this product we got a knee jerk reaction, understandably, from people who were worried," says Mr Dards. "We actually took out a £10m worldwide insurance policy to protect people in case anything bad happened, but we've never had to touch it. We haven't had any incidents.
"The idea is not to trap people, but when the bad guy breaks into the buildings a wall of fog comes at him and drives him out."
Of course even if you accept that the fogging devices are safe it's not easy to assess just how effective they are at stopping crimes.
Many will be installed in businesses that already have excellent security precautions, says Prof Matthews.
"A lot of the places that use these kinds of devices are the better organised institutions, which are already very well protected."
And in the case of high value targets like prestige jewellers a different kind of thief is in operation.
"At that end of the market, you are not talking about amateur people who thought about doing it in the morning and went in the afternoon. They will know there is a smoke device and they will find some way to deal with that."
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