The Big Question: Will body scanners in airports reduce the threat from terrorism?

(PA )

Why are we asking this now?

Gordon Brown has said that the "gradual" introduction of airport scanners across British airports is a necessary response to "the emerging terrorist threat" from Yemen and Somalia. They will be used at first alongside metal detectors. Hand- luggage checks will be stepped up too, to search for traces of explosives.

Umar Farouk Abdulmutallab, a 23-year-old Nigerian educated in London, and thought to have been radicalised in Yemen, was foiled in his attempt to blow up a flight to Detroit on Christmas Day. Scanners are described by the Prime Minister as "essential". They are being trialled at Manchester airport, following tests at Heathrow from 2004 to 2008. BAA, the airports operator, has said they will be introduced as soon as is practicable.

Would they have stopped the Christmas bomber?

Abdulmutallab allegedly concealed a package containing nearly 3oz (80g) of the chemical powder PETN (pentaerythritol tetranitrate) in his underpants. He is also alleged to have carried a syringe containing a liquid accelerant, intended to detonate the explosive. He botched the detonation, and was left only with severe burns.

It is impossible to state conclusively if his intentions, and his package, would have been picked up by airport scanners, were they in use at Amsterdam's Schiphol airport (where he boarded). But early indications are that "passive millimetre scanners" would not have picked up the substance Abdulmutallab was carrying, thought to be favoured by al-Qa'ida operatives at present.

Ben Wallace, the Tory MP who used to work at QinetiQ, one of the companies making the technology, said: "It probably wouldn't have picked up the very large plot with liquids in 2006 at Heathrow or indeed... bombs that were used on the Tubes."

So are they ineffective?

Scanners can certainly pick up metal objects including knives, but whether they could have detected powder plastic explosive such as the 3oz of PETN is extremely doubtful. The kind of explosive Abdulmutallab used was low-density and so probably wouldn't have shown up on the scanner.

This weekend, The Independent on Sunday reported that officials at the Department for Transport and the Home Office have already tested the scanners and were not persuaded that they would work comprehensively against terrorist threats to aviation. Wallace added, "scanners cannot provide a comprehensive solution on their own", and are not "a big silver bullet". On Sunday, the Prime Minister accepted there was no way of being certain the devices were 100 per cent effective, and said: "We have got to go further".

How do they work?

Essentially, the scanners use a kind of radar. The emitted millimetre waves are reflected back from the body, so that the reflected waves create a 3D image. The scanner detects high-density objects such as knives, guns and dense plastics such as C4 explosive, but the waves pass through low-density objects such as clothing or organic material, so these appear translucent.

In this way the scanner creates a real-time, "naked" image, revealing concealed weapons without the need for invasive and ineffective "pat-downs".

Are there different types?

There are two main types of scanner – passive millimetre scanners and X-ray scanners. X-ray scanners generate a D image which is sharper than the 3D images generated by passive millimetre scanners, although passengers must be scanned from different angles.

Passive millimetre scanners are potentially safer and easier to use than X-rays, which require a passenger's permission and cannot be used on pregnant women. There are concerns about the health risks caused by the frequent use of X-ray machines, which have been investigated but on which there is no consensus.

What are the objections?

Those who oppose the introduction of airport scanners have two main complaints. First, that they lead to lengthy delays (some of which may themselves add to security problems, causing large crowds to develop in concentrated areas). Most scanners will process between two and three people a minute. Critics have pointed out that this is approximately how long a conventional frisk takes too.

Second, the scanners themselves are ineffective. This latter point is a subject of contention (see above), but in all instances those who object to scanners say that their benefits are at best speculative and do not, in any case, justify the practical and financial cost.

How much do they cost?

Depending on the type of scanner, the manufacturer, and the quantity purchased, scanners will cost between £80,000 and £100,000. That's much more than the £5,000 of a standard metal detector. Across Britain's major international airports, this could lead to a cost of tens of millions of pounds from Treasury coffers (costs include the hiring of extra staff to operate them). Proponents say the first job of government is to protect its citizens, and the fight against terrorism justifies these costs. BAA declined to put a specific cost or time-scale on the move to scanners.

Who stands to gain commercially?

Manufacturers of scanners will gain; the airline industry, and those companies who make a profit through travellers flying, are likely to lose from anything that makes boarding a plane more arduous. Scanners, given their intrusive nature and time-consuming operation, will certainly make travelling less easy.

One indication of the potential for commercial gain is the jump in shares for airport security firms yesterday. Smiths Group, the world's biggest manufacturer of airport detection devices, rose 3.7 per cent, up 38p to a 16-month high of 1052p. America's OSI Systems, parent company to scanner manufacturer Rapiscan Systems, has seen shares go up by 24 per cent since the foiling of the Christmas plot.

Do our American allies recommend it?

American security agencies are rolling out the scanners across the US, with 40 different machines so far being used across 19 airports.

But support from across the Atlantic is far from unanimous. Anthony Fainberg, former director of science at the Department of Homeland Security, argues that swab tests on airline passengers are easier, cheaper, and more effective than full body scanners. Such technology, which can also harness the benefits of sniffer dogs and hand-held vapour detectives, are already in use in many countries. Larry Johnson, former deputy director of counter-terrorism at the State Department, is another influential figure who says swabs are a better option than scanners.

What about my privacy?

There is no way round it: scanners are very intrusive, and members of some religions in particular may consider them an unacceptable affront to deeply held views. There are gender sensitivities to consider: it's conceivable, for example, that some people may insist on being seen only by members of the same sex. Simon Davies, director of Privacy International, said the emphasis should be on "ordinary and quite boring measures that actually do work".

Are body scanners more effective than alternative detection methods?

Yes

*We know they do pick up high-density objects such as the metals in guns and knives

*They are already in use in several countries, including the US, who have not seen fit to abandon them

*They could complement, rather than substitute, other measures such as swabbing

Yes

*They may not pick up low-density objects, such as the explosive carried by the Christmas bomber

*They are very expensive; that money should be used on techniques that work better

*Inevitable lengthy delays and intrusion into privacy may end up doing more harm than good