As Australian airports roll out full-body scanners, our science columnist looks at the scanners already in use in the US.
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Last time I travelled with my family to the United States, my biggest worries were whether the children would sleep on the plane and whether Baby On the Move would deliver the car seats and portacots I had ordered. In planning our next trip, I’ve found myself worrying about America’s airport body scanners. Should I expose my children to unnecessary radiation to reassure the Transportation Security Administration (TSA) that they’re not carrying weapons or explosives onto the plane?
The TSA, which introduced the body scanners in 2007, says they are designed to detect “a wide range of threats to transportation security” and present “no health or safety concern for any passenger, including children and pregnant women”. It boasts that 99% of passengers walk through the body scanners rather than opt for the alternative of a thorough pat-down by TSA staff. Given the choice, I would rather avoid any unnecessary x-rays, but do I really want to subject my children – and myself – to an invasive pat-down?
So how much radiation do the body scanners deliver? The TSA uses two types: a millimetre wave unit that bounces electromagnetic waves off the body, and a backscatter unit that reflects low-energy x-rays off the body.
The millimetre wave scanner does not use ionising radiation – the sort of radiation known to cause DNA mutations that can lead to cancer – but the backscatter unit does, though at very low doses. A diagnostic x-ray can provide radiation doses from a few millisieverts (mSv) to less than 0.1mSv; the x-ray body scanners give a radiation dose of just 0.00001mSv per scan, which the TSA points out is about the equivalent of spending a few minutes in an aeroplane at cruising altitude.
But whereas medical x-rays project through the body to create a picture generated by the different levels of absorption of different tissue types, the body scanners bounce x-rays off the body to create a picture of the body surface and anything on it. In April 2010, a group of University of California scientists and physicians, in a letter to President Barack Obama’s science and technology assistant, pointed out that the x-ray scanners, by depositing all their energy into the skin and immediately adjacent tissue, present “potential serious health risks … While the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high.”
The US Food and Drug Administration, along with many other scientists, challenged these claims, saying there is only a “minuscule risk” to the people being scanned. Other doctors, including the chief medical officer of the American Cancer Society, have raised concerns about whether the machines are being appropriately calibrated and inspected. But although the US has continued to roll out more x-ray body scanners – there are now more than 500 scanners, of both designs, in American airports – the European Commission banned x-ray body scanners in airports in November, opting for other scanning technologies instead.
To put the radiation dose in perspective, more than three-quarters of New Zealanders’ average annual radiation exposure comes from background radiation – from the sun, from cosmic radiation and from radioactive elements in the soil and rocks. Most of our additional exposure comes from medical procedures such as x-rays and nuclear medicine, with a very small contribution from bomb fallout, air travel, consumer goods, such as luminous dials, and occupational exposure.
The dose from airport body scanners, even if they’re used frequently, doesn’t even register. Tony Cotterill, a senior medical physicist from New Zealand’s Institute of Environmental Science and Research, says there are “no significant radiation health risks posed by backscatter airport body scanners when they are properly maintained and operated”.
But in choosing to have medical radiation, it’s a matter of weighing up any small risk that the radiation might pose against the risk of not having the treatment. “Both diagnostic and therapeutic ionising radiation doses to individual patients are required to be justified – that is, to do more good than harm,” says Cotterill. However, there is no medical benefit to an airport body scan; rather, the scanners are promoted as having a societal benefit. But is there really a benefit? It has been suggested the most effective aircraft safety measure since 9/11 is locking cockpit doors; the body scanners have not led to the apprehension of potential terrorists.
Last year, New Zealand Customs tried out millimetre wave scanners at Auckland Airport, but under the Radiation Protection Act, any person using ionising radiation, such as a full-body x-ray backscatter scanning unit, must be licensed for that use by the Ministry of Health. “The first application for a licence for one of these scanners would initiate an assessment with the purpose of identifying specific radiation issues and safety conditions,” says Cotterill. No applications have yet been received, says the ministry.
Although there’s probably no health risk from airport body scanners, things that have been considered safe in the past are no longer so. My mother says one of the highlights of a trip to town with her own mother was getting to play with the shoe-shop x-ray machine – the last machines were removed in 1969. We know a lot more about the health effects of x-rays now than we did in the 1950s and 60s, but as long as there is uncertainty and debate over the safety of x-ray body scanners, I’m opting for the pat-down.
WORLDLY WHYS
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Question: When my cat sees a bird, it emits a distinctive miaowing noise that has a vibrato quality. It’s as if the cat is trying to chirp (to fool the bird into thinking it has nothing to fear). Yet to my ear it sounds nothing like a bird. How or why has evolution saddled cats with this reaction? It seems as if nature has “put a bell round the cat’s neck” to inhibit the cat’s chances of making a kill. If this is the case, nature seems to be working back to front in establishing a behaviour that reduces the chance of survival. Or is it that birds are less critical of a cat’s attempt at making conversation with them? – Richard Matthews, Napier
Answer: I asked Professor Kevin Stafford, of Massey University’s Institute of Veterinary, Animal and Biomedical Sciences, why cats chirp at birds. “Domestic cats are quite vocal and use a wide range of calls. Some cats, when watching birds they cannot get at, pull back the corners of their mouths and make a chattering noise with their teeth. We don’t know why they do this but it is probably frustration or excitement. Some scientists call it a displacement activity, and this particular chattering noise is probably only made under those circumstances. The chirruping mentioned by your reader is probably also a result of frustration, but cats chirrup as a greeting to humans and to one another.
“We don’t know why a cat would chirrup or chatter at a bird but it is unlikely to assist with predation, although some people suggest it is a decoy sound to attract birds. We also don’t know if cats that chatter or chirrup are more or less successful predators than those that remain silent. I wonder whether cats chirrup at mice or skinks?”
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