As we look ahead to the upcoming winter holiday season, traveling circuits are buzzing. Families anxiously watch weather reports, airplane ticket prices soar, and security becomes the question on everyone’s 21st century minds.
Some of you may have heard about the recent controversy over the implementation of rigorous new screening requirements at select airports. Cities across the world are beginning to install full body scanners, primarily dominated by the Z-backscatter X-ray machine. Concerns have been voiced over both the privacy violations that these detailed scans represent as well as potential safety concerns over the long-term health effects of the X-ray. In light of these concerns, TSA has allowed individuals to decline scans if they subject themselves to a thorough physical pat-down, though for many this option doesn’t present any appeal either.
In the interest of keeping my topics relevant, I have changed my plans for this week and elected to go with something that has had a bit more news-presence. When I initially learned about backscatter X-ray scanners, I dug around for more information, but quickly realized that the technology behind these devices is not complicated. People focused primarily on the concerns of privacy violation, as these scanners produce detailed images of a person’s naked, organic exterior. Transgendered individuals fear embarrassing exposure and harassment, airport security has been communicating misleading information regarding their ability to save and recall images of specific individuals, and some people just simply don’t like the idea of being seen naked by a stranger. While I find that debate to have plenty of merit, that’s not the intention to my column, so while I won't keep my opinions completely to myself, I’ll try to stay on topic when it comes to how it works and what it might mean for your health.
How X-rays were discovered, and how they are made
Almost everyone has had at least one medical or a dental X-ray. Luggage is subject to X-ray screening in airports. We use X-rays to inspect crystal structures and welded metals. They provide us with the simplest and most ubiquitous form of medical imaging, implemented not more than a month after Wilhelm Conrad Röntgen discovered them and took a picture of his wife’s skeleton.
It was 1895, and pioneering minds such as Nikolai Tesla and Heinrich Hertz were experimenting with vacuum tube equipment for sound production and manipulation, and Röntgen was studying the external effects of these devices. He discovered that when a high enough voltage was applied, X-rays came out. This was later discovered to be a result of two related phenomena, and today they are produced in nearly the same way that they were discovered. A voltage is applied across a hollow tube under vacuum, creating an electric potential. On one end, an electron-rich substance (like a tungsten filament) is heated up, causing electrons to slide free and go careening across the tube towards the positively charged end. On that end is a metal target, and when these fast-flying electrons strike its surface, they quickly decelerate, the energy released is given off as X-rays. These are called Bremsstrahlung (which is basically German for “breaking”) X-rays. Using this same setup, X-rays can also be produced by the second process called fluorescence, but I don’t have the space to go into that process here.
X-rays then are essentially light particles that fall in the wavelength range of 0.01-10 nanometers. “Soft” X-rays are those in the range of 1-10 nm range and don’t have the ability to penetrate the skin very well, while “Hard” X-rays fall in the range of 0.01-0.1 nanometers – the range we use for imaging.
Traditional X-ray imaging
Remember how traditional film photographs were made before the digital age – light reacts with a chemical on film and exposes it. Different strengths of light will expose the film more or less, creating contrast. That is how traditional X-rays work. A beam of X-rays is pointed at a subject, and after passing though, they will strike a film. The more the exposure, the darker the film becomes, so if these X-rays are absorbed minimally (for example, by passing through air in the lungs) you will see dark areas. However, in very dense tissue such as bone, most of the X-rays will be completely absorbed, and the film will not be exposed, leaving it bright.
Inorganic or metallic substances – like calcium or metal weapons – tend to absorb X-rays very well, while organic materials – like skin, fruit, or liquid explosives – tend to let most X-rays pass through them, so this technique is not particularly effective at identifying these types of materials. However, when organics do interact with X-rays, they tend to scatter them.
Backscatter X-ray imaging – the application of Compton scattering
The above process works on the assumption that when light hits a particle, it is either completely absorbed or it passes directly through it as if it nothing happened. This attenuation has to do with the type of material, its density, and the energy of the incoming beam. But light can also collide with a particle and bounce off in a different direction. This bouncing-off process is called Compton scattering.
It is Compton scattering which provides the explanation for backscatter X-ray devices. Instead of placing a detector directly opposite the incoming beam, a subject could be placed in an 360-degree detector that collects scattered X-rays. The energy of the incoming beam, the outgoing beam, and the angle that it scatters at are all related by a simple mathematical equation. Additionally, because the scattering pattern is specific to the type of material, backscatter detection systems can distinguish between things like carbon, lithium, or hydrogen. What that means is a detector is capable of precisely identifying the positions different substances, and even potentially identify their composition.
Because the locations of these materials can be pinpointed with reliable accuracy, the result is very detailed pictures of anything organic or inorganic that is on or outside of our bodies, which is why these scanners are so popular for travel security.
Ionizing ability and exposure – what is “safe”?
In nearly every post leading up to this one, I have mentioned the electromagnetic spectrum; X-rays are just another form of light. But they have a relatively small wavelength, which means a high frequency. The higher frequency, the higher the energy. High-energy light can knock out exterior electrons or even particles in the nucleus from molecules in our bodies. For their ability to do some significant altering of the subatomic particles that carry out essential functions in the body, both X-rays and gamma rays fall under the classification of what we call ionizing radiation: the kind that everyone is afraid of.
You may have heard of different ways to measure exposure levels, but currently, most people use the rem. A whole rem is a pretty huge amount of radiation, so millirems (one-thousandth of a rem) are more common. According to the American Nuclear Society, the average person is exposed to about 620 mrems of ionizing radiation per year just from living on this planet, but it’s not uncommon to be exposed to more. It’s part of the natural environment; it comes from space, from naturally occurring radon, and from the food and water we consume (like in potassium). A dental X-ray exposes you to about 0.5 mrem, a chest X-ray to about 10 mrem, an abdominal X-ray or CT scan to about 700 mrem. Flying on an airplane exposes you to approximately 0.5 mrem per hour.
What is generally agreed upon is that exposure levels below about 1,000 mrem per year are difficult to interpret, and don’t seem to directly correlate with any significant increased risk of developing cancer, which is currently the most focused-on health link. However, a theoretical model simply based on the knowledge that ionizing radiation can damage cell nuclei and DNA suggests a linear relationship with no threshold. In laymen’s terms, any increase in dose, no matter how small, results in an increase in risk. This has been accepted by the Nuclear Regulatory Commission, the EPA, and the National Academy of Sciences Committee.
The concerns, counters, and subtle opinions of your friendly Science Corner author
When most of us get X-rayed, we’re usually provided with a dense shielding material like a lead vest, the purpose of which is to absorb all those X-rays before they get to our vital areas. But there are no lead vests in airport scanners – after all, what’s to stop determined terrorists from hiding weaponry in vital or private areas? So will these scans expose our vulnerable bodies to dangerous levels of radiation? Most experts are tentatively saying no.
Though the results coming from different groups are mixed, most agree that a person will be exposed to approximately 0.005-0.009 mrem per scan. At the lower limit, it would take 200 scans per year to receive one mrem of exposure, which is equivalent to what you would get from about two hours of flying, or two days spent in Denver. Most of us don’t fly often enough for this to add up, but many groups have not forgotten the linear exposure model. True, the risk is hardly statistically significant, and for that reason, backscatter manufacturing companies are having little trouble proving them safe. But for many people, it’s difficult to casually accept yet another man-made radiation source in a world where the FDA recently released its Initiative to Reduce Unnecessary Radiation Exposure. Additionally, it has been argued that certain areas of the body are much more vulnerable to smaller doses of radiation, such as vital organs or the oft-targeted reproductive system, and that numbers based on “whole-body” exposure are misleading.
While I have often been one to roll my eyes at the panic of the uninformed masses when it comes to new science and technology, this time I will not hide that this has given me pause. Despite the multitude of times I have voiced my support for statistical data from a large study population, ionizing radiation seems different to me. It is not unnatural, and at times it is exceptionally useful; I do not condemn its use. And yet it is clear to me that it is destructive to living material. Even if you can’t prove that it adds up, it cannot be denied that damage occurs at the cellular level. And that to me distinguishes it from the other hand-waving, high-anxiety health threats out there. I feel that there is cause for concern. Yet, given my hesitation and the similar hesitations of many others, the question still remains – is it worth it? Full body scans take much less time than full pat downs, and metal detectors are limited. Is the risk of a miniscule amount of radiation exposure to the greater population worth the dramatic increase in our efficient safety?
It’s certainly a question worth discussing. I hope you can enter into it armed with some background knowledge.
See you back again next Thursday. Email suggestions or requests to firstname.lastname@example.org!