The hubbub (if I can use my old nickname) about the dangers of the TSA body scanners is quieting down as people refocus on the political and social ramifications of our latest “weapon” in the “war on terrorism”, and I plan to take an airplane ride in March. This makes it a perfect time for me to review the basic physics of these machines in order to reasonably gauge their safety. Since I am not an electrical, nor any other kind of engineer, it took me a while to collect some facts and understand them.
There are two different TSA body scan machines, a millimeter machine and a back-scatter machine. The millimeter machine uses short (hence “millimeter”) frequency radio waves which are produced by electromagnets and are NOT ionizing radiation. Ionizing radiation is the kind that can cause breaks in cell structures and is one of the culprits causing cancer. These millimeter waves are very close to infra-red waves, are invisible, and at a very extreme duration of exposure might just warm up your clothes or your skin a bit. The millimeter machine is usually grey with clear windows. The image produced is not clear enough to see your facial features. Enough said about that one.
The back-scatter machine uses weak X-rays that do just that. They go through your clothes, stop at your skin, and “back scatter” to a reading device. All X-rays are ionizing radiation, and so these rays could cause cell structure breakage in your skin. They do NOT go beyond your skin. They will not show your hip joint replacements as you set off the metal detector. (So I will probably still get a pat down every time I fly. If I traveled more I could apply for “TSA Pat Down Poster Man”.) The back-scatter machines are usually grey and blue or black and have no windows. The amount of radiation from them is about 1/100 of a chest X-ray, and remember these back scatter X-rays do NOT penetrate the skin. Just like a 60 watt bulb that can’t put out 100 watts of energy “by mistake”, so these weak X-ray machines can’t burn or cause internal damage like the high energy diagnostic and therapeutic radiation machines we have read about. Unless you stand around inside one of these back-scatter machines for a day or so, they are pretty mistake-proof sources of radiation.
The University of California, San Francisco scientists who wrote the President urging caution about using these machine on the general population included physicians who are dermatologists concerned about melanoma and other skin cancers. Melanoma is a serious disease with unclear causes (multiple teen-age sunburn episodes seem to be one of the most accepted risk factors). The letter to the President stated that we don’t know what this low energy X-ray will do to the skin over the years. Will it do anything to our skin? There is no evidence that it causes skin cancer, but to declare it 100% safe “you (the US government) will have to prove that it won’t cause skin cancer”. (See previous blog on how hard it is to prove a negative; the “purple dust story”)
Unlike many new drugs approved by the FDA which are tested on small populations over relatively short periods, we have lots of data over the last ten years about skin exposed to ionizing radiation and the resultant relative risks of skin cancer. People in airplanes receive more high energy radiation and cosmic ray bombardment (from the sun) than those who stay on the ground. Of course, these ionizing radiations are strong enough to go through the aluminum fuselage, and so are hugely more penetrating than the TSA body scanner radiation. As you might imagine the Association of Flight Attendants have paid a lot of attention to this issue. Their review indicates that if a flight attendant (either a father or a mother) who flies 900 hour a year for 10 years has a 0.01% increased risk of having a child with a birth defect ( up from the 2.50% risk for the general population to 2.51%). The Federal Aviation Agency puts the risk of dying from a fatal cancer for a flight attendant who flies 900 hours a year for 30 years as just 1% higher than the general population. The incidence of malignant melanoma in flight attendants in Swedish, Icelandic, U.S, and eight European countries’ studies was only slightly higher than the general public.
The amount of radiation JUST TO YOUR SKIN from a back-scatter machine is the same amount you receive in two minutes of flying in an airplane. So, if you are still worried about the radiation from the TSA machines, you probably shouldn’t be flying anyway.