In the name of increased airport security, the TSA (Transportation Security Administration) is installing full body scan machines in an increasing number of airports. Critics argue that these systems invade the traveler's privacy. The TSA counters that the security measures are needed and that safeguards are taken to guard privacy. How is a full body scan image made? Do the images violate personal privacy?
Millimeter Wave Technology Full Body Scan
One type of full body scan machine uses millimeter wave technology. Millimeter radio waves are a type of electromagnetic waves having wavelengths of about a millimeter. These wavelengths are the shortest wavelength radio waves and are just longer than the border between infrared waves and radio waves.
Ordinary clothing is transparent to millimeter wavelength radio waves, but flesh is not. Hence a millimeter wave full body scan images through a passenger's clothing to reveal hidden weapons or explosives. The resulting full body scan is essentially a radar image of the passenger's body.
Backscatter X-Ray Technology Full Body Scan
Medical diagnostic X-rays image the interior of a patient's body by passing through the body. To X-ray a broken arm, for example, the arm is placed between the source of X-rays and the film.
Backscatter X-ray full body scans, on the other hand, use lower energy X-rays. The low energy X-rays still penetrate clothing, but reflect, in physics terminology backscatter, off the passenger's skin. The resulting images resolve small details and very efficiently detect nonmetallic weapons and organic explosives.
Atoms of light elements with fewer protons in the nuclei (scientifically called low atomic number or low Z) more effectively backscatter the X-rays. Organic materials contain large amounts of carbon, oxygen, and hydrogen, which are light. Hence backscatter X-ray full body scans very efficiently detect organic materials, such as organic explosives, that metal detectors would miss.
Health Radiation Issues
Millimeter waves have a very low energy. Hence, millimeter wave technology exposes travelers during a full body scan to an amount of electromagnetic energy that is too low to pose a health hazard.
X-rays, on the other hand are much more energetic, and exposure to high doses of X-rays pose a health hazard. According to an article by Julia Layton posted at How Stuff Works, a person receiving a full body scan with backscatter X-rays receives a radiation dose of only about 9 microrems. A passenger on a three hour flight typically receives a dose of about 1,000 microrems of radiation from the plane's altitude. The radiation dose from backscatter X-rays is therefore quite negligible.
Privacy Issues
Backscatter X-ray technology, in particular, allows very detailed images underneath people's clothing. Millimeter wave technology can also produce detailed images. Privacy advocates therefore call full body scans a virtual strip search.
The TSA and other security advocates counter that as currently configured full body scan technology used by the TSA slightly blurs the images to reduce the resolution. This blurring, while not offering complete personal privacy, improves passengers' privacy. Additionally the TSA screener reading the scans is in a separate room and cannot directly see the passengers. The TSA claims scanned images are not stored.
In the case of full body scans, laws and regulations have not kept up with the pace of physics and technology. Full body scanners are being adopted with relatively little public debate. Society and lawmakers need to actively debate this new technology to insure that TSA procedures adequately balance the competing needs for airport security and personal privacy.
Further Reading
Layton, Julia, "Do backscatter X-ray systems pose a risk to frequent fliers?" How Stuff Works, accessed May 10, 2010.
TSA Imaging Technology, Tsa.gov, Accessed May 10, 2010.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
