Laura Covarrubias, Cancer Prevention and Treatment Fund
Following the September 11th attacks in 2001, the Transportation Security Administration (TSA) was created and given responsibility for protecting the public from security threats in transportation systems, such as airports. Although metal detectors were once the main security devices used at American airports, the TSA introduced new technologies after terror attempts were made using hidden explosives (in shoes and underwear, for example). These backscatter and millimeter scanners have begun replacing metal detectors and are designed to scan a person to determine what weapons or explosives they may have beneath their clothing. Currently, there are about 250 backscatter and 264 millimeter wave scanners in the United States. The TSA hopes to have 1,800 scanners of either type installed by the end of 2014 – which would mean that nearly every airport in the country will have one.
Backscatter scanners look like two large blue boxes. People raise their arms and stand sideways between these two boxes when they are scanned.
In contrast, millimeter wave scanners look like circular glass phone booths, and the person being scanned stands with their arms raised while part of the scanner rotates around them:
If you are not sure which scanner is in use at your airport, ask a TSA official at the security checkpoint.
When a millimeter scan is used, the machine determines if the person has any potentially harmful items on his or her body. If so, only the outline of a standard human body is shown with potentially dangerous objects highlighted in yellow. If no dangerous objects are detected, the security officer will only see an empty green screen. The security official does not see an image of the actual individual when the millimeter scan is used.
In contrast, when a backscatter scan is used, a blurred, colorless image of the individual (without clothing) is produced. A security officer views the image and determines if further screening is necessary. In an effort to increase passenger privacy, the TSA updated the software on backscatter machines to make it harder to see details in the images. Still, some people argue that both types of machines violate their privacy.
While metal detectors and millimeter scans both use non-ionizing radiation, which until recently was assumed to be safe (see our article Can Cell Phones Harm our Health?), backscatter scans use ionizing radiation, which is used in x-rays and known to potentially increase the risk of cancer. Backscatter scans work a little differently from x-rays. X-rays work by sending high-energy radiation to the body and recording the radiation that passes through the body. Dense parts of the body (like bones) block some of the radiation, resulting in lighter areas on the recorded image. Backscatter scanners also send radiation toward the body, but at much lower energy than an x-ray. Because it is not as strong as the radiation used in x-rays, the radiation does not pass through the body. Instead, the outer layers of the body “scatter” the radiation, which bounces off the body and back toward the machine. Most of the radiation that is absorbed by the body is deposited in the outer layers (like the skin and ribs), although a 2012 study showed that radiation from these scans may penetrate to other organs. Because the radiation is concentrated in the skin, there are concerns that this could cause skin cancer.
All data on backscatter scans are provided by TSA, a government agency that does not allow independent researchers to examine the machines they use. Researchers must therefore make educated guesses using data provided by the TSA, or they must make models of the scanners based on information that the agency releases.
Scientists differ in their opinions regarding whether small doses of radiation increases the risk of cancer. Some scientists think that very small doses-like those received during backscatter scans-pose zero risk to the individual. Other scientists think that there isn’t enough research on the effects of such low doses of radiation to be able to say how it will affect a person’s cancer risk. However, radiation risk accumulates during a person’s lifetime. This means that even though a single exposure may be very small, it is “added” to every other exposure the person has ever encountered.
The TSA states that backscatter scans use such low doses of radiation that estimating the potential effects of the scan is extremely difficult.  2011 report using information from the TSA found that these backscatter scans expose people to the same amount of radiation that they receive from 3 to 9 minutes of normal daily life or from 1 to 3 minutes of flight. To put this into perspective, we would expect only 6 of the 100 million airline passengers each year to develop a cancer in their entire lives due to the backscatter scans.
Dr. David Brenner, a researcher at Columbia University, produced a different estimate based on the risk that the scanners are to the entire population, not just to an individual. Dr. Brenner multiplied the risk associated with one scan by the number of scans conducted each year to estimate the number of people who may develop cancer in one year because of the scanners. Because up to one billion scans may be performed each year, Brenner estimated that each year 100 people would develop cancer because of their exposure.
In April 2010, a group of scientists from the University of California, San Francisco wrote a letter of concern to Dr. John Holdren, the Assistant to President Obama for Science and Technology, about the backscatter scans. These researchers pointed out that because backscatter scans only penetrate outer layers of the body, it is possible that these layers receive a higher concentration of radiation than previously believed. Because of this, conventional estimates would be inaccurate. This higher concentration of radiation could pose particular risk to certain groups such as:
- The elderly (due to their susceptibility to skin cancer)
- Women who have the BRCA gene mutation
- People with weakened immune systems (such as those with HIV or cancer)
- Children and adolescents (who have smaller bodies but receive the same amount of radiation as adults with larger bodies)5
The scientists also expressed concern that sperm may mutate because the testicles are close to the surface of the skin and are exposed to radiation during these backscatter scans. In addition, they noted that the effects of radiation on the cornea (the outer surface of the eye) and the thymus (a part of the immune system located in the chest) have not been studied. While this letter only outlined concerns of the scientists and did not present new data, it called for further testing of backscatter scans. The scientists called for more rigorous and independent studies to ensure that the scans are safe for the entire population, as well as for all parts of the body. When Dr. Holdren received the letter, he sent it along to the Food and Drug Administration (FDA). The Food and Drug Administration does not have authority to regulate backscatter scanners because the devices are classified as electronic devices, not medical devices. However, the FDA has experience in regulating radiation-emitting devices used in medicine, such as mammography devices. In a joint reply with the TSA, the FDA stated that the radiation exposures from the backscatter scans were within established legal limits, even for frequent fliers. In reply to the scientists’ concerns that the radiation dose to the skin would be higher, the FDA wrote that their calculations showed that a person would have to pass through the scanner 1000 times in a year in order to begin to absorb the annual limit of what is considered safe.
Not everyone agrees with the FDA, and some people have pointed out that TSA agents operating the scanners may improperly manage the devices or that mechanical errors may occur, either of which could cause the machines to emit more radiation than they are supposed to. From May 2010 to May 2011, there were 3,778 calls for mechanical problems on backscatter machines, but only 2% of those machines were evaluated for radiation safety. Anyone who is concerned about the radiation from the scanners has the legal right to refuse to undergo a scan, as long as they agree to a full-body pat-down by a security officer.
For more information on other types of radiation, see our article Everything You Wanted to Know About Radiation and Cancer, But Were Afraid to Ask.
- Schmidt T, Hoppe M. Estimation of organ and effective dose due to Compton backscatter security scans. Medical Physics. 2012;39(3396).
- Mehta P, Smith-Bindman R. Airport full-body screening: what is the risk? Archives of internal medicine. 2011;171(12):1112-5. Available at: http://www.ncbi.nlm.nih.gov/pubmed/21444831. Accessed March 17, 2012.
- Brenner DJ. Are x-ray backscatter scanners safe for airport passenger screening? For most individuals, probably yes, but a billion scans per year raises long-term public health concerns. Radiology. 2011;259(1):6-10. Available at: http://www.ncbi.nlm.nih.gov/pubmed/21436091
- Cerra F. Assessment of the Rapiscan Secure 1000 Body Scanner for Conformance with Radiological Safety Standards. 2006. Available at: http://scholar.google.com/scholar?hl=en&btnG=Search&q=intitle:Assessment+of+the+Rapiscan+Secure+1000+Body+Scanner+for+Conformance+with+Radiological+Safety+Standards#0. Accessed June 18, 2012.
- Sedat J, Agard D, Shuman M, Stroud R. Letter to Dr. John P. Holdren. 2010.
- Rabin R.C.. X-Ray Scans at Airports Leave Lingering Worries. The New York Times. August 6, 2012. Available at http://well.blogs.nytimes.com/2012/08/06/x-ray-scans-at-airports-leave-lingering-worries/. Accessed August 7, 2012.
- McCrohan J, Shelton Waters K. Letter to Dr. John P. Holdren. 2010.