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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
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1

Introduction

ADVANCED IMAGING TECHNOLOGY IN U.S. AIRPORTS

Passenger screening at commercial airports in the United States has gone through significant changes since the events of September 11, 2001. In response to increased concern over terrorist attacks on aircrafts, the Transportation Security Administration (TSA), a component of the Department of Homeland Security (DHS) responsible for the security of the transportation systems in the United States, has deployed security scanners of advanced imaging technology (AIT) to screen passengers at airports. According to TSA, AIT systems provide enhanced security benefits by detecting both metallic and non-metallic threat items, including weapons, explosives, and other concealed objects, on passengers that would not be detected by walk-through metal detectors.1 This report does not address the benefits of AIT systems. The committee was not asked to address such issues.

TSA started deploying AITs in 2008 for use as a secondary2 screening of airport

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1 Government Accountability Office, Transportation Security Administration: Progress and Challenges Faced in Strengthening Three Key Security Programs, Statement of Stephen M. Lord, Director, Homeland Security and Justice Issues, Testimony Before the Committee on Oversight and Government Reform and Committee on Transportation and Infrastructure, House of Representatives, released March 26, 2012, http://oversight.house.gov/wp-content/uploads/2012/03/3-26-12-Joint-TI-Lord-Testimony.pdf.

2Primary screening is screening to which all passengers are subject. Secondary screening is additional screening that may be used based on the response to the primary screening, randomization, or other metrics that suggest that further information is needed about a passenger (e.g., to possibly resolve an alarm during primary screening).

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

BOX 1.1
Ionizing Radiation

Radiation consists of elementary particles that are capable of interacting with material and transferring energy to this material. Ionizing radiation is a category of particles that deposit energy which is sufficiently concentrated to eject electrons from individual atoms. Although the amount of energy is quite small, the patterns of ionization create localized damage that can cause malfunctions in inanimate objects such as microelectronics or injury to biological systems.

For purposes of radiation protection in humans, the exposure to ionizing radiation is measured in terms of effective dose termed a sievert (Sv). A single large exposure to ionizing radiation of greater than 6 Sv (6,000,000,000 nSv) could result in fatality soon after the exposure; whereas low doses of radiation around 100 mSv (100,000,000 nSv) might result in excess risk of carcinogenesis many years following the exposure.

Continuous exposure of the general population to ionizing radiation originates from ubiquitous natural background and utilization of man-made sources. Sources of natural background, that are located outside of the body, consist of penetrating cosmic rays (muons and neutrons), and gamma rays originating from radioactivity in terrestrial minerals containing uranium (238U), Thorium (230Th), and potassium (40K). There are also sources of natural radioactivity that accumulate within the body through ingestion of food and liquids, such as potassium (40K), carbon (14C) and through inhalation (radon). The magnitude of these sources varies considerably with geographical location, elevation above sea level and lifestyle. Man-made activities include voluntary medical procedures and exposure from other sources such as consumer products, commercial aviation and nuclear power generation.

The National Council on Radiation Protection and Measurements (NCRP) has published a comprehensive report summarizing the exposure to radiation of the U.S. population. The average effective dose in 2006 was 6 mSv per year (6,000,000 nSv per year). Figure 1.1.1 illustrates the relative contributions to the average annual exposure from the various sources. It is interesting that radon is the largest contributor to natural background (solid colors) whereas the largest contribution to the overall average comes from diagnostic imaging procedures such as conventional X-rays, CT examinations, and nuclear medicine.

security. Their deployment was accelerated, and usage was switched to primary3 screening after the December 25, 2009, event when Northwest Airlines Flight 253 was the target of a failed attempt by a passenger to set off plastic explosives hidden in his underwear.

Current procedures require that all passengers be screened using an AIT system deployed at the airport.4 Passengers can choose to opt out of such screening and be subjected to a patdown search by a TSA security officer. In some cases, passen-

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3 S. Mahoney, Laying down the law, Security Today, April 01, 2010, http://security-today.com/articles/2010/04/01/laying-down-the-law.aspx?admgarea=ht.airport.

4 With certain exceptions, as noted on www.tsa.gov, such as travelers over the age of 75, travelers with certain types of disabilities, and those approved for the TSA Preimages™ program.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 1.1.1 Average annual exposure to radiation of the U.S. population. SOURCE: Data from National Council on Radiation Protection and Measurements, NCRP Report No. 160: Ionizing Radiation Exposure of the Population of the United States, Bethesda, Md., 2009.

gers opting for an AIT screening during primary screening may subsequently be required to undergo a patdown search if anomalies5 are detected.

To date (December 2014), TSA has deployed in U.S. airports two different types of AIT systems that use different types of radiation to detect threats:6 the first technology (see Figure 1.1a) deployed by TSA, is based on the X-ray Compton scattering effect of X rays, a type of ionizing radiation (see Box 1.1 for a short de

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5Anomalies in this context indicates some type of object(s) that will require further screening to resolve.

6 There are two types of radiation: ionizing radiation and nonionizing radiation. Ionizing radiation has enough energy to remove electrons from atoms and create ions. Nonionizing radiation does not have enough energy to remove electrons from atoms but can cause them to move or vibrate. Ionizing radiation and nonionizing radiation have different biological effects.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

scription of ionizing radiation). The deployed equipment, a Rapiscan Secure 1000 manufactured by Rapiscan Systems, projects a fast-moving, narrow, low-energy X-ray beam7 over the body surface to detect the radiation reflected (or backscattered) from the person being screened as well as any objects on that person. These X rays are of lower energy and intensity than medical X rays are and do not penetrate the body as deeply. Objects on the surface and shallow subsurface of the body can be detected with this technology.

The second technology (see Figure 1.1b), and the only one currently deployed at airports by TSA, uses millimeter-length radio waves, a type of nonionizing radiation, and the equipment deployed is the L-3 ProVision manufactured by L-3 Security and Detection Systems. Millimeter wave technology measures the electromagnetic waves backscattered from the human body or objects on the human.

X-ray backscatter and millimeter wave AITs have the capacity to create detailed images of those being screened and reveal genitalia, breasts, fat creases, and all types of prosthetics and piercings8 (see Figure 1.2). In the early phases of deployment of AIT, TSA took several steps to protect the privacy of passengers. For example, TSA decided neither to save nor transport the images acquired from screening, and it eventually isolated the operator viewing the images from the individuals being screened. In 2011, in response to ongoing privacy concerns, TSA piloted a program for manufacturers to develop automatic target recognition (ATR) software for AITs to enable display of anomalies on a generic figure (see Figure 1.3) that is identical for all persons being screened, as opposed to a backscatter image of the individual’s body, removing the need for a TSA agent to review the images generated.

As part of the Federal Aviation Administration Modernization and Reform Act of 2012 (P.L. 112-95), Congress mandated that all AIT units be equipped with ATR by June 1, 2012, a deadline later extended to June 1, 2013. While all of the millimeter wave units have been equipped with the ATR software, Rapiscan Systems did not develop ATR software for its X-ray backscatter AIT units that passed testing. As a result, TSA canceled its contract with Rapiscan and removed all 250 X-ray backscatter units manufactured by Rapiscan from U.S. airport checkpoints by May 31, 2013.

Currently, there are no X-ray backscatter AIT units deployed in U.S. airports, but 740 AIT millimeter wave units are located at about 160 airports in the United States.9 In November 2012, TSA issued a contract award to American Science and Engineering, Inc. (AS&E) to deliver next-generation X-ray backscatter AIT systems

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7 In the context of X-ray AIT systems, the operating potential of the X-ray tube is 50 kV.

8 P. Mehta and R. Smith-Bindman, Airport full-body screening: What is the risk?, Archives of Internal Medicine 171(12):1112-1115. 2011.

9 Transportation Security Administration (TSA), “Advanced Imaging Technology,” https://www.tsa.gov/traveler-information/advanced-imaging-technology-ait, accessed December 4, 2014.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 1.1 Advanced imaging technologies deployed by TSA: (a) X-ray-based Rapiscan Secure 1000. (b) Millimeter-wave-based L-3 ProVision ATD. SOURCE: (a) Courtesy of Scott Olson/Getty Images; (b) Courtesy of L-3 Communications.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

images

FIGURE 1.2 Images from AITs not equipped with automatic target recognition: (a) X-ray backscatter AIT (b) millimeter wave AIT. SOURCE: (a) Courtesy of the Transportation Security Administration; (b) Courtesy of L-3 Communications.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
images

FIGURE 1.3 Images from a millimeter wave AIT equipped with automatic target recognition. NOTE: If potential threat items are detected on the person being scanned, they are indicated on a generic outline of a person that is identical for all passengers screened (a). Areas identified as containing potential threats will require additional screening. (b) If no potential threat items are detected, an “OK” appears on the monitor with no outline. SOURCE: Courtesy of the Transportation Security Administration.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

BOX 1.2
Statement of Task

An ad hoc committee will review previous studies as well as current processes used by the Department of Homeland Security (DHS) and equipment manufacturers to estimate radiation exposures resulting from backscatter X-ray advanced imaging technology (AIT) system use in screening air travelers, and provide a report with findings and recommendations on:

  1. Whether exposures comply with applicable health and safety standards for public and occupational exposures to ionizing radiation, and
  2. Whether system design (e.g., safety interlocks), operating procedures, and maintenance procedures are appropriate to prevent over exposures of travelers and operators to ionizing radiation.

This study will not address legal, cultural or privacy implications of this technology.

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NOTE: The committee’s work plan further stipulated that as part of this study, the Academies will subcontract with an appropriate independent testing organization or qualified consultants to conduct field studies measuring the radiation dosage emitted by the machines as they are used.

for testing. TSA also issued a contract award to L-3 Security and Detection Systems to deliver next-generation millimeter wave AIT systems for testing. Both systems are equipped with ATR capabilities. The National Research Council (NRC)10 has been asked by DHS to evaluate the X-ray backscatter AIT systems as specified in the statement of task (Box 1.2). In response, the NRC appointed the Committee on Airport Passenger Screening: Backscatter X-Ray Machines.

SAFETY CONCERNS RELATED TO X-RAY BACKSCATTER AIT

Although it was privacy concerns and lack of ATR that led to the removal of the Rapiscan X-ray backscatter AITs from the airports, another concern relates to their safety. X-ray backscatter systems emit ionizing radiation. While some of the X rays emitted during a scan11 are reflected back, some are absorbed by the body,

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10 Effective July 1, 2015, the institution is called the National Academies of Sciences, Engineering, and Medicine. References in this report to the National Research Council are used in a historical context identifying programs prior to July 1.

11 In this report scan and scanning refers to the AIT system using X-rays to inspect one view of the passenger (front or back) whereas “screening” refers to the complete inspection with a scan of both the front and the back (two scans).

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

and a very small fraction is transmitted through the body. Ionizing radiation is high-frequency electromagnetic radiation that has enough energy to potentially damage the DNA in cells. This in turn may lead to cancer. Based on the linear-nonthreshold (LNT) dose-response model of radiation effects12,13 currently used for radiation protection purposes, any dose,14 no matter how small, is assumed to be linked to an increase in the risk15 of contracting cancer.

Safety concerns related to the X-ray backscatter AIT systems are often discussed at the individual as well as the societal level. At the individual level, two special populations are suggested to be at higher risk of developing cancer from being screened with the X-ray backscatter AIT systems:

  1. Children and the developing fetus are generally believed to be more sensitive to the effects of radiation for a given radiation dose compared to the general population,16 and
  2. Frequent flyers and airline and airport staff whose cumulative dose, and therefore risk, increases because they are screened more frequently compared to the general population.

At the societal level, the concern about risks from exposure17 to the X-ray backscatter AIT systems is as follows: Even though the dose to an individual from screening with the X-ray backscatter AITs may be small, and therefore the risk of developing cancer for that particular individual is also small, the collective dose18 to the more than 815 million passengers traveling annually within the United States19 and likely to be screened with this technology may be high. Based on the LNT model currently used for radiation protection purposes, the concept of col-

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12 International Commission on Radiological Protection (ICRP), Recommendations of the ICRP, Publication 26, Annals of the ICRP 1(3), 1977.

13 National Council on Radiation Protection and Measurements (NCRP), Evaluation of the LinearNonthreshold Dose-Response Model for Ionizing Radiation, Report No. 136, Bethesda, Md., 2001.

14 Dose is a term used to express how much radiation exposure a person has received. The word dose without a modifier (such as effective dose or absorbed dose) refers to energy deposition in a general sense, rather than a specific amount.

15Risk is defined as the probability of an adverse health effect to occur.

16 UNSCEAR, Effects of radiation exposure of children, Annex B in Sources, Effects and Risks of Ionizing Radiation, UNSCEAR 2013 Report, Volume II, United Nations, New York, N.Y., 2013.

17Exposure is here used as a general term indicating that a subject has been exposed to radiation. Unless otherwise stated, it is not intended to imply any relationship to the ICRU (International Commission on Radiation Units) notion of exposure.

18Collective dose is the sum of all individual effective doses over the time period under consideration.

19 Department of Transportation, Bureau of Transportation Statistics, “Total Passengers on U.S. Airlines and Foreign Airlines U.S. Flights Increased 1.3% in 2012 from 2011,” Press Release BTS 16-13, April 4, 2013, http://www.rita.dot.gov/bts/press_releases/bts016_13.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

lective dose does appear to justify this concern. However, the National Council on Radiation Protection and Measurements (NCRP)20 and the International Commission on Radiological Protection (ICRP)21 point out that application of collective dose to large populations requires an extrapolation of risk estimates to doses far below values where any data exist (see discussion in Chapter 2) and thus cannot be justified. It is impossible to determine if there is any increase in the number of cancer occurrences in a large population exposed to a low dose, because baseline cancer rates are high (around 40 percent in the United States),22 and the variation in the risk of developing cancer is considerably high because of individual lifestyle and environmental factors. The baseline cancer rate far exceeds any increase in individual risk that might be induced by the radiation exposure from the X-ray backscatter AIT.

The dose received from the X-ray backscatter AIT was tested in a number of investigations, including those of the Food and Drug Administration (FDA),23 the Johns Hopkins University/Applied Physics Laboratory (JHU/APL),24 the U.S. Army Public Health Command (USAPHC),25 the National Institute of Standards and Technology (NIST),26 and the American Association of Physicists in Medicine (AAPM).27 These investigations made measurements of the radiation emitted by the AIT systems and calculated the dose to the whole body, which was then compared to the current standard for usage of X-ray backscatter AIT systems, namely, the American National Standards Institute/Health Physics Society (ANSI/HPS) N43.17-2009 standard,28 to determine whether the calculated dose complies with that standard. The FDA, JHU/APL, NIST, and AAPM studies made measurements

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20 NCRP, Principles and Application of Collective Dose in Radiation Protection, Report No. 121, Bethesda, Md., 1995.

21 ICRP, The 2007 Recommendations of the International Commission on Radiological Protection, ICRP Publication 103, Annuls of the ICRP 37(2-4), 2007.

22 National Research Council, Health Risks From Exposure to Low Levels of Ionizing Radiation: BEIR VII—Phase 2. The National Academies Press. Washington, D.C., 2006.

23 F. Cerra, Assessment of the Rapiscan Secure 1000 Body Scanner for Conformance with Radiological Safety Standards, Food and Drug Administration, Arlington, Va., July 21, 2006.

24 Johns Hopkins University Applied Physics Laboratory, Radiation Safety Engineering Assessment Report for the Rapiscan Secure 1000 in Single Pose Configuration, NSTD-09-1085, Version 2, Laurel, Md., August 2010.

25 U.S. Army Public Health Command (USAPHC), Radiation Protection Consultation No. 26-MF-0E7k-11 Rapiscan Secure 1000 Single Pose Dosimetry Study, Aberdeen Proving Ground, Md., 2012.

26 Glover et al., Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, 2012.

27 American Association of Physicists in Medicine, Radiation Dose from Airport Scanners: Report from Task Group 217, College Park, Md., 2013.

28 The ANSI/HPS N43.17-2009 standard, “Radiation Safety for Personnel Security Screening Systems Using X-Ray or Gamma Radiation,” is available at the Health Physics Society website at http://hps.org/hpssc/index.html.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

using portable instruments and calculated eye and skin equivalent doses from computer modeling; the USAPHC study made measurements using commercially available personnel dosimeters. These five studies are discussed in more detail in Chapter 6.

The standard governing radiation exposure from X-ray backscatter AIT systems—from ANSI/HPS N43.17-2009—sets a maximum whole body dose of 250 nanosieverts (nSv) per screening. The five investigations mentioned above showed that a single screening with the Rapiscan Secure 1000 SP (single pose)29 unit is associated with an effective dose to a person being screened, roughly in the range of one-twenty-fifth to one-eighth of the standard limit (11 to 33 nSv) per screening (see the Table 1.1 column, “Reference Effective Dose to Standard Man Being Screened”). The NIST study also provided estimates of dose to sensitive subpopulations such as young children. The estimated dose to these sensitive subpopulations also was less than one-eighth of the standard (30 nSv, see Table 1.1 column “Effective Dose to Sensitive Populations Considered”). Also, most of the studies investigated dose to bystanders—that is, to persons such as the operators and persons waiting in line to be screened—who may receive some dose because of their proximity to the inspection area. In most studies, the dose to bystanders30 was found to be about one-tenth of the dose received by the person being scanned, if it could be measured at all with the available equipment.

The studies shown in Table 1.1 were in agreement that the dose to persons being screened is below the standard with which X-ray backscatter AIT systems need to comply. According to NCRP, any dose of 100,000 nSv or less per source or practice is considered negligible.31 Dose from an X-ray backscatter scan, even when considering the highest dose estimate from the previous studies (30 nSv), is less than one-hundredth of what NCRP describes as a negligible level. Cancer risks at effective doses of the order of “100,000 nSv or less” are currently unknown.

Despite the general agreement among the studies reviewed by the committee that doses received from the X-ray backscatter AIT systems are very low, concerns were raised by individuals within the scientific community. There were concerns,

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29Single-pose (SP) system refers to an AIT system that can screen a person without him/her having to turn to scan both sides. In this report any reference to the Rapiscan Secure 1000 relates to the SP system unless otherwise stated.

30 Here, bystanders that are waiting in line can include passengers, operators, and airline personnel. These bystanders can be men, women, pregnant women, and children.

31 According to NCRP, the level of average annual excess risk of fatal health effects attributable to radiation exposure below this level is so low that effort to further reduce the exposure to an individual is not warranted. See NCRP, Limitation of Exposure to Ionizing Radiation, Report No. 116, Bethesda, Md., 1993.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

TABLE 1.1 Summary Findings from Selected X-ray Backscatter Investigations of the Rapiscan Secure 1000

Study Number of Units Tested Reference Effective Dose to Standard Man Being Screened (nSv) Effective Dose to Sensitive Populations Considered Absorbed Dose to Skin or Eye Lens Air Kerma in Region Surrounding Inspection area per screening Dose rate to bystanders from stray radiationa
JHU/APL 1 4.6 No No 8.4 nGy Up to 1285 nSv/hour
NIST 1 at NIST facility 18.5 Newborn infant: 29.8 nSv 5 year-old: 21.9 nSv 111 nGy to skin and eyeb <0.1-1.8 nGy Up to 276 nSv/hourc
AAPM 3 at Factory and 6 at one airport 11.1d No 40.4 nGy to skin Below detection limit Below detectable level
FDA Multiple 29 N/A N/A 1.9-2.6 nGy N/A

a This value is based on 180 screenings per hour.

b Value based on dose calculated for the position of maximum exposure, which was at a height of 185 cm.

c This value would apply to an operator who spends 2,000 hours per year at the edge of the inspection zone, which cannot occur in light of current work practices.

d Calculated by the committee from kerma and half-value layer (HVL) reported by AAPM.

NOTE: The four studies presented here were selected because they are the most comprehensive studies to be performed on the Rapiscan Secure 1000 system.

The ANSI/HPS N43.17 standard limits the dose per screening for a general-use X-ray backscatter system to a reference dose of 250 nSv.

SOURCE: American Association of Physicists in Medicine, Radiation Dose from Airport Scanners: Report from Task Group 217, College Park, Md., 2013.

primarily by faculty members of the University of California, San Francisco,32,33 that data supporting this conclusion came from studies34 that were not independent,35 and that the studies did not carefully examine the dose to the skin, which they

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32 J.W. Sedat, Ph.D., Marc Shuman, M.D., David Agard, Ph.D., and Robert Stroud, Ph.D., Letter to Dr. John P. Holdren, Assistant to the President for Science and Technology, April 6, 2010, available at http://www.whitehouse.gov/sites/default/files/microsites/ostp/ucsf-jph-letter.pdf.

33 P. Mehta and R. Smith-Bindman, 2011, Airport full-body screening: What is the risk?, Archives of Internal Medicine 171(12):1112-1115.

34 See, for example, Cerra, Assessment of the Rapiscan Secure 1000 Body Scanner for Conformance with Radiological Safety Standards, 2006; JHU/APL, Radiation Safety Engineering Assessment Report for the Rapiscan Secure 1000 in Single Pose Configuration, 2009.

35 The authors do not define independence.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

believe could be high even though the effective dose is low.36 Another concern was that the studies did not address issues regarding AIT system malfunctions that could potentially cause a high radiation dose to be concentrated on a single spot.37 Such studies were described as “difficult-to-impossible” to conduct using publicly available information.38

To ensure that malfunctions would not lead to overexposure39 to radiation, manufacturers of the AIT systems include safety interlocks that, for example, prevent the AIT system from emitting X rays if the X-ray beam is not moving across the passenger being screened. These interlocks can be implemented in hardware and/or software. Hardware interlocks for AITs include switches that prevent X-ray generation if the X-ray electronics cabinet doors are open or if a number of other safety-related conditions are not met. With regard to software interlocks, at the highest level, the software provides password protection against changing AIT system settings that would increase the radiation dose. Within the AIT system, software interlocks monitor critical voltages and currents and prevent the generation of X rays if the operating conditions vary from nominal conditions. For AIT system maintenance and repair, the X-ray tube must sometimes be turned on with the cabinet doors open or in other non-standard AIT system configurations. For these cases, each AIT software design includes a “maintenance mode” or “engineering mode” that is restricted to trained and certified maintenance personnel.

THE STUDY REQUEST

Congress introduced legislation calling for an independent investigation of the safety of the X-ray backscatter AIT systems.40 In response to this call, in November 2011, TSA made a commitment at the Senate Homeland Security and Governmental Affairs Committee hearing that it would conduct an independent

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36 In their April 6, 2010, letter, Sedat et al. express a number of concerns about the safety of backscatter X-ray inspection. Some of the points raised relate to the physics of energy deposition by ionizing radiation and the quantities used to quantify radiation exposure. However, some of the issues raised are based on mischaracterization of radiation concepts. For the committee’s discussion of the letter and clarification of some of these misconceptions, see Chapter 6.

37 D.J. Brenner, 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 259(1):6-10. 2011.

38 J.E. Moulder, Risks of exposure to ionizing and millimeter-wave radiation from airport whole-body scanners, Radiation Research 177(6):723-726, 2012.

39 For the purpose of this report, overexposure is defined as exposure to radiation above the dose limit set by the ANSI, 2009, ANSI/HPS N43.17-2002, Radiation Safety for Personnel Security Screening Systems Using X-Ray or Gamma Radiation standard.

40 U.S. House of Representatives, 112th Congress, H.R. 4068, introduced February 16, 2012, https://www.congress.gov/bill/112th-congress/house-bill/4068/text.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

study on the effects of X-ray backscatter AIT.41 Around May 2012, DHS engaged the NRC to review current processes used by TSA and equipment manufacturers to estimate radiation exposures resulting from X-ray backscatter AIT use in screening air travelers and announced its intention to award a contract to the NRC in December 2012.42 As specified in the statement of task (Appendix A) the request was to consider only the use of X-ray backscatter AIT technology and to address only the radiation issues related to standards and malfunction. Specifically, it was not to address the following:

  1. The issue of justification of the use of X-ray backscatter AIT systems when an alternative technology that does not use ionizing radiation exists, or
  2. Whether the standard this technology needs to comply with is appropriate and/or adequate to protect human health.

The NRC study started in May 2013. However, during that month, TSA completed the removal of the Rapiscan X-ray backscatter AIT systems from airports because of Rapiscan’s inability to develop the ATR privacy software by the deadline directed by Congress. Despite removal of the X-ray backscatter AIT systems from all airports, DHS, the study sponsor, expressed their continuing interest for the NRC to perform an independent examination of the Rapiscan Secure 1000 previously deployed in U.S. airports. Given TSA’s intention to potentially procure the second-generation X-ray backscatter AITs manufactured by AS&E, which are now in testing, DHS also expressed an interest in an independent examination of these second-generation AIT systems.43

STRATEGY TO ADDRESS THE STUDY CHARGE

This study was carried out by a committee of experts appointed by the NRC. The committee consisted of 14 members with expertise that spans the disciplines relevant to the study task: radiation physics and dosimetry, radiation biology, diagnostic and therapeutic radiology, materials science, systems and electrical engineering, manufacturing testing and evaluation, aviation safety, software safety, and statistics. In selecting the membership of the committee, the NRC sought to

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41 TSA, “TSA to Conduct an Independent Study with National Academy of Sciences on Backscatter X-ray Advanced Imaging Technology,” press release, December 18, 2012, http://www.tsa.gov/es/node/1671.

42 Federal Business Opportunities (FedBizOps.gov), “Airport Passenger Screening: Backscatter X-Ray Machines,” Solicitation Number RSEN-13-00004, posted December 13, https://www.fbo.gov/?s=opportunity&mode=list&tab=list&tabmode=list2012.

43 Oral communication with TSA representatives from the Office of Security Capabilities on February 3, 2014.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

balance members who have direct experience with the design and operation of the X-ray backscatter AIT and members who have relevant disciplinary expertise but no direct experience. Biographical sketches of the committee members are provided in Appendix E.

As part of the NRC independent evaluation, DHS requested that the NRC subcontract with an appropriate testing organization or qualified consultants to conduct field studies to measure the radiation emitted by the AIT systems as they are used. According to the DHS-NRC agreement, the work of the subcontractors would be overseen by the NRC.

To identify appropriate subcontractors to conduct field studies to measure the radiation emitted by the AIT systems as they are used and to estimate doses received by the populations of interest, NRC staff solicited nominations for candidates by drawing upon a vast network of contacts and resources and invited the candidates to respond to two separate requests for proposals (RFPs) issued by the NRC on February 21, 2014. In both RFPs (field measurements and dose estimations), the technical requirements and specifications, statement of work, and deliverables were defined by the NRC in consultation with the study committee. Decision on the awards was made by NRC staff in consultation with a subgroup of committee members who declared no conflict of interest related to the individuals or companies that responded to the RFPs. When making a decision on the subcontractors, the following criteria were considered: expertise and knowledge on radiation physics and dosimetry; adequacy of response to the technical aspects of the RFP; availability of personnel and timely ability to respond to the task; absence of conflict of interest; and cost of services. Subcontracts were awarded in April 2014 to David Hintenland and Wesley Bolch, both in the Advanced Laboratory for Radiation Dosimetry Studies, J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, to perform the field measurements and dose estimations, respectively. Brief biographical information for the subcontractors is presented in Appendix E.

Results of the subcontractors’ work was used by the committee to draw its findings and recommendations for the first part of the statement of task related to whether exposures from the AIT systems comply with the applicable health and safety standard, ANSI/HPS N43.17-2009. The committee identified three possible approaches to respond to the second part of the statement of task related to whether system design, operating procedures, and maintenance procedures are appropriate to prevent overexposures:

  1. Carry out a system design reliability analysis by reviewing the mechanical and electrical diagrams and other documentation of how critical AIT safety systems work.
  2. Inspect and test AIT safety systems.
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
  1. Identify a worst-case scenario for the AIT that could lead to exposure higher than intended and determine whether this scenario could lead to overexposure of the person being screened.

The committee did not have access to the needed material for the first approach (i.e., to carry out an AIT system design reliability analysis) because this material is proprietary information and is protected from public release. However, the committee was able to inspect and test some AIT safety systems and performed a detailed analysis of the dose received by the person being screened in a situation where safety interlocks fail and overexposure may be possible. The committee’s findings and recommendations are described in Chapter 7.

At the time this study was conducted, all X-ray backscatter AIT systems manufactured by Rapiscan had been removed from airports. Also, the second-generation AS&E AIT systems were undergoing initial evaluations at TSA’s Transportation Systems Integration Facility (TSIF), and none were deployed at airports. Therefore, the committee and its subcontractors were unable to test the AIT systems at airports. Instead, they had access to two AIT systems:

  1. A Rapiscan Secure 1000 unit located at NIST, Gaithersburg, Maryland, that was previously deployed at LaGuardia Airport. This AIT system, now the property of NIST, was set up by Rapiscan to operate as previously used at airport checkpoints. Measurements on this unit took place July 14-16, 2014.
  2. A second-generation AS&E SmartCheck AIT system located at the TSIF testing laboratory, Arlington, Virginia, that was undergoing product certification/qualification procedures for checking that the system meets TSA’s requirements and specifications. The committee is not aware how this unit undergoing certification/qualification differs from the units possibly deployed to airports in the future. Measurements on this unit took place May 20-22, 2014.

The manufacturers of the units (Rapiscan and AS&E), TSA personnel, and NIST personnel had no oversight during the measurements process. The subcontractors who performed the measurements, with committee oversight, and dose estimations provided their expert opinions to the committee about the interpretation of the measurements and calculations of doses but did not have input on how the results were used in this report.

When analyzing the results from the measurements, the committee requested that the subcontractor conduct a sensitivity analysis determining how the dosage received by a person being screened with the X-ray backscatter AIT would be affected by differences in design and radiation emission from various possible AIT

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

systems. This analysis makes the results from the NRC study more broadly applicable than for only the two AIT systems studied.

INFORMATION GATHERING AND CHALLENGES

The committee held six meetings, three of which were information gathering, to receive briefings and documentation from TSA and DHS and subject-matter experts. The committee also visited NIST and TSIF to inspect the X-ray backscatter AIT systems and learn about their design and operation by engaging in discussions with experts and operators.

In addition, the committee requested and received written information from DHS, NCRP, JHU/APL, NIST, and AAPM. Requests for information were also sent to Rapiscan and AS&E, the manufacturers of the units examined. However, even though there was some dialog, the companies were unable to meet all of the requests from the committee for information.44

REPORT ORGANIZATION

Chapter 1 (this chapter) provides an introduction to the study. Chapters 2 through 7 address the statement of task (Appendix A) in its entirety.

  • Chapter 2 describes the radiation physics behind the X-ray backscatter AIT.
  • Chapter 3 describes the different backscatter AIT implementations.
  • Chapter 4 describes dosimetric considerations for measurements of ionizing radiation emitted by the X-ray backscatter AITs.
  • Chapter 5 provides an overview of the pertinent radiation standards.
  • Chapter 6 provides a review of previous studies on the radiation emitted from X-ray backscatter AIT systems and an investigation of interlocks put in place.
  • Chapter 7 describes the committee’s measurements of radiation emitted and calculation of dose as well as the committee’s observations of the fail-safe mechanisms.

Appendixes A-E provide detailed information about organ dose calculations and statistical considerations from previous studies as well as abbreviations, the statement of task, and committee biographical information.

_______________

44 For example, the committee requested the mechanical and electrical diagrams of the AIT systems and information about safety interlocks. The companies could not respond to all of the committee’s requests for a variety of reasons (national security, proprietary information, trade secrets, etc.).

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×

There are two chapters in this report that lead to key findings and recommendations: Chapters 6 and 7. Chapter 6 contains the review of previous studies, while Chapter 7 contains the measurements and computations the committee and the subcontractors undertook during the course of the study. For the analysis of the previous studies in Chapter 6, the committee focused on the methodologies used to measure radiation emitted by the AIT systems as well as on the results reported. For the reports or papers in which assessments of the effective dose are based on estimates of AIT system characteristics rather than measurements, the committee provides a summary at the end of Chapter 6. Chapter 7 describes the committee’s measurement procedures, provides the measurements for the Rapiscan Secure 1000 and the second-generation AS&E AIT systems, and discusses the dose computations used. Descriptions of possible failures related to interlocks, subcomponents, and other issues that are fundamental to a functional AIT system are assembled in Tables 6.5, 6.6, 7.5, and 7.11.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 9
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 10
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 11
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 12
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 13
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 14
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 15
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 16
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 17
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 18
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 19
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 20
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 21
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 22
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 23
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 24
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
Page 25
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2015. Airport Passenger Screening Using Backscatter X-Ray Machines: Compliance with Standards. Washington, DC: The National Academies Press. doi: 10.17226/21710.
×
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Passenger screening at commercial airports in the United States has gone through significant changes since the events of September 11, 2001. In response to increased concern over terrorist attacks on aircrafts, the Transportation Security Administration (TSA) has deployed security systems of advanced imaging technology (AIT) to screen passengers at airports. To date (December 2014), TSA has deployed AITs in U.S. airports of two different technologies that use different types of radiation to detect threats: millimeter wave and X-ray backscatter AIT systems. X-ray backscatter AITs were deployed in U.S. airports in 2008 and subsequently removed from all airports by June 2013 due to privacy concerns. TSA is looking to deploy a second-generation X-ray backscatter AIT equipped with privacy software to eliminate production of an image of the person being screened in order to alleviate these concerns.

This report reviews previous studies as well as current processes used by the Department of Homeland Security and equipment manufacturers to estimate radiation exposures resulting from backscatter X-ray advanced imaging technology system use in screening air travelers. Airport Passenger Screening Using Backscatter X-Ray Machines examines whether exposures comply with applicable health and safety standards for public and occupational exposures to ionizing radiation and whether system design, operating procedures, and maintenance procedures are appropriate to prevent over exposures of travelers and operators to ionizing radiation. This study aims to address concerns about exposure to radiation from X-ray backscatter AITs raised by Congress, individuals within the scientific community, and others.

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