Guidelines establishing limits for millimeter wave exposure1 have been issued by several national and international organizations, most of which are nongovernmental.2 In the United States, the main federal agency responsible for radio frequency (RF) health and safety standards is the Federal Communications Commission (FCC), with the Food and Drug Administration (FDA) responsible for medical devices and radiation-emitting products. The RF exposure guidelines published by these various organizations are similar to one another in terms of average power density (W/m2)3 across the RF spectrum and are based on the principle of protecting individuals against potentially adverse effects resulting from tissue heating. The guidelines for average power density fields (for public exposure) are remarkably consistent, always given as 10 W/m2. The main difference arises from variations in the time prescribed for averaging the intensity of the electromagnetic waves, with proposed averaging times ranging from 3 min to much longer. (See Boxes S.1 and S.2 for details on important terms and time averaging.)
The Transportation Security Administration requested a study by the National Research Council (NRC) to establish the Committee on Airport Passenger Screening: Millimeter Wave Machines to evaluate two models of active millimeter wave
1 In this report the RF electromagnetic waves of interest (emitted by the ProVision portals) are in the 24 to 30 GHz frequency range.
2 See for example: Institute of Electrical and Electronics Engineers (IEEE), 2006, “IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Field, 3 kHz to 300 GHz,” IEEE Std C95.1™-2005 (Revision of IEEE Std C95.1-1991), ISBN 0-7381-4835-0-SS95389.
scanners: the L3 ProVision 1 and L3 ProVision 2. The Committee on Airport Passenger Screening: Millimeter Wave Machines was formed to provide findings and recommendations on (1) compliance with applicable health and safety guidelines and (2) appropriateness of system design and procedures for preventing over exposure.
The main issue for the committee to address during this study was whether millimeter wave machines used at airports comply with these existing guidelines. That is, do they expose a person to more or less than 10 W/m2? Also, is there anything that can go wrong with the machine so that, by mistake, it exposes a person to more than 10 W/m2?
To answer these questions, the committee looked at some previous reports discussing millimeter wave AIT machines. The committee took a limited set of available documentation describing the machines and, using that information, it guided a team in the field to measure actively used machines at four U.S. airports selected by the committee to include both ProVision 1 and ProVision 2 machines and some with high as well as low throughput of people being screened.
At the airports, measurements were done in positions 1, 2, and 3 according to Figure S.1. To appreciate the distance from the subject to the antenna masts, consider the fact that a typical American male has a bust depth of approximately 0.3 m and is asked to stand in the center of the machine on a set of marked footprints. In order to reach scan line 1, and thus getting closer to the antennas and
receiving a higher exposure, a positioning error of 15 cm would require a subject to have significant parts of both feet outside the footprint decals, a case that would be clearly out of compliance with the operator’s instructions. For a detailed description of how the machine works, see Chapter 2, section “System Design and Operating Procedures.”
The measurements at four airports4 show that the peak intensity of the pulsed signal at scan plane 1 is only in the range of 0.0001 W/m2, and for plane 2, it is 0.00003 W/m2, representing 100,000 and 320,000 lower than the 10 W/m2 limit mentioned above. Hence even an “out of position” person will receive a continuous power density during a scan that is no more than what is in scan plane 1—that is,
4 The airports were chosen by the committee and the subcontractor from a set of 30 airports made available to the committee by Transportation Security Administration; the selection enabled variation in portal use and type as well as timely travel to each.
100,000 times below the applicable guideline exposure limit—and certainly will receive much less standing in the center of the portal. According to these measurements, the value of 0.0001 W/m2 can be considered an upper limit on the peak value of the exposure.
It will become clear from the measurements and descriptions in this report that the transmitter is active for about 6 microseconds (µs) and then off for the remaining 3 µs of the period in which an antenna element transmits. Furthermore,
during the 1.3 s that the scan of a person lasts, some antenna is transmitting for ~0.5 s, producing a duty cycle of 37 percent.
The question now is what time should be used to average the signal, 1.3 s, or as the different guidelines prescribe, 3 min, 6 min, or 30 min, which in turn would give intensity that is approximately 270,000, 38 million, 75 million, or 375 million times below the recommended limit, respectively. See Box S.2 for a description of averaging according to the IEEE standard. Even with an average time of 1.3 s, a person that is standing too close to the antennas would receive an exposure that is 270,000 times below the guidelines. Therefore, the committee offers the following finding:
Finding: The National Academies’ committee-led measurements at airports indicate that even an “out of position” person will receive an average pulsed power density during a scan that is 270,000 times below the applicable recommended exposure limit of 10 W/m2 and will receive even less standing correctly in the center of the portal. (Chapter 6)
Additionally, measurements at the entry point of the portal and further away, where a bystander might be, similarly indicate that the average power density is at least several million times below the recommended limit.
Just as the 24 to 30 GHz frequency range, referred to as in-band emissions, has been described above, the committee also looked at the subharmonic (lower) out-of-band frequencies and briefly described their results in this report. For the out-of-band measurements, a standard dual polarized horn antenna was positioned in close proximity to the ProVision scanner in an attempt to detect harmonics. Radiated emissions were sampled in the 25 percent harmonic band and the 50 percent harmonic band while the ProVision scanner was operating in the mid-scan position, and in repetitive sweep mode. Despite placing the spectrum analyzer antenna right up against the ProVision Lexan radome, no radiation was detected. This indicates that if any radiation is present in these frequency bands, the intensity is too low to be measured using the instruments that were available for the measurements. This allows for an upper limit on the intensity of the out-of-band emissions to be determined, which is considerably lower than that of the in-band emissions.
Another part of the committee’s task was related to the system design and the possibility that a portal malfunctions and produces a signal of a higher strength than during normal operation. The committee looked at the transceiver and antenna system in detail. The transceiver employed is based on a gallium arsenide (GaAs) monolithic microwave integrated circuit; the specific device has a maximum output power of 0.032 W, and attempting to drive the device above its maximum output for more than 1 s may cause permanent damage to the device. This information, in combination with the fact that the current transceiver design
has the output amplifier operating in full saturation, will make it highly unlikely that the signal generation is increased beyond the normal value. Furthermore, the cables and switches between the transceiver and the antenna array are such that any damage or accidental, unintended alteration of them would decrease the signal strength at the antenna, not increase it. The committee has made the following finding:
Finding: The signal power cannot be higher than during normal operation due to the system transceiver operating at full saturation. (Chapter 7)
Engineers often determine the performance and safety of complex systems by performing a Failure Modes and Effects Analysis (FMEA). An FMEA effort includes methodologies designed to identify potential failure modes for a product or process, and it is used to assess the risk associated with those failure modes, rank the issues in terms of importance, and identify and carry out corrective actions to address the most serious concerns. In the material received by the committee from the TSA, no formal FMEA existed in the traditional sense, but upon request, the manufacturer, L3 Technologies (formerly, L-3 Communications Holdings), provided a table describing some of the subsystems. As discussed in Chapter 8, this information was enough for the committee to execute the statement of task for this report. However, there are many published guidelines and standards for the conduct and recommended reporting format of FMEAs. Some of the main published standards for this type of analysis include SAE J1739,5 AIAG FMEA-4,6 and MIL-STD-1629A.7 In addition, many industries and companies have developed their own procedures to meet the specific requirements for their products and processes. The conduct of a FMEA gives the designers insight into the possible failure modes of complex machines such as the AIT scanners and thus valuable insight into any potential safety issues. Therefore, this committee has found and recommends the following:
Finding: It appears that no formal FMEA following established procedures was conducted while the active millimeter wave AIT portals were designed and manufactured. (Chapter 7)
7 See Department of Defense, “Procedure for Performing a Failure Mode, Effects and Criticality,” Military Standard MIL-STD-1629A, issued November 24, 1980, cancelled in August 1994.
Recommendation: The Transportation Security Administration should require a Failure Mode and Effects Analysis following established procedures to be done on any complex systems they plan to put into operation. (Chapter 7)
During the review of the documentation and procedures followed during installation, maintenance, and operation of the systems, the committee made the following observations:
Finding: The installation procedure leaves the ProVision system in a normal operating condition in which both the radio frequency and mechanical systems are operating as designed. (Chapter 7)
Finding: Maintenance of the ProVision systems returns the system to its nominal operating condition. (Chapter 7)
Finding: Practically all potential malfunctions of the ProVision 1 and ProVision 2 are detected during the cycle of scanning just one individual. (Chapter 7)
Finding: During normal operation of the ProVision system, and also considering the potential failure modes of the system design, it is not possible for the scanned subject to be exposed to a power density exceeding the recommended maximum exposure limit of 10 W/m2. (Chapter 7)
The committee also reviewed previous reports that relate to the analysis of millimeter wave AIT. Available to the committee were the following three reports:
- Assessment of Health Risks Related to Use of the ProVision 100 “Millimetre-Wave”Body Scanner.8 Published by the French Agency for Environmental and Occupational Health Safety (AFSSET), this report discusses the ProVision 100, the predecessor to the ProVision 1 and ProVision 2.
- Report of Measurements and Assessment for Potential Electromagnetic Interference Effects on Personal Medical Electronic Devices from Exposure to Emissions from the L3Provision MillimeterWaveAdvancedImagingTechnology
8 French Agency for Environmental and Occupational Health Safety (AFSSET), 2010, Assessment of Health Risks Related to Use of the ProVision 100 “Millimetre-Wave” Body Scanner, Collective Expert Assessment Report, Maisons-Alfort, France, February.
(AIT) Security System.9 This FDA report also discusses the predecessor design.
- Underwriters Laboratories Test Report E240592-A1-IT-5.10
In short, the following findings are made by the committee:
Finding: The committee finds previous reports having recorded values between 3,000 and 250,000 times lower than the recommended exposure limit of 10 W/m2 from active AIT millimeter wave portals. (Chapter 4)
Finding: Previous reports applied various assumptions on how to measure and average the pulsed signal from an active AIT millimeter wave portal, which might explain some of the differences in their findings. (Chapter 4)
Another aspect that some of the previous reports looked at, which this report has considered as well, is related to implantable medical devices. The committee reviewed information available for patients as well as physicians and discussed manufacturer’s recommendations regarding the use of these devices when proceeding through millimeter wave AIT scanners. Recalling that the power density of 26 GHz waves entering tissue will decrease to approximately 13 percent of the surface value at a depth of approximately 0.65 mm, it is possible that implantable medical devices that are not placed in their entirety deep inside the body are of high interest. The following findings highlight the committee’s work:
Finding: There seems to be no available documented evidence of any deviation or effect from normal operation of preemptive medical devices when being scanned with a millimeter wave AIT. However, there seems to be many cautionary warnings of potential deviations. (Chapter 5)
Finding: The frequency range for current millimeter wave portals is such that the power is mostly deposited in the outer surface or skin of the body and as
9 Center for Devices and Radiological Health, Food and Drug Administration, 2011, Report of Measurements and Assessment for Potential Electromagnetic Interference Effects on Personal Medical Electronic Devices from Exposure to Emissions from the L3 Provision Millimeter Wave Advanced Imaging Technology (AIT) Security System, April 4, in Department of Health and Human Services (DHS), 2012, Compilation of Emission Safety Reports on the L3 Communications, Inc. Provision 100 Active Millimeter Wave Advanced Imaging Technology (AIT) System, Draft, DHS/ST/TSLICC4711, Washington, D.C., January 31.
10 Underwriters Laboratories, Inc., 2010, Test Report E240592-A1-IT-5, for L-3 Communications Safeview Full-Body Scanner, July 2.
such devices that are not deeply, or only partially, embedded might be of greater concern. (Chapter 5)
Recommendation: The Transportation Security Agency gives travelers with implanted electronic devices, whether life sustaining or therapeutic, a choice between hand screening and millimeter wave advanced imaging technology (AIT). Travelers may weigh the advice of their physicians and the device manufacturers against the absence of evidence of any deviation or effect from normal operation of preemptive medical devices when being scanned with a millimeter wave AIT. (Chapter 5)
The findings and recommendations are available within each chapter and also in full in Chapter 9.