The Transportation Security Administration (TSA) is responsible for keeping weapons and explosives off commercial airliners. A part of its strategy involves screening passengers prior to boarding aircraft at U.S. airports using advanced imaging technology (AIT) to identify concealed objects on passengers.
In 2008, the TSA introduced X-ray backscatter AIT systems manufactured by Rapiscan for airport security. However, there were public health concerns raised concerning the use of ionizing radiation.1 Moreover, the use of full-body images raised privacy concerns in the public sector. In response, Congress mandated that all AIT units be equipped with Automatic Target Recognition (ATR) software that displays potential threats on a generic figure rather than an image of the individual’s body. When Rapiscan failed to meet ATR specifications by the congressional deadline (June 2013), all 250 Rapiscan units were removed from U.S. airports.
TSA currently relies on millimeter wave AIT systems, which emit non-ionizing radio frequency (RF) waves. These systems are manufactured by L3 Security and Detection and are equipped with ATR software. L3 has provided two types of systems to the TSA, the ProVision ATD, where ATD stands for Automatic Target
1 Ionizing radiation is radiation that carries enough energy to free electrons from atoms and molecules so they become ionized. Non-ionizing radiation is only able to excite electrons to a higher energy state but not free the electrons.
Both TSA and the equipment manufacturers state that the millimeter wave AIT systems in use are safe under normal conditions, with exposures received by both frequent passengers and machine operators being below currently accepted safety standards.4 Further, they claim that both operating procedures and equipment safety interlocks assure safety and contain provisions for avoiding accidental radiation overexposures, even in the event of equipment malfunction.
Millimeter Wave Body Scanners
Active millimeter wave scanners, the type being used in U.S. airports, emit RF electromagnetic waves. The waves pass through clothing, bounce off the skin (and potential threats), and return to an array of receiving antennae. The reflected signals are post processed by computer ATR software that identifies potential targets, creates two generic figures (front and back), including spatially identified targets, then sends this information to an operator station. Active millimeter scanners such as the L3 ProVision operate in non-continuous modes (chirped pulse signal), with typical parameters (standard operating settings) involving 10 to 100 µW of total output power from antennas operating at 24-30 GHz.5
Measurement and Relevant Quantities
Biological effects relevant for the evaluation of potential hazards or risks from radiation exposures are generally related to the amount of energy absorbed from the radiation per unit mass in units of joules per kilogram (J/kg), which is called the absorbed dose or, simply, the dose. For RF electromagnetic fields, the dose is usually inferred from parameters defining the fields themselves, such as intensity, frequency, and power-density distributions. The intensity of RF electromagnetic fields is most conveniently expressed in terms of the root mean square (rms) field strength of the electric and magnetic components. These are expressed in units of volts per meter (V/m) and Teslas (T), respectively. For frequencies above 300 MHz, power density is often used to express intensity. It is defined as power per unit area
2 There is only a footprint size difference between the two systems, so in the remainder of the report no difference will be made between the two types.
3 Transportation Security Administration, “Technology,” factsheet, https://www.tsa.gov/sites/default/files/resources/technology_factsheet_051916-508.pdf, accessed September 27, 2016.
4 The equipment manufacturer has limited liability because they are covered under the Safety Act of 2001.
5 In the United States, the L3-produced ProVision system operates at 24.25 to 30 GHz with −11.6 dBm (70 µW) at array output (Doug McMakin, presentation to the committee, February 25, 2015).
and is typically expressed in milliwatts per square centimeter (mW/cm2) or watts per square meter (W/m2).6
Power density is most accurately used when the point of measurement is far enough away from the radiation source7 (far-field region)—for example, more than several wavelengths distance from a typical RF emitter. In the far field, the electric and magnetic fields have simple mathematical relationships to power density. Thus it is only necessary to measure one of these quantities in order to determine the other. In the near-field region,8 the physical relationships between the electric and magnetic components of the field are usually complex, and it is necessary to determine both the electric and magnetic field strengths to fully characterize the RF environment. In the case of the L3 AIT, the RF emitter is at least 10 cm from the subject (the distance from the emitter to the Plexiglas separating the emitters from the subject). This corresponds to at least about 10 wavelengths, so the far-field criterion is satisfied. In addition to intensity, the frequency of an RF electromagnetic wave, defined as the number of cycles through which an alternating field passes per second (1 cycle/s = 1 hertz), can be important in determining how much energy is absorbed and, therefore, the potential for harm. The quantity used to characterize this absorption is called the specific absorption rate (SAR), which measures the rate at which the body absorbs RF energy and is usually expressed in units of watts per kilogram (W/kg). However, at frequencies greater than 10 GHz, at which energy absorption occurs primarily at the body surface (and where the AIT systems under consideration here operate), the SAR is not a good measure for assessing absorbed energy. The incident power density of the field is a more appropriate dosimetric quantity. Chapter 2 of this report addresses the relevant radiation physics in greater detail.
It has been known for many years that exposure to high levels of RF radiation can be harmful due to thermal effects. Exposure to high RF power densities (on the order of 1,000 W/m2 or more) can result in heating of biological tissue and an increase in temperature that can cause tissue damage, due to the body’s inability to quickly dissipate the excessive heat generated. Two areas of the body, the eyes and the testes, are known to be particularly vulnerable to heating by RF energy because of the relative lack of available blood flow to dissipate the excessive heat load.
6 The SI unit is W/m2, but the unit has commonly been expressed as mW/cm2. To convert mW/cm2 to W/m2, simply multiply by 10.
7 Far-field field strength decreases inversely with distance from the source, resulting in an inverse-square law for the radiated power intensity of electromagnetic radiation.
8 Near-field field strength decreases more rapidly with distance—for example, with an inverse-distance squared or cubed relation.
In the far field of an RF source, whole-body absorption of RF energy by a standing human adult has been shown to occur at a maximum rate when the frequency is between about 80 and 100 MHz, depending on the size, shape, and height of the individual. In other words, the SAR is at a maximum under these conditions. Because of this “resonance” phenomenon, RF safety standards have placed the most restrictive limits on exposure in this VHF (very high frequency) range.
Aside from thermal interactions and the so-called “microwave heating effect,”9 little is known about millimeter wave interactions with biological systems. With regard to nonthermal interactions, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) examines available data related to potential nonthermal effects and concluded, “Whilst it is in principle impossible to disprove the possible existence of non-thermal interactions, the plausibility of various non-thermal mechanisms that have been proposed is very low.”10 Nonthermal effects reported have included changes in the immune system, and neurological and behavioral effects.11Chapter 3 of this report addresses the relevant radiation protection guidelines and standards in detail.
Interaction with Personal Electronic Medical Devices (PMEDs) and Volatile Materials
Concerns have been raised that signals from RF devices could cause electromagnetic interference (EMI) and disrupt the operation of implanted electronic pacemakers and other medical devices. Electromagnetic shielding has been incorporated in the design of modern pacemakers, and the Food and Drug Administration requires manufacturers to test their devices for susceptibility to EMI. This issue is addressed in detail in Chapter 6 of this report.
10 International Commission on Non-Ionizing Radiation Protection, 2009, “Exposure to high frequency electromagnetic fields, biological effects and health consequences (100 kHz-300 GHz),” Review of the Scientific Evidence and Health Consequences, Munich, Section II.6.2.
11 Scientific Committee on Emerging and Newly Identified Health Risks, 2013, Preliminary Opinion on Potential Health Effects of Exposure to Electromagnetic Fields, November 29, http://ec.europa.eu/health/scientific_committees/emerging/docs/scenihr_o_041.pdf.
The TSA requested a study by the National Research Council12 (NRC) to evaluate two models of active millimeter wave scanners: the L3 ProVision 1 and L3 ProVision 2. Specifically, it has asked the NRC 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 study request includes supporting measurements at four different airport locations.
With regard to the statement of task (Box 1.1), it is noted that all millimeter wave radiation is non-ionizing but it is stated to emphasize the difference to X-ray radiation, which has also been used in AIT and is ionizing.
This study was carried out by a committee of experts appointed by the NRC. The Committee on Airport Passenger Screening: Millimeter Wave Machines consisted of 14 members with expertise that spans the disciplines relevant to the study task: aviation security, systems engineering, manufacturing testing and evaluation,
12 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 to refer to activities before July 1.
mechanical engineering, electrical engineering, physics, health physics, materials, epidemiology, and non-ionizing radiation physics and biology.
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. NRC staff issued a request for proposals (RFP) for this subcontract. The statement of work and deliverables were defined by the NRC in consultation with the study committee. Decision on the award 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 RFP.
This report is organized into a summary and the following eight chapters that address the statement of task:
Chapter 1 (this chapter) provides an introduction to the study.
Chapter 2 describes millimeter wave advanced imaging technology.
Chapter 3 describes radiation protection guidelines and standards.
Chapter 4 reviews previous studies of millimeter wave AIT.
Chapter 5 addresses personal implants and medical devices.
Chapter 6 describes the NRC-led measurements.
Chapter 7 describes the system design.
Chapter 8 gives findings, conclusions, and recommendations.