3

Radiation Protection Standards

In this chapter the committee discusses relevant guidelines and standards with a focus on U.S. regulatory efforts. The chapter lays out the details for dosimetry aspects for millimeter wave advanced imaging technology (AIT) and for health effects of millimeter wave AIT, specifically, the interaction of AIT millimeter waves with tissues, before describing the relevant standards.

DOSIMETRY ASPECTS FOR MILLIMETER WAVE AIT

Dosimetry is the study of the transfer of energy to matter. The transfer of energy to and within the body is required to do work, ranging from maintaining the temperature of human bodies, through heating and cooling, breathing, and such microscopic processes as constructing DNA (deoxyribonucleic acid) and chromosomes. Likewise, our interactions with the environment—including, for example, seeing, hearing, and feeling to communicating orally—requires external energy. Essentially all chemical and mechanical processes involve work and energy. The majority of that energy comes to Earth from the Sun in the form of electromagnetic waves (described in Appendix C). Because chemical processes and their biological consequences, including those at both the molecular and clinical levels, require energy, the amount of energy transferred to a biological system is related to, although not necessarily directly proportionally to, observed biological changes associated with externally supplied energy. If the changes are undesired or unhealthy, the sources are considered hazardous and the probability of the effects is defined as the risk. Frequently, the risk may be proportional, or at least a function of the

dose. Practically, it is difficult to measure or assess the relationship between dose and effects, particularly when the amount of energy absorbed is small (i.e., it does not produce changes that can be observed above the natural rates with sufficient certainty). Moreover, the magnitude or level of risk for equivalent amounts of energy per unit mass may be associated with other confounding factors, such as the length of exposure time, variations in the intensity with time, the amount of the body exposed, and biological factors, such as the age and weight of the individual. This is particularly the case with small doses of energy transferred by millimeter waves at intensities used for AIT where most of the energy is deposited at the surface. For these reasons, most recommendations and standards are usually stated in terms of the parameters of the radiation—that is, energy of the waves, wavelength, and their incident intensity—in the assessments of risks from an external source of energy rather than dose.

HEALTH EFFECTS OF MILLIMETER WAVES FROM AIT

There have been many scientific studies investigating the correlation between exposure to non-ionizing radiation and subsequent health effects. Because of the widespread use of cell phones operating in the 1 to 5 GHz range, many of these studies have focused on this radio frequency (RF) in the microwave range of the spectrum. However, the scientific literature concerning millimeter waves is very sparse; the most relevant studies are discussed below.

Deviations in the normal functioning of individuals or a population that result from external influence (i.e., exposure to microwave or millimeter wave energy) are referred to as health effects. This may include improvement and degradation of organism function, direct injury or healing, development of disease, and effects on reproductive status or life span.

These effects can by classified into the following three groups: deterministic effects, which include direct injury or healing observed in individuals; stochastic effects, which are changes that are only apparent as patterns in the study of large populations; and teratogenic effects, or changes that are expressed in the reproductive process (i.e., as birth defects).

Deterministic effects are readily observed in single beings and are often acute in nature, and the severity of their effects is readily predicted based on exposure. Typically, deterministic effects are associated with high exposure levels.

Stochastic effects are commonly those occurring at low exposure levels. These effects require study of large numbers of potentially affected individuals over a relatively long time to develop predictive models.

Teratogenic effects are associated with effects on the embryonic or fetal development due to exposure to the parent. Typically, these effects are observed as spontaneous abortion or congenital abnormalities. Teratogenic effects may, in extreme cases, be detected over a single gestation period, but generally also require a relatively long period of study.

Not all phenomena observed under experimental conditions result in what could be called health effects. Physical and biochemical changes induced under laboratory conditions might be impossible or unlikely to occur in the living organism, or, in some instances, the changes induced in the laboratory are offset by normal biological processes without undue stress upon the organism. In these cases, the observed changes are not considered health effects. Furthermore, effects observed in animal or artificial models often do not predict similar effects in exposed humans.¹

The millimeter wave AIT system emits pulse trains of electromagnetic waves that have frequencies from 24 to 30 GHz. Large number of studies have been conducted at frequencies used in mobile communications, but only few studies have been done in the gigahertz range, mostly at around 100 GHz. Almost no work has looked specifically at the 24 to 30 GHz range, so extrapolation from lower or higher frequencies are used.

INTERACTION OF AIT MILLIMETER WAVES WITH TISSUES

In the 24 to 30 GHz frequency range, electromagnetic energy incident upon tissues generally deposits maximum energy at the surface, and the amount of energy deposited, which decreases exponentially with depth, is inversely related to frequency; therefore in this part of the radio spectrum, high-frequency waves are less penetrating, and penetration increases as frequency is decreased. The power level of 26 GHz waves entering tissue will decrease to approximately 13 percent of the surface value at a depth of approximately 0.65 mm.² Because of the shallow, penetrative nature of the waves in this frequency range, the organs of concern are the skin and the eye.

The well understood deterministic effect on tissues in this frequency range is heating resulting from currents induced by the electric field interaction with conductive tissue, which is largely due to the interaction of the electromagnetic waves

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¹ National Council on Radiation Protection and Measurements, 1998, “Commentary No. 15—Evaluating the Reliability of Biokinetic and Dosimetric Models and Parameters Used to Assess Individual Doses for Risk Assessment Purposes,” ISBN 0-929600-58-4, https://www.ncrppublications.org.

² 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.

with water molecules that are present in abundance. Heating may also have a direct effect in the form of thermal damage to cells. As most biochemical reactions are thermally moderated, a less direct heating effect is biochemical deregulation. In a living organism, heating of tissues may be offset by blood flow to the immediate vicinity, which acts as a cooling mechanism. Therefore poorly perfused organs, such as some structures of the eye, may be at greater risk of thermal damage from a given exposure than other well-perfused regions of the body.³

STRUCTURE OF THE HUMAN EYE

The adult human eye (Figure 3.1) is approximately spherical in shape, with an average diameter of 24 to 25 mm. In children, the diameter is approximately 70 percent of an adult, or 17 mm. The anterior or front portion of the eye is translucent, and light entering passes through the cornea and fluid-filled anterior chamber, through the opening defined by the iris and the lens into the vitreous humor before striking the receptors at the posterior or rear portion of the eye. The remainder of the eye globe is covered by the whitish and opaque sclera. Thickness of the average

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³ Ibid.

cornea is 0.57 mm, and the anterior chamber and lens are typically 3.3 to 3.5 mm (although these two values vary among individuals). The thickness of the sclera varies from approximately 0.3 to 1.0 mm at different locations in the eye.⁴

AIT MILLIMETER WAVE EFFECTS ON THE HUMAN EYE

Induction of cataracts, or the formation of opacities in the lens, is understood to be a significant health effect to the eye due to exposure to electromagnetic energy. A temperature increase of greater than 3°C within the lens is generally regarded as the threshold for induction of cataracts.⁵ Current maximum permissible exposure (MPE) levels to the eye are based on limiting temperature increase.

One 2012 animal study investigated the ocular damage threshold in rabbits for millimeter waves with a frequency of 40 GHz and found that permanent ocular changes began to occur at incident power density levels between 100 and 200 mW/cm² (1,000 and 2,000 W/m² ), with transient changes observed at 100 mW/cm² (1,000 W/m²). The temperature changes reported at these power densities were up to 4°C at the anterior corneal surface at 200 mW/cm² (2,000 W/m²).⁶

Results of computational model investigated in 2013 indicated that changes in aqueous humor fluid flow in the anterior chamber were possible for electromagnetic exposure at frequencies from 40 GHz to 100 GHz at incident power densities of 10 to 160 mW/cm² (100 to 1,600 W/m²). Liquid flow was reversed as power levels were increased, because of the temperature differential with depth in the anterior eye at thermal values largely below the cataract threshold.⁷

STRUCTURE OF HUMAN SKIN

Covering greater than 95 percent of the body surface, skin is the largest human organ. It varies in thickness with location, from about 1.8 to 3 mm, and on average is composed of approximately 65 percent free water, 25 percent proteins, and 9 percent lipids. For purposes of electromagnetic modeling and calculation, skin can be thought of as having three main layers: the epidermis (0.06 to 0.1 mm), the

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⁴ A. Karampatzakis and T. Samaras, 2013, Numerical modeling of heat and mass transfer in the human eye under millimeter wave exposure, Bioelectromagnetics 34:291-299.

⁵ P. Bernardi, M. Cavagnaro, S. Pisa, and E. Piuzzi, 1998, SAR distribution and temperature increase in an anatomical model of the human eye exposed to the field radiated by the user antenna in a wireless LAN, IEEE Transactions on Microwave Theory and Techniques 46(12):2074-2082.

⁶ M. Kojima, N. Hasanova, Y. Suzuki, K. Sasaki, K. Wake, S. Watanabe, M. Taki, Y. Kamimura, A. Hirata, K. Sasaki, and H. Sasaki, 2012, Investigation of acute ocular damage threshold of 40 GHz millimeter wave on rabbit, 37th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), doi:10.1109/IRMMW-THz.2012.6380275.

⁷ Karampatzakis and Samaras, 2013.

dermis (1.2 to 2.8 mm), and a fat layer or hypodermis (1.1 to 5.6 mm). Epidermis is the outermost layer, and consists of three sublayers comprised of the stratum corneum (layers of keratinized/“dead” skin cells), the keratinocyte layer, and the basal epidermal layer. Approximately 25 percent of the outermost layer of the epidermis is stratum corneum, or 25 to 30 layers of “dead” cells. The remaining 75 percent is comprised of basal cells. As outer layers of cells slough off, they are replaced by epidermal cells that arise in the basal layer and become keratinized over a 10- to 14-day cycle. There is little to no blood supply or enervation of the epidermis, save at the interface of the basal layer with the dermis, which is well-vascularized and enervated. Collagen makes up approximately 75 percent of the dermis, arranged in fibrous bundles that give skin its elasticity. A simplified schematic representation of human skin is shown in Figure 3.2.⁸

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⁸ M. Zhadobov, N. Chahat, R. Sauleau, C. Le Quement, and Y. Le Drean, 2011, Millimeter-wave interactions with the human body: State of knowledge and recent advances, International Journal of Microwave and Wireless Technologies 3(2):237-247, doi:10.1017/S1759078711000122.

MILLIMETER WAVE EFFECTS ON HUMAN SKIN

The penetrative property of millimeter waves from AIT is such that approximately 90 percent of the energy is deposited at depths less than 0.6 mm. Therefore, the epidermis and dermis are the most affected skin structures. However, with regard to heating effects, it has been observed that heating from exposure is not limited to the depth of energy deposition, but may occur at slightly greater depths. This process depends strongly on time and local cooling mechanisms, such as external air temperature and regional blood flow (perfusion). Box 3.1 describes some papers in this field.⁹

MILLIMETER WAVES AND CANCER

Carcinogenesis, or the induction of cancer, is usually regarded as a stochastic effect, beause the causative factors for individuals are not always evident. The International Agency for Research on Cancer has classified all RF electromagnetic radiation as a “possible human carcinogen” (Group 2B) based on “limited evidence” from both human and animal studies.¹⁰

In a recent study from the National Toxicology Program of the National Institutes of Health, rats received lifelong (2-year) exposure to 0.9 GHz waves that were pulsed according to either CDMA (code division multiple access) or GSM (Global System for Mobile communication) cell phone modulation schemes. Exposure power density level was adjusted as the rats grew, to maintain a fixed specific absorption rate (SAR). The authors observed a “slight increase” in the presence of two types of cancer cells in a statistically significant number of the exposed animals, compared to those with no exposure.¹¹

SUMMARY OF AIT MILLIMETER WAVE HEALTH EFFECTS

For the millimeter wave AIT frequency range (24 to 30 GHz), there is little in the published record regarding health effect phenomena. Additionally, few studies consider the low levels of exposures in the range of the stated power density for

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⁹ T. Wu, T.S. Rappaport, and C.M. Collins, 2015, The human body and millimeter-wave wireless communications systems: Interactions and implications, arXiv:1503.05944v2[cs.ET].

¹⁰ International Agency for Research on Cancer, 2011, “IARC Classifies Radiofrequency Electromagnetic Fields as Possibly Carcinogenic to Humans,” Press Release 208, May 31, http://www.iarc.fr/en/media-centre/pr/2011/pdfs/pr208_E.pdf.

¹¹ M. Wyde, M. Cesta, C. Blystone, S. Elmore, P. Foster, M. Hooth, G. Kissling, et al., 2016, “Report of Partial Findings from the National Toxicology Program Carcinogenesis Studies of Cell Phone Radiofrequency Radiation in Hsd: Sprague Dawley SD Rats (Whole Body Exposure),” preprint, June, doi: https://doi.org/10.1101/055699.

AIT. Heating is the most well understood mechanism for this frequency range, and existing exposure guidelines are designed to prevent heating.

The existence of nonthermal effects remains controversial. Numerous experiments conducted at much lower cellular phone frequencies (1 to 6 GHz) and much higher millimeter wave frequencies (66 to 110 GHz) than AIT seem to indicate that there may be nonheating phenomena observable in vitro.¹² Results also seem to indicate that such athermal effects may be highly frequency dependent, making extrapolation to AIT frequencies extremely challenging. There seems to be no

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¹² A. Gurliuc, M. Zhadobov, and R. Sauleau, 2014, Dosimetric Aspects Related to the Human Body Exposure to Millimeter Waves, Report Deliverable D1.3 to the MiWaveS consortium of the European Union CEA, http://www.miwaves.eu/deliverables.html.

indication of mechanism or preponderance of reproducible scientific results that equivocally describe a health effect resulting from athermal mechanisms.¹³

The task for the study committee is focused on whether exposures comply with the current applicable health and safety standards for public and occupational exposures to non-ionizing millimeter wave radiation. The study committee is not mandated to examine the adequacy of the official radiation exposure standards currently in place.

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¹³ K. Sasaki, T. Nagaoka, K. Wake, and S. Watanabe, 2014, Dielectric property measurements of skin and dosimetry for millimeter wave irradiation up to 100 GHz, 2014 International Symposium on Electromagnetic Compatibility, Tokyo (EMC’14/Tokyo), pp. 537-540, http://ieeexplore.ieee.org.

STANDARDS

Guidelines establishing limits for millimeter wave exposure have been issued by several national and international organizations, most of which are nongovernmental (see Box 3-2). Among the most influential guidelines for the use and application of millimeter waves are those set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Institute of Electrical and Electronics Engineers (IEEE). Because there are no compulsory international safety standards for the exposure to nonionizing radiation, various international limit guidelines are adopted in each country, and in some cases by each government agency, into its national recommendations or legally binding regulations. Additionally, in the United States, each state has rights to establish its own laws and regulations. In the United States, the main federal agency responsible for RF health and safety is the Federal Communications Commission (FCC), with the Food and Drug Administration (FDA) responsible for medical devices and radiation-emitting products.

The AIT scanners being considered operate under FCC Part 15 rules for unlicensed low-power devices and are subject to certification under FCC Part 2 equipment authorization rules. The purpose of the Part 15 rules is to prevent interference

by unlicensed transmitters using the same spectrum. These are not exposure limits and typically specify a peak and average field strength limit at 3 m from the source. Certain Part 15 rules were waived in the public interest.¹⁴ Nevertheless, the scanners have to comply with FCC exposure limits.

Because the FCC is not a health and safety agency, it defers to other organizations and agencies with respect to interpreting the biological research necessary to determine what levels are safe. The FCC adopted present exposure limits in 1996, based on guidance from federal safety, health, and environmental agencies using recommendations published separately by the National Council on Radiation Protection and Measurements (NCRP) and the IEEE. The ICNIRP has developed a recommendation (published in 1998) supported by the World Health Organization (WHO), which is currently being revised. The IEEE has revised its recommendations several times, while the NCRP has continued to support its recommendation. Because of these developments, the FCC has opened an inquiry to determine whether current exposure limits remain appropriate. In addition to the FCC, the FDA is responsible for the safety of individuals with medical devices. See the section below on FDA recommendations for individuals with medical devices that are exposed to millimeter waves.

Guidelines often allow for higher exposure for the occupationally exposed population, because it consists of adults who are generally exposed under known conditions and are trained to be aware of potential risks and to take appropriate precautions. By contrast, the general public comprises individuals of all ages and of varying health status and may include particularly susceptible groups or individuals, such as children and pregnant women. For TSA workers, as well as other airline and airport personnel not specifically trained in RF exposure risks, general public exposure limits should apply, and thus only limits for general public exposure are presented in this report.

Hareuveny et al. have stated in the Journal of Radiological Protection that the RF exposure limits published by various organizations are similar to one another in terms of power density (W/m²) across the RF spectrum and are based on the principle of protecting individuals against potentially adverse effects resulting from tissue heating.¹⁵ An extensive literature of animal experimentation indicates disrupted food behavior when the SAR is above about 4 W kg⁻¹ for roughly an

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¹⁴ Federal Communications Commission, 2006, Order in the Matter of SafeView, Inc. Request for Waiver of Sections 15.31 and 15.35 of the Commission’s Rules to Permit the Deployment of Security Screening Portal Devices That Operate in the 24.25-30 GHz Range (ET Docket No. 04-373), FCC document DA-06-1589, released August 4, available at https://apps.fcc.gov/edocs_public/.

¹⁵ R. Hareuveny, R. Kavet, A. Shachar, M. Margaliot, and L. Kheifets, 2015, Occupational exposures to radiofrequency fields: Results of an Israeli national survey, Journal of Radiological Protection 35(2): 429-445.

hour¹⁶ and the guidelines and standards limit exposure levels such that theSAR for the general public limit is 0.08 W kg⁻¹ (i.e., a safety factor of 50). The limits recognize that the efficiency of RF energy absorption rate is body size and shape dependent, with an “average”-sized adult maximally resonant at about 60 to 80 MHz. To accommodate persons of all geometries, the guidelines/standards power density limits have a minimum at frequency spans of up to 10 to 400 MHz (ICNIRP) (Figure 3.3). Beyond about 100 kHz, coupling of the field to the body starts to transition from quasi-static induction to the production of thermal energy.¹⁷

Hareuveny et al. additionally stated in the Journal of Radiological Protection that as frequency increases toward the resonant trough, the body becomes a more efficient receiving antenna; also, the decrease in tissue permittivity produces a larger

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¹⁶ J.A. D’Andrea, E.R. Adair, and J.O. de Lorge, 2003, Behavioral and cognitive effects of microwave exposure, Bioelectromagnetics S39–S62 (suppl. 6).

¹⁷ R. Hareuveny, R. Kavet, A. Shachar, M. Margaliot, and L. Kheifets, 2015, Occupational exposures to radiofrequency fields: Results of an Israeli national survey, Journal of Radiological Protection 35(2): 429-445.

induced electric field and, thus, an increased SAR for the same power density. As frequency increases beyond resonance, the body becomes a less efficient receiving antenna, and skin effect becomes dominant, which decreases coupling efficiency to internal tissues, resulting in relaxed exposure limits in the gigahertz range. Although the International Agency for Research on Cancer (IARC) has classified RF as a “possible human carcinogen” (Group 2B) based on “limited evidence” from both human and animal studies, the weight of evidence has not risen to a level that would change the basis for RF exposure limits.¹⁸

As was made clear in the previous section, the scientific data on possible health effects of millimeter waves is sparse. Aside from thermal effects, little is known about mechanisms that may govern millimeter wave interactions with biological systems. Therefore, exposure limits are based on the avoidance of adverse effects caused by whole body heat stress and/or tissue damage caused by excessive localized heating. Power density, rather than SAR, is used as both a primary metric and maximum permissible exposure (MPE) at frequencies of 24 to 30 GHz, due to the shallow depth of penetration at these high frequencies.

The power density is spatially averaged over contiguous area that varies from guideline to guideline. More relaxed spatial peak power density is allowed for localized exposures. In addition, fields are averaged over a certain time period that decreases with increasing frequency (5 min for IEEE and around 2 min for ICNIRP at 30 GHz). For pulsed fields, as is the case here, the peak incident power density is averaged over the pulse width. It is not clear whether any time averaging should apply to the airport scanning, because the FCC recommends time averaging mostly for occupational exposures and states that time-averaging provisions of the MPE guidelines may not be used in determining typical exposure levels for portable devices intended for use by consumers. Instead, “source-based” time-averaging “based on and inherent property or duty-cycle” would be appropriate in this case (47 CFR 2.1091(d) (2)).

In the present case, this source-based time averaging may be more appropriate. The inherent duty cycle is somewhat difficult to define, but a worst-case scenario can be envisioned in which a passenger remains inside the AIT system continuously and is imaged repeatedly. In that case, the duty cycle is given by

Here, T_rad is the total amount of time during which the millimeter waves are being emitted during a single scan, and T_scan is the minimum reset time between scans. The manufacturer of the ProVision systems provided the following approximate numbers: T_rad = 0.48 s, while T_scan = 1.3 s. Therefore the approximate duty

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¹⁸ Ibid.

TABLE 3.1 Reference Levels for Average and Peak Exposure for the General Public in the 25 to 30 GHz Range

NOTE: fG, frequency in GHz; fM, frequency in MHz.

^a Federal Communications Commission (FCC), Office of Engineering and Technology, “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields,” OET Bulletin 65, Edition 97-01.

^b Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz,” IEEE Std C95.1-2005, copyright 2006 by IEEE, New York, N.Y.

^c Instantaneous peak E field.

^d For pulsed fields measure the instantaneous peak.

^e International Commission on Non-Ionizing Radiation Protection (ICNIRP), 2010, Guidelines for Limiting Exposure to Time-Varying Electric and Magnetic Fields (1 Hz - 100 kHz), Health Physics 99(6): 818-836.

^f Averaged over pulse width.

cycle DC = 0.37. To apply this time-averaging procedure, one would divide the peak intensity on the target by this duty cycle in order to obtain a numerical value that can be compared to the applicable power density standard (shown in Table 3.1). For a more detailed description of the pulse shape, see the section related to the airport measurements in Chapter 6 and the system description in Chapter 2.

As can be seen in Table 3.1, limits set on average power density fields are remarkably consistent. The main differences arise from differences in averaging time; ranging from about 3 to 6 min to 30 min. Limits on peak pulsed-RF fields vary substantially. Because little information is available on the relation between biological effects and peak values of pulsed fields, there is a lack of harmonization among standards. The FCC inquiry is considering whether to adopt peak pulsed-field limits for RF sources regulated by the commission and, if so, what limits, would be appropriate.

Additionally, induced current and/or contact current reference levels might be relevant. These levels, for example, in the Health Canada Safety Code 6, are depending on frequency, 0.3 to 40 mA for induced current and 0.6 to 20 mA for contact current.

For simultaneous exposure to multiple frequencies (not relevant in this case)

and where exposure is estimated in terms of field strength, each of the squares of the field strength frequency component amplitudes shall be divided by the square of the corresponding field strength reference level for that frequency, and the sum of all these ratios shall not exceed unity. In the AIT systems, there is never more than one frequency of millimeter wave radiation present at any time, so this procedure is not relevant here.

SUMMARY

With small doses of energy transferred by millimeter waves at intensities used for AIT, most of the energy is deposited at the surface. Numerous experiments have been conducted at much lower cellular phone frequencies (1 to 6 GHz) and a few at higher millimeter wave frequencies (66 to 110 GHz). Additionally, few, if any, studies consider the low levels of exposures in the range of the stated power density for millimeter wave AIT. Heating is the most well understood mechanism for this frequency range.

Guidelines establishing limits for millimeter wave exposure have been issued by several national and international organizations, most of which are nongovernmental. In the United States, the main federal agency responsible for RF health and safety is the FCC, with the FDA responsible for medical devices and radiation-emitting products.

The RF exposure limits published by various organizations are similar to one another in terms of power density (W/m²) across the RF spectrum and are based on the principle of protecting individuals against potentially adverse effects resulting from tissue heating. Limits set on average power-density fields are remarkably consistent and are set as 10 W/m². The main differences arise from differences in averaging time. Limits on peak pulsed-RF fields vary substantially. Because little information is available on the relation between biological effects and peak values of pulsed fields, there is a lack of harmonization among standards. The committee considers source-based time averaging as most appropriate in this case. The inherent duty cycle is somewhat difficult to define, but a worst-case scenario can be envisioned in which a passenger remains inside the AIT system continuously and is imaged repeatedly. In that case, the duty cycle is approximately 0.37.

Finding 3.1: Few studies have looked at millimeter waves at frequencies and/or intensities used for AIT.

Finding 3.2: In the United States, the main federal agency responsible for RF health and safety is the FCC, with the FDA responsible for medical devices and radiation-emitting products.

Finding 3.3: The RF exposure limits published by various organizations are similar to one another in terms of power density (W/m²) across the RF spectrum and are based on the principle of protecting individuals against potentially adverse effects resulting from tissue heating. Limits on peak pulsed-RF fields vary substantially.

Finding 3.4: Limits set on average power density fields are set as 10 W/m². However averaging times used might differ.

Recommendation 3.1: The Transportation Security Administration should adopt source-based time averaging.