The Transportation Security Administration (TSA) has required that the X-ray backscatter advanced imaging technology (AIT) systems approved for deployment conform to the American National Standards Institute/Health Physics Society (ANSI/HPS) Accredited Standards Committee N43 (Equipment for Non-Medical Radiation Applications) standard N43.17.1,2,3 The ANSI/HPS N43.17 standard is a consensus standard that provides guidelines for both manufacturers and users of the systems and covers dose to subject, interlocks, operational procedures, and information to be provided to the travelers by the operators. Prior to this standard, there was little guidance for this type of intentional, nonmedical radiation
3 There is a comparable international standard, International Electrotechnical Commission (IEC) 62463-2010, “Radiation Protection Instrumentation-X-Ray Systems for the Screening of Persons for Security and Carrying of Illicit Items,” Geneva, Switzerland.
Manufacturers of electronic products that emit radiation also need to comply with the Federal Food, Drug and Cosmetic Act, Chapter V, Subchapter C; and manufacturers of personnel security screening X-ray AIT systems must comply with applicable requirements of Title 21 of the Code of Federal Regulations 1000-1005.6 In addition, system operators need to comply with Occupational Safety and Health Administration ionizing radiation safety limits as promulgated in Title 29 of the Code of Federal Regulations, Part 1910.1096.7 This regulation specifies radiation dose limits for the whole body, lens of the eye (same as whole body), and skin for occupationally exposed individuals.
The ANSI/HPS N43.17-2009 standard refers to the airport X-ray backscatter AIT systems as “general use” X-ray security screening systems and limits the dose per screening to 250 nSv, referred to as the reference effective dose. This description for dose is a simplified version of effective dose and is easier to calculate but still considered sufficiently accurate to ensure radiation safety (see Chapter 4 for a comprehensive discussion of reference effective dose.) Together with establishing a per-screening dose limit for the person being screened, the standard also recommends annual radiation dose limits of 250,000 nSv over a 12-month period (1,000 screens per year).
The annual dose limit of the ANSI/HPS N43.17-2009 standard is based on dose limit recommendations for the general public published in National Council on Radiation Protection and Measurements (NCRP) Report No. 116,8Limitations of Exposure to Ionizing Radiation, and endorsed in a later NCRP commentary,9 “Screening of Humans for Security Purposes Using Ionizing Radiation Scanning Systems,” prepared at the request of the Food and Drug Administration. NCRP10 recommends limiting the annual effective dose from all sources (excluding back-
4 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.
6 Food and Drug Administration, “CFR—Code of Federal Regulations Title 21,” last updated September 1, 2014, http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?CFRPart=1000.
7 Department of Labor, Occupational Safety and Health Administration, Standard 1910.1096, “Ionizing Radiation,” https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10098.
8 National Council on Radiation Protection and Measurements (NCRP), Report No. 116, Limitation of Exposure to Ionizing Radiation, Bethesda, Md., 1993.
9 NCRP, “Screening of Humans for Security Purposes Using Ionizing Radiation Scanning Systems,” Commentary No. 16, Bethesda, Md., 2003.
10 NCRP, Report No. 116, 1993.
ground and medical)11 for members of the public to 1,000,000 nSv. If a detailed assessment of all sources is not conducted, NCRP recommends that no single source or set of sources “under one control” should result in an individual being exposed to more than 250,000 nSv annually. For backscatter systems, “under one control” refers to the use of ionizing radiation AITs at one or more security checkpoints at a given venue (e.g., multiple checkpoints at a given airport).12 In other words, NCRP recommends that a passenger not receive a dose of more than 250,000 nSv in 1 year from being scanned in a single airport. However, if this same passenger received a dose of more than 250,000 nSv in 1 year from being scanned in airports in different cities, NCRP’s recommended administrative control is still met. Administrative controls are essential for keeping radiation exposure levels in compliance with the radiation safety principle of as low as (is) reasonably achievable (ALARA).13
The ANSI/HPS N43.17-2009 standard also addresses the issue of various subgroups (for example, pregnant women and children) of the general population being more susceptible to the radiation-induced health effects compared to others. Again, the standard refers to NCRP Report No. 116,14 which recommends limiting the annual effective dose from all sources (excluding background and medical) for members of the public to 1,000,000 nSv. It does not issue different limits for these subgroups of the general population, implying that the 1,000,000 nSv dose limit offers sufficient protection to them also. However, NCRP issues a different limit for the embryo or fetus of an occupationally exposed woman and recommends a maximum occupational dose15 of 500,000 nSv per month. For comparison, the occupational dose limit is 50,000,000 nSv per year.
ANSI/HPS N43.17-2009 provides detailed guidance not only for acceptable radiation dose levels for individuals and radiation workers but also for radiation-producing systems, manufacturing, installation, safety performance, and regular maintenance. Radiation-producing instruments are required to include fail-safe mechanisms that would halt the operation in case of major failures in mechanical
11 These sources include activities such as nuclear power generation, decommissioning of radioactive waste, industrial and research activities, and security inspection systems.
12 D.A. Schauer, Does security screening with backscatter x-rays do more good than harm?, Radiology 259:12-16, 2011.
13As low as (is) reasonably achievable is defined in Title 10, Part 20.1003 of the Code of Federal Regulations (CFR) to mean “making every reasonable effort to maintain exposures to radiation as far below the dose limits in this part as is practical consistent with the purpose for which the licensed activity is undertaken, taking into account the state of technology, the economics of improvements in relation to state of technology, the economics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations, and in relation to utilization of nuclear energy and licensed materials in the public interest.”
14 NCRP, Report No. 116, 1993.
15Occupational dose refers to dose of ionizing radiation received by workers (often referred to as radiation workers) in the course of employment. See OSHA 29 CFR 1910.1096.
or electronic components. Periodic maintenance and inspection of such instruments must include testing of those fail-safe mechanisms.16
Some background on the potential health effects and associated risks of the use of X-ray backscatter AIT systems may help to put dosimetric concerns in context. Current understanding of the potential health risks at very low doses of ionizing radiation is far from complete. Some things, however, are clear. First, the doses that an X-ray backscatter AIT system is capable of producing are far below the levels required to produce symptoms of acute radiation syndrome,18 even in unusually sensitive individuals. Similarly, the doses are well below the threshold required for the production of late effects, such as cardiovascular or neurological damage, which only appear to occur at higher doses (approximately 500,000,000 nSv),19 so these are also not of concern for X-ray backscatter AIT system exposures. In contrast, carcinogenesis is thought to be a stochastic event, meaning that exposure to a low level of a carcinogen could slightly increase the chance of cancer occurrence. Therefore, radiation protection limits for X-ray backscatter AITs have been designed with a focus on protecting against increased cancer risks to the exposed public.
Projections of cancer risks from exposure to radiation are based largely on epidemiological studies, with much of the information coming from studies of the atomic bomb survivors in Hiroshima and Nagasaki, Japan. More than 27,000 atomic bomb survivors had exposures in the 5,000,000 to 100,000,000 nSv range, and statistically significant excess cancer incidence has been detected.20 In the Oxford Survey of Childhood Cancers, a study of 15,000 case control pairs, an increase in childhood cancer was found after in utero X-ray exposure to a mean dose of about 6,000,000 nSv.21 Very large studies such as these are needed to detect effects
16 ANSI/HPS N43.17-2009, Section 8.1.6.
17Low dose in this report refers to a dose lower than that used in radiation therapy. The amount of radiation used in photon radiation therapy is usually measured in gray (Gy) and varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy. In this report, low is in the range nanogray (nGy), a billion times lower.
18 For acute syndrome to manifest, doses need to be high, possibly higher than 0.7 Gy. See Centers for Disease Control and Prevention, “Acute Radiation Syndrome: A Fact Sheet for Clinicians,” http://www.bt.cdc.gov/radiation/arsphysicianfactsheet.asp, accessed March 3, 2014.
19 International Commission on Radiological Protection (ICRP), ICRP statement on tissue reactions/Early and late effects of radiation in normal tissues and organs—Threshold doses for tissue reactions in a radiation protection context, Publication 118, Annals of the ICRP 41(1/2), 2012.
20 D.L. Preston, E. Ron, S. Tokuoka, S. Funamoto, N. Nishi, M. Soda, K. Mabuchi, and K. Kodama, Solid cancer incidence in atomic bomb survivors: 1958-1998. Radiation Research 168: 1-64, 2007.
21 R. Doll and R. Wakeford, Risk of childhood cancer from fetal irradiation, British Journal of Radiology 70: 130-139, 1997.
from low-dose exposures because of the high background incidence of cancer (around 40 percent in the United States) and the relatively small expected increase in cancer due to small radiation doses such as those from screening with an X-ray backscatter AIT. Linear extrapolation indicates that measurable increases in cancer incidence will not occur at the doses used in X-ray backscatter AITs.
Epidemiological studies are limited in detecting increases in cancer risks following small doses of radiation (less than 100,000,000 nSv). Therefore, alternative ways to characterize risks at low doses are used, such as projecting the risks of very small exposures by extrapolating from existing studies where doses are higher. However, characteristics of the exposure or the populations exposed in these existing studies may vary substantially from those of interest.22 Risk estimates at low doses (often defined as less than 100,000,000 nSv)23 may be greatly affected by the choice of the model used to extrapolate to low doses. Currently, the linear nonthreshold dose-response model for extrapolation to low and very low doses is used as the basis for U.S. radiation protection standards.24 However, the possibility of other shapes for dose-response curves at low doses cannot be ruled out because there are insufficient low-dose data.
In addition to providing guidance on radiation dose limits as discussed earlier in this section, ANSI/HPS N43.17-2009 also provides guidance as to the acceptable protection systems (interlocks to protect against accidental excess radiation) and operational procedures. Below, the requirements for general use systems are outlined because the X-ray-emitting passenger screening AIT systems fall into that category.
Although the radiation levels emitted from general use systems are low, ANSI/HPS N43.17-2009 requires a certain level of protection against accidental excess radiation as well as suitable notifications that the X-ray-emitting AIT system is in operation (Box 5.1).
22 For example, atomic bombing survivors received a dose acutely while other low-dose exposure may happen chronically; the exposed individuals were Japanese, who may vary in their genetic makeup and susceptibility to cancer from the U.S. populations.
23 NRC (National Research Council), Health Risks from Exposure to Low Levels, of Ionizing Radiation: BEIR VII—Phase 2, The National Academies Press, Washington, D.C., 2005.
24 NRC, Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII—Phase 2, 2006.
ANSI/HPS 43.17-2009, Sections 7.2.1 and 7.2.2
7.2.1 Requirements for All Systems: The requirements of this subsection apply to all the systems regardless of category or type of radiation source. In addition to these requirements systems must comply with the requirements of one of the sections 7.2.2 through 7.2.5 as appropriate.
a. There shall be at least one indicator, clearly visible from any location from which a scan can be initiated, that indicates when a scan is in progress.
b. There shall be at least one lighted indicator clearly visible from the inspection zone. For portal systems the indicator shall be visible from any approach to the inspection zone to indicate that a scan is in progress.
c. Power to the system shall be controlled by a key switch. The key shall be captured (unable to be removed) whenever it is in a position that allows exposures to be initiated. Turning on the key switch shall never result in the external emission of radiation.
d. Each system shall have a means for the operator to initiate the emission of radiation other than the function of an interlock or the main power control.
e. Each system shall have a means for the operator to terminate the emission of radiation other than the function of an interlock.
f. Means shall be provided to ensure that operators have a clear view of the scanning area. This can be a direct, mirror view, or real-time video of the scanning area. Engineering controls should be provided to ensure that individuals do not reenter the scanning area from the exit while x-rays are being produced (e.g., one way turnstile, see also specific requirements in Sections 7.2.2 through 7.2.5).
g. A ground fault shall not result in the generation of x-rays or activate a scan beam from a sealed radioactive source.
h. Failure of any single component of the system shall not cause failure of more than one safety interlock.
i. A tool or key shall be required to open or remove access panels. Access panels shall have at least one safety interlock.
j. For stationary-subject systems, the scanning motion of the x-ray beam relative to the subject shall be interlocked and the exposure shall terminate when the rate of motion of the beam in any direction falls below a preset minimum speed. The minimum speed shall be chosen so that the dose during the exposure period is within the applicable limit.
k. For portal systems, the minimum walking or driving velocity through the inspection zone shall be determined by the manufacturer. The minimum speed shall ensure that the dose during the exposure period is within the applicable limit.
l. Operational interlocks shall terminate the primary beam in the event of any system problem that could result in abnormal or unintended radiation emission. This shall include, but is not limited to, unintended stoppage of beam motion, abnormal or unintended x-ray source output, computer safety system malfunction, termination malfunction, and shutter or beam stop mechanism malfunction.
m. In the event of a malfunction, the system shall terminate radiation exposure rapidly enough so that no location on the subject’s body shall receive an ambient dose equivalent (H*10) exceeding 250 µSv (25 mrem), regardless of the size of the exposed area.
n. Following interruption of x-ray production or external gamma emission by the functioning of any safety interlock, resetting the interlock shall not result in the production of x-rays or emission of gamma radiation. Use of the normal control sequence shall be necessary for resumption of x-ray generation or gamma radiation emission.
7.2.2 Requirements for General-use Systems Using X-ray Sources: In addition to the requirements of Section 7.2.1, “Requirements for All Systems,” the following requirements apply to general-use systems using x-ray sources:
a. For any x-ray system that normally keeps high voltage applied to the x-ray tube at times other than during a scan, there shall be at least one lighted “xray on” indicator at the control console where x rays are initiated indicating when x-rays are being produced.
b. Technique factors1 for each mode of operation shall be preset by the manufacturer and shall not be alterable by the system operator. If there is more than one mode, prior to each scan, a mode indicator shall be clearly visible to the operator.
c. Each access panel to the x-ray source shall have at least one safety interlock to terminate the x-ray production when opened.
d. The following warning label shall be permanently affixed or inscribed on the x-ray system at the location of any controls used to initiate x-ray generation: “CAUTION: X-RAYS PRODUCED WHEN ENERGIZED.”
e. X-ray emission shall automatically terminate after a preset time or exposure.
f. For portal systems, motion sensors shall monitor the speed of pedestrians or vehicles through the inspection zone (in the forward direction) and the radiation exposure shall terminate when the speed drops below the minimum (as determined according to Section 7.2.1k).
1 Technique factors relate to the X-ray beam configuration and include accelerating potential, beam current, beam filtration, and stand-off distance.
SOURCE: 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.
In addition to the technical requirements related to radiation levels and the engineering of an airport screening AIT system, ANSI/HPS N43.17-2009 lays out the following requirements on the operation of the AIT system as well as the operators using the AIT system:
- The compliance with required operational procedures must be ensured by a designated responsible individual. The operational procedures for the particular installation must be documented and be provided to the
- operators of the AIT system. ANSI/HPS N43.17-2009 outlines the minimal amount of information that must be included in this operational procedures document. In addition, all operators must receive appropriate training, including information about the types of radiation present, comparison to other radiation sources, units of measurement, hazards with the system, and security procedures.
- The manufacturer must provide adequate installation procedures so that the installed AIT system complies with its operational specification. This includes a qualified individual to perform a radiation survey to verify that the dose to the scanned passenger, to the operators, as well as to the bystanders25 is within acceptable guidelines. Such radiation surveys must also be performed at least once every 12 months to ensure that the AIT system stays within its operation parameters. Finally, radiation surveys shall also be performed after maintenance and after any incident that might damage the system to prevent unintended radiation emission.
- The facility operating the system must provide the person to be screened with information that the system emits radiation, basic information about the dose expected in the screening, and information so that the dose can be compared to other radiation sources (for example, background radiation or a chest X ray).
- Finally, the institution operating the AIT system must maintain records for at least 5 years, documenting operator training, maintenance and upgrade, radiation surveys, and the number of scans performed. Current information on the responsible individual designated for the site as well as a complete set of operating procedures must be readily available.
25 In general, this report differentiates only between the scanned passenger and anyone else using the term bystander. An operator can be seen as a bystander that spends more time and in closer proximity to the equipment than anyone else.