The United States continues to use conventional units for radiation measurements despite the long history of U.S. government policies supporting the adoption of SI and 30-year-old national and international recommendations to exclusively use SI units for radiation measurements (see Chapter 1). Mr. Michael Boyd, Environmental Protection Agency (EPA), provided an overview of practices in radiation unit use in the U.S. federal and state governments. Mr. Boyd co-chairs the Interagency Steering Committee on Radiation Standards’ Federal Guidance Subcommittee, whose goal is the development of consistent national radiation protection standards. He primarily described practices of agencies with regulatory authorities in radiation protection. Information provided in the following sections is attributed to a workshop participant if other than Mr. Boyd. Table 2.1 summarizes current practices in radiation unit use.
Dr. William Blakely, Armed Forces Radiobiology Research Institute, who declared himself to be in favor of adopting the SI units for radiation measurements, proposed that agencies and organizations take the leading role to provide rewards or certificates of good practice to federal agencies, organizations, and other stakeholders that already exclusively use or are taking steps toward exclusively using SI units for radiation measurements. A commenter suggested that NIST would be the appropriate agency to play that role.
TABLE 2.1 Current (as of September 2016) Practices in Use of Units for Radiation Measurements
|Agency/Organization||Current Practices in Use of Radiation Measurements (Score 1-5)a|
|Environmental Protection Agency||3|
|Nuclear Regulatory Commission||4|
|Occupational Safety and Health Administration||4|
|Department of Transportation||1|
|National Institute of Standards and Technology||1|
|Department of Energy (DOE)||3|
|Department of Defense|
|Department of Homeland Security—Federal Emergency Management Agency’s Radiological Emergency Preparedness (REP) Program||5|
|Food and Drug Administration—Center for Devices and Radiological Health||1|
|Centers for Disease Control and Prevention (CDC) (overall)||2|
|CDC Agency for Toxic Substances and Disease Registry||3|
|National Aeronautics and Space Administration||1|
|DOE National Laboratories||3|
|State and local governments||3|
|Nuclear power industry||5|
a Use of units for radiation measurements is described by the rapporteur of this proceedings by a 1-5 score. Information for the scoring was extracted solely from discussions that took place during the workshop. 1 = Exclusively SI; 2 = Primarily SI; 3 = Mixture of SI and Conventional; 4 = Primarily Conventional; 5 = Exclusively Conventional. The term “exclusively” is used to denote the predominant use of SI (or conventional) units and that some exceptions may apply.
This section discusses current practices in use of SI units for radiation measurements in the federal government and, when it was discussed, the intention to exclusively use SI units in the future.
Environmental Protection Agency (EPA)
EPA uses a mix of SI and conventional units in its regulatory documents. Regulatory documents published in the mid-1970s and before are written using conventional units because they are based on older recommendations from international agencies on permissible dose limits given in conventional units. For example, the 1976 drinking water regulations (40 Code of Federal Regulations [CFR] Part 141) and 1977 uranium fuel cycle regulations (40 CFR Part 190) are based on International Commission on Radiological Protection Publication 2 (ICRP, 1959) recommendations on permissible dose for internal radiation and are written using conventional units. ICRP Publication 2 was superseded by Publication 30 (ICRP, 1979), but the EPA regulations have not been updated.
In contrast, the 1983 uranium and thorium mill tailings regulations (40 CFR Part 192), the National Emissions Standards for Hazardous Air Pollutants, Standards for Radionuclides (40 CFR Part 61), and the 1994 Spent Fuel and High Level Waste standards (40 CFR Part 191) are mostly written using SI units or at least provide SI units in parentheses.
EPA also issues scientific and technical publications and federal guidance reports that provide methods to other agencies for performing radiation dose and risk assessments. These publications use SI units only. Examples include Federal Guidance No. 11 on radionuclide intake and air concentration and dose conversion factors1 (Eckerman et al., 1988) and Federal Guidance No. 12 on External Exposure to Radionuclides in Air, Water, and Soil (Eckerman and Ryman, 1993). Other EPA documents that are revised and updated based on new national and international recommendations also use SI units.
EPA’s environmental radiation monitoring program (RadNet) currently reports measurements in conventional units but, according to EPA officials who were at the workshop, newer data will be reported in SI units. EPA’s Protective Action Guide (PAG) manual (EPA, 2016), a planning guide for emergency responders, uses conventional units with SI units in parentheses. The 1992 version of the PAG manual (EPA, 1992) used conventional units only.
1 Now called dose coefficients.
Nuclear Regulatory Commission (NRC)
The NRC’s current radiation protection regulations (10 CFR Part 20, 1991) were issued approximately a year before publication of the agency’s metrication policy and have not been updated to reflect that policy. Most dose limits in 10 CFR Part 20 are listed in conventional units followed by SI units in parentheses. Some NRC regulations list SI units first followed by conventional units in parentheses. Section 20.2101c of Part 20 requires that information be recorded on shipping manifests in SI units (or SI first and conventional in parentheses).
Occupational Safety and Health Administration (OSHA)
The U.S. Department of Labor’s OSHA regulation on occupational exposure limits was issued in 1971 and redesignated in 1996 as 29 CFR § 1910.1096. It references the NRC’s 10 CFR Part 20 regulations (1969 version) and is written using conventional units. Some limits are still based on the 1959 ICRP Publication 2 on permissible dose for internal radiation.
Department of Transportation (DOT)
DOT uses SI units exclusively (Pipeline and Hazardous Materials Safety Administration, 2008) to be compatible with international transportation standards and commerce purposes. Conventional units may appear in parentheses following the SI units for “informational purposes” only.
National Institute of Standards and Technology (NIST)
NIST, a nonregulatory agency of the U.S. Department of Commerce, uses SI units exclusively in its processes and has played a central role in facilitating the adoption of SI units by federal agencies and other stakeholders. NIST’s special publications SP330 (Taylor and Thompson, 2008) and SP811 (Thompson and Taylor, 2008), currently in 2008 editions, provide information about SI, including its legal interpretation and guidelines for its use in the United States. Both publications discourage the use of conventional units. NIST also has a dedicated website explaining and promoting SI.2
Department of Energy (DOE)
Ms. Rajah Mena, DOE, said that the agency uses a mix of conventional and SI units, depending on the application and the stakeholder request-
ing services and information. For example, reporting of outcomes from atmospheric dispersion models is done using a mix of SI and conventional units when DOE supports the states and federal agencies, but reporting is in SI units when DOE supports international counterparts or international organizations. Reporting domestically on environmental radiation monitoring data from field surveys and fixed-point sensor arrays in databases such as the RadResponder is primarily done in conventional units. However, RadResponder is “prepared” to use SI units, if requested. DOE officials who support early and intermediate phase response to a radiological incident primarily use conventional units. DOE also supports the NRC and its licensees by providing dose and dose rate information; this information is provided in conventional units for radiation measurements.
Department of Defense (DOD)
Col. John Cuellar, DOD, said that the agency’s three services (Air Force, Army, and Navy) follow their own practices related to unit use:
- Air Force Instruction 48-148 (Department of the Air Force, 2014) uses SI units followed by conventional units in parentheses.
- Army Pamphlet 385-24 (Department of the Army, 2015) uses conventional units followed by SI units in parentheses.
- Navy NAVMED P-5055 (Department of the Navy, 2011) uses conventional units followed by SI units in parentheses.
The three services follow DOD’s overall program for occupational ionizing radiation protection, which is governed under Instruction 6055.08 (Under Secretary of Defense, 2009) and is written using conventional units followed by SI units in parentheses. DOD’s operations in radiological or nuclear environments are governed by Joint Publication 3-11 (Joint Chiefs of Staff, 2013), which is written in SI units but provides conversion factors for converting to conventional units. Col. John Cuellar said that for “simplicity,” Joint Publication 3-11 uses centigray throughout the document as a unit of absorbed dose of radiation (1 centigray = 1 rad).
DOD uses SI units exclusively in operational settings to report doses and track exposures. Current radiation detection instruments use a mix of SI and conventional units. Col. Cuellar said that moving forward (2018 and beyond), the joint dosimeter and joint detection equipment will be able to display units in both conventional and SI units, and the SI unit will be the default setting.
Joint medical training is done using both sets of units, with SI as the primary unit. In operational training, the Air Force and the Army use both units, with SI as the primary unit, and the Navy uses conventional units exclusively.
Despite the differences in current practices, DOD’s three services are all taking steps toward adopting the exclusive use of SI units for radiation measurements. Col. Cuellar explained that some of the impetus for adopting SI units comes from the department’s continuous interactions with its international counterparts and the need for harmonization with the North Atlantic Treaty Organization (NATO).
Current practices of federal agencies without regulatory authorities in radiation protection were mentioned during the workshop and are summarized in Table 2.1.
Overall, state regulations are written in conventional units in some cases followed by SI units in parentheses. This practice was described as “necessary” by some workshop participants for harmonization with the NRC and Federal Emergency Management Agency (FEMA) emergency preparedness and response planning documents, which are in conventional units. Local emergency responders including police officers, firefighters, and emergency managers are trained in conventional units.
Dr. Stephen Musolino, Brookhaven National Laboratory, a member of DOE’s radiological assistance program who advises local authorities on protective actions following a radiological incident, observed that the use of SI units is growing in prevalence within state and local jurisdictions. Entities regulated by states have relationships with international product and service vendors who adopted SI units many years ago; this reinforces these entities to use SI units. For example, state regulators license and inspect users of radiation-producing machines and radioactive materials. These machines and materials are often manufactured for the international market and therefore display SI units, have operational procedures described in SI units, and are serviced by technicians who communicate in SI units. State regulators will often interact directly with these SI-using device manufacturers and service providers during old device replacement and new device installation and maintenance. At these times, the exchange of information requires use of SI units for radiation measurements.3
3 Detailed information was provided to National Academies staff member Dr. Ourania Kosti by Dr. William Irwin, Vermont Department of Health, following a request for clarification (December 1, 2016).
The Conference of Radiation Control Program Directors, Inc. (CRCPD)4 supports the adoption of SI units for radiation measurements (see Table 1.1 in Chapter 1). States, through the CRCPD, engage in joint projects with international agencies, and their communications during these projects is exclusively in SI units. For example, CRCPD is currently working with the International Atomic Energy Agency (IAEA) on joint projects on radon exposure, naturally occurring radioactive materials, and medical radiation exposures; it is also working with the Heads of the European Radiological Protection Competent Authorities (HERCA) on radon-related exposure policies.5
Ms. Ellen Anderson, Nuclear Energy Institute (NEI),6 said that nuclear workers are exclusively using conventional units for radiation measurements. Only a small fraction of workers, those dealing with transportation of radioactive waste under DOT regulations, are trained to use both SI and conventional units. The nuclear power industry has no plans currently to adopt SI units for radiation measurements. In fact, Mr. Willie Harris, Exelon Nuclear, said that, to the best of his knowledge, the four commercial nuclear power reactors currently being built (Plant Vogtle Units 3 and 4 in Georgia and Virgil C. Summer Nuclear Generating Station Units 2 and 3 in South Carolina) also plan to use conventional units for in-plant instrumentation.
Dr. Fred Mettler, professor emeritus at the University of New Mexico School of Medicine, provided an overview of current practices in medicine related to use of units for radiation measurements. Dr. Mettler noted that, on average, members of the public receive most of their radiation dose from diagnostic and therapeutic medical procedures. This dose is much higher than the dose they receive, on average, from nuclear power industry operations. Therefore medical staff, radiation technologists, and professional
4 CRCPD was created to provide a common forum for the exchange of information among state and local radiation control programs and a mechanism for states to communicate with the federal government on radiation protection issues.
5 Detailed information on CRCPD’s engagement with international agencies was provided to National Academies staff member Ourania Kosti by Dr. William Irwin, Vermont Department of Health, following a request for clarification (December 1, 2016).
6 NEI represents the nuclear power industry’s positions and formulates positions on key legislative and regulatory issues affecting the industry.
medical societies7 concerned with the safety of the patient, are important stakeholders in discussions on adoption of SI units for radiation measurements in the United States.
Dr. Mettler said that the adoption of SI units in medicine has been promoted for at least 30 years and, although SI unit use is increasing, medical practitioners continue to use a mix of SI and conventional units. For example,
- in diagnostic radiology, total air kerma (the cumulative dose to a reference position) is in milligrays; the American College of Radiology’s (ACR’s) requirements for reporting overdose events are in rad; exposure index8 (an indirect measure of digital image quality) is unitless.
- in nuclear medicine, radioisotope dose calibrators (used to determine the activity of radioactive dose administered to the patient) can be in SI or conventional units; prescription labels from commercial nuclear pharmacies and labels on syringe for administration to the patient are in millicuries; nuclear medicine practice guidelines from the Society of Nuclear Medicine and Molecular Imaging (SNMMI) are typically in SI units followed by conventional units in parentheses, whereas guidelines from the American Board of Nuclear Medicine are in conventional units followed by SI units in parentheses.
- radiation oncology uses SI units exclusively. However, activity of brachytherapy9 seeds is given in conventional units.
Dr. Mettler stated that, in his opinion, there are good reasons to use a mix of SI and conventional units in medicine and that many medical staff find the dual unit expression helpful. He argued that dual use rarely contributes to inadvertent radiation overexposures in medicine, and that most common reasons for radiation overexposures are lack of training, miscalibrations, and mechanical failures. He recognized, however, that brachytherapy has the potential for introduction of errors in dose distribution calculations because of the incorrect use of radiation measurements (and their units) as required by the dose calculation software. He mentioned two such events that were reported to the IAEA database: Event No. 53, which resulted in doses to five patients that were 14 percent higher than
7 For example, the American College of Radiology (ACR), Society of Nuclear Medicine and Molecular Imaging (SNMMI), and the American Society for Radiation Oncology (ASTRO).
8 These measures are typically created by digital radiography manufacturers.
9 Brachytherapy is short-distance cancer treatment with radiation emanating from small sealed sources known as “seeds.”
The Radiation Emergency Assistance Center/Training Site (REAC/TS) at Oak Ridge Institute for Science and Education has maintained a radiation accident registry since 1944. REAC/TS’s director, Dr. Nicholas Dainiak, stated that there has been no reported inadvertent radiation overexposure of a patient because of a unit conversion error. There was, however, a medical misadministration of 198Au to a cancer patient because of miscalculation of “micro” to “milli” curies.
Soon after SI units for radiation measurements were introduced and endorsed by the ICRU and ICRP in the mid-1970s, articles were published in scientific journals to introduce these units to the scientific and user communities and explain their correct use. This likely resulted in the overall quick acceptance of SI units for radiation measurements by the scientific community.
Dr. Mark Mendonca, editor-in-chief of the journal Radiation Research, analyzed the adoption of SI units (using gray as an example) in the literature and found an increased use per year that started immediately after the ICRP 1977 recommendation. Scientific publications such as Radiation Research stopped using conventional units in publications in 1987, which coincided with the end of the recommended transition period by ICRP.
Health Physics started requiring all manuscripts to use the SI units for radiation measurements in 1985. Other organizations (e.g., the American Association of Physicists in Medicine [AAPM]) have editorial policies to report exclusively in SI units.11
Medical journals have been slower to adopt the exclusive use of SI units in their publications. For example, the Journal of Thoracic and Cardiovascular Surgery published an article using the unit “rad” in 2008 (Brown et al., 2008), and the Journal of Nuclear Medicine published an article using the unit “Ci” in 2016 (Erwin et al., 2016).
Dr. Fred Mettler described his own experience with suggesting to a publisher that the next edition of his book on nuclear medicine use SI units exclusively. The publisher was against this change because the book would
10 The IAEA has removed identifiers such as the countries the events occurred in and year they occurred. Therefore, it is not possible to know whether any of these events occurred in the United States.
11 AAPM’s editorial policy was mentioned by a workshop participant, but the rapporteur of this proceedings was unable to confirm this policy with the organization’s management despite a request for information.
not “sell.” The publisher suggested instead that Dr. Mettler use conventional units followed by SI units as he had done for previous book editions.
The workshop discussions summarized in this chapter can be organized in the following theme (Theme 2):
SI units for radiation measurements are finding increasing use in the United States. However, the variability in the degree of current use of these units in the U.S. government is considerable. Some agencies (for example, the Department of Transportation and the National Institute of Standards and Technology) use SI units exclusively; others (for example, the Nuclear Regulatory Commission, the Department of Defense, and the Department of Energy) use a mix of SI and conventional units, and a few (for example, the Federal Emergency Management Agency’s Radiological Emergency Preparedness program and the Occupational Safety and Health Administration) still use only conventional units for radiation measurements. The use of SI units for radiation measurements is also increasing in state and local governments, as well as in medicine and scientific publications. The nuclear power industry exclusively uses conventional units for radiation measurements and has no plans currently to adopt SI units.