According to NRC’s website, radiation doses, such as those received by survivors of the atomic bombs in Japan, can cause cancers such as leukemia and colon cancer and, if levels are high enough, acute radiation syndrome. The symptoms of this syndrome range from nausea, fatigue, and vomiting to death within days or weeks. The higher the radiation dose, the sooner the effects of radiation will appear, and the higher the probability of death. For example, according to NRC’s website, 134 of the plant workers and firefighters battling the fire at the 1986 Chernobyl nuclear power plant accident received high doses of radiation and suffered from acute radiation syndrome. Of these, 28 died within the first 3 months from their radiation injuries. In contrast, the effects of low-dose radiation are more difficult to detect. In particular, below about 100 millisieverts (mSv) (10 rem)—the level below which the National Academies’ 2006 report on radiation and human health considered radiation to be low dose—data do not definitively establish the dose-response relationship between cancer and radiation exposure. It is often not possible to determine the extent to which a health outcome such as cancer is caused by low dose radiation because of the potential confounding effects of other chemical and physical hazards and lifestyle factors, such as smoking and diet. In addition, much of the data on health effects of radiation exposure come from non-U.S. populations, such as Japanese atomic bomb survivors, who received a large exposure to radiation over a short period of time (an acute exposure), and there is uncertainty about the extent to which the health effects for these populations can be extrapolated to a U.S. population that is regularly (chronically) exposed to low-dose radiation. The roles of federal agencies in developing and applying radiation protection requirements and guidance vary depending on the setting in which radiation exposure occurs. For the four settings in our review— operation and decommissioning of nuclear power plants, cleanup of sites with radiological contamination, use of medical equipment that produces radiation, and accidental or terrorism-related exposure to radiation—the key agencies for establishing dose limits and guidance levels are EPA, NRC, DOE, and FDA. EPA advises federal agencies about radiation matters that affect public health and provides technical information for conducting radiation risk assessments; federal and state agencies use such assessments to develop and implement radiation protection regulations and standards. EPA also develops requirements and guidance for particular settings in which radiation exposure can occur. For example, EPA has developed regulations to limit discharges of radioactive material affecting members of the public from operations associated with use of nuclear energy to produce electrical power for public use, such as nuclear power plants. In addition, EPA has developed guidance on establishing protective cleanup levels for radioactive contamination at sites cleaned up under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). It has also developed guidance on levels of radiation exposure that would trigger public safety measures, such as evacuation, to minimize or prevent radiation exposure during an emergency. NRC is responsible for protecting people and the environment from unnecessary exposure to radiation as a result of civilian uses of nuclear materials. Among other things, NRC has established dose limits for workers and the public exposed to radiation from the operation and decommissioning of nuclear power plants, as well as minimum requirements for emergency plans for protecting members of the public from exposure in the event of a radiological emergency. NRC also has the primary responsibility for licensing, inspecting, and regulating medical uses of nuclear material. DOE is responsible for ensuring that its facilities are managed to protect workers and the public. As part of this responsibility, DOE has established radiation dose limits for workers at its facilities and public dose limits for DOE radiological activities, including cleanup of radioactive contamination at DOE sites. In addition, under the Atomic Energy Act of 1954, DOE is the federal agency that currently has primary responsibility for research related to nuclear energy. This responsibility includes the protection of health during activities that can result in exposure to radiation. DOE addresses this requirement through research to determine if DOE workers and people living in communities near DOE sites are adversely affected by exposures to hazardous materials from site operations. DOE’s National Nuclear Security Administration (NNSA) assists in emergency response to accidental or terrorism-related exposure to radiation by characterizing radiation levels in the area of an accident or terrorist event and providing information to emergency-response decision makers. FDA has issued radiation safety regulations for medical equipment, such as diagnostic X-ray systems. According to FDA officials, FDA’s regulations generally do not limit the dose to the patient but instead prescribe mandatory performance standards for most radiology medical devices, such as standards for the display of cumulative time that an X-ray system is activated. FDA has also developed guidance for state and local agencies to aid in emergency response planning for accidental or terrorism-related radioactive contamination of human food and animal feeds. Other federal agencies also have roles in radiation protection. For example, ionizing radiation is addressed in specific OSHA standards for general industry, shipyard employment, and construction. According to DOD officials, DOD operates facilities and engages in activities where radiation exposure can occur and implements occupational and public dose limits established by NRC and states in which these facilities and activities are located. NASA sets radiation exposure limits for space flight and supports research on the health effects of cosmic radiation to better manage health risks to astronauts. DHS’s Federal Emergency Management Agency provides guidance on responding to incidents involving release of radioactive material and has established procedures for review and approval of state and local emergency plans for the offsite effects of a radiological emergency that may occur at a commercial nuclear power facility. Two U.S. scientific advisory bodies—the National Academies’ Nuclear and Radiation Studies Board and the National Council on Radiation Protection and Measurements (NCRP)—and one international body—the International Commission on Radiological Protection (ICRP)—are involved in analyzing scientific developments regarding the health effects of radiation exposure and advising federal agencies. The National Academies’ Nuclear and Radiation Studies Board conducts studies on safety and other issues associated with nuclear and radiation-based technologies. The board has published a series of seven reports to advise the U.S. government on the relationship between exposure to radiation and human health, with the most recent report published in 2006. NCRP, a congressionally-chartered, nonprofit educational and scientific body, seeks to formulate and disseminate information, guidance, and recommendations on radiation protection and measurements that represent the consensus of leading scientific thinking. NCRP issues reports on specific issues of concern to federal agencies, such as on the use of medical equipment that produces radiation. ICRP, an independent, international organization with members consisting of scientists and policymakers in the field of radiological protection, offers recommendations to regulatory and advisory agencies on protection against radiation. In addition to addressing particular areas within radiological protection, its publications describe an overall system of radiological protection. Several other organizations are involved in scientific research and standards setting for protection against radiation. For example, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), which includes 27 United Nations states as members of its scientific committee, has a mandate to assess and report on levels and effects of exposure to radiation. Its summaries of basic scientific studies, along with scientific developments reported by the National Academies and other national organizations, serve as a primary source of information for NCRP and ICRP. The International Atomic Energy Agency, in collaboration with other organizations, has issued basic safety standards for protecting people and the environment from harmful effects of radiation. It has also issued safety requirements for preparedness and response for a nuclear or radiological emergency. The World Health Organization, one of the organizations that has collaborated with the International Atomic Energy Agency, also supports research on the health effects of radiation. EPA, NRC, DOE, and FDA have generally used the advice of scientific advisory bodies to develop and apply radiation protection requirements and guidance for workers and the public for the four radiation settings in our review. Three scientific advisory bodies—ICRP, NCRP, and the National Academies’ Nuclear and Radiation Studies Board—have supported the use of the linear no-threshold model for such requirements and guidance; this model assumes that the risk of cancer increases with every incremental increase in radiation exposure. The requirements and guidance the four agencies have developed and applied vary depending on the settings, in part because the scientific advisory bodies on which the agencies relied have also developed recommendations specific to the settings we reviewed. In developing and applying radiation protection requirements and guidance for workers and the public—specifically, developing limits on dose or increased health risk and guidance levels on exposure—EPA, NRC, DOE, and FDA have generally taken the advice of scientific advisory bodies. This advice includes the use of the “linear no-threshold model,” which assumes that the risk of cancer increases with every incremental increase in radiation exposure. The model is used to estimate the risk of cancer when the overall level of exposure is in the range considered to be low dose. At this level of exposure, data from epidemiological studies of individuals exposed to radiation provide evidence of increased risk to cancer, but with uncertainties about the extent of this risk. Under this model, federal regulations set dose limits for radiation exposure that are below the level in the National Academies’ 2006 report on radiation and human health for defining low-dose radiation. For example, NRC’s annual dose limit for members of the public (excluding natural, or background, sources of radiation) is 1 mSv (0.1 rem), or a hundredth of the level the National Academies considers low dose. Three key scientific advisory bodies—ICRP, NCRP, and the National Academies’ Nuclear and Radiation Studies Board—have supported use of this model for development of radiation protection requirements and guidance. For example: ICRP, in its 2007 update to its recommendations on radiological protection, stated that at low doses of radiation, it considers the linear no-threshold model to be the best practical approach to managing risk from radiation exposure.