GLOSSARY

Abbreviations, acronyms and definitions


ABCC (Atomic Bomb Casualty Commission)

—the research program sponsored by the Japanese Ministry of Health and Welfare and the US Department of Energy charged with the responsibility for the study of the survivors of the atomic bombings from the inception of the investigations in 1947 until it became the Radiation Effects Research Foundation (RERF) in 1975. The successor was called RERF (see below).

Absorbed dose

—radiation energy deposited in a volume of tissue or other material divided by the mass of this volume. The SI unit of absorbed dose is J/kg; its special name is gray (Gy). In the USA, the unit 100 erg/g is often used with the special name rad. The conversion is 100 rad=1 Gy.

Absorbed dose rate

—absorbed dose per unit of time. High dose rates are usually more damaging to humans and animals than low dose rates. This is because repair of damage is more efficient when the dose rate is low. For the very low doses and dose rates that were associated with fallout from nuclear testing around the world, there are large uncertainties about risks of harm from radiation exposure, because these radiogenic contributions are at most very small when compared to contributions from other sources, e.g., “spontaneous” normal processes of living.

Activity

—the number of nuclear transformations occurring in per unit of time. The SI unit of radioactivity is the becquerel (Bq) (see below). AEC (Atomic Energy Commission)—the agency of the U.S. government that became the Department of Energy and the U.S. Nuclear Regulatory Commission (USNRC).

Alpha particle

—a charged particle having the same mass and charge as that of a helium nucleus stripped of its electrons. Alpha particles are emitted by some heavy elements that include uranium, plutonium, and radon. These particles have a very short range (about 60 µm in tissue) and in most cases will not penetrate a piece of paper.

Atom

—the smallest particle of an element that cannot be divided or broken up by chemical means. It consists of a central core of protons and neutrons, called the nucleus. Electrons revolve in orbits in the region surrounding the nucleus.

Ataxia telangiectasia

—a hereditary disease connected with decreased DNA repair capacity and, thus, with an increased sensitivity to ionizing radiation.

Atmospheric testing

—detonating a nuclear weapon or device in the atmosphere or close to the earth’s surface with release of radioactivity to the atmosphere as part of the testing program.

Atomic bomb

—a term sometimes applied to a nuclear weapon using fission energy only.



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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests GLOSSARY Abbreviations, acronyms and definitions ABCC (Atomic Bomb Casualty Commission) —the research program sponsored by the Japanese Ministry of Health and Welfare and the US Department of Energy charged with the responsibility for the study of the survivors of the atomic bombings from the inception of the investigations in 1947 until it became the Radiation Effects Research Foundation (RERF) in 1975. The successor was called RERF (see below). Absorbed dose —radiation energy deposited in a volume of tissue or other material divided by the mass of this volume. The SI unit of absorbed dose is J/kg; its special name is gray (Gy). In the USA, the unit 100 erg/g is often used with the special name rad. The conversion is 100 rad=1 Gy. Absorbed dose rate —absorbed dose per unit of time. High dose rates are usually more damaging to humans and animals than low dose rates. This is because repair of damage is more efficient when the dose rate is low. For the very low doses and dose rates that were associated with fallout from nuclear testing around the world, there are large uncertainties about risks of harm from radiation exposure, because these radiogenic contributions are at most very small when compared to contributions from other sources, e.g., “spontaneous” normal processes of living. Activity —the number of nuclear transformations occurring in per unit of time. The SI unit of radioactivity is the becquerel (Bq) (see below). AEC (Atomic Energy Commission)—the agency of the U.S. government that became the Department of Energy and the U.S. Nuclear Regulatory Commission (USNRC). Alpha particle —a charged particle having the same mass and charge as that of a helium nucleus stripped of its electrons. Alpha particles are emitted by some heavy elements that include uranium, plutonium, and radon. These particles have a very short range (about 60 µm in tissue) and in most cases will not penetrate a piece of paper. Atom —the smallest particle of an element that cannot be divided or broken up by chemical means. It consists of a central core of protons and neutrons, called the nucleus. Electrons revolve in orbits in the region surrounding the nucleus. Ataxia telangiectasia —a hereditary disease connected with decreased DNA repair capacity and, thus, with an increased sensitivity to ionizing radiation. Atmospheric testing —detonating a nuclear weapon or device in the atmosphere or close to the earth’s surface with release of radioactivity to the atmosphere as part of the testing program. Atomic bomb —a term sometimes applied to a nuclear weapon using fission energy only.

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Background radiation —the type of ionizing radiation to which a person is exposed from natural sources such as cosmic radiation originating in space, or terrestrial radiation due to naturally occurring radionuclides in the soil or air, or radiation from natural radionuclides incorporated in the makeup of the body. Becquerel (Bq) —the special name for the unit of radioactivity equal to the number of disintegrations per second (s−1). The older special name for a different unit of activity was the Curie (Ci), which was equal to 3.7×1010 disintegrations per second; the becquerel introduced with the system international (SI) in 1981 is equal to 1 disintegration per second. Thus, 1 Ci is equal to 3.7×1010 Bq. BEIR —a series of National Academy of Sciences’ studies conducted by committees on the biological effects of ionizing radiations (BEIR VII is the current study). Benign —a general category of tumors that does not invade surrounding tissue. Benign tumors are characterized by slow growth through expansion. Such tumors are usually not referred to as cancer. Beta particle —an electron (positive or negative) emitted during decay of some isotopes. Beta particles have a short range in air (less than a few meters) and even shorter range in more dense material (less than a few mm in tissue). Beta-emitting isotopes (e.g. strontium-90) deposited on the skin in large amounts can cause skin burns and other more serious effects. Many of the radioactive isotopes found in fallout are beta emitters. CDC —Centers for Disease Control and Prevention (http://www.cdc.gov/). Cesium-137 —a beta-particle and gamma-ray emitting isotope of cesium. Cesium-137 is a major long-term pollutant of fallout from nuclear testing. It has a physical half-life of 30 years. Chain reaction —a reaction where fissile atoms (e.g., atoms of uranium-235) absorb neutrons, then split, releasing energy and more neutrons, which in turn cause additional atom splitting and neutron releases so that the process continues. Nuclear weapons tests that were conducted around the world were based on uncontrolled chain reactions. Fission fragments arising from the atom splitting are radioactive and appear in fallout. CI (confidence interval) —a range about a parameter estimated in a trial that, in some predetermined proportion of trials, will include the true value of that parameter. Commonly, this proportion is selected to be 67, 90 or 95%. Collective dose —see collective effective dose. Collective effective dose —quantity obtained by multiplying the average effective dose by the number of people exposed to a given source of ionizing radiation. It is usually expressed in man sievert (man Sv).

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Correlation —the degree of association between two variables, such as height and weight. It is customarily measured by a unitless coefficient, known as the correlation coefficient that varies between −1 and +1. A correlation coefficient of 0 implies no association, whereas a value of −1 or +1 implies a perfect association, one variable increasing while the other decreases in the first instance, and both increasing in the second. It should be noted that the existence of a non-zero correlation coefficient is not sufficient (nor, strictly, is it necessary) to demonstrate causality. Curie (Ci) —originally, the special name for the amount of radioactivity in 1 gram of radium-226. One curie was subsequently (for precision) redefined to equal 37 billion radioactive disintegrations per second. To convert the specification of an amount of activity in curie to that in microcurie, multiply by one million. Conversely, to convert from a specification in microcurie to curie, divide by one million. The curie is not an SI unit. The corresponding SI unit is the becquerel. One microcurie equals 37,000 becquerel. Decay constant —the average fraction of atoms of a radioactive nuclide that decays per unit of time divided by that unit of time (s−1). DHHS —Department of Health and Human Services-(http://www.dhhs.gov/). DOD —Department of Defense (http://www.dod.gov/). DOE —Department of Energy (http://www.doe.gov/). The DOE is responsible for developing and producing nuclear weapons and for the sites at which weapons materials have been produced and handled. Dose —general term for the quantity of energy absorbed from ionizing radiation in a volume of a material divided by its mass. See absorbed dose, equivalent dose, effective dose, and collective effective dose. Often used in risk assessment as an abbreviation for effective or organ dose. Dose coefficient —a factor used to derive an organ or effective dose from radionuclide intake by members of the general public. It is usually expressed as a dose in Sv per unit intake in Bq, e.g., Sv Bq−1. Dose, equivalent —a quantity that expresses an equal biological effectiveness of a given absorbed dose on a common scale for all kinds of ionizing radiation. The equivalent dose is now most commonly expressed in sieverts (Sv), but has, in the past, been stated in rem. See rem and sievert. Dose reconstruction —a scientific activity that estimates doses to people from releases of radioactivity or other pollutants. The reconstruction is usually done by determining how much material was released, how it was transported, and how people came in contact with it and the amount of radiation energy absorbed by their bodies.

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Dose-response curve (or relationship) —the mathematical representation of the change in an effect due to irradiation with increasing dose. For cancer induction, the most commonly used relationships assume this increase to be either linear, linear-quadratic, or quadratic with dose (see below for definitions of these terms). DDREF —dose and dose rate reduction effectiveness factor, a measure of the extent to which radiation-related damage accruing at a high dose rate is ameliorated when the dose rate and/or dose is low. This value will presumably vary with the endpoint measured, but it is not known precisely for such endpoints as incidence of, or death due to, cancer or any of the other effects seen also among the atomic bomb survivors. DS86 —the currently employed system of dosimetry used to describe the exposure of the survivors of the atomic bombings of Hiroshima and Nagasaki; introduced in 1986. Effective dose —a calculated dose quantity obtained by multiplying the equivalent doses to various tissues and organs by a tissue/organ weighting factor appropriate to each and summing the products. The unit is J/kg; its special name is sievert (Sv). The expression “effective dose” has replaced the earlier “effective dose equivalent”. Electromagnetic radiation —the propagation of energy in the form of electromagnetic waves through space. Examples: light, radio-waves, gamma rays, x rays. All can be transmitted through a vacuum. Electron —an elementary particle with a mass 1836 times smaller than that of a proton and which carries negative charge. Electrons orbit around the nucleus and determine the chemical properties of an atom. Electrons are emitted by radioactive atoms in a beta decay. EML —Environmental Measurements Laboratory in New York City, formerly known as the Health and Safety Laboratory (HASL). Equivalent dose —a quantity obtained by multiplying the absorbed dose by a radiation-weighting factor to allow for the different effectiveness of the various types of ionizing radiations in causing late effect harm in tissue. The equivalent dose is theoretical and has replaced the earlier dose equivalent. The equivalent dose is often expressed in sievert (Sv). It is also sometimes expressed in rem (an older unit). One hundred rem equals 1 Sv. Erg —Unit of work done by the force of one dyne acting through a distance of one cm. (units, dyne-cm or gm-cm2/sec2). Excess mortality —the amount by which the number of deaths observed in a group exceeds the number expected. Expected baseline mortality —a baseline standard of mortality that would be expected absent the exposure under study; can be calculated from standard population rates or from an unexposed comparison population.

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Exposure (radiation) —is a measure of the amount of ionization produced by x rays or gamma rays as they travel through air. The unit of radiation exposure is the roentgen (R), named for Wilhelm Roentgen, the German scientist who in 1895 discovered x rays. Fallout —radioactive material (particles, vapors, and gases) from a nuclear explosion (e.g. atomic weapon or nuclear accident such as occurred at Chernobyl). The particles gradually fall from the sky and deposit on the ground and on other surfaces, while vapors may condense to form particles, and vapors and gases may stick to particles and similarly deposit. Small fractions of the materials may also be inhaled. Animals such as cows and goats can ingest the radioisotopes from fallout through grazing and pass the isotopes such as iodine-131 to humans via the food supply. Rainfall may help scour particles, vapors, and gases from the atmosphere as washout and rainout, which are efficient modes of radioactivity transfer from the atmosphere to the biosphere. Fissile —isotopes that can split (break up) when hit by a low-energy neutron. The fissile isotopes uranium-235 and plutonium-239 are used in nuclear weapons. Fission —the splitting (breaking apart or fissioning) of the nucleus of a heavy atom such as uranium-235 or plutonium-239. The fission, usually caused by the absorption of a neutron, releases large amounts of energy. Nuclear bombs used in Hiroshima and Nagasaki were based on nuclear fission induced by neutrons. Fusion —a nuclear transformation characterized by the joining together of two light nuclei under extreme pressure and heat that results in the release of a substantially larger fraction of mass-energy conversion than occurs in fission. Gamma rays —high-energy, penetrating electromagnetic radiation emitted in the radioactive decay of most radionuclides. Gamma rays are similar to x rays, but x rays have lower energy. Gray —special name of the unit of absorbed dose J/kg (joules per kilogram), equal to 100 rad, named after the English biophysicist, Louis Harold Gray. Half-life, physical —the average time it takes for one-half of any given number of unstable atoms to decay. Half-lives of isotopes range from small fractions of a second to more than a billion years. As an example, if on average 100 out of 200 radioactive atoms of a specified kind decay in 1 day (half-life=1 day), then of the remaining 100 atoms, 50 would be expected to decay during the second day. Similarly, 25 of the remaining 50 atoms would be expected to decay during the third day. This type of decay is called exponential. Half-life, biological —the time required for the elimination of half of a substance, such as a chemical, from the body when the fraction of the remaining material removed each hour is approximately constant. For a biological half-life of 1 hour, 1/2 of the material would be expected to be eliminated during the first hour. Of the material that remained, 1/2 would be expected to be eliminated during the second hour. This represents 1/4 of the

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests initial material present. Thus, for each successive hour, the expected fractions of the initial material present that are removed would be 1/2, 1/4, 1/8 and so on. This type of decrease over time is called exponential. Half-life, effective —the time required for the activity of a radioactive substance in the body to decrease to 1/2 its value due to the combined effects of biological elimination and radioactive decay. The effective half-life facilitates evaluating radiation dose from inhaled and ingested radionuclides and applies when the biological and physical half-lives are constant. For an effective half-life of 1 hour, 1/2 of the radioactivity would be expected to be eliminated during the first hour. Of the radioactivity that remained, 1/2 would be expected to be eliminated during the second hour. This represents 1/4 of the initial radioactivity present. Thus, for each successive hour, the expected fractions of the initial radioactivity present that are eliminated would be 1/2, 1/4, 1/8 and so on. This type of decrease over time is called exponential. Half-life, radioactive —see half-life, physical. High-LET radiation —radiation with linear energy transfer (LET) values above, say, 10 keV/µm. It produces much damage over a short distance in tissue or other material. In contrast, low-LET radiation produces only a small amount of damage when evaluated over the same amount of deposited energy. Alpha particles represent high-LET radiation. Gamma and x rays represent low-LET radiations. To produce a given amount of damage, it generally takes a larger absorbed dose of low-LET radiation than for high-LET radiation. Also, biological damage produced by low-LET radiation is often more efficiently repaired than damage produced by high-LET radiation. IAEA —the International Atomic Energy Agency, one of the specialized bodies of the United Nations charged with the responsibility of overseeing and setting standards and recommendations for the operation of nuclear activities and for radiation safety in the member states. It is headquartered in Vienna, Austria, and its members have played a major role in the accumulation and dissemination of the information derived from the Chernobyl accident as well as other accidents involving exposure to ionizing radiation. ICRP —the International Commission on Radiological Protection, a nongovernmental agency headquartered in the Sweden and the United Kingdom, concerned with radiation protection in the workplace and of the general population. It was founded by the International Congress of Radiology in 1928. It is generally viewed as the world’s leading source of authoritative statements on radiation protection. ICRU —the International Commission on Radiation Units and Measurements, a nongovernmental agency headquartered in Bethesda, MD, USA, concerned with recommendations regarding harmonized measurement of radiation and responsible for recommending nomenclature for quantities, units and their special names, e.g., Bq, Gy, Sv.

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Incidence —the number of new cases of a specific disease occurring during a certain period of time. See prevalence. Integrated intake —the estimated intake of a radionuclide summed over time in an area having a specific deposition density. Commonly specified in Bq per Bq m−2. Iodine-131 (131I) —a radioactive isotope of iodine. Iodine is an element required in small amounts for healthy growth and development. It is mainly concentrated in the thyroid gland where it is needed to synthesize thyroid hormones. 131I is used as a radioactive tracer in nuclear medicine and is found in fallout from nuclear testing. 131I has been demonstrated to cause thyroid cancer in humans after moderate and high doses following the Chernobyl accident. Whether very low radiation doses causes thyroid cancer is uncertain. Iodine-131 has a relatively short half-life (8 days). Ionizing radiation —any form of radiation that produces ionizations. Examples are alpha, beta, and gamma radiation. When ionizing radiation passes through tissue of the body, molecular, cellular, and tissue damage can occur depending on the dose and dose rate. Irradiate —to expose to radiation. Isotope —forms of the same chemical element that have different numbers of neutrons. Many isotopes are produced in nuclear reactors (e.g. radioactive iodine-131 and strontium-90) and particle accelerators. Stable iodine in the from of potassium iodide (KI) pills can be ingested to prevent humans from incorporating radioactive iodine in the thyroid following nuclear accidents. The field of nuclear medicine depends on a constant supply of radioactive isotopes (i.e., radioisotopes). Joule —SI unit of energy, corresponding to the work done by one newton moving through one meter. (Symbol J) Kerma —an acronym derived from the expression “kinetic energy released in materials”; these materials could be air, a body organ, or a structural component of a building. Kerma is expressed in gray. Kriging —in the present context, an interpolation technique used to estimate the amount of 131I deposition in areas (counties) where measurements were not available. LET —short for Linear Energy Transfer. LET represents the average amount of radiation energy lost when traversing a small distance. It has units of energy divided by the short distance. For low-LET radiations such as X rays, gamma rays, and beta particles, little energy is lost when traversing a sheet of paper. For high-LET alpha particles emitted by plutonium isotopes, essentially all of the particle’s energy is lost in the sheet of paper.

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Leukemia —the term used to describe a group of malignant, commonly fatal blood diseases with certain common findings, notably a progressive anemia, internal bleeding, exhaustion, and a marked increase in the number of white cells (generally their immature forms) in the circulating blood. Life Span Study (LSS) —ongoing follow-up of the population exposed to atomic-bomb detonations in Hiroshima and Nagasaki, Japan, and progeny; conducted by the Radiation Effects Research Foundation (RERF). Linear model —the effect of exposure to ionizing radiation is assumed to be directly and simply proportional to dose. Linear non-threshold model —often referred to as the LNT model, is an empirical equation used to assign risk for cancer induction by a specified genotoxicant (including ionizing radiation). The equation has the form, risk=A+kD, where k is a risk coefficient, D is a measure of dose, and A represents the baseline risk, absent radiation. With this empirical model, any dose in excess of zero is presumed associated with an increased risk of cancer. Further, use of this model implies that doubling the dose will double the calculated excess risk. For low radiation doses and dose rates, there are large uncertainties about what are the true risks to humans and how they relate to dose. Linear-quadratic model —the effect of exposure is assumed to be not only related to the dose received but to the square of the dose as well. Malignancy/malignant neoplasm —a general category of neoplasm that invades surrounding tissue. A malignant tumor is generally characterized by invasive growth and is able to metastasize via the lymphatic and blood systems to distant tissue sites. Microcurie —one curie divided by one million. To convert microcuries to curies, divide by one million. To convert microcuries to becquerels, multiply by 37,000. To convert microcuries to kilobecquerels, multiply by 37. A kilobecquerel is 1000 becquerels. Model —a construct (generally mathematical) that attempts to describe the events that underlie some biological or physical phenomenon of interest, such as the occurrence of cancer following exposure to ionizing radiation. Model uncertainty —for low-dose radiation-risk assessment, the linear, non-threshold (LNT) model is often used. However, the LNT model is empirical so that it is unknown where the true risk lies for a given radiation dose to a specified organ. Uncertainty about what is the true risk model is called model uncertainty. Some researchers try to account for model uncertainty when evaluating risks. This can be achieved, for example, by using a variety of acceptable risk models and characterizing the variability of the risk derived using the different models.

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Mortality —a measure of the number of people who die of a disease or condition within a specified population in a certain period. This measure is usually a count or a rate (frequency). NAS —National Academy of Sciences. The National Academy of Sciences is a private, non-profit, self-perpetuating society of distinguished scholars engaged in scientific research. Upon the authority of the charter granted by the Congress in 1863, the NAS has a mandate that requires it to advise the federal government on scientific and technical matters. NCI —National Cancer Institute (http://www.nci.gov/). NCRP —the National Council on Radiation Protection and Measurements, a nongovernmental agency based in Bethesda, Maryland, with a charter similar to that of ICRP, but focusing in particular on issues related to radiation protection in the United States. Neutron —an uncharged particle that makes up part of an atomic nucleus. Uranium-235 and plutonium-239 atoms split (fission) when they absorb neutrons. This causes additional neutrons (and heat, in the form of motion of the neutrons and the fission fragments) to be released, potentially leading to a chain reaction. The controlled chain reactions that occur in nuclear power reactors and uncontrolled chain reactions that occur in nuclear weapons are due to neutrons being absorbed by atoms that then fission. Nevada Test Site (NTS) —the region in Nevada set aside for the continental atmospheric nuclear weapons testing program. Also referred to as the Nevada Proving Ground (NPG). NRC —National Research Council. The NRC is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering to serve the federal government and other organizations. NTS —see Nevada Test Site. Nuclide —a species of atom characterized by the constitution of its nucleus. The composition is principally specified by the number of protons and neutrons, although is it strictly also necessary to take account of more esoteric factors (energy and spin states). ORERP —the acronym for the Offsite Radiation Exposure Review Project that sought to determine the exposures of individuals residing in a limited number of counties near the Nevada Test Site.Organ dose—the energy absorbed in a specific organ divided by its mass. This quantity is expressed in gray or its submultiples. Prevalence —the number of cases of a specific disease existing in a particular population or area at a certain time. The value is different numerically from incidence. Proton —an elementary particle with a single positive charge. Protons along with neutrons make up the nucleus of an atom.

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests Quadratic model —the effect of exposure to ionizing radiation is assumed to be related to the square of the dose received. Quality factor —a factor, dependent upon the linear energy transfer, by which absorbed doses are multiplied to obtain a quantity that expresses the effectiveness for radiation protection purposes of an absorbed dose on a common scale for all forms of radiation. See also RBE. Rad —a unit of absorbed dose. It is equivalent to 100 ergs of energy per gram. In the more recent radiobiological literature the rad has been replaced by the gray (1 Gy=100 rad). See also gray. Radiation —energy transferred as particles or waves through space or other media. Sunlight is a form of radiation. Without that radiation, we would not exist. Thus, radiation is necessary for life on our planet, as we know it. However, moderate and large amounts of ionizing radiation are known to cause harm to humans and to the environment. Whether very small amounts of radiation cause harm is uncertain. All of us are exposed to low-level radiation originating within our bodies, from our planet, and from outer space. Many of us have been exposed to additional low-level radiation via smoke from forest fires, from cigarette smoke (direct or side stream), and via fallout from nuclear weapons tests. Radioactive —exhibiting radioactivity. Radioactive decay —the spontaneous disintegration of the nucleus of a radionuclide. Radioactivity —the spontaneous emission of radiation from unstable atoms. Radionuclide —a radioactive species of an atom. Radionuclides loose particles (e.g., alpha or beta) and energy through radioactive decay. Radon-222 —a radioactive inert gas that arises from the decay of radium. Radon occurs naturally in many minerals and is a chief hazard of uranium mill tailings. Some radon can also be found in our homes. Radon decays into other isotopes that emit alpha radiation. RBE (relative biological effectiveness) —a factor used to compare the biological effectiveness of absorbed radiation doses due to different types of ionizing radiation for a defined biological endpoint, e.g., cell killing; this factor is experimentally determined using X- or gamma rays as the standard of comparison. Thus, if 1 Gy of fast neutrons produced the same amount of cell killing as 5 Gy of gamma rays, the RBE of neutrons for cell killing would be 5. Note the RBE will vary with the biologic endpoint used. Regression —in the present context, this word is used to indicate a mathematical procedure whereby a presumed mathematical relationship between two variables (e.g., Y=a+bX; variables X, Y; parameters a, b) is used along with data for the two variables to fit the mathematical relationship to the data. The regression procedure yields estimates of the parameters and their associated statistical error. After parameters have been estimated,

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests the equation can then be used to predict expected values for one variable (e.g., Y) for specific values of the independent variable (e.g. X). The reliability of the prediction will depend on the appropriateness of the model used and on uncertainties associated with model parameter estimates. Rem —a unit of dose equivalent. The dose equivalent in rem is equal to the absorbed dose in rad multiplied by the quality factor, the distribution factor (which accounts for a nonuniform distribution of internally deposited radionuclides), and other necessary modifying factors. See rad, sievert, and dose equivalent. RERF (Radiation Effects Research Foundation) —the nonprofit research foundation sponsored by the governments of Japan and the United States that currently supervises the studies of the atomic-bomb survivors; the successor in 1975 of the Atomic Bomb Casualty Commission. Risk —probability of injury, harm, or damage. Risk, absolute —the excess number of deaths (or cases) above that “normally” expected in some population in the absence of exposure to ionizing radiation beyond that to which everyone is subjected because of the radiation emanating from the earth’s crust or originating in outer space. Alternatively (depending on context) the increase in probability of injury, harm or damage due to some specified factor. Risk, attributable —the percentage of deaths or cases ostensibly assignable to a specific cause, for example to ionizing radiation, asbestos, chemical exposures, etc. Risk coefficient —slope of the linear, no-threshold risk curve (that is, the increase in risk per unit increase in dose). Is sometimes called a risk factor, which is different in meaning from its use in epidemiological studies. The term risk factor is often used in epidemiological research to indicate exposure co-hazards such as alcohol, cigarettes, radiation, etc. Risk, relative —the ratio of the risk in one population to that in another; for example, the ratio of the risk among individuals exposed to 2 Gy as contrasted with the background risk. Roentgen —the international unit of measurement of gamma or x-irradiation. It is based upon the number of ionizations produced by x-rays or gamma rays in a standard mass of air, and unlike the rad, it is not a measure of absorbed energy. It is named after Wilhelm Roentgen, the discoverer of X-rays. SEER —The Surveillance, Epidemiology, and End Results Program of the National Cancer Institute (http://seer.cancer.gov/). This program currently collects and publishes cancer incidence and survival data from 11 population-based cancer registries and three supplemental registries covering approximately 14 percent of the US population. Sievert —the sievert (Sv) is the special name of the SI unit for the quantity of dose equivalent. The dose equivalent is a hypothetical dose unit that in theory makes different radiations

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Exposure of the American Population to Radioactive Fallout from Nuclear Weapons Tests appear to be equally effective in producing biological effects in a given organ of the body even though the radiations may not be equally effective. Special weighting factors (i.e., radiation weighting factors) are used to convert absorbed dose to equivalent dose. Other weighting factors are used to convert equivalent doses to effective dose, which is more often used in risk assessment. With effective dose, a single number applies to the total body. With equivalent dose, different doses may be attributed to different organs. The name is derived from the Swedish physicist, Rolf M.Sievert. SI units —the international system of units. Special SI units for radiation include the becquerel (Bq), gray (Gy), and sievert (Sv). Standard deviation —square root of the variance (see below). Strontium-90 —a beta-emitting isotope of strontium which element is chemically similar to calcium. Strontium-90 has a half-life of 28 years, is a common isotope produced in nuclear reactors, and is contained in fallout from nuclear testing. Threshold —some minimal dose to a population required to produce a specified biological effect; below this dose the effect dose not occur among any member of the population. Biological effects requiring threshold radiation doses are called deterministic effects. TOA —time of arrival. Uncertainty —the term used to describe the lack of precise knowledge in a given estimate based on the amount and quality of the evidence or data available. UNSCEAR —the United Nations Scientific Committee on the Effects of Atomic Radiation, one of the specialized bodies of the United Nations charged with the responsibility of evaluating the effects of exposure to atomic (ionizing) radiation on behalf of the member nations. Variance —a measure of the spread or dispersion of a variable about its mean value. It is by definition the mean of the squared deviations of the values of the variable about their mean. The standard deviation is the square root of the variance. X-rays —High-energy lectromagnetic radiation with a wavelength that is much shorter than that of visible light, historically the rays produced in an electrical device, e.g., such as a diagnostic x-ray machine. Yield —the total effective energy released in a nuclear detonation and measured in tons of trinitrotoluene (TNT) equivalent.