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Health Risks of Radon and Other Internally Deposited Alpha-Emitters: BEIR IV (1988)
Commission on Life Sciences (CLS)

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577
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Glossary Absorbed dose. The mean energy imparted to the irradiated medium, per unit mass, by ionizing radiation. Units: gray (Gy), red. Activity. The mean number of decays per unit time of a radioactive nuclide. Units: becquere! (Bqj, curie (Ci). Activity median aerodynamic diameter (AMAD). The diameter of a unit-density sphere with the same terminal settling velocity in air as that of the aerosol particulate whose activity is the median for the entire aerosol. Additive interaction model. This model is used to find the combined risk for risk factors which have no interaction with each other. For example, the combined mortality risk of cigarette smoking and automobile accidents is the sum of the separate risks. Adenosarcoma. A mixed tumor which consists of a substance like embroyonic connective tissue together with glandular elements. Alpha particle. Two neutrons and two protons bound as a single particle that is emitted from the nucleus of certain radioactive isotopes in the process of decay or disintegration. Aneuploid. Having numbers of chromosomes not equal to exact multiples of the haploid number. Down syndrome is an example. Background radiation. Radiation arising from radioactive material other than that under consideration; background radiation due to cosmic rays and natural radioactivity is always present; there may also be background radiation due to the presence of radioac- tive substances in building material. 577

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578 GLOSSARY Bayesian analysis. Analysis in which Bayes' theorem is used to derive posterior probabilities from assumed prior knowledge together with observational data. For example, biological information on the relationship between species and hazardous substances can be combined with data on interspecies dose response to calculate the response of human populations. Becquerel (Bq). SI unit of activity. (See Units.) Bremsstrahiung. The production of electromagnetic radiation (pho- tons) by the acceleration (positive or negative) that a fast, charged particle (usually an electron) undergoes from the ef- fect of an electric or magnetic field; for instance, from the field of another charged particle (usually a nucleus). Bronchioles. The small branches of the tracheobronchial tree of the Jung. Cell culture. The growing of cells in vitro, in such a manner that the ceils are no longer organized into tissues. Chromosomal nondisjunction. Either a gain or a loss of chromosomes that occurs when cell division leading to either egg or sperm production goes awry. This results in aneuploidy. Ciliated mucosa. The mucous membrane in the lung covered with small hairlike structures which serve to move the mucus. Competing risks. Other causes of death which affect the value of the risk being studied. Persons dying from other causes are not at risk of dying from the factor in question. Constant-relative-risk model. A risk mode} which assumes that, after a certain time, the ratio of the risk at a specific dose to the risk in the absence of the dose does not change with time. Contact inhibition. The cessation of migratory activity and some- times other functions, including mitosis, when adjacent cells establish firm contact. Con proportional hazards model. A relative-risk mode! that permits the use of internal comparison groups as controls for confounding variables such as cigarette smoking and age. Curie (Ci). A unit of activity equal to 3.7 x 10~° disintegrations/s. (See Units.) Daughter product. An isotope formed as a result of radioactive decay. One daughter atom is formed for each particle emitted.

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GLOSSARY 579 Decay chain or decay series. A sequence of radioactive decays of the same nucleus. An initial nucleus, the parent, decays into a daughter nucleus that differs from the first by whatever particles were emitted during the decay. If further decays take place, the subsequent nuclei are also usually called daughters. Sometimes, to distinguish the sequence, the daughter of the first daughter is called the granddaughter, etc.; ordinarily, however, this quickly becomes too complicated. Diffusion. The random path followed by very small particles due to the impact of surrounding molecules. This Brownian motion governs the transport of submicrometer-size particles in air. Dominant mutation. The mutation is dominant if it produces its effect in the presence of an equivalent normal gene from the other parent. Dose-distribution factor. A factor which accounts for modification of the dose effectiveness in cases in which the radionuclide distri- bution is nonuniform. Dose equivalent. A quantity that expresses, for the purposes of radiation protection and control, the assumed effectiveness of dose on a common scale for all kinds of ionizing radiation. ST unit is the Sievert. (See Units.) Doubling dose. The amount of radiation needed to double the natural incidence of a genetic or somatic anomaly. Electron volt (egg. A unit of energy = 1.6 x 1o-~2 ergs = 1.6 x 10-~9 J; 1 eV is equivalent to the energy gained by an electron in passing through a potential difference of 1 V; 1 keV = 1,000 eV; 1 MeV = 1,000,000 eV. Epithelium. A membranous cellular tissue that covers the surface of some organ or part of the body. Equilibrium fraction. In equilibrium, the parents and daughters have equal radioactivity, that is, as many decay into a specific nucTide as decay out. When fresh radon enters a volume, the daughter products have not yet accumulated, and there IS disequilibrium. The working-level definition of radon does not take into account the amount of equilibrium. Equilibrium, radioactive secular. The condition in which the activities of a parent and daughter in a radioactive decay chain are (very nearly) equal.

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580 GLOSSARY Equilibrium, radiation. The condition in a radiation field where the energy of the radiations entering a volume equals the energy of the radiations leaving that volume. Euploid. Having uniform exact multiples of the haploid number of chromosomes. Gamma ray. Short-wavelength electromagnetic radiation of nuclear origin (range of energy, 10 keV to 9 MeV). Gray (Gy). ST unit of absorbed dose. (See Units.) Half-life, biologic. Time required for the body to eliminate half of an administered dose of any substance by regular processes of elimination; it is approximately the same for both stable en c! radioactive isotopes of a particular element. Half-life, radioactive. Time required for a radioactive substance to lose 50~o of its activity by decay. impaction. As air is taken into the Jung, it follows a tortuous path, changing direction many times. At each change of direction, the momentum of the particles carried in the airstream causes them to impact on the bifurcations of the lung. The force on the particle causing it to move and impact on the lung surface is the Stokes force, which is proportional to the velocity of the air moving with respect to the particle. Impaction is important for particles with large aerodynamic diameters. Incidence. The rate of occurrence of a disease within a specified period; usually expressed in number of cases per 100,000 persons per year. Ionization. The process by which a neutral atom or molecule acquires a positive or negative charge. ionization density. Number of ion pairs per unit volume. ionization path (track). The trail of ion pairs produced by ioniz- ing radiation in its passage through matter. Isotopes. Nuclides that have the same number of protons in their nuclei, and hence the same atomic number, but that differ in the number of enutrons, and therefore in the mass number; chemical properties of osotopes of a particular element are almost identical. The term should not beused as a synonym for nuclide.

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GLOSSARY 581 Latent period;. The period of time between exposure and expression of the disease. After exposure to a dose of radiation, there is a delay of several years (the latent period) before any cancers are seen. Life-span study {LSS). Life-span study of the Japanese atomic-bomb survivors; the sample consists of 120,000 persons, of whom 82,000 were exposed to the bombs, mostly at low doses. Lifetime risk. The lifetime probability of dying of a specific disease. Lifetime risk ratio. The ratio of the lifetime risk (Re) of an exposed person to the lifetime risk of an unexposed person (Ro). This number minus 1 is the proportional increased risk associated with exposure (Re - Ro). Linear dose model. This mode} postulates that the excess risk is linearly proportional to the dose. Linear energy transfer fLET9. Average amount of energy lost per unit track length. Low LET. Radiation characteristic of electrons, x rays, and gamma rays; the distance between ionizing events is large on the scale of a cellular nucleus. High [ET. Radiation characteristic of protons and fast neutrons; the distance between ionizing events is small on the scale of a cellular nucleus. Average LET is specified to even out the effect of a particle that is slowing down near the end of its path and to allow for the fact that secondary particles are not all of the same energy. Lymphosarcoma. A sarcoma of the lymphoid tissue. This does not include Hodgkin's disease. Minute volume. The amount of air moving through the Jung per minute; the product of the breathing rate times the volume of air per breath. Multiplicative interaction model. A mode! in which independent risk factors interact so that the combined risk is the product of the relative risks due to each factor alone. Neoplasms. Any new and abnormal growth, such as a tumor; neo- plastic disease refers to any disease that forms tumors, whether malignant or benign. Nonstochastic. Describes effects whose severity is a function of dose; for these, a threshold may occur; some nonstochastic somatic

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582 GLOSSARY ejects are cataract induction, nonmalignant damage to skin, hematological deficiencies, and impairment of fertility. Nuclide. A species of atom characterized by the constitution of its nucleus, which is specified by its atomic mass and atomic number (Z), or by its number of protons (Z), number of neutrons (N), and energy content. Oncogenes. Genes which carry the potential for cancer. Person-gray. Unit of population exposure obtained by suIruning individual dose-equivalent values for all people in the exposed population. Thus, the number of person-grays contributed by 1 person exposed to 1 Gy is equal to that contributed by 100,000 people each exposed to 10 ,uGy. Person-years-at-risk (PYAR}. The number of persons exposed times the number of years after exposure minus some lag period during which the dose Is assumed to be unexpressed (latent period). Prevalence. The number of cases of a disease in existence at a given time per unit population, usually 100,000 persons. Progeny. The decay products resulting after a series of radioactive decays. Progeny can also be radioactive, and the chain continues until a stable nuclide is formed. Quadratic-dose model. A mode! which assumes that the excess risk is proportional to the square of the dose. Quality factor (Q}. A linear energy transfer dependent factor by which absorbed doses are multiplied to obtain (for radiation- protection purposes) a quantity which corresponds more closely to the degree of biological effect produced by x or low-energy gamma rays. Rad. A unit of absorbed dose. Replaced by the gray in SI units. (See Units.) Radioactivity. The property of some nuclides of spontaneously emit- ting particles or gamma radiation, emitting x radiation after orbital electron capture, or undergoing spontaneous fission. ATtipCia' Ta~ioaCtiVit9. Man-made radioactivity produced by fission, fusion, particle bombardment, or electromagnetic irradi- ation. Natural radioactivity. The property of radioactivity exhibited by more than 50 naturally occurring radionuclicles.

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GLOSSARY 583 Radioisotopes. A radioactive atomic species of an element with the same atomic number and usually identical chemical properties. Radionuclide. A radioactive species of an atom characterized by the constitution of its nucleus. Radiosensitivity. Relative susceptibility of cells, tissues, organs, and organisms to the injurious action of radiation; radiosensitivity and its antonym, radioresistance, are used in a comparative sense rather than an absolute one. Recessive gene disorder. This requires that a pair of genes, one from each parent, be present in order for the disease to be manifest. An example is cystic fibrosis. Relative biological effectiveness (RBE). Biological potency of one ra- diation as compared with another to produce the same biological endpoint. It is numerically equal to the inverse of the ratio of absorbed doses of the two radiations required to produce equal biological effect. The reference radiation is often 200 kV x rays. Relative mutation risk. The ratio of the risk of a genetic mutation among the exposed population to that in the absence of exposure. Risk coefficient. The increase in the annual incidence or mortality rate per unit dose: (1) absolute risk coefficient is the observed minus the expected number of cases per person year at risk for a unit dose; (2) the relative-risk coefficient is the fractional increase in the baseline incidence or mortality rate for a unit dose. Risk estimate. The number of cases (or deaths) that are projected to occur in a specified exposed population per unit dose for a defined exposure regime and expression period: number of cases per person-Gray or, for radon, the number of cases per person cumulative working-level month. Rem. A unit of dose equivalent. Replaced by the sievert. (See Units.) Sedimentation. The gravitational force on a particle is partially balanced by the viscous force of the air. The resultant velocity toward the earth is the sedimentation velocity. Important for particles with intermediate aerodynamic diameters. Sex-linked mutation (OT X-linked). A mutation associated with the X chromosome. It will usually only manifests its effect in males (who have only a single X chromosome). S] units. The International System of Units as defined by the General Conference of Weights and Measures in 1960. These units are generally based on the meter/kilogram/second units, with spe- cial quantities for radiation including the becquerel, gray, and sievert.

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584 GLOSSARY Sievert. The ST unit of radiation dose equivalent. It ~ equal to dose in grays times a quality factor times other modifying factors, for example, a distribution factor; 1 sievert equals 100 rem. Specific activity. Total activity of a given nuclide per gram of a compound, element, or radioactive nuclide. Specific energy. The actual energy per unit mass deposited per unit volume in a given event. This is a stochastic quantity as opposed to the average value over a large number of instances (i.e., the absorbed dose). Squamous cell carcinoma. A cancer composed of cells that are scaly or platelike. Standard mortality ratio {SMR). Standard mortality ratio is the ratio of the disease or accident mortality rate in a certain specific population compared with that in a standard population. The ratio is based on 100 for the standard so that an SMR of 200 means that the test population has twice the mortality from that particular cause of death. Stochastic. Describes random events leading to effects whose proba- bility of occurrence in an exposed population (rather than sever- ity in an affected individual) is a direct function of dose; these effects are commonly regarded as having no threshold; hereditary effects are regarded as being stochastic; some somatic effects, es- pecially carcinogenesis, are regarded as being stochastic. Stopping power. The average rate of energy loss of a charged particle per unit thickness of a material or per unit mass of material traversed. Straggling. The statistical variation in the range of a particle caused by the large number of interactions and scatterings within the material being traversed. Surface-seek~ng raclionuclide. An internal emitter that is deposited and remains on the surface of bone for a long period of time. This contrasts with a volume seeker, which deposits more uniformly throughout the bone volume. Target theory (hit theory). A theory explaining some biological effects of radiation on the basis that ionization, which occurrs in a discrete volume (the target) within the cell, directly causes a lesion that later results in a physiological response to the damage at that location; one, two, or more hits (ionizing events within the target) may be necessary to elicit the response.

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GLOSSARY 585 Threshold hypothesis. The assumption that no radiation injury oc- curs below a specified dose. Time-since-exposure (TSE) model. A mode} in which the relative risk is not constant but varies with the time after exposure. Transformed cell. Tissue culture cells changed in vitro from growing in an orderly pattern and exhibiting contact inhibition to growing in a pattern more like that of cancer cells, resulting in the loss of contact inhibition. Transiocation. A chromosome aberration resulting from chromosome breakage and subsequent structural rearrangement of the parts between the same or different chromosomes. Tumorigenicity. Ability of cells to proliferate into tumors when inoc- ulated into a specified host organism under specified conditions. Unattached fraction. That fraction of the radon daughters, usually memo (Radium A), which has not yet attached to a particle. As a free atom, it has a high probability of being retained within the lung and depositing alpha energy when it decays. Units" Conversion Factors Becquere} (SI) 1 disintegration/s = 2.7 x 10-~t Ci Curie 3.7 x 10~° disintegrations/s = 3.7 x 10~° Bq Gray (SI) 1 J/kg = 100 red Rad 100 erg/g = 0.01 Gy Rem 0.01 Sievert Sievert (SI) 100 rem international Units are designated (SI). Working level (WL). Any combination of short lived radon daughters in 1 liter of air that will result in the ultimate emission 1.3 x 105 MeV of potential alpha energy. This number was chosen because it is approximately the alpha energy released from the decay of daughters in equilibrium with 100 picocuries of 222Ra. Working-leve! month (W[M}. Exposure resulting from inhalation of air with a concentration of 1 working level of radon daughters for 170 working hours. Years of life lost. The expected years of life for a nonexposed person minus the expected lifetime of an exposed person.

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Representative terms from entire chapter:

lifetime risk