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2 Natural Radioactivity and Radiation This chapter describes the behavior of selected natural radionuclides in the environment, the sources and variability of natural radiation, and the doses received by humans. Its purpose is to provide background information for discussions of the mechanisms by which exposures to natural sources can be increased by technologic activities, that is, can become exposures to TENORM. A more detailed account of natural radiation can be found in Eisenbud and Gesell (1997), which was used as a guide to prepare parts of this chapter. Natural radiation comprises cosmic radiation and the radiation arising from the decay of naturally occurring radionuclides. The natural radionuclides include the primordial radioactive elements in the earth's crust, their radioactive decay products, and radionuclides produced by cosmic-radiation interactions. Primordial radionuclides have half-lives comparable with the age of the earth. Cosmogenic radionuclides are produced continuously by bombardment of stable nuclides by cosmic rays, primarily in the atmosphere. Humans are exposed to natural radiation from external sources, which include radionuclides in the earth and cosmic radiation, and by internal radiation from radionuclides incorporated into the body. The main routes of radionuclide intake are ingestion of food and water and inhalation. A particular category of exposure to internal radiation, in which the bronchial epithelium is irradiated by alpha particles from the short-lived progeny of radon, constitutes a major fraction of the exposure from natural sources. In most places on the earth, natural radiation from external sources varies within about a factor of 4; but in some localities, the variation is greater because of abnormally high or low soil concentrations of radioactive minerals. Cosmic radiation alone varies by about a factor of 2 over the range of elevation that encompasses most of the world's population (0-2,000 m) and to a much smaller degree with latitude because of the variation in the earth's magnetic field. Particularly high concentrations of radioactive minerals in soil have been 25

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26 NO TURAL R24DIOA CTIVITY AND RADIA TION reported in Brazil, India, and China. Variations of radon concentrations in buildings are responsible for the largest variations in doses received by the public from natural internal sources. NATURALLY OCCI)~RING RADIONUCLIDES The origin of the primordial natural radionuclides of the earth is associated with the phenomenon of nucleosynthesis in stars (Fowler 1967~. The fact that the uranium, thorium, and actinium decay chains are found in nature is directly related to the very long half-lives of the parents of these chains. The absence of the neptunium decay chain is due to the lack of sufficiently long- lived members of this chain; complete decay of the parent radionuclides and their progeny has already occurred. Naturally occurring radionuclides with long half-lives that are not members of decay chains also exist in relatively high isotopic abundance. For purposes of discussion, the naturally occurring radionuclides are divided into those which occur singly (tables 2.1 and 2.2) and those which are components of three chains of radioactive elements. The uranium chain (table 2.3) originates with MU; the thorium chain (table 2.4), with 232Th; and the actinium chain (table 2.5), with 235U. Each table shows the nuclide, half-life, and principal radiations associated with each important branch of the chain. Minor branches, (less than 1%) and natural fissions are not listed, nor do they make any important contribution to the radiation dose from these chains. Tables 2.1 and 2.2 also show typical concentrations in various environmental media. 2In nature, 235U and a few other nuclides of uranium and thorium undergo fission spontaneously or as a result of interactions with neutrons that originate in cosmic rays or other natural sources. The half-life of 235U owing to spontaneous fission is 10~5-10~6 y, so decay by this process is at a rate less than 10-7 of that due to alpha-particle emission.

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GUIDELINES FOR EXPOSURE TO TENORM Table 2.1 Radionuclides Induced in Earth's Atmosphere by Cosmic Raysa 27 Radio- Half- Major Target nuclide life Radiations Nuclides Typical Concentrations, Bq/kg Air (troposphere) Rain Water Ocean Water ~Be 1,600,00 ,B N. O -- -- 2 x 10-8 Oy 26Al 716,000 ~Ar -- -- 2 x y 1 0-' 36C1 300,000 ,B Ar -- -- 1 x 10-5 y 8'Kr 229,000 K x rays Kr -- -- - y 14C 5730 Y ,B N,O - 5 x 10-3 32si 172y ~Ar -- -- 4 x 10-7 39Ar 269 y ~Ar -- -- 6 x 10-8 3H 12.33 y ,B N,O 1.2x10-3 7x10-4 22Na 2.60 y ,B+ Ar lx10 ~2.8x10-4 35S 87.51 d ,B Ar 1.3 x 10-4 7.7x 10-3 107x 10-3 7Be 53.29 d ~N. O 0.01 0.66 - 37Ar 35.0 d K x rays Ar 3.5 x 10-s 33p 25.3 d ~Ar 1.3 x 10-3 32p 14.26 d ~Ar 2.3 x 10-4 - 28Mg 20.91 h ~Ar 24Na 14.96 h ,B Ar -- 3.0x10-3 5.9 x 10-3 38s 2.84 h ,B Ar -- 6.6x 10-2 21.8 x 10-2 aAdapted from NCRP (1987a) and NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997.

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28 Table 2.1 (continued) NATURAL RADIOACTIVITYAND RADIATION Radio- Half- Major Target nuclide life Radiations Nuclides Typical Concentrations, Bq/kg AirRain Water Ocean (troposphere)Water 31 18F 2.62 h ,B Ar 1.83 h p+ Ar -- -- - 39C1 55.6 m ~Ar -- 1.7 x 10-i 8.3 x 10-~ 38C1 37.24 m ~Ar -- 1.5 x 10-~ 25 x 10-~ 34mcl 32.0 m ,B + Ar

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GUIDELINES FOR EXPOSURE TO TENORM Table 2.2. Nonchain Primordial Radionuclidesa Radionuclide Half-life, y Major Radiations Typical Crustal Concentration, Bq/kg 40K 1.28 x 109 0, y sov 1.4 x 10~7 4.75 x 10' 87Rb 't3Cd 9 x 1O's 630 2 x 10-5 70 <2 x 10-6 ~sIn 6x 10'4 ~2x 10-5 123Te 1.24 x 10'3 x rays 2 x 10-7 ~3sLa 1.05 x 10t' ~, ~y 2 x lo-2 42ce >5x 10~6 ~<1 x 10-5 i44Nd 2.29 x 10~5 a 3 x 10-4 '47Sm 1.06x 10" a 0.7 .s2Gd 1.08 x 10'4 a 7 X 10-6 174Hf 2.0 x 10~5 a 2 x 10-7 ~76Lu 3.73 x 10~ ,B, 1t 0~04 ~S7Re 4.3 x 10~ ,B 1 x 10-3 '90Pt 6.5 x 10" a 7 x 10-8 29 aAdapted from NCRP (1987a) and NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997.

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30 Table 2.3 Uranium-238 Chaina NATURAL RADIOACTIVITYAND RADIATION Nuclide Historical Name Half-life Major Radiations 23su Uranium I 4.47 x 109 y a, < 1% y aData from NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997. Minor branches, <1%, not shown. 234Th Uranium X, 24.1 d ,B 234mpa Uranium X2 1.17 m 0, <1% y 234u Uranium II 2.46 x 105 y a, < 1% 230Th Ionium 7.54 x 104 y a, < 1% 226Ra Radium 1600 y a, y 222Rn Emanation 3.82 d a, < 1% y 2~8po Radium A 3.10 m a, < 1% ~y 214pb Radium B 26.8 m ,B, y 2~4Bi Radium C 19.9 m 0, ~ 2~4po Radium C 164.3 ~s a, < 1% ~y 2~0Pb Radium D 22.3 y ,B, y 2~0Bi Radium E 5.01 d 2~0po Radium F 138.4 d a, < 1% ~y 206pb Radium G Stable None

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GUIDELINES FOR EXPOSURE TO TENORM Table 2.4 Thorium-232 Chaina Nuclide Historical Name Half-life Major Radiations 232Th Thorium 1.41 x 10l y a, <1%y 228Ra Mesothorium I 5.75 y 0, <1 228Ac Mesothorium II 6.15 h ,B, ~ 228Th Radiothorium 1.91 y a,~y 224Ra Thorium X 3.66 d a, ~ 220Rn Emanation 55.6 s a, <1% y 2,6po Thorium A 0.145 s a, <1% 2l2pb Thorium B 10.64 h ,B, ~ 2~2Bi Thorium C 1.01 h a,y 212po (64%) 208TI (36%) Thorium C' / 0.300 ms I a / ,B, Thorium C'' 3.05 m 208pb Thorium D Stable None 31 aData from the NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997. Minor branches, <1%, not shown.

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32 Table 2.5 Uranium-235 (Actinium) Chaina NA TUR,4L RADIOA CTIVI7YAND RADIA TION Nuelide Historical Name Half-life Major Radiations 235u Aetinouranuim 7.04 x 1Os y a, 23lTh Uranium Y 1.06 d 0, y 23lpa Protoaetinium 3.28 x 104 y a, y `^ Aetinium 21.77y ~,<1%Y 227Th 223Fr Radioaetinium / 18.72 d / a, y 1 ~, (98~62%) (1.38%) Aetinium K 22.0 m 223Ra Aetinium X 11.44 d a, 2~9Rn Aetinon 3.96 s a, 2.5po Aetinium A 1.78 ms a, < 1% 2'~Pb Actinium B 36.1 m 0,y 2"Bi Actinium C 2.14 m a, ~y 2o7Tl Actinium C' 4.77 m 0, e 1% 2o7pb Actinium D Stable None aData from NuDat online database maintained by Brookhaven National Laboratory, September 9, 1997. Minor branches, <1%, not shown.

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GUIDELINES FOR EXPOSURE TO TENORM 33 The three chains of radioactive elements and the long-lived primordial nuclide potassium-40 account for much of the external background radiation dose from radionuclides to which humans are exposed. Of the 22 nuclides identified as cosmogonic (table 2.1) only two, carbon-14 and tritium (3H), are of any consequence from the perspective of dose to humans. Only two of the 15 nonchain primordial nuclides, 40K and rubidium-87, are of particular interest (table 2.2~. Uranium and thorium can be concentrated in rocks by igneous and sedimentary processes (Bliss 1978~. Where uranium and thorium concentrations are high enough, rocks constitute ores to industrial societies. In the western United States, uranium ores have been extensively mined and milled to produce nuclear fuels. The biogeochemical behavior of a radionuclide in a given decay chain can be expected to vary with atomic number (that is, the element). For example, in the uranium decay chain, isotopes of uranium, thorium, radium, radon, and other elements occur. Chemically they range from an inert gas (radon) to a readily sorbed, tetravalent cation (thorium). Those properties determine the fate of the radionuclides in fuel and mineral processing, their transport in soil or surface disposal environments, and ultimately Heir biologic availability and uptake; a knowledge of their behavior is essential for defining source terms and assessing doses. Regulations for controlling exposure of the public to radionuclides are often dose-based. Because the doses result from interaction of humans with radionuclides contained in environmental media air, water, soil, and biota a knowledge of the behavior of naturally occurring radionuclides in these media is needed (Landa 1980~. It is important to know: The different mobilities of the various radionuclides in the decay chains. How technologic processes have changed the physical and chemical form of radionuclides and the release rates of radionuclides to the various media. How naturally occurring radioactive materials evolve with time (weathering reactions). . The concentrations and physical and chemical forms of the radionuclides. The following sections discuss the naturally occurring radionuclides that are potentially important contributors to human exposure to TENORM.

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34 NATURAL RADIOACTIVITYAND RADIATION Other natural radionuclides that are contributors to background radiation dose but not necessarily to exposure to TENORM are discussed for completeness, but in less detail. Uranium The primordial uranium found ubiquitously in nature consists of two isotopes with mass numbers of 235 and 238. In the earth's crust, 23sU constitutes 99.27% of the uranium by mass, and 235U, the parent isotope of the actinium chain, 0.72%. 234U, a shorter-lived member of the 238U chain, is usually in radioactive equilibrium or near-equilibrium with the parent isotope. Geochemistry Oxidation-reduction processes play a major role in the occurrence and behavior of uranium in aqueous environments. The dominant uranium valence states that are stable in geologic environments are the uranous (U4+) and uranyl (U6+) states, the former being far less soluble. Uranium transport generally occurs in oxidizing surface water and groundwater as the uranyl ion, UO22+, or as uranyl fluoride, phosphate, or carbonate complexes. UO22+ and uranyl fluoride complexes dominate in oxidizing, acidic waters, whereas the phosphate and carbonate complexes dominate in near-neutral and alkaline oxidizing waters, respectively. Hydroxyl, silicate, organic, and sulfate complexes might also be important, the sulfate complex being important especially in mining and milling operations that use sulfuric acid as a leaching agent. Maximum sorption of uranyl ions on natural materials (organic matter; iron, manganese and titanium oxyhydroxides; zeolites, and clays) occurs at pH 5.0-8.5. The sorption of uranyl ions by such natural media appears to be reversible; for uranium to be "fixed" and thereby accumulate, it requires reduction to U4+ by the substrate or by a mobile phase, such as H2S. Occurrence and Doses Uranium is found in all rocks and soils. Typical concentrations in the more prevalent types of rock and average concentrations in the earth's crust and in soil are listed in table 2.6. In the common rock types, the uranium concentrations range from 0.5 to 4.7 ppm, corresponding to activity concentrations for 238U of 7-60 Bq/kg (0.2-1.6 pCi/g). The overall effect of soil development results in an average soil concentration of uranium less than the average rock concentration. Some ores mined and processed for nonradioactive materials can produce residues with elevated concentrations of radionuclides. A well-known example is phosphorus ore, which contains uranium at up to 120 ppm and has also been used as a commercial source of uranium (NCRP 1993b). Natural materials that contain uranium at over 500 ppm are considered to be uranium ores. Uranium also occurs in air, water, and food and so is present in human tissues. The average annual intake of uranium from all dietary sources is about

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GUIDELINES FOR EXPOSURE TO TENORM 35 Table 2.6 Rangesa and averages of concentrations of 40K, 232Th, and 238U in Typical Rocks and Soilsb Material 40K 232Th 238u %total K Bq/kg ppm Bq/kg ppm Bq/kg Igneous rocks Basalt 0.8 300 3-4 10-15 0.5-1 7-10 (crustal) 1.1 300 2.7 10 0.9 10 Mafic Salic 4.5 1400 20 80 4.7 60 Granite(crustal) >4 >1000 Sedimentary rocks 17 70 3 40 Shale 2.7 800 12 50 3.7 40 Sandstones Clean quartz <1 <300 <2 <8 <1 <10 2? 400? 3-6? 10-25? 2-3? 40? Dirty quartz 2-3 600-900 2? <8 1 -2? 10-25? Arkose Beach sands <1 <300 6 25 3 40 Carbonate 0.3 70 2 8 2 25 rocks All rocky 0.3-4.5 70-1400 2-20 7-80 0.5-4.7 7-60 Continental 2.8 850 crust Soil 1.5 400 10.7 44 2.8 36 9 37 1.8 22 l aExamples of materials outside ranges can be found, but quantities are relatively small. bAdapted from NCRP (1987a).

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50 ~- a 1 'C . ~ . . ~ S a. rat - e_ ~ = o eg o O ~ .z ^ . ~ ~ m O ~ 0 ~ O 0 ~ "^ : O 0 ~ - m ._ O ._ O ~1 O O O ^ F E .E F~ O o ~ O

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GUIDELINES FOR EXPOSURE TO TENORM Cal [ o = o ~ no c, ~o . Ct _ ~ ~fi Ct V: . . . . . . o o CO UD o o ~ Cal suo!~oo1 Jo ~eq~nN 51 .= V) sit .s ~a' o ~ a) o Cal U) o Ct Ct o o ED .b Cal a> ~ . a _ Cal ~ ~ m O 5- ._ rT. ~ G

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52 NATURAL RADIOACTIVITY AND RADIATION Laboratory at its rural background monitoring station in Chester, NJ (Klemic 1996~. Figure 2.4 shows short-term variations and the effects on dose rate of diurnal variations in radon concentration, soil moisture, and rainout of radon decay products. Diurnal variations in radon concentration are caused by diurnal changes in atmospheric stability. Rainout of radon decay products briefly increases the dose rate, whereas accumulated soil moisture decreases it as a result of attenuation of the gamma-ray flux. Figure 2.5 shows long-term variations, which are influenced mostly by the attenuating effects of soil moisture and snow cover. In addition to calculations and direct ground-level measurements of external dose, measurements can be made with sensitive gamma-ray detectors in aircraft (IAEA 1991~. Many such surveys have been made, either to explore for uranium or to provide information about the radiation in the vicinity of proposed nuclear facilities. The data were analyzed by Oakley (1972), who estimated the population dose distribution in the United States. The data are grouped by geographic region: (1) the Atlantic and Gulf coastal plain, for which the mean annual absorbed dose is 0.23 mGy (23 mrad); (2) a portion of the eastern slope of the Rocky Mountains, where the annual absorbed dose averages 0.9 mGy (90 mrad); and (3) the remainder of the United States, where the average annual absorbed dose is 0.46 mGy (46 mrad). Cosmic Radiation The primary radiation that originates in outer space and impinges isotropically on the top of the earth's atmosphere consists of 87% protons, 11% alpha particles, about 1% nuclei of elements of atomic number 4-26, and about 1% electrons of very high energy. An outstanding characteristic of the cosmic radiation is that it is highly penetrating, with a mean energy of about 10~ eV and maximum energy of as much as 102 eV. The primary radiation predominates in the stratosphere above an altitude of about 25 km (NCRP 1987a). Most cosmic radiation originates outside the solar system. However, the solar component is important outside the atmosphere after flares associated with sunspot activity that follows an 11-y cycle. The interactions of the primary particles with atmospheric nuclei produce electrons. gamma rays, neutrons, plans and muons. At sea level, muons account for about 80% of the cosmic-radiation charged-particle flux, and electrons account for about 20%. The neutron flux is comparable with the electron flux.

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GUIDELINES FOR EXPOSURE TO TENON a) o a, ~ o o ~ , ~ U) ~ a,' CL Q 1x _ CD o _ _ - E :3 _ I \ ~ I o I ~ I o 3' ~ ' o 11 ~ ~ , o . I Cal ~ I =o o ;' I ~ no' it' ~ ~ , . . . . . . . ~ .. . . . . . . . o ~ o Us o ~ o ~ Cal ~ ~ o o CD ~ on . . ~ . . (mu) aped So 53 Ct _ ~ 4~ ._ UD O ~ A ~ ~ m ~ u, ~ == cO ~ ~ ~ ~ 3 o ~ . - .~ ~ _^ ~ _ ~ =, C~ . o o ~ ~ ~ ~ ~ ~ o ~ 3 ~ ~ ~ c o _ R ~ c, ~ 3 ~ .= o ~ ~o Cd o ~ ~ ~ ~ ~ C=~! ~ o -c Y ;: ~ C U.- . ~ ~ 3 , ~ o C~

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54 :_2 ~. ~. , ,, ! i , ., . . . i : : 1 ! 1 : : 1 : . :C - . 1 : : 1 : ! . I 1 1 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ .1 o o o o o C ~o oc) CO ~ C ~C~ NATURAL RADIOACTIVITYAND RADIATION ., ! ~ ., Lf) 1 w) ~ C~; ~, .o i . ~ ~oe~ . i q : od 1 i i . 1 : : . i i - o ~ o ~o o b o ~o p . ~o goo oo .o ,~ - _ C~ _ CD _ ~ ~ ~o _ ~ . ,~b ~ .~' Oo - `2 C~ ( ~ <' o*< oc) oo oc) CD CO ao C~ LO oo CS) ~ ~ en C~> _ ~ o _ oc) _ oc) C5) .e o ~ .~C]Ol ~- ~ : ~ o_ ~ . [g ~ ~ ~ o o o o oo CO Il/ASU - o C~ I.4 ~ o ~ Cq ~; ._ .= ~ ~_` - CO ~o =,.~e ~ A_ ~ .s ~ o o 3= ~ ~ d ,~ o ~ 'e ~ ~ ~ ~ o c': ~ ~ .= o c~ ~ - o ~ .N S uO.u ~ ~ ~ u, cd ~ ~ ~ ~'X ~ ~ - a~ ~ ~ c~ d o ~ o

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GUIDELINES FOR EXPOSURE TO TENORM 55 The dose from cosmic radiation is markedly affected by elevation. The annual cosmic-ray dose equivalent is about 0.29 mSv (29 mrem) at sea level. For the first few kilometers above the earth's surface, the cosmic-ray dose rate doubles for each 2,000-m increase in altitude (figure 2.6~. With the development of high-altitude aircraft and manned space flight, the dose from primary cosmic radiation attracted interest (O'Brien and McLaughlin 1972; Curtis and others 1966), which continues to the present (NCRP 1995; Reitz and others 1993; NCRP 1989b). A transcontinental flight has been estimated to result in a dose of about 0.025 mSv (2.5 mrem), or 0.05 mSv (5 mrem) per round trip (NCRP 1987a). Air crews who work an exceptionally heavy schedule (1,100 h/y) can receive annual doses of 0.3-9 mSv (30-900 mrem), depending on the routes flown (O'Brien and others 1992~. Once or twice during the 11-y cycle, a giant solar event can deliver dose equivalents at very high altitudes (15-25 kin) of 10-100 mSv/h (1-10 rem/h), with a peak as high as 500 mSv (5 rem) during the first hour (Upton and others 1966~. During a well-documented solar flare in February 1956, dose rates in excess of 1 mSv/h (100 mrern/h) existed briefly at altitudes as low as 10,000 m (Schaefer 1971~. SUMMARY OF HUMAN EXPOSURES TO NATURAL IONIZING RADIATION The annual effective dose equivalent received by persons living in areas of normal background radiation is estimated at 2.4 mSv (240 mrem) for the world population (UNSCEAR 1988~. The annual external effective dose equivalent is estimated at 0.36 mSv (36 mrem) from cosmic sources and 0.41 mSv (41 mrem) from terrestrial radiation. 222Rn and its short-lived decay products contribute about 40% of the total effective dose equivalent. The natural sources of dose are shown in more detail in table 2.9. A somewhat larger total annual dose of 3 mSv (300 mrem) is estimated for residents of the United States and is shown in detail in table 2.10 (NCRP 1987a). The US estimates are 0.27 mSv (27 mrem) for cosmic sources and 0.28 mSv (28 mrem) from terrestrial radiation. The major difference between the two estimates, however, is the average effective dose equivalent due to 222Rn, which is 55% of the total in the US estimate but 40% of the total for the UNSCEAR estimate. That is a difficult quantity to estimate, because world average 222Rn concentrations are not well known and several models are used to convert 222Rn exposure to lung dose (chapter 8~. The population distribution of external dose in the United States from terrestrial and cosmic sources combined is shown in figure 2.7 and is seen to range over a factor of about 4. The variation in radon exposure would be

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56 NATURAL RADIOACTIVITYAND RADIATION ABSORBED DOSE RATE IN AIR ( mGyly ) 0 0 O 0 _ . . ~I I _ TIT~ 1 1. 1_.- 111.] 1 1 1 ~ I 0 0 up call ad _ 0 0 (l MASH ~ 31~8 lN31VAlt)03 3soa 3nssl1 cO u) c o J fir lo ~ ~ o ~ Ho i% o CHID In -I ~ . ~ ~ ~ qua ha.,= 'e ~ ;^ l .e Do ~ ~ ~ o ~ ~ - ~ o ~ ~ ~ o cd 4 ~ - ~ =. v . o c) ~ - ~ ~ .~3 ~ `,, U. o ~ ~ ~ cd ~ c) ~ ~ m cq ~ o - c~ 'e ~ o ~ ~^ o ~ ~ ~ ~ c) cd ~ ~ 3 o ~o - ~ .= ~ 3o = ~ - CO ~; o V C) ~ ~, o _ Ct oo

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GUIDELINES FOR EXPOSURE TO TENORM Table 2.9 Estimated Effective Dose Equivalents From Natural Sources in Normal Regionsa Source 57 Annual Effective Dose Equivalent mrem mSv External Internal Total External Internal Total -- 0.36 0.015 0.015 Cosmic, including neutrons Cosmogenic nuclides Primordial nuclides 40K 87Rb 238U chain 238U ~ 234U 230Th 226Ra 222Rn ~ 214Pb 2l0Pb ~ 210po 232Th chain 232Th 228Ra = 224Ra 220Rn = 208Pb Total (round) aAdapted from UNSCEAR (1988). 36 -- 36 0.36 1.5 1.5 18 -- 0.6 0.5 0.7 0.7 110 12 16 0.3 1.3 16 160 33 0.6 0.5 0.7 10.7 110 12 0.3 17.3 16 240 0.15 0.1 0.18 0.006 0.005 0.007 0.007 1.1 -- 0.12 0.16 0.013 0.006 0.005 0.007 0.107 0.12 0.003 0.003 0.173 0.16 0.16 0.8 1.6 2.4 80

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58 NATURAL RADIOACTIVITYAND RADIATION ~e o ._ e. o U. C~ o CJ ._ C} ._ U. o CJ - C) C, a: ._ - o . - E~ Ct ._ o C, ._ C) - C) ~4 Ct C) au C~ o V) V) 0 oo o. ~ 0 0 0 0 o 0~ _` V - U' 0~ C~ _ ~0 ~._ Ct~ 0c> ._ _~ CtU) 0 3v o oo o o 5 Ct o U. Ct 3 CD ._ - o _ Ct s~ ._ C~ o ra O .~ Ct _ Ct - Ct .s U. r~ ~ r~ ._ ~ ~: C ~ $- o .O _ U. ~ O O V E Ct oo C~ V Z ~o C~

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GUIDELINES FOR EXPOSURE TO TENORM ! I 1 1 N 00 . ~9 - - l ! - ~o N N ~ (SUo!ll!U~) U!~lndd 1~) 0 59 U. Ct a' ~ so, En a_ N CS to IL a) 00 8 ~5 to N to to o - o .e U. o C) Ct - Ct .~ U. - Ct U. o - Ct au a - Ct o o o . ~. ~ =\ _1 Cal fi ~ ~ Ct o ~ o = ~

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60 NATU~L ~DIOACTIVI~AND RADIATION expected to be nearly proportional to the distribution of indoor radon levels in figure 2.2, which implies a range of factor of more than 20. CONCLUSIONS The main conclusions drawn from the foregoing review are as follows: All natural media earth, air, water, and biota, including humans are radioactive to some degree, and the concentrations of radionuclides in these media are highly variable, both between and within media. Humans receive radiation exposure from natural sources outside and inside the body, averaging about 1 mSv (100 mrem) per year in the United States. Humans receive radiation exposure from radon averaging about 2 mSv (200 mrem) per year in the United States. Doses received by humans from sources of natural radiation in the environment are quite variable, with a range of a factor of about 4 for external sources except radon and about 20 for radon. As a practical matter, the implications of existing levels and the variability of natural radionuclides and doses received by humans should receive careful consideration as efforts to regulate TENORM are contemplated.