What is Radon? Radon is a gas produced by the radioactive decay of the element radium.

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1 Radon What is Radon? Radon is a gas produced by the radioactive decay of the element radium. Radon itself is radioactive, losing an alpha particle and forming the element polonium. Each of the three naturally-occurring series-decaying radionuclides releases radon (only the uranium-238 series contributes significantly to the hazard). Uranium is everywhere (soils, building materials, etc.) Uranium decays eventually to radium. Radium, the parent of radon is very soluble. Therefore radon is also everywhere. The major source of radon is the soil, but ground water, natural gas, and building materials also contribute. What is the Hazard? The half-life of radon is only 3.8 days. It is the daughter products of radon decay that are more of a health risk. Polonium, bismuth, and lead (among others) are produced by radon decay in the air and in the lung. Alpha particles are released in the decay of the radon daughters. The biological effects of very low doses of alpha particles are uncertain (extrapolated from high doses to miners). Radon Page 1 of 22

2 Radon History: As early as the 1500s an unusually high mortality from respiratory disease ( black lung disease ) was documented among underground metal miners in the Erz Mountains of Eastern Europe. In the 1800s pulmonary malignancy was documented in miners from that area, and in the early 20 th century shown to be primary carcinoma of the lung. Rutherford and Owens studying the emanations from thorium in 1899 wrote the radiation from thorium was not constant but varied in a most capricious manner. Rutherford made the key observations that the emanation acts like an ordinary gas and that the intensity of the radiation has fallen to one half of its value after an interval of about one minute. Rutherford and Owens had discovered radon-220 (thoron), half life 51 sec. Pierre and Marie Curie, studying the emanations from radium in 1899, concluded that..it stayed radioactive for several days. The Curies had detected radon-222, half life 3.8 days, but they still had doubts as to whether the activity was really in the form of a gas. Later work by Rutherford (1901) and also by Ernst Dorn (1900) confirmed that the radiations emitted by radium were a radioactive gas. In the 1920s high levels of radon were detected in mines in Czechoslovakia, where miners also had high lung cancer rates. It was proposed that the radon was the cause, but the hypothesis was not generally accepted until the reports of epidemiological studies on other groups of radon-exposed miners in the 1950s and 1960s. The mining industry begins to monitor mine air in the 1950s and 1960s and limit exposure. Radon itself was initially assumed to be the direct cause of the lung cancers in miners. In the 1950s, Harly, in a doctoral thesis at Rensselear Polytechnical Institute, proposed that it was the decay products of radon that delivered the pertinent dose to the lung cells. Radon Page 2 of 22

3 Table B-4 in [BEIR VI]. National Research Council. Health Effects of Exposure to Radon: BEIR VI. Washington DC: National Academies Press, See [WL, working level, is a measure of radon daughter concentration in air.] Radon was found to be a constituent of indoor air, but the potential health implications received little attention until the late 1970s. High indoor radon levels were found in Scandinavia and traced to building materials containing a particular type of shale. Sparse data from the United States reported similar levels. Only since the 1970s that radon has been recognized as a major source of radiation exposure to the general public. Home construction practices (in response to the energy crisis) in the 1970s led to better insulated and more tightly sealed homes. In 1984, a nuclear power plant worker in Pennsylvania set off a radiation monitor on his way IN to work. His home was found to be contaminated with radon at levels higher that the limits for underground mines. Other homes were later found to contain similarly high radon levels. As the database grew, the problem was recognized as widespread. Radon Page 3 of 22

4 Radon Uranium: The source All rocks contain some uranium: average values between 1 and 3 parts per million (ppm). In general, the uranium content of a soil will be about the same as the uranium content of the rock from which the soil was derived. Some types of rocks have higher than average uranium contents (lightcolored volcanic rocks, granites, dark shales, sedimentary rocks that contain phosphate, and metamorphic rocks derived from these rocks). These rocks and their soils may contain as much as 100 ppm uranium. Layers of these rocks underlie various parts of the United States. The amount of radon in a house is greatly affected by other factors in addition to the presence of uranium in the underlying soil. Radon Page 4 of 22

5 Source: U.S. Geological Survey. Geologic Radon Potential of the U.S. [updated 13 October 1995, cited 29 March 2004]. Courtesy of the U.S. Geological Survey. Radon Page 5 of 22

6 Uranium Radium Radon Alpha recoil is the most important factor affecting the release of radon from mineral grains. Some radon escapes the grain into the pore space. The presence of water in the pore space can slow the radon and increase the likelihood that it will stay in the pore space. Source: U.S. Geological Survey. The Geology of Radon [updated 13 October 1995, cited 29 March 2004]. For most soils, only 10 to 50 percent of the radon produced actually escapes from the mineral grains and enters the pores. Most soils in the United States contain between 0.33 and 1 pci of radium per gram of mineral matter and between 200 and 2,000 pci of radon per liter of soil air. Radon Page 6 of 22

7 Soil conditions affect radon movement Radon can more easily leave the rocks and soils by escaping into fractures and openings in rocks and into the pore spaces between grains of soil. Factors affecting radon movement: the amount of water present in the pore space (the soil moisture content) the percentage of pore space in the soil (the porosity) the soil permeability, the "interconnectedness" of the pore spaces that determines the soil's ability to transmit water and air. Source: U.S. Geological Survey. The Geology of Radon [updated 13 October 1995, cited 29 March 2004]. Courtesy of the U.S. Geological Survey. Radon moves more rapidly through permeable soils, such as coarse sand and gravel, than through impermeable soils, such as clays. Fractures in any soil or rock allow radon to move more quickly. The distance that radon moves before most of it decays is less than 1 inch in water-saturated rocks or soils, but it can be more than 6 feet, and sometimes tens of feet, through dry rocks or soils. Radon Page 7 of 22

8 Radon entry into buildings Source: U.S. Geological Survey. The Geology of Radon: Preface [updated 13 October 1995, cited 29 March 2004]. Courtesy of the U.S. Geological Survey. Characteristics of homes with elevated radon levels Houses with basements rather than crawlspaces. Houses with walls below grade. Houses with exposed earth in basements/sumps. Houses on ridges/slopes rather than in valleys. The soil on slopes is usually better drained and more permeable. Houses with hollow concrete block foundation walls rather than solid poured concrete walls. Tightly sealed rather than drafty houses. Radon usually escapes into the atmosphere. Where a house is present, however, soil air often flows toward its foundation for three reasons: differences in air pressure between the soil and the house, the presence of openings in the house's foundation increases in permeability around the basement (if one is present). Radon Page 8 of 22

9 Radon concentrations Radon levels in outdoor air, indoor air, soil air, and ground water can be very different. Radon in outdoor air ranges from less than 0.1 pci/l to about 30 pci/l, average 0.2 pci/l. Radon in indoor air: ranges from less that 1 pci/l to about 3,000 pci/l, averages between 1 and 2 pci/l. Radon in soil air (the air that occupies the pores in soil) ranges from 20 or 30 pci/l to more than 100,000 pci/l; most soils in the United States contain between 200 and 2,000 pci of radon per liter of soil air. Radon dissolved in ground water ranges from about 100 to nearly 3 million pci/l. Approximate contributions to indoor radon concentration in a single family home. Soil 1.5 pci/l Public water supplies 0.01 Private wells 0.4 Building materials 0.05 Outdoor air 0.2 Radon Page 9 of 22

10 20 GM Geometric Mean = 0.96 pci/l Geometric Standard Deviation = 2.84 Arithmetic Mean = 1.66 pci/l Percent of Houses AM > Rn Concentration (pci/l) Figure by MIT OCW. Distribution of indoor radon concentrations in 552 U.S. homes in data from 19 different studies (above) and in similar data from Sweden (below). (A concentration of 4 pci/l is equal to 160 Bq/m 3 ).

11 Radon Decay Fig in Alpen, E. L. Radiation Biophysics, 2 nd ed. San Diego, CA: Academic Press, Radon itself, due to its fairly short half-life ( 222 Rn) is not a major concern. Radon is also an inert gas and is typically exhaled after breathing it in (although some will dissolve in the blood). The concern is over the daughter products of radon that are particulate (attached to aerosol particles), α-emitting, and decay within hours to 210 Pb (T 1/2 = 22 years). Radon Page 10 of 22

12 The Working Level By convention, the concentration of radon daughters is measured in working levels (WL), and cumulative exposures over time are measured in working-level months (WLM). The working level (WL) is a single value used to describe the radon daughter concentrations in a way that also reflects their biological hazard. The hazard is most directly related to the total amount of energy that will be deposited in the lung tissue by the alpha decays of 218 Po and 214 Po. The working level is that combination of radon daughters in one liter of air which will result in the emission of 1.3 x 10 5 MeV of alpha particle energy. 1 WL = 100 pci/l of radon daughters at equilibrium (each = 100 pci/l) 1 WL = 1.3 x 10 5 MeV/L Typical working level concentrations of radon daughters: Outdoors Indoors, Basement Indoors, First Floor WL WL WL The Working Level Month is a cumulative exposure to radon daughters. It results from an exposure to a radon daughter concentration of 1 WL for one working month (170 hours). Exposure (WLM) = Concentration (WL) x 170 exp osure time (hrs) Radon Page 11 of 22

13 Equilibrium Fig. B-1 in [BEIR VI]. See Given enough time, a closed system initially containing only 222 Rn will come into equilibrium with the daughter products. Most atmospheres show an equilibrium factor of about 0.5 [ daughters] = 0.5 [ radon] All daughters at 100 pci/l = 1 WL. Or, radon at 200 pci/l, with equilibrium factor of 0.5 = 1 WL. Radon Page 12 of 22

14 Potential Alpha Energy Concentration (PAEC) The convention for description of radon progeny concentration is moving towards more standardized SI units expressed in joules/m 3. PAEC refers to radon daughters, not radon, which is inert and not retained in the lungs. Cp(J/m 3 ) = (5.79 C C C 3 ) x Where: Cp = PAEC in J/m 3 C 1 = activity concentration of 218 Po (RaA) in Bq/m 3 C 2 = activity concentration of 214 Pb (RaB) in Bq/m 3 C 3 = activity concentration of 214 Bi (RaC) in Bq/m 3 Potential Alpha Energy (13.69 MeV) (7.69 MeV) (7.69 MeV) Exposure is expressed as Jh/m 3 1 WLM = 3.5 x 10-3 Jh/m 3 Fig. 4.7 in Turner J. E. Atoms, Radiation, and Radiation Protection, 2 nd ed. New York: Wiley-Interscience, Radon Page 13 of 22

15 Attachment to aerosols When radon decays the daughter product ( 218 Po) is usually stripped of several electrons and left with a net positive charge. The 218 Po most likely combines with oxygen or becomes hydrated as water condenses around it. This unattached 218 Po may be anywhere from µm in size. The unattached 218 Po suffers one of three possible fates: it remains unattached until decay, it attaches to ambient aerosols or it plates out on some available surface. Estimates place the unattached fraction between 1 and 10%; commonly quoted average values are 4% in mines and 7% in homes. The higher the concentration of ambient aerosols, the larger the attached fraction. Radon Page 14 of 22

16 Lung Physiology Source: U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program. Respiratory System: Conducting Passages. [cited 29 March 2004.] Courtesy of the U.S. National Cancer Institute. Source: U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program. Respiratory Tract. [cited 29 March 2004.] Used with permission. Radon Page 15 of 22

17 Aerosol particle size affects lung deposition pattern. Fig. B-19 in [BEIR VI]. See Fig. B-8 in [BEIR VI]. See Smallest particles (unattached fraction) are retained in the nose and throat (long considered a major dose contributor). ~5 nm: deposit in bronchial region 3-10 nm: dose to lung increases greatly >10 nm: deep lung deposition ~100 nm: deposition effectiveness decreases due to less effective diffusion of larger particle >500 nm: impaction helps deposit in respiratory tract >2500 nm: deposition in nose and throat Radon Page 16 of 22

18 Dose to the lung: the dosimetric approach There are several ways to compute the dose from radioactive dust deposited in the lung. The simplest is to assume that the energy is deposited uniformly throughout the entire lung mass (taken to be 1000 grams in the adult male). The dose to the bronchial epithelium is orders of magnitude higher than that calculated on the assumption that the energy is absorbed in the whole lung. Most lung cancers arise in the bronchial epithelial tissue. Factors that influence the dose to target cells in the respiratory tract from radon exposure. The unattached fraction Aerosol characteristics Breathing pattern: Tidal volume; Nose or mouth breathing Characteristics of the lung: mucus thickness; morphology of bronchi The particle size distribution can affect the dose distribution. For radon the energy is assumed to be absorbed to a depth of µm, which is the range of the most energetic alpha particle emitted in the radon decay scheme. Dose estimation is still experimental. Active research in this area continues. Not used yet for risk estimation by radiation protection agencies. Radon Page 17 of 22

19 The special nature of the biologic damage induced by alpha particles. Fig. 2-1 in [BEIR VI]. See For any particular cell the probability of being hit is VERY low. Target cells for lung cancer induction are still not known. What is the effect of exactly one alpha particle traversal? Table 2-1 in [BEIR VI]. See Documented increased risk of lung cancer in miner cohorts. The problem is how to extrapolate this risk down to the exposure levels seen by the general public. Typical exposure to the general public: ~ 0.2 WLM/year ~ WLM/lifetime. An average lifetime exposure to 200 Bqm -3 (5.4 pci/l), which is about 4 times the average indoor exposure, would result in, on average, about 1 alpha particle traversal per bronchial epithelial cell nucleus (0.6 for the bronchial basal cell nucleus, and 2-4 for the secretory cell nucleus). Radon Page 18 of 22

20 Lung Cancer and Estimation of Risk: The Epidemiological Approach Risk factors for lung cancer (from BEIR VI Table 1-1) Active cigarette-smoking Passive cigarette-smoking Radon Occupational carcinogens Arsenic Asbestos Chromates Chloromethyl ethers Nickel Polycyclic aromatic hydrocarbons Family history Fibrotic lung disorders Ambient air pollution Table 3-2 in [BEIR VI]. See Figure 3-6 in [BEIR VI]. See ERR: excess relative risk A model that describes the risk from exposures as a multiplicative increment to the excess disease risk above the background level. Figure G-1 in [BEIR VI]. See Radon Page 19 of 22

21 The EPA Action Level In the U.S and Europe the indoor radon concentration varies from Bq/m 3 (0.5 to 1.6 pci/l) Finland, Norway and Sweden average about 100 Bq/m 3 (2.7 pci/l) Conversion of radon concentration to a dose has many uncertainties. 20 Bq/m 3 (0.5 pci/l) will give a dose to the bronchial epithelial cells of 1 msv/year (100 mrem/year). The U.S. EPA has set a recommended action level of 160 Bq/m 3 (4 pci/l). This is ~ 4 x the average radon concentration in U.S. homes (1 in 12 homes exceed this level). This is the most stringent action level in the world. Germany and the U.K. are 2x higher. Finland, Sweden and Norway are 5x higher. The following are estimated doses based on radon exposures: 1 WLM = 14.2 rem, (0.71 rads) adult male 1 WLM = 12.6 rem, adult female 1 WLM = 25 rem, 10 year old child Typical cumulative exposures: 0.2 WLM/year ~3 rem/year to the bronchial endothelium WLM/lifetime BEIR VI estimate: reducing all homes to < 4 pci/l will reduce lung cancer incidence by 3-4%. Radon Page 20 of 22

22 Risk Assessment How to predict the risk from low levels of exposure? Figure 1-3 in [BEIR VI]. See Dosimetric approach: focuses on computing the doses deposited in lung target cells by alpha particles. The approach relies on applying risk estimates from populations exposed to other radiations, such as the Japanese atomic bomb survivors, to the doses estimated in lung cells. Biological approach: would use data from molecular, cellular and animal studies to generate parameters which could be used to refine models of carcinogenesis. The miner data could be used in conjunction with the models to test reliability. Empirical approach: Uses statistical methodology to analyze data from miner and residential studies. Problems: 1. Use of gamma-ray-exposed groups requires assumptions about extrapolation from acute low-let exposure to prolonged high-let localized exposures. 2. Residential radon exposure data limited in statistical power. Few studies, small groups. 3. Dosimetric approach has many uncertainties in the extrapolation of effects of acute whole body exposure to gamma rays to localized exposure to high- LET alphas. 4. Biological approach: limited data on mechanisms of radiation induced carcinogenesis. BEIR VI summary: choose the empirical approach. Radon Page 21 of 22

23 Reference: National Research Council. Health Effects of Exposure to Radon: BEIR VI. Washington DC: National Academies Press, Viewable online at Radon Page 22 of 22