Health Physics Pergamon Press 1976. Vol. 31 (August), pp. 119-125. Printed in Northern Ireland

RESPIRATORY EXPOSURE IN BUILDINGS DUE TO RADON PROGENY*? J. A. AUXIER

Director, Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 (Received 30 September 1975; accepted 23 January 1976) Abstract-The alpha radiation dose to the lungs of people who live in buildings constructed of some granites, low density concretes, and gypsum boards is higher than for residents of most other types of dwellings due to the airborne progeny of radon. There is evidence that sealing the interior surfaces with epoxy paint, for example, can reduce the alpha dose to the lung significantly without a compensating increase in whole-body exposure to the gamma rays from radon progeny. Based on the incidence rates for lung cancer in uranium miners, a concentration of radon of the order of l-SpCi/l. appears to be a reasonable limit for “lifetime” exposure indoors for “typical” home ventilation conditions.

stations cause any sort of biological damage, then the NRE shbuld receive intensive study. DURINGthe last 2 decades there has been a The dose equivalent in the area of nuclear growing concern, outside the health physics power stations is increased only on the order profession, over the somatic and genetic of a few mrem/yr or less, and people have effects of low levels of ionizing radiation on been exposed for only a matter of years. Dose man. This increased awareness by others than equivalent due to the NRE ranges from a health physicists has grown until, in some minimum of about 75 mrem/yr to a maximum quarters, the same health physics policies and of perhaps 10.000 mrem/yr, and people have procedures which were taken by many people been exposed to the NRE for all generations. 20 yr ago to be far too conservative are consiThe NRE as a field of study has been redered to be far too liberal in 1975. This viewed extensively in the course of two symincreased awareness or concern, especially by posia (Ad64; Ad74). Numerous others have those outside the health physics profession, reviewed the special problems of lung irradiacommenced with the controversy over fallout tion by radon daughters, especially as related from weapons’ tests and has been well nurto uranium miners (e.g. Ba5.5; BaS6; Ja64). tured by intervenors in the nuclear power However, if the NRE is of importance in arena. One result of the overall controversy human exposure at low levels, it appears that has been a greatly increased interest in the the lifetime exposure of the lungs of persons natural radiation environment (NRE), the living in certain types of homes should be normal geographical variations in it, and evaluated. Several preliminary and related the technological enhancement of it studies have been made (Lo71 ; To72), but (UN58,62,66,69,72; Ad64; Ad74; Ge7S). further experimental studies appear warIt has been abundantly clear for several years that if the radiation doses received by ranted. This paper is intended to stimulate interest in the problem in a wider geographic people residing in the area of nuclear power area and assortment of structures than has been feasible to date. No specific discussion of natural emitters other than the uranium series *Research sponsored by the U.S. Energy Kesearch and Development Administration, in con- Will be given; the contribution to dose by potassium-40 and other naturally occurring tract with Union Carbide Corporation. +Some of this research was accomplished as par- radionuclides can be found in the proceedings tial fulfillment of the requirements for the Ph.D. of the two symposia on the NRE (Ad64; degree at the Georgia Institute of Technology. Ad7 4). INTRODUCTION

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RESPIRATORY EXPOSURE IN RUlLDlNGS DUE TO RADON PROGENY

URANIUM DECAY SERlES IN MATERIAL

Radon-222 is present in all of the air on earth to some extent, the concentration depending largely on the distribution of radium-226 in the local soils. Inside buildings, the sources of radon are the outside air and that emanating from the building materials. The buildup of radon daughters toward equilibrium a5 a function of time is well known, the early calculations having been reported by Lord Rutherford (Ru04); hence, the effect of known ventilation rates can be computed. Therefore, the chief interest here is in the concentrations o f radium-226 in some building materials and the emanation of radon-222 from them. It has long been recognized that there is little radium in wood and most other materials from which “light-frame” houses are constructed. Therefore, there is little exposure to radon and its daughters within these structures unless there are high levels of radon in the outside air and some degree of ventilation. In this case, the minimum ventilation necessary to bring the radon into the structure is probably adequate to maintain as great an average degree of disequilibrium as exists outside. Therefore, the part of the radon problem to be cxamined here is that associated with structures built of materials having relatively high concentrations of radium-226 or, in the special case of uranium mine tailings, those structures built upon such tailings. HUMAN EXPOSURES TO RADIATION FROM CONSTRUCTION MATERlALS

The dose rates inside buildings constructed of materials relatively rich in radium-226 have two chief components, i.e. the gamma dose from radon daughters trapped in the material and the dose to the lungs of occupants from inhaled radon daughters. The building materials having the greatest concentrations of radium include granite, certain low-density concrete blocks, and certain gypwm boards (“dry-wall’’ board used for internal walls). From studies at ORNL (Au72) and from data from several sources, especially Lowder ef a!. (Lo71). Oakley (Oa71), and Spiers et al. (Sp64), it appears that the external gamma dose rate for human exposures inside buildings constructed of granite or of the low-

density concretes incorporating expanded Chattanooga shale both average approx 200 mradlyr. The dose rate range in both cases appears to be 100-500 mrad/yr. This external gamma ray dose is independent of the dose rate due to cosmic radiations, and the internal dose due to potassium-40 is independent of the type of dwelling. The internal exposures due to radon daughters are related most directly to the radium-226 concentrations in the building materials, and the radium-226 and -224 concentrations determine most strongly the gamma ray dose rate inside structures built o f granite or concrete. In buildings in which the daughters of radium-224 are the chief sources of gamma rays, the total dose to man would be very nearly the world average doses given in UNSCEAR reports (UNS8,62,66,69,72), plus the increase in the’ gamma ray dose due to the thorium series, especially that part from radium-224 to lead208, because of the short half-life of radon220 compared to radon-222. However, if the gamma dose rate is elevated due to increased concentrations of radium-226. additional internal exposures due to radon-222 and its daughters would be expected, especially to the lung. One possibility for limiting the lung dose could be to “seal” the building materials in such a way as to prevent radon-222 emanation. If the materials are sealed, the external uncollided gamma fluence would increase in the typical granite or shale-aggregate concrete structure, and recent studies have shown that the increase in gamma ray exposure from radon daughters after scaling amounts to as. much as 25% (Au72). However, this increase in uncollided photons from the radon daughters provides, in general, a small to negligible part of the total gamma ray fluence. Further, the lungs may frequently receive doses due to the alpha rays from radon daughters which are much higher than the “whole body” gamma doses. Therefore, sealing thc interior surfaces to prevent or decrease radon emanation appears generally desirable, especially if good ventilation is not feasible. Similar reasoning applies to uranium mines except that sealing exposed mine surfaces appears to be impractical. if not impossible, at present due to the

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extended fissures, water seepage, and large areas generally existing in mines. Radon emanation from building materials is controlled strongly by the moisture content of the radium-bearing material (Au72). There are limited data from other sources which support the findings here (Ha36; Ki32; Bar60; St.57). Tanner (Ta64) provides a fine review of the transport and diffusion theory for radon, but it is not clear that any existing theory can predict the behavior of radon in a complex, part-crystalline, part-non-crystalline, porous, and inhomogeneous solid. However, of the two mechanisms for explaining the enhancement due to moisture in the material, it appears that the relatively high solubility of radon and its subsequent diffusion, through water containing pores, to the surface is the most likely, but this cannot be put on a quantitative basis at present. The failure of a finegrained stucco of 0.25411. thickness to lower the emanation significantly lends credence to this hypothesis (Au72). The fact that an adhesive layer of asphalt cement was ineffective, also, in decreasing the emanation significantly is probably due to the general diffusibility of the noble gases through hydrogenous materials. Epoxy paint applied over a “stucco” finish of cement is the most effective of the liquid or plaster type sealants studied to date (Au72). It is clear from the data on radon emanation from concrete that a home constructed of concrete containing only radium-bearing shale aggregate could have radon levels that reach several tens of pCi/l. under equilibrium conditions. Considering that homes are generally only partially constructed of concrete and that equilibrium conditions are seldom met in occupied structures due to some minimal ventilation, it is still evident that a few picocuries per liter would not be unusual in the East Tennessee area, as an example. Actually, in a limited survey (Lo71), values of the order of 1 pCi/l. were obtained in East Tennessee homes under ventilation conditions; the air exchange rates were not measured. For low ventilation rates such as might be found in cold climates, it is reasonable to assume that 10 pCi/l. would not be uncommon. In an extensive study involving 841 measurements, Toth (To72) found average concentrations in living rooms

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of houses in Hungary of 3.05, 2.64 and 2.49 picocuries of RaA, RaB and RaC, respectively. These measurements were made in 14 different towns in Hungary in houses which had not, in general, been ventilated for at least 8 hr. However, the average is not of as great interest as the distributions; Toth found that the concentration of RaA exceeded lOpCi/l. in 6% of all measurements. Further, he shows that his values are not atypical of other areas of the world having “radioactivity not higher than normal.” In structures of interest here, there are above average concentrations of uranium in the building materials; considering that all occupied structures have some ventilation, the commonly observed levels of up to 10 pCi/l. in granite and low density concrete structures appear reasonable. DOSE TO THE RESPIRATORY SYSTEM

Early contributors to the understanding of respiratory system exposure due to radon and its daughters were Bale and Shapiro (Ba55; Ba56) who recognized the importance of the attachment of radon daughters to dust particles. The radon daughters, whether formed in free air or in the lung cavity, have a high probability of attachment to particulates which are abundant in all air. Inspired radon gas, which is soluble in water, blood, and most other body fluids and especially fatty tissue, may be either expired or sorbed into body fluids. Once in body fluids, especially the circulatory system, it is free to circulate throughout the body or to rediffuse into the lung and be expired. However, the daughters, which are heavy metals, whether attached to aerosols or free ions or atoms, may be deposited in the tracheobronchial tree. Bale and Shapiro (Ba55) calculated that for equilibrium of radon and its daughters, 95% of the dose to the tracheobronchial epithelium was due to the daughters. Of course, there are betas and gammas emitted by the daughters, but the major part of the energy absorbed in the basal cells of the tracheobronchial epithelium is from alpha particles. The relative biological effectiveness (RBE) of alphas compared to gamma rays is not accurately known for any radiation effect in man, including lung carcinogenesis, but it is generally assumed to be

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in the range of 5-20 (Sa70), thu5 increasing the effectiveness of the dose due to alpha rays. Standard anatomy texts show the details of the human respiratory system (So39). However, experimental workers with models have used various simplifications, and an excellent recent study (Ma72) has demonstrated some of the important physical phenomena using a plastic model of the upper bronchial tree. With the aid of this model and radioactively tagged aerosols, they have shown evidence that the important sites for deposition of aerosols are at and near the first few bifurcations of the bronchi. This deposition is that which would be expected due to impaction of particles for both inspiration and expiration. However. the inhomogeneous deposition 111cludes additional particulates in locations near the impaction areas, which indicate that diff usion in non-laminar flow regions is important, also. Even so, the authors were not convinced that these two phenomena alone could explain the concentrations of activity in the bifurcation areas of the upper bronchial tree. Another important facet of respiratory exposure was explored by Chamberlain and Dyson (Ch56) and by Bale and Shapiro (RaS5; Ba.56). A primary point of concern in those 5tudies was the ratio of unattached ions or atoms of the radon daughter?, mostly radium-A, to the total ions or atoms, generally referred to as the unattached fraction. Though the various measurements and estimates of the unattached fraction varied from near 1 % ‘ to as much as SO%, a mean value of 10% was generally assumed. This unattached fraction is estimated to produce about 90% of the dose to the basal cells of the bronchial epithelium because of the enhanced attachment (faster diffusion) of the atoms or ions to the bronchi and, to a minor extent, the absence of self-absorption by a particulate (ICKP59). Although numerous researchers have investigated the problems of the alpha ray dose to the bronchi, generally by basing a set of calculations on a chosen deposition model, the recent work of Martin and Jacobi (Ma72), and Harley (Har7 1) permits not only an evaluation of the range of uncertaintie3 in the bronchial dose due to radon but also a narrowing of the

limits of uncertainty for a given set of exposure parameters. Harley evaluated many of the physical parameters, including stopping power, and computed the dose to the largest part of the respiratory tree (thc trachea with 1.8 cm diameter) and the smallest bronchi (0.6 mm diameter). Because the differences were small, she did not compute the values for the intermediate diameters. She used experimentally determined values for the attached fraction (attached to particulates) and particle size distribution and assumed a 15-pm-thick mucous layer lining the bronchial tree. She used a particle size diameter of 0.3 pm,which is larger than generally used, but showed that selfabsorption in it is negligible; hence, her data can be applied with some confidence to smaller particles. An important conclusion drawn hy Harley is that the dose to the bronchial tree where lung cancers are observed in uranium miners is lower for a given radon concentration than assumed by lCRP (ICRP§9), though the difference is strongly dependent on the lung model used. In Harley’s analysis of Weibel’s model (We63), she has computed a dose to the basal cells ( 2 2 - ~ mdepth) which is nearly an order of magnitude lower than similar calculations based on a model by Altshuler et al. (A164). Modifying Harley’s conclusions by a different spatial distribution of deposited activity, for example, using the results of Martin and Jacobi, for 0.3 p m particles, it is reasonable to conclude that the dose to the regions of bifurcation is much higher than the average lung dose and is in the range of 10-20 radslyr for exposure to 100 pCi/l. of radon in equilibrium with its daughters for 2000 hr/yr, i.e. a worker’s general exposure time of 40 hriweek for 50 weekslyr. If it is assumed for the present that the working level unit (WL) used in monitoring for radon in uranium mines is numerically equal to 100 pCi/l., then the dose to the bifurcation regions of the bronchi is of the order of 10-20 rads per 12 working level months (WLM) (i.e. -2000 working hours). Lundin et al. (Lu71) report that exposure to 120-3.59 WLM causes an increase in respiratory cancer deaths of about a factor of four over control populations though they concluded that there was no significant increase in

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cancer below 120 WLM. The latent period for bronchial epithelium. Although an attempt lung cancer depends on the exposure, but was made in developing the above to be as generally decreases with increasing dose. realistic as possible, some conservative pruAlso, no data are available on the relative dence may have influenced the results unduly. sensitivity of women or children to the induc- The uncertainties in the calculations are not tion of this form of carcinoma, but from other readily estimated but may be of the order of a human studies it might be expected that chil- factor of 5. dren would be more sensitive. Therefore, it SUMMARY appears prudent to assume the “lifetime It appears that it is not unlikely that the doubling dose” to be about 100 WLM or 85- overall dose to the bronchi of people 170 rads to the bronchi. If for convenience a occupying homes built of uranium-bearing “low exposure” latent period of 10 yr is as- materials over a 50-yr period at an average of sumed to be reasonable, a 2-fold increased 15 hr/day would approach that at which the risk of death due to radon (daughters) induced incidence of lung cancer in uranium miners is cancer before age 60 would represent too doubled. If a sensitivity factor for children is large a “cost” to an individual for living in a assumed, then a broader segment of the popumasonry house as compared to a wooden lation is of concern. The dose to the basal cells building (a highly suspect assumption). Then of the bronchial epithelium is reduced by venwe may ask, what concentration of radon tilation or by use of a sealant to prevent radon would yield a dose for-exposure from birth to diffusion from the structural materials of age 50 without doubling the respiratory homes. It appears that further physical and cancer risk? There are 600 months for expos- epidemiological studies are warranted and ure and, averaged over the 50 yr, something would be scientifically interesting. If no admore than 8 hr per day and less than 24 verse effects of doses of this magnitude can be hr/day spent in the home; at present, there is demonstrated, it should be clear that the addino way of arriving at a satisfactory average, tion of a few mremlyr to the doses of small especially for any specific type of home. For segments of the population has not caused convenience, assume 15 hr/day over a period extensive deleterious effects. Contrarily, if adof 600 months which is equal, compared to verse effects are detected due to the “rem/yr” the miner’s 8 hr shifts for 5 daydweek, to levels, further control of “mrem releases” 1575 exposure months. In order to limit ex- might be justified. For all exposures, some posure to 100 WLM, the maximum WL would cognizance of the “risk-benefit’’ relation must be 100 WLM/1575 M or 0.06 WL. Assuming be taken and analyses updated as data on equilibrium of radon and its daughters, this is 6 low-level effects, positive or negative, become pCi/l. of radon in air. However, considering available. the effect of even minimal ventilation, it is Acknowledgements-The author gratefully acexpected that the dose due to particulates in knowledges the assistance given in the thesis rethe bronchi is seldom as high as it would be if search by Professors F. W. Chambers, R. H. the radon daughters were in equilibrium. At Fetner, D. S. Harmer, K. Z. Morgan and E. C. present it is not possible to ascertain the ad- Tsivoglou of the Georgia Institute of Technology. ditivity of other doses to the bronchi, e.g. the A special note of appreciation is due Professor G. 200-400 mradslyr of whole body exposure Eichholz, the Chairman of the Thesis Committee, due to penetrating gamma rays from the who contributed most extensively to the thesis promasonry structure materials, the dose due to ject from beginning to end. In addition, extensive diagnostic X-rays, etc. Consequently, pru- assistance and collaboration was given by D. J. G. D. Kerr, P. T. Perdue, W. H. Shindence would make desirable an average radon Christian, paugh and J. H. Thorngate. level of less than 6 pCi, perhaps 1 pCi/l. for residences, especially in view of the childhood REFERENCES exposure. This 1 pCi/l. corresponds to a dose Ad64 Adams J. A. S. and Lowder W. M. (Eds.), 1964, in: The Natural Radiation Environment to age 50yr of 5-10 rads or a dose rate of (Chicago: Univ. of Chicago Press). 100-200 mrads/yr to the basal cells of the

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