Radon and Its Decay Products - ACS Publications - American

Chapter 10

Population Doses in Ireland

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J. P. McLaughlin Physics Department, University College of Dublin, Belfield, Dublin 4, Ireland

Indoor a i r radon concentrations measured i n a randomly selected sample of 220 I r i s h houses have been found to range from about 20 Bq/m to as high as 1740 Bq/m with a median value of 61 Bq/m . Using current dose estimation methods the estimated e f f e c t i v e dose equivalents due to radon daughter inhalation i n these houses are 1.6 mSv/year (median value) and 46 mSv/year (maximum value). Integrating alpha track based passive detectors, which y i e l d both a measurement of the mean radon concentration and of the radon daughter equilibrium (F) i n each house, are being used i n this national survey. 3

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As part of the current Radiation Protection Research Programme of the Commission of the European Communities (C.E.C) a national survey of indoor radiation exposure i n Ireland a r i s i n g from natural radiation i s being carried out. Most of the e f f o r t i n this survey i s being directed towards an assessment of the doses to the general population a r i s i n g from radon and i t s daughters. In this paper unless otherwise stated the terrn'Mose" i s taken to refer to e f f e c tive dose equivalent. The emphasis on radon has been adopted because a p i l o t survey of 278 houses, i n various parts of Ireland, carried out during 1983-85 (McAulay and McLaughlin, 1980) revealed that indoor doses from natural radiation were dominated by those a r i s i n g from radon daughter inhalation. In the p i l o t survey the estimated median value of the e f f e c t i v e dose equivalent from radon daughters was about 1 mSv/y while that from penetrating radiation (gamma plus cosmic) was about 0.6 mSv/y. The doses from penetrating radiation covered a r e l a t i v e l y short range about the median value from approximately 0.3 mSv/y to 1 mSv/y. Those from radon daughters were found to cover a large range having a log normal d i s t r i b u t i o n with maximum value of 31.7 mSv/y. The corresponding indoor radon concentrations ranged from 3 Bq/m-* to a maximum value of about 1200 Bq/m^. Approximately 10% of the dwellings had a radon concentration i n excess of 100 Bq/m and about 4% had greater than 200 Bq/rn^. I t thus appears i n Ireland that while mean values of doses from radon daughters are l i k e l y to be low a small number 0097-6156/87/0331-0113$06.00/0 © 1987 American Chemical Society

Hopke; Radon and Its Decay Products ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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of individual households i n some areas may be receiving s i g n i f i c antly high doses. The results of the p i l o t survey also indicated that indoor radon concentrations i n Ireland are correlated with the geological nature of the s o i l subjacent to a dwelling with construction characteristics playing a subsidiary r o l e . On the basis of the findings of the p i l o t survey i t was decided to carry out a nationwide survey of indoor radon i n a random manner.


The p r i n c i p a l s p e c i f i c objectives of this study are:(i) (ii)

(iii)

to obtain a representative d i s t r i b u t i o n of indoor radon concentrations i n Ireland. to ascertain the extent to which the radon concentrations correlate with various parameters such as geological charact e r i s t i c s , construction type, household energy usage etc. to determine the magnitude of individual exposures and the l i k e l y number of households i n which exposures might be so high as to j u s t i f y the development of p o l i c i e s aimed at exposure l i m i t a t i o n or reduction.

Survey Structure In the 1983-85 p i l o t survey about half of the 278 houses i n v e s t i g ated were chosen i n selected areas of the country where uraniferous deposits were known to e x i s t . Within the selected areas the houses were chosen at random. Because of the use of selected areas the indoor radon concentration data suite obtained i n the p i l o t survey cannot be considered as representative of the country as a whole. In the new national survey, i n which i t i s planned to investigate indoor radon concentrations i n approximately 2000 households, the households are being selected i n a random manner and are thus expected to y i e l d data which i s representative of the national housing stock. The selection i s being made i n collaboration with An Foras Forbartha (AFF), the National Institute for Physical Planning and Construction Research, who are carrying out a C.E.C. supported study of energy usage i n I r i s h dwellings. Access to the professional competence of the AFF i n the housing construction f i e l d and also to i t s housing energy usage study w i l l be of considerable advantage to the analysis phase of the indoor radon survey. The selection of households i s being made as follows:- A l i s t of 3,000 names and addresses of voters has been chosen randomly by computer from the national e l e c t o r a l r e g i s t e r . This selection i s considered to be reasonably representative of the population d i s t r i b u t i o n i n Ireland both on a geographical and on a s o c i a l category b a s i s . Each selected voter i s v i s i t e d by an interviewer from the AFF. Those agreeing to participate in the AFF house energy study are then invited to participate i n the indoor radon survey. By the end of the 1985 over 1500 households had agreed to participate i n the radon survey. The magnitude of this figure should be considered i n the context of the population size which i s presently about 3.6 m i l l i o n for the Republic of Ireland. Passive radon detectors for the p a r t i c i p a t i n g households are both dispatched and recovered by post. Each household i s sent one



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detector together with instructions regarding placing i t i n the house. Participants are requested to place the detector i n the p r i n c i p a l bedroom or a l i v i n g area of the home. In t h i s survey i t i s considered that an exposure time of about six months i s the optimum f o r a detector to be placed i n a house. It i s a s u f f i c i e n t l y long time to obtain a "representative" measure of the indoor radon concentration i n a dwelling. The small number of dwellings found to have a mean radon concentration greater than about 200 Bq/m^ are sent a second detector f o r a further s i x month period thereby obtaining a measure of the annualised mean radon l e v e l i n such houses. Each household i s requested to complete a detailed questionna i r e . This i s designed to obtain information regarding the house and i t s occupants which i s considered as relevant to an assessment of the r a d i o l o g i c a l impact of radon and i t s daughters on the household. The questions range from those on the age d i s t r i b u t i o n and smoking habits of the household to b u i l d i n g construction type, v e n t i l a t i o n and energy conservation practices used. I t i s made clear to each p a r t i c i p a t i n g household that a l l such information obtained and the measured value of the radon concentration w i l l be treated as s t r i c t l y confidential and w i l l be used only f o r s t a t i s t i c a l purposes. Measurement Techniques The radon detectors used i n both the p i l o t and the main survey are of the passive time integrating alpha track p l a s t i c v a r i e t y . In the p i l o t study each detector consisted of a single piece of CR-39 alpha track p l a s t i c mounted inside a closed small p l a s t i c container or cup. Alpha track v i s u a l i s a t i o n and counting was by means of chemical etching and o p t i c a l microscopy. This CR-39 based type of passive radon detector has the advantages of s i m p l i c i t y , r e l i a b i l i t y and excellent alpha p a r t i c l e r e g i s t r a t i o n c h a r a c t e r i s t i c s . In common with most other similar passive detectors presently available however, i t only y i e l d s information on the mean radon concentration during the exposure period. No information i s obtainable from these type of detectors on the degree of equilibrium existing between radon and i t s short l i v e d daughters i n the indoor a i r of a house. An accurate value of the degree of equilibrium or F factor of the daughters i s however of importance i n estimating the dose to members of a household where only the mean concentration of radon i s known. Here the equilibrium factor (F) i s considered i n respect to potential alpha energy and i s defined as the r a t i o of the equilibrium equivalent radon concentration to the actual concentration of radon i n the a i r . In the national surveys of indoor radon being presently carried out i n the European Community countries an assumed value of F between 0.35 and 0.5 i s being used f o r dose estimation purposes. Some recent measurements i n Germany (Keller and Muth, 1985) have shown, f o r the houses investigated, that the F factor ranged from 0.3 to 0.35. I t should however be noted that most F factor measurements given i n the l i t e r a t u r e are based on short time integrating instrumental measurements i n a small number of houses.



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For houses i n a survey found to have low radon concentrations (say less than 100 Bq/m^) a lack of precise knowledge of the F factor may not be important as the values of the estimated doses w i l l be towards the lower end of the dose d i s t r i b u t i o n for any value of F. From a dose assessment perspective an important problem may arise due to a lack of knowledge of the F factor for houses with a high radon concentration (several hundred Bq/nr* and upwards). I t i s recognised that both the radon concentration and F factor values in a house are complex functions of the radon source, building c h a r a c t e r i s t i c s , v e n t i l a t i o n regimes, aerosol size d i s t r i b u t i o n s etc. (Porstendorfer, 1984). I t i s however reasonable to expect f o r similar houses i n a given area that a general tendency may exist for both the highest radon concentrations and highest F values to be co-associated with low v e n t i l a t i o n rates. The use of an assumed mean F f a c t o r , of say 0.4, for such houses where the actual F factor may be as high as 0.6 or greater may severely underestim ate the doses being received by the household. I t i s therefore of some importance i n population dose assessment that passive time integrating detectors should be developed capable of measuring simultaneously both the radon concentration and the radon daughter F factor ( i . e . some form of passive "Working Level" meter i s required). Investigations both experimental and t h e o r e t i c a l , have been made i n Poland (Domanski et a l . , 1982) i n this d i r e c t i o n using alpha track p l a s t i c detectors. ~~The general conelusion from these studies was that the p a r t i c u l a r alpha track devices used were r e l a t i v e l y insensitive to changes i n the F factor. I t has been recently reported (Urban and Piesch, 1981; Put and Meijer, 1985) that with modifications i n the design and etching procedures of the standard Karlsruhe passive radon dosemeter i t i s possible to measure simultaneously radon and radon daughter concentrations. This approach has been used i n a radon survey i n the Netherlands and appears promising but w i l l require extensive f i e l d testing and c a l i b r a t i o n so that i t s performance may be properly assessed. Some recent developments i n radon daughter dosimetry (Vanmarcke e_t al_., 1986; James, 1986) suggest that an approximate inverse v a r i a t i o n of the F factor and the unattached f r a c t i o n i n indoor a i r leads to a r e l a t i v e l y constant lung dose per unit concentration of radon gas. These developments raise the question of the necessity of making F factor measurements i n addition to radon measurements i n a dwelling. To abandon F measurements, i f they can be conveni e n t l y and accurately made, purely on the basis of a currently popular lung dose model i s not however advisable. Dose models apart the value of F f o r a dwelling may be considered as a useful and informative c h a r a c t e r i s t i c property of dwelling a i r and aerosol dynamics. It i s related to the interplay of the various radon daughter production and removal mechanisms present. Where remedial or preventative techniques are used i n a dwelling the changes i n F value that occur can be used i n conjunction with radon concentration changes as a useful c h a r a c t e r i s t i c i n assessing the effectiveness of the techniques used. In order to address the F factor problem i n Ireland a "new" passive detector i s being used i n the national radon survey. I t consists of a small closed c y l i n d r i c a l container or cup, approximately 7cm i n height and 5.5 cm i n diameter, f i t t e d with two



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pieces of LR-115 alpha track p l a s t i c . One piece of LR-115 i s mounted inside the closed cup and the other i s mounted on the outside of the l i d . This detector i s thus conventional i n terms of construction but i t i s i n i t s c a l i b r a t i o n and the method of alpha track analysis used that a new approach has been adopted. Following exposure to radon and i t s airborne daughters i n a house the inner piece of LR-115 y i e l d s a track density which i s proportional to the radon exposure and thus the radon concentration may be determined. The outer piece of LR-115 yields a track density which has a component a r i s i n g from the radon a c t i v i t y i n the a i r and also has a component a r i s i n g from the alpha emitting airborne radon daughter a c t i v i t i e s i n the a i r . A method of track density analysis has been developed, based on laboratory c a l i b r a t i o n s , by which these two components can be separated. In this way the detector y i e l d s both the mean radon concentration (using the inner LR-115 track density) and the mean F factor (using both the inner and outer LR-115 track d e n s i t i e s ) . A f u l l description of this method w i l l be published i n due course. The p l a s t i c LR-115 was chosen as the alpha p a r t i c l e detecting medium, instead of the more sensitive CR-39 used i n the p i l o t survey, to avoid obtaining a track density component on the outer piece of p l a s t i c due to plateout of radon daughters or other alpha emitters. Preliminary results from the f i r s t phase of the national survey using t h i s new detector approach are encouraging and, as anticipated, high F values are being found to be associated with some of the highest indoor radon concentrations. I t i s recognised that t h i s F factor determination method requires refinement and interference from other alpha emitters may need to be eliminated i n some situations. On the other hand the current practice i n Ireland as i n other countries of using a single assumed F factor value i n a survey i s not the most s a t i s f a c t o r y . The mean radon concentrations determined by the passive detectors are based on calibrations using NBS standard radium-226 solutions and also from p a r t i c i p a t i o n i n the OECD (Nuclear Energy Agency)/CEC radon dosimeter intercomparisons (Commission of the European Communities, 1986) held at the U.K. National Radiological Protection Board (NRPB). Recent calibrations of the new LR-115 based detectors, i n terms of response to radon concentrations and F factors have been carried out at the NRPB, which assistance i s greatly appreciated. For the etching and track counting procedures used the inner LR-115 piece i n the detectors has a mean s e n s i t i v i t y of approximately 1.6 tracks cm"^ kBq~l is? h r " . 1

Exposure to Dose Conversion In the radon surveys the primary quantity determined i s the indoor a i r mean radon a c t i v i t y concentration. From a r a d i o l o g i c a l health perspective i t i s the dose a r i s i n g from the inhalation of radon daughters that i s of i n t e r e s t . The conversion from radon exposure to annualized e f f e c t i v e dose equivalent for the survey was carried out using the factors given i n Table I which are similar to those being used i n other European surveys. The occupancy and e q u i l i b rium factors given i n t h i s table are assumed mean values for I r i s h


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conditions and attempts are currently being made to obtain more representative values of these factors. The occupancy factor used here should be considered as an e f f e c t i v e value, adjusted f o r a l l members of a household. Ideally i t should take into account not only the f r a c t i o n of time exposure to radon i n a p a r t i c u l a r house but also the time f r a c t i o n spent i n other buildings (work place, school etc.) and outdoors exposed to mean radon concentrations appropriate to these locations. Table I: Factors used i n Radon Daughter Dose Estimations Occupancy Factor:

0.85

Equilibrium Factor (F):

0.45

Dose Conversion Factor =

5 mSv/WLM 3

3

(1 WLM = 1 Working Level Month = 3.5 χ 10" J.h.m" ) The dose conversion factor of 5 mSv/WLM being used i s that considered by a number of expert groups (OECD, 1983; UNSCEAR,1982) as appropriate for members of the public exposed to airborne radon daughters. I t i s however interesting to note that the Inter national Commission on Radiological Protection (ICRP, 1984) seems to consider a dose factor as high as 10 mSv/WLM as appropriate to members of the p u b l i c . Recent studies i n lung dosimetry (Cohen et a l . , 1985) taking into account the observed enhanced deposition of inhaled radon daughters at lung airway b i f u r a t i o n s indicate that lung doses could be 20% greater than currently estimated. In addition doses to the upper respiratory tract (URT) from inhaled radon daughters are now beginning to receive close study. While i t i s d i f f i c u l t to predict the l i k e l y change i n the dose conver sion factor when URT deposition i s taken into account i t i s clear that an increase i n the conversion factor w i l l occur. Survey Results The national indoor a i r radon survey commenced i n the autumn of 1985. In this f i r s t phase of the work radon detectors were sent to 400 randomly selected households throughout Ireland. A further 400 households are scheduled to receive detectors by the end of A p r i l 1986. At the time of writing radon detectors from a t o t a l of 220 households have been recovered and processed. The p r i n c i p a l results obtained are shown i n Table I I , together with the results of the e a r l i e r p i l o t survey and the cumulative results for both sets of data. The results for the f i r s t phase of the national survey are also presented i n histogram form i n Figures 1 and 2 together with the annualised e f f e c t i v e dose equivalents estimated using the factors given i n Table I. I t i s evident from the data that i n the majority of households surveyed the radon concentrations and ass ociated doses are low. In a small percentage of cases however individual households have been found with very high radon


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exposure levels for which the individual r i s k may be higher than what should be accepted by the individual or society. 3

Table I I Radon concentrations (Bq/m ) i n I r i s h Houses


>v

SURVEY

Radon (Bq/m ) 3

National (1985-) (220 Houses)

Pilot 1983-85 (278 Houses)

Combined results (498 Houses)

\

Median Value Min Value Max. Value 95% F r a c t i l e

53 3 1740 222

37 3 1189 200

61 17 1740 296

Some of the 220 detectors recently recovered have been analysed not only for radon exposure but also to determine the value of F (the equilibrium factor) i n the houses. A preliminary set of such F factor r e s u l t s , obtained by analysing the inner and outer LR- 115 track densities of each detector, are presented i n Table III for 12 houses with mean indoor radon concentrations greater than 200 Bq/m . In Table III are also presented radon daughter doses estimated using the i n d i v i d u a l l y determined equilibrium factor values F^ together with the doses estimated on the basis of an assumed mean F factor value of 0.45. 3

Table H I : F Factor Values f o r 12 Houses

Radon Cone. (Bq/m ) 3

Equil Factor F.

Dose (mSv/y) Using F

i

Dose (mSv/y) Using F = 0.45

1

203 217 244 266 414 522 576 607 750 815 1487 1740

0.36 0.36 0.37 0.42 0.43 0.29 0.82 0.27 0.28 0.39 0.76 0.61

4.4 4.7 5.5 6.8 10.7 9.2 28.4 9.8 12.4 18.9 67.5 63.9

5.5 5.9 6.6 7.2 11.2 14.1 15.6 16.5 20.3 22.0 40.2 47.0

It i s of interest to note that the houses with the two highest radon concentrations have determined F factor values substantially above the assumed mean value of F = 0.45 for I r i s h


RADON AND ITS DECAY PRODUCTS 3

RADON CONCENTRATION Bq/m

1800

1200

600 — I —

40


30

20

10 -H

4 " . 20

10

JLL 30

40

50

ANNUAL DOSE mSv/year Figure 1.

Results of Phase I National Survey ( F u l l Range) J

RADON CONCENTRATION Bq/m

0

74

148

222

2

4

6

370

8

10

ANNUAL DOSE mSv/year Figure 2.

Results of Phase I National Survey (0-*10mSv/y)



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Population Doses in Irefand

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houses. These high F values result i n increased values of the estimated doses. The mean F factor value determined for the 12 houses equals 0.45 which i s equal to the assumed values used i n the survey. This i s probably fortuituous. In dealing with such a small sample i t i s d i f f i c u l t to draw any general conclusions except to r e i t e r a t e that where possible for a house the mean value of the F factor should be determined simultaneously with the mean radon concentration so that a dose estimate representative of each p a r t i c u l a r household may be obtained. The method now being used to determine the F factor i n the I r i s h survey using a passive detector i s being subjected to increased laboratory testing so that i t s r e l i a b i l i t y and accuracy may be improved. Conclusions A general picture of radon exposure i n contemporary I r i s h houses i s emerging from the survey date obtained to date. For a t y p i c a l house the indoor a i r radon concentration i s l i k e l y to be between 25 and 70 Bq/m . I t i s also evident that a small number of houses exist with radon levels substantially above the mean value range. Both i n the p i l o t survey and i n the ongoing present survey i t appears that the p r i n c i p a l factor governing high radon levels i n I r i s h houses i s the radon exhalation c h a r a c t e r i s t i c s of the s o i l or geological structure subjacent to a house with construction c h a r a c t e r i s t i c s playing a minor r o l e . It i s recognised that substantial uncertainties s t i l l exist i n our knowledge of the microdosimetry of inhaled radon daughters not least because s i g n i f i c a n t inter - as well as i n t r a - subject v a r i a b i l i t i e s of the human lung exist (Hofmann, 1982). Notwithstanding this i t i s becoming increasingly obvious, from the present study and studies elsewhere, that i n d i v i d u a l members of the public are receiving continuous doses of some tens of mSv per year from radon daughters i n their homes. This i s a matter of substantial r a d i o l o g i c a l health importance which must be addressed both from the regulatory p o l i c y stance and also from the technical remedial and preventative viewpoints. In common with other member states of the European Community (Sinnaeve et_ a l . , 1986) this problem of population radiation exposure i s being seriously considered by the competent authorities i n Ireland but as yet no firm p o l i c y has emerged. The findings and eventual analysis of the national radon survey described here, when i t i s completed, w i l l be an essential input to the p o l i c y making process. 3

Acknowledgment s The radon survey of Ireland i s being j o i n t l y funded by the Commission of the European Communities under Contract BI6.0117.IRL. The assistance given by the NRPB (UK), An Foras Forbartha (AFF) and M.Burke (UCD) i s greatly appreciated.


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Literature Cited Cohen, B.S., Harley, N.H., Schlesinger, R.B. and Lippman, Μ., Nonuniform Particle Deposition on Tracheobronchial Airways: Implications for Lung Dosimetry. Presented at Second International Workshop on Lung Dosimetry, Cambridge, England, (September 1985). Commission of the European Communities., Results of the Second CEC Intercomparison of Active and Passive Dosemeters for the Measurement of Radon and Radon Decay Products, EUR Report 10403 EN (1986). Domanski, T., Chruscielewski, W., Swiatnicki, G., The Performance of passive differentiating track detectors containing a diffusion barrier, Rad. Prot. Dos. 2:27-32 (1982). Hofmann, W., Cellular Lung Dosimetry for Inhaled Radon Decay Products as a Base for Radiation - Induced Lung Cancer Risk Assessment. Radiat. Environ. Biophys. 20:95-112 (1982). International Commission of Radiological Protection, Principles of Limiting Exposure of the Public to Natural Sources of Radiation. ICRP Publication 39 (1984). James, A.C., A Reconsideration Cells at Risk and other Key Factors in Radon Daughter Dosimetry. Presented at ACS Symposium on Radon and its Decay Products, New York (April 1986). Keller, G. and Muth, H., Radiation Exposure in German Dwellings, some results and proposed formula for dose limitation. Sci. Total. Environ. 45:299-306 (1985). McAulay, I.R. and McLaughlin, J.P., Indoor Natural Radiation Levels in Ireland. Sci. Total. Environ. 45:319-325 (1985). OECD Nuclear Energy Agency., Dosimetry Aspects of Exposure to Radon and Thoron Daughter Products. Expert Report, Paris September (1983). Pörstendorfer, J . , Behavious of Radon Daughter Products in Indoor Air. Rad. Prot. Dos. 1: 107-113, (1984). Put, L.W. and de Meijer, R.J., Measurements of Time-averaged Radon Daughter Concentrations with Passive Dosemeters. Sci. Total. Environ. 45:389-395 (1985). Sinnaeve, J., Olast, M. and McLaughlin, J . , Natural Radiation Exposure Research in the Member States of the European Community: State of the Art and Perspectives. Presented at APCA Speciality Conference on Indoor Radon Philadelphia, U.S.A. (Feb. 1986). UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation)., Sources and Effects of Ionizing Radiation. Report to UN General Assembly, New York (1982).


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Urban, M. and Piesch, E., Low Level Environmental Radon Dosimetry with a Passive Track Etch Detector Device, Rad. Prot. Dos. 1:97-109 (1981).


Vanmarcke, H., Janssens, Α., Raes, F., Poffijn, Α., Berkvens, P. and Van Dingenen, R., On the Behaviour of Radon Daughters in the Domestic Environment and its Effects on the Effective Dose Equivalent. Presented at ACS Symposium on Radon and its Decay Products, New York (April 1986). RECEIVED October 9, 1986


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