Chapter 23 The Behavior of Radon Daughters in the Domestic Environment Effect on the Effective Dose Equivalent 2
H. Vanmarcke1,A. Janssens1,F. Raes1,A. Poffijn, P. Berkvens1,and R. Van Dingenen1 1Nuclear Physics Laboratory, State University of Gent, Proeftuinstraat 86, B-9000 Gent, 2
Belgium Physics Laboratory (II), State University of Gent, Proeftuinstraat 86, B-9000
Belgium
Gent,
Simultaneous measurements of the radon daughter concentrations, the ventilation rate and the size distribution of the inactive aerosol have been performed in two bedrooms, a living room and a cellar. The measured radon daughter concentrations were fitted by the room model to optimize the deposition rate of the unattached daughters. The mean value was 18/h in the rooms and 8/h in the cellar. Then the unattached fraction was calculated in each measurement and was found to be between .05 and .15 without aerosol sources in the room and below .05 in the presence of aerosol sources. The effective dose equivalent was computed with the Jacobi-Eisfeld model and with the James-Birchall model and was more related to the radon concentration than to the equilibrium equivalent radon concentration. On the basis of our analysis a constant conversion factor per unit radon concentration of 5.6 (nSv/h)/(Bq/m3) or 50 (µSv/y)/(Bq/m3) was estimated. The radiation problem due to the presence of radon (Rn-222), thoron (Rn-220) and their respective daughter products in underground mines has been recognised many years ago and has been extensively studied. There has been a growing consensus that in a mining environment the quantity, exposure to radon daughter potential alpha energy (thoron daughter), can be transformed into adequate radiation protection. On historical grounds, this way of assessing the dose is known as "the working level concept". Some years ago it was realized that the indoor inhalation of the short-lived radon daughters constitutes the most important contribution to the radiation exposure of the general population (Unscear, 1982). The working level concept has been introduced in the domestic environment due to the success of the concept in the occupational environment and due to a lack of experimental data on the relative and absolute magnitudes of the transformation and 0097-6156/87/0331-0301 $06.75/0 © 1987 American Chemical Society
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removal processes determining the fate of the radon daughters i n the domestic a i r . These processes are : radioactive decay, v e n t i l a t i o n , attachment on and detachment from the ambient aerosol and deposition on walls and furniture. The rate of deposition i s strongly dependent on the physical c h a r a c t e r i s t i c s of the daughters. Although the description of the physico-chemical transformation of the daughters is very complex (Raes et al.,1985a; Raes,1985b) we s t i l l s t i c k to the simple c l a s s i f i c a t i o n into a f r a c t i o n that i s attached to the ambient aerosol and a f r a c t i o n that contains small clusters which for convenience i s c a l l e d the unattached f r a c t i o n . The deposition of the l a t t e r and the attachment to the room aerosol are fast competitive processes. They determine to a large extend the internal radon daughter equilibrium i n houses. The other processes : v e n t i l a t i o n , deposition of the attached f r a c t i o n and detachment of Pb-214 from the room aerosol due to the r e c o i l energy, are less c r i t i c a l to the equilibrium. According to Mercer (1976) a r e c o i l factor .83 i s used. In turbulent mixed a i r , the concentrations of the radon daughters can be expressed as a series of d i f f e r e n t i a l equations. The steady-state equilibrium version i s known as the room model. The model was f i r s t applied to underground mines (Raabe,1969;Jacobi,1972) and l a t e r on adapted to the domestic environment. The Porstenddrfer version (1984) includes p r e f i l t e r i n g of the unattached concentrations i n the incoming a i r . In the present paper h i s version has been changed s l i g h t l y , assuming the r a t i o ' s of the "outroom" to the "inroom" daughter concentrations to be equal. The error coming from this assumption i s small due to the l i m i t e d impact of v e n t i l a t i o n on the daughter equilibrium. The term "outroom" i s preferred to outdoor because part of the incoming a i r comes from adj acent rooms. Our research project has the ambition to contribute to the knowledge of the parameters of the room model and to calculate the a c t i v i t y median diameter of the attached daughters (AMD) which i s an important parameter i n the lung dosimetry models. However the main reason for application of the room model i s to investigate the unattached f r a c t i o n , which y i e l d s a much higher dose to the bronchial epithelium than the attached f r a c t i o n . A d i r e c t measurement of the unattached f r a c t i o n i n dwellings i s only possible with a high volume sampler, since the concentrations of radon daughters i n dwellings are i n general rather low. This could disturb the steady-state conditions i n the room. Besides there are d i f f i c u l t i e s i n achieving an adequate separation of the attached and unattached fractions by c o l l e c t i o n on a gauze. The working l e v e l concept evaluates the unattached f r a c t i o n and the a c t i v i t y median diameter i n an i n d i r e c t way, through the dose conversion factor. This paper w i l l show that i n the domestic environment this i s mostly inaccurate to estimate the dose.
Experimental apparatus The apparatus for measuring the low radon daughter concentrations, occuring i n normal indoor conditions, involves sucking a i r at a constant rate through a f i l t e r and counting the a c t i v i t y by means of alpha spectroscopy during sampling, and during a decay time interval.The detector configuration has been c a l i b r a t e d through
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simultaneous α and γ counting (Vanmareke,1984) and the analysis of the spectrum was standardised. The timing has been optimized by minimizing the mean of the minimum measurable radon daughter concentrations (MMC), which i s defined by Nazaroff (1984), as the concentrations at which the r e l a t i v e standard deviation i n the measurement due to counting s t a t i s t i c s i n 20%. The flow rate and the detector e f f i c i e n c y are taken to be 28 1/min and 0.127 respectively, our standard conditions. I t was found that a long sampling time and a delay time, longer than the minimum time needed to transfer the spectrum to a storage medium, are favourable. As an example Figure 1 shows the optimized MMCs as a function of the sampling and delay time for a measuring time of 60 min. The same kind of optimization has been performed f o r the thoron daughters. In the calculations the sampling period was set at 30 min and the f i r s t decay time i n t e r v a l i s started after the decay of the radon daughters (270 min). For a t o t a l measurement time of 16 hours the optimized MMC of Pb-212 and Bi-212 are respectively 0.02 Bq/m and 60 Bq/m (270-370 min, 540-960 min). Better results f o r Bi-212 are obtained with only one decay time i n t e r v a l and an estimation of the r a t i o of Pb-212 to Bi-212 out of the removal processes ( v e n t i l a t i o n and deposition of the attached thoron daughters). The influence of the removal rate on the potential alpha energy concentration i s small. For the decay i n t e r v a l (270-960 min) the MMC of Pb-212 i s 0.014 Bq/m , assuming the sum of the removal rates to be 0.6+0.5/h. Measuring the Po-218 and Po-214 alpha decay during sampling improves s i g n i f i c a n t l y the p r e c i s i o n (Cliff,1978), i n p a r t i c u l a r for Po-218 and thus for the unattached f r a c t i o n . However, at the same time a part of the unattached a c t i v i t y i s l o s t i n the complex sampling-head geometry. This loss has been determined i n our 1 m radon chamber as a function of the flow rate during sampling (Figure 2). The daughters were kept i n the unattached state by a forced v e n t i l a t i o n of 0.46/h over an absolute f i l t e r . The turbulence, induced by the v e n t i l a t i o n , reduces the mean residence time of the daughters i n a reproducible way. The daughters were alternately sampled with and without the complex sampling-head and counted during decay. In Figure 2 i t can be seen that, at 28 1/min the loss of the unattached daughters equals 22+2%. An independent way of assessing the order of magnitude of this loss i s through an analysis of the results to the l a s t q u a l i t y assurance excercise at NRPB (Miles et al.,1984). Comparing our measurements with the measurements of participants using a bare f i l t e r gives a loss of the unattached a c t i v i t y of about 30%. The deposition v e l o c i t i e s of the unattached daughters were calculated from the measurements i n the radon chamber with the bare f i l t e r (Figure 2) and found to equal .095+.007 cm/s f o r Po-218, .085+.012 cm/s f o r Pb-214 and .045+.015 cm/s f o r Bi-214. This decrease i n deposition v e l o c i t y i s one of the most important sources of error i n the room model. Together with the radon daughter measurements, nearly continuous measurements of the v e n t i l a t i o n rate are performed by means of the release of N2O tracer gas and observation of i t s decay with an infrared spectrometer (Miran 101). Furthermore the aerosol concentration and size d i s t r i b u t i o n are monitored every 20 to 30 min with an automated aerosol spectrometer (Raes et al.,1984). 3
3
3
3
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RADON AND ITS DECAY PRODUCTS
10
20 30 AO Sampling time (min)
50
Figure 1. Optimized minimum measurable concentrations (MMC's) of radon progeny as a function of sampling time. The mean of the MMC's i s used as optimisation parameter and the t o t a l measurement time i s fixed at 60 min.
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Radon Daughter Behavior in the Domestic Environment 305
Flow
1
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Figure 2. Ratio of the unattached Po-218 concentration to the radon concentration for d i f f e r e n t flow rates during sampling. The daughters are alternately sampled with (·) and without (O) the detection head.
RADON AND ITS DECAY PRODUCTS
306 Measurements
The measurements were performed at four d i f f e r e n t locations with radon concentrations ranging from 7 to 111 Bq/m and with low to moderate v e n t i l a t i o n rates. The main c h a r a c t e r i s t i c s of the locations are l i s t e d i n Table I.
Table I. Main c h a r a c t e r i s t i c s of the four locations studied. Number Type Location Floor l e v e l Type No days No meas. Vent. (1/h) range Rn (Bq/m ) range 3
1
2
det. house quiet area gr. f l o o r bedroom 7 32 .28 .13-.67 58 31-103
lab. busy road basement cellar 4 12 .74 .35-2.5 75 44-111
3 det. house quiet area 1st f l o o r bedroom 4 15 .17 .11-.20 30 20-41
4 semi-det. railway 1st f l o o r l i v . room 3 13 .46 .38-1.26 9 7-21
The f i r s t house and the room chosen for the measurement are the same as r e f e r r e d to i n e a r l i e r studies (Raes et al.,1984 ;Vanmarcke et al.,1985). Eightythree radon daughter measurements were c a r r i e d out on 18 d i f f e r e n t days. Eleven measurements were rejected from the optimization excercise because they were performed within 2 hours after the generation of a high aerosol concentration, so that no steady-state was reached. As an example, Figure 3 shows the data determined during one day i n each of the four locations. The indicated v e n t i l a t i o n rates are best f i t s to the N2O decay curves and are considered as representative f o r the indicated time periods. There i s some a r b i t r a r i n e s s i n the f i t t i n g s leading to d i s c o n t i n u i t i e s i n the r e s u l t s . The accuracy i s believed to be of the order of 20%. High aerosol concentrations with d i f f e r e n t size d i s t r i b u t i o n s were produced by burning a j o s s - s t i c k , a b i t of paper or by smoking or cooking. The calculated active size d i s t r i b u t i o n s corresponding with these disturbancies are p l o t t e d i n Figure 4. The attachment rate i s calculated out of the aerosol size d i s t r i b u t i o n as explained i n (Raes et al.,1984) and has a systematic uncertainty of about 50%. The lower part of Figure 3 show the measured radon daughter concentrations.
Analysis The deposition rate of the attached f r a c t i o n , p l o t t e d i n Figure 3, is calculated from the aerosol size d i s t r i b u t i o n assuming d i f f u s i o n and electrophoresis to be the most important deposition mechanisms (Raes et al.,1985a). The accuracy of the absolute values was checked by forming the aerosol mass balance after the generation of a high aerosol concentration.In Table II i s compared the decay of the
VANMARCKE ET AL.
Radon Daughter Behavior in the Domestic Environment 307
TIME (h) Figure 3. Evolution of the attachment rate, the deposition rate of the attached daughters, the v e n t i l a t i o n rate and the radon daughter concentrations ( • Po-218 • Pb-214 · Bi-214 measured, Δ V Ο f i t t e d ) during one day i n each of the four locations.
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RADON AND ITS DECAY PRODUCTS
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