Chapter 39 Remedial Measures To Reduce Radon Concentrations in a House with High Radon Levels K. D. Cliff, A. D. Wrixon, J. C. H. Miles, and P. R. Lomas National Radiological Protection Board, Chilton, Didcot, Oxfordshire 0X11 ORQ, United Kingdom Measures to reduce radon concentrations have been studied in an old house in which the radon decay-product concentration initially exceeded 0.3 Working Level (WL). Some of the measures were only partially successful. Installation of a concrete floor, designed to prevent ingress of radon in soil gas, reduced the radon decay-product concentration below 0.1 WL, but radon continued to enter the house through pores in an internal wall of primitive construction that descended to the foundations. Radon flow was driven by the small pressure difference between indoor air and soil gas. An under-floor suction system effected a satisfactory remedy and maintained the concentration of radon decay products below 0.03 WL. Before standards for indoor exposure to radon can be formally established, work is necessary to determine whether remedies are feasible and what is likely to be involved. Meanwhile, the Royal Commission on Environmental Pollution (RCEP) in the UK has considered standards for indoor exposure to radon decay products (RCEP, 1984). For existing dwellings, the RCEP has recommended an action level of 25 mSv in a year and that priority should be given to devising effective remedial measures. An effective dose equivalent of 25 mSv per year is taken to correspond to an average radon concentration of about 900 Bq m" or an average radon decay-product concentration of about 120 mWL, with the assumption of an equilibrium factor of 0.5 and an occupancy factor of 0.83. The range of radon decay-product concentrations found in houses in southwest England spans three orders of magnitude. A few dwellings have been found where levels exceed 0.6 WL. It is clear that there are likely to be an appreciable number of houses in this region with indoor concentrations higher than 120 mWL. In 1982, the Department of the Environment identified two local authorities in the county of Cornwall that were willing to lend council property to test 3
0097-6156/87/0331-0536$07.00/0 Published 1987 American Chemical Society
39.
CLIFF ET AL.
Remedial Measures To Reduce Radon Concentrations
537
various remedial measures. At the end of 1983 a suitable house with a radon decay-product concentration t y p i c a l l y exceeding 0.3 WL was selected for detailed study. This report presents the results of a range of remedial measures c a r r i e d out through 1984 and 1985 i n co-operation with the Building Research Establishment of the Department of the Environment. Our objectives were to i d e n t i f y the main sources of radon i n the dwelling, to test the effectiveness of remedial measures not requiring alterations to the f a b r i c of the b u i l d i n g , and to provide a permanent means of reducing the radon concentration to the value t y p i c a l of UK dwellings, i . e . 25 Bq m" , which corresponds to 3 mWL. 3
Measurement techniques 2 2 2
The concentrations of the decay products of R n were determined using a Radon Decay Products Monitor, RDPM ( C l i f f , 1978a). Room a i r i s sampled through a glass f i b r e f i l t e r by means of an external pump. Radon decay-product a c t i v i t y c o l l e c t e d on the f i l t e r i s counted by a diffused junction alpha-particle detector during sampling and twice afterwards. The concentrations of P o , *Pb and Bi( *Po) in a i r can then be calculated ( C l i f f , 1978a). Two additional gross alpha counts enable the concentrations of the thoron decay-products ( P b and B i ) to be determined from the same a i r sample. Continuous measurements of the potential alpha energy concentration of the radon decay products were made with a Continuous Working Level Monitor (WLM-300) (EDA Instruments Inc., Toronto). Concentrations of radon gas were measured using s c i n t i l l a t i o n c e l l s of 150 ml capacity f i t t e d with two s e l f closing vacuum connectors (EDA Instruments Inc., Toronto). The c e l l s were f i l l e d by flushing with f i l t e r e d room a i r . Flushing was c a r r i e d out for several minutes to ensure complete f i l l i n g . The c e l l s were l e f t for 3 hours to allow radon and i t s short-lived decay products to approach secular equilibrium before counting with a photomultiplier and scaler assembly. Measurements of the radon gas concentration i n room a i r , i n s o i l gas beneath the f l o o r and i n the voids i n the internal walls of the dwelling were made. Ventilation rates were determined with a nitrous oxide tracer. The gas was released into room a i r . After allowing time for uniform mixing, the decay i n concentration was followed using an i n f r a - r e d gas analyser, type Miran-101 (Foxboro/Wilks Inc. USA). 2 1 8
2 1 2
21
2 1 4
2 l
2 1 2
Characterisation of the dwelling The dwelling was a detached building some 100 years o l d with exterior walls constructed of granite blocks and mortar. Internal load bearing walls were of b r i c k and stone, cement rendered, and plastered. The material under the house contained a high proportion of mine s p o i l a r i s i n g from l o c a l t i n mining operations. Although, at present, there are just a few operating t i n mines i n Cornwall, the number exceeded 200 i n the eighteenth century. Spoil t i p s from d e r e l i c t mines have provided the construction industry i n Cornwall with a p l e n t i f u l and cheap source of material for hardcore or as an
RADON AND ITS DECAY PRODUCTS
538
aggregate i n concrete. Mine s p o i l from these sources often has elevated uranium concentrations. One ground floor room of the dwelling had been retained by the l o c a l Council as an o f f i c e to c o l l e c t l o c a l taxes. The remainder of the dwelling had been occupied by a family. A f l o o r plan of the dwelling showing the room usage i s given i n Figure 1. The o f f i c e , s i t t i n g room and h a l l had wooden suspended f l o o r s , whereas the f l o o r was of s o l i d concrete i n the s c u l l e r y (a room for rough kitchen work), kitchen and behind the s t a i r s . The o f f i c e and s i t t i n g room each had an open fireplace and associated chimney. Two a i r g r i l l e s were set below floor level i n the front exterior wall of the house; one was intended to ventilate the space under the o f f i c e f l o o r , the other the space under the s i t t i n g room f l o o r . This arrangement, with just two a i r g r i l l e s i n one wall and with no v e n t i l a t i o n access to the other three sides of the underfloor spaces, did not provide adequate underfloor v e n t i l a t i o n . S o i l gases could accumulate i n the underfloor spaces and reach high concentrations, except when the wind v e l o c i t y was very high. Measurements of gamma dose rates over the ground f l o o r indicated values i n the range 0.15 yGy h" to 0.24 pGy h" . These values are higher than the average (0.12 pGy h" ) found i n stone houses i n Cornwall, but are within the normal range (Wrixon et a l . , 1984). Localised higher dose rates were detected close to the floors of the o f f i c e , s i t t i n g room, h a l l and kitchen. When the wooden suspended floors were later removed, areas of high dose rate i n the o f f i c e , h a l l and s i t t i n g room were found to be associated with discrete lumps of uraniferous material i n the oversite f i l l . The kitchen had the highest density of hot spots over the f l o o r . Since the f l o o r was s o l i d , no attempt was made to raise i t to investigate the immediate sources of high gamma dose rate. Gamma dose rates measured close to the walls within the dwelling were unremarkable. It was concluded that the building materials did not contribute s i g n i f i c a n t l y to the indoor radon concentrations. I n i t i a l studies of the indoor concentrations of radon and i t s decay products were carried out with a l l exterior doors and windows closed, but with a l l internal doors f u l l y opened. The radon production rate for the whole house was determined by measuring the P o concentration i n indoor a i r and i n the a i r outside the dwelling and by determining the v e n t i l a t i o n rate. The radon production rate, K, i s given by ( C l i f f , 1978b): 1
1
1
2 1 8
3
Κ = j ( l + 0.0748j)(C - C ') A
where
j C C' A
A
Bq m"
A
h"
1
1
i s the v e n t i l a t i o n rate, h" i s the concentration of P o i n indoor a i r , Bq m" i s the concentration of P o i n outside a i r , Bq m" . 2 1 8
2 1 8
3
3
The radon production rate for a dwelling, or for an individual room, i s not constant with time, as i t i s affected by meteorological and other conditions. Average radon production rates based on longer term integrated measurements of radon gas concentration would have resulted i n s l i g h t l y different values from those reported here. However, t h i s parameter i s less variable than "grab" sample determinations of the concentration of radon or i t s decay products.
539 Concen 39. CLIFF ET AL. Remedial Measures To Reduce Radon It best indicates the effectiveness of different strategies to reduce the concentration of indoor radon. The radon production rate for the whole dwelling was measured as 280 Bq m" h' , with all internal doors open. When internal doors were closed, large differences were found in the concentrations of radon and its decay products in different rooms. All ground floor rooms had much higher concentrations than those upstairs. Radon production rates were determined for each of the ground floor rooms. In the scullery, kitchen, office and sitting room, the production rates were in the ratio 1 : 5 : 65 : 130. The radon production rate for the sitting room was 7800 Bq m" h" . Evidently radon was entering the dwelling predominantly in the sitting room and office. The likely source of indoor radon was accumulation from soil gas in the poorly ventilated underfloor spaces. The ease with which gases could enter the dwelling was demonstrated by injecting nitrous oxide into the underfloor space. This was detected in both the sitting room and the office within minutes of injection under the floor. It appeared earlier and reached higher concentrations in the sitting room, in keeping with the difference in the radon production rates of the two rooms. Figure 2 shows the variation in the radon decay-product concentration in the sitting room over a 10 day period at the beginning of the study, together with the wind speed. The ventilation rate will increase with increasing wind speed. If the radon production rate is reasonably constant, the concentration of radon decay products in room air should therefore decrease with increasing wind speed. This effect is seen between days 6 and 8 in Figure 2, but for other periods wide fluctuations in the radon decayproduct concentration occur for little change in wind speed. The reason for this variable behaviour was later shown to be due to the Venturi effect of the wind on the two chimneys. 3
1
3
1
Remedial measures Some of the remedial measures tested in this study were not regarded as likely to form part of a long-term control stategy. For example, the installation of a mechanical ventilation system, with a heat recovery unit, would not be used in a dwelling of this type, because of the very high installation cost. Nevertheless, the availability of the dwelling enabled devices to be tested under real housing conditions, rather than in the laboratory. Four remedial measures that did not entail altering the building fabric were tested with the following results. Forced underfloor ventilation. Two attempts were made to forcibly ventilate the spaces below the suspended wooden floors of the office, hall and sitting rooms. At first, a 30 W centrifugal fan was installed in ducting connected between a hole in a floor-board in the sitting room and the chimney. The ducting was sealed through a plate covering the fire place aperture that prevented air discharged to the chimney from entering the room. The hole in the floor-board from which the ducting was connected was only some 0.6m from the front exterior wall in which the underfloor ventilation grilles were fixed. When operated, the fan produced a reduction in the radon decay
R A D O N A N D ITS D E C A Y P R O D U C T S
540
Solid j
concrete floor
:
I S u s p e n d e d wooden floor
Chimney Grille
« π
—
I
-Office-
Hall
.
C
Landing
j
Right bedroom
Sitting room-*
Bath room
WC
ι Side bedroom,
/ Left
Centre bedroom
bedroom
First floor lb
Ground floor la Front
Figure 1. Floor plans of the study house. 700r-
L
600h
Ε 500h
0
12 25
0
12 26
0
12 27
0
February
12 28
0
12 29
0 I
12 1
0
12 2
0
12 3
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12 4
0
12 5
0
March
Figure 2. Variation in radon decay-product concentration over a 10 day period and wind speed.
39.
CLIFF ET AL.
Remedial Measures To Reduce Radon Concentrations
541
products concentration i n the s i t t i n g room by a factor of about four. The radon concentration i n the exhaust a i r discharged to the chimney was 2600 Bq m" . In the second configuration the fan was connected i n ducting fixed over the v e n t i l a t i o n g r i l l e beneath the o f f i c e f l o o r , such that a i r was drawn from the underfloor spaces and discharged to the outside. In t h i s case the radon concentration i n the exhaust a i r from the fan was again 2600 Bq m~ , but the radon decay products concentration i n the s i t t i n g room was reduced by only a factor of about two. With only two v e n t i l a t i o n g r i l l e s i n the same exterior wall, a i r movement under the f l o o r s was largely r e s t r i c t e d to the area close to the front wall. Low resistance a i r paths existed between the two exterior g r i l l e s and only a small proportion of the t o t a l underfloor volume was e f f e c t i v e l y v e n t i l a t e d . This explains the r e l a t i v e l y small reductions i n the radon decay products concentration achieved i n the s i t t i n g room. The lower reduction i n concentration when the fan was connected d i r e c t l y to the a i r g r i l l e under the o f f i c e f l o o r was due, i n part, to a i r leakage d i r e c t l y from the o f f i c e to the underfloor space v i a gaps behind the s k i r t i n g boards i n that room. At a l a t e r stage, when the suspended wooden floors were removed, i t was found that the oversite f i l l , consisting of coarse sand, gravel and some stones, was very uneven and i n places was higher than the bases of the j o i s t s supporting the wooden f l o o r s . Had the oversite f i l l been l e v e l l e d , or excavated, to leave a clear space below the j o i s t s , the reduced pressure on the underfloor space r e s u l t i n g from the operation of the fan would have been e f f e c t i v e over a larger area. A more e f f e c t i v e forced v e n t i l a t i o n of the underfloor space could have been effected by having a d i s t r i b u t e d system of v e n t i l a t i o n extract points within the underfloor space, connected by a manifold to a single fan system. Forced underfloor v e n t i l a t i o n i s not considered an ideal permanent remedy, and i t i s preferable to f i n d a passive solution to the problem. A passive method i s one requiring no energy source and minimal maintenance by the occupants of the dwelling. 3
3
Polymeric b a r r i e r s . The UK b u i l d i n g codes require that when a building i s constructed on a concrete r a f t (slab-on-grade) the concrete base must incorporate a vapour barrier to prevent ingress of moisture. The vapour b a r r i e r i s usually a 125 pm or 250 ym thick polythene sheet. Provided the polythene sheet i s free from holes, i t should reduce the d i f f u s i o n of radon from the underlying s o i l through the concrete and into the b u i l d i n g . An intact sheet of 250 pm polythene i n a 10 cm thick concrete base would reduce the f l u x of radon into a b u i l d i n g by about 40% ( C l i f f , Miles and Brown, 1984). Unfortunately, with normal construction practices i n the UK, the polythene vapour b a r r i e r i s r a r e l y i n s t a l l e d free of perforations. To test the effectiveness of an intact polythene sheet as a radon b a r r i e r , two layers of 125 pm sheet were l a i d across the s i t t i n g room f l o o r . The sheets were dressed up the walls to a height of about 60 cm and fixed with adhesive tape. Polythene sheets were also used completely to cover the f i r e p l a c e . Figure 3a shows the radon decay-product concentration i n the s i t t i n g room, together with the wind speed, for the 25 day period
RADON AND ITS DECAY PRODUCTS
542
b e f o r e t h e p o l y t h e n e sheets were l a i d . L a r g e e x c u r s i o n s i n c o n c e n t r a t i o n o c c u r r e d , p a r t i c u l a r l y between days 13 and 18. These are n o t w e l l c o r r e l a t e d w i t h wind speed. F i g u r e 3b shows t h e d a t a for the p e r i o d immediately f o l l o w i n g the l a y i n g o f the polythene sheets on day 26. I t i s seen t h a t q u i t e s u b s t a n t i a l changes o c c u r i n wind speed, e.g. d u r i n g day 3 1 , b u t t h a t t h e changes i n t h e c o n c e n t r a t i o n o f radon d e c a y - p r o d u c t s a r e modest compared w i t h t h o s e of F i g u r e 3a. The peak i n r a d o n d e c a y - p r o d u c t c o n c e n t r a t i o n d u r i n g days 43 and 44 i s n o t r e l a t e d t o wind speed. The mean radon d e c a y - p r o d u c t c o n c e n t r a t i o n b e f o r e l a y i n g t h e p o l y t h e n e sheets was 240 mWL ( F i g u r e 3 a ) , whereas t h a t a f t e r l a y i n g was 160 mWL ( F i g u r e 3b). T h i s r e d u c t i o n , by a f a c t o r o f 1.5, was l e s s t h a n a n t i c i p a t e d . The main reasons f o r t h e l i m i t e d success o f p o l y t h e n e sheets were d i f f i c u l t i e s i n a c h i e v i n g a p e r f e c t s e a l between t h e p o l y t h e n e and t h e w a l l s , and a l s o t h e f a c t t h a t t h e p o r e s i n t h e i n t e r n a l w a l l s o f t h e d w e l l i n g p r o v i d e d a p a t h f o r radon i n s o i l gas t o e n t e r t h e room above t h e p o l y t h e n e l i n i n g . I t was a l s o d i f f i c u l t t o prevent c o m p l e t e l y a i r from elsewhere i n t h e house ( n o t a b l y t h e o f f i c e ) m i x i n g w i t h t h a t i n t h e s i t t i n g room. Spot measurements w h i l e t h e p o l y t h e n e s h e e t was i n p o s i t i o n i n t h e s i t t i n g room showed t h a t t h e c o n c e n t r a t i o n o f radon decay p r o d u c t s i n t h e o f f i c e were t y p i c a l l y 10 t i m e s h i g h e r . F i g u r e 4 shows t h e r a p i d i n c r e a s e i n t h e c o n c e n t r a t i o n o f r a d o n decay-products i n t h e s i t t i n g room on removal o f t h e p o l y t h e n e s h e e t s . T h i s was e f f e c t e d i n t h r e e s t a g e s . I n i t i a l l y , the sheets were removed a l o n g t h e f r o n t w a l l o f t h e room and f o r a d i s t a n c e o f 1.5 m a c r o s s t h e f l o o r . A s h a r p i n c r e a s e o c c u r r e d i m m e d i a t e l y . An hour l a t e r t h e r e m a i n i n g p o l y t h e n e , e x c e p t t h a t c o v e r i n g t h e f i r e p l a c e , was removed. The c o n c e n t r a t i o n was s t i l l r i s i n g a t 21:00. When measurements were resumed a t 10:30 t h e f o l l o w i n g day, t h e c o n c e n t r a t i o n had s t a r t e d t o f a l l and c o n t i n u e d t o f a l l u n t i l 15:15. The p o l y t h e n e c o v e r i n g t h e f i r e p l a c e was t h e n removed, p r o d u c i n g a marked i n c r e a s e i n t h e c o n c e n t r a t i o n o f P o , b u t l i t t l e change i n the aggregate radon d e c a y - p r o d u c t c o n c e n t r a t i o n (mWL). F r e s h r a d o n gas was thus drawn i n t o t h e room when t h e f i r e p l a c e was uncovered because o f t h e V e n t u r i e f f e c t on t h e chimney. T h i s reduced t h e p r e s s u r e s l i g h t l y i n t h e s i t t i n g room and i n c r e a s e d t h e p r e s s u r e d r i v e n f l o w o f s o i l gas i n t o t h e room. An i n t a c t p o l y t h e n e membrane w i t h i n t h e c o n c r e t e base o f a b u i l d i n g w i l l p r e v e n t p r e s s u r e d r i v e n f l o w o f radon i n t o t h e b u i l d i n g from t h e s o i l , even i f t h e c o n c r e t e i s c r a c k e d . D i f f u s i v e flow o f radon i n t o t h e b u i l d i n g w i l l a l s o be r e d u c e d because o f t h e c o m p a r a t i v e l y low d i f f u s i o n c o e f f i c i e n t o f radon i n p o l y t h e n e (% 1 0 " cm s " ) . No s i g n i f i c a n t improvement was a c h i e v e d by s u b s t i t u t i n g a 50 ym sheet o f mylar f o r p o l y t h e n e (mylar d i f f u s i o n c o e f f i c i e n t ^ 1 0 " cm s " ) . I n t h i s c a s e a d d i t i o n a l d i f f i c u l t i e s were experienced i n s e a l i n g the l e s s f l e x i b l e m a t e r i a l t o the w a l l s . 2 1 8
2
7
1
1 1
2
1
E l e c t r o s t a t i c p r e c i p i t a t o r s . Two e l e c t r o s t a t i c p r e c i p i t a t o r s , o f t h e type d e s i g n e d f o r domestic p r e m i s e s o r s m a l l o f f i c e s , were o p e r a t e d i n t h e s i t t i n g room. Each p r e c i p i t a t o r was r u n a t i t s maximum nominal a i r throughput o f 270 m h " . The c o n c e n t r a t i o n o f radon i n room a i r was n o t measured d u r i n g t h e s e t e s t s , b u t d i s - e q u i l i b r i u m 3
1
39. CLIFF ET AL.
Remedial Measures To Reduce Radon Concentrations
Figure 3. Variation in radon decay-product concentration and wind speed, (a) before and (b) after laying polythene.
543
544
RADON AND ITS DECAY PRODUCTS
Polythene sheet covering f i r e place removed
Polythene sheet removed along window wall
Upstairs window and inner door opened
-no
4
