Radon Levels in Swedish Homes: A Comparison of the 1980s with the

Chapter 7 R a d o n Levels i n Swedish Homes: A C o m p a r i s o n of the 1980s with the 1950s

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Gun Astri Swedjemark, Agneta Burén, and Lars Mjönes National Institute of Radiation Protection, Box 60204, S-104 01 Stockholm, Sweden In 1980-82 a study was carried out on Swedish homes built before 1976. The aim was to provide averages and distributions for the radon exposure of the Swedish population. Correlations with parameters such as building materials and building periods were also investigated. In 1955-56, a study of homes built before 1946 was carried out in four towns in central Sweden by Hultqvist with the aim of obtaining results which were representative for the homes in the areas that were studied. The radon concentrations in homes from approximately the same regions in the two studies are compared. The aver age was found to be four times higher for the homes measured in 1980-82 than for those measured in 1955-56. The reason for this increase is discussed including a thorough evaluation of the sampling and measurement methods used in the two studies. Indica tions that there has been an increase in the radon concentrations in other Swedish homes and also in other countries in temperate regions are also discussed. There are many indications of an increase in radon concentration in homes located in temperate regions during the last decades. In some of the nordic countries the ground has a high potential for radon exhalation and in Sweden we also have used a building material containing a higher concentration of Ra-226 than usual building materials. These conditions give a rather high concentration of radon indoors and is therefore easy to measure. Increases will also be higher resulting in more severe health effects than in regions with low concentration of Ra-226 in the building materials or in the ground. In Sweden measurements of the radon concentrations in homes were made in the 1980s and was studied also in the 1950s: two studies, which are possible to compare. 0097-6156/87/0331-0084$06.00/0 © 1987 American Chemical Society

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

7.

SWEDJEMARK ET AL.

Radon Levels in Swedish Homes

85

Measuring methods


The equipment For the country-wide study 1980-82 we used integrating instruments. Applying the ideas from the Environmental Laboratory i n New York (Breslin and George, 1979) the radon group at our laboratory designed and b u i l t passive radon monitors named Integrating Radon Measuring Apparatus (IRMA) based on thermoluminescence dosemeters, TLD (Burén et a l , 1982). The devices were calibrated during every summer period i n our radon chamber No I (Falk, et a l , 1982; Swedjemark, 1985) when the country-wide investigation was carried out. The standard deviation of the measurement values between the devices for a p a r t i c u l a r c a l i b r a t i o n period was found to be 5 - 6 % for radon concentrations between 100 and 3 800 Bq/m . Humidity, 20 60 %, temperature, 18-26 C and atmospheric pressure, 970 - 1 040 mbar, were not found to influence the c a l i b r a t i o n factors. The c a l i b r a t i o n s were made with an ion chamber with an estimated o v e r a l l uncertainty of 10 % (1 s ) . This uncertainty includes that of the c a l i b r a t i o n factor of which the proportion of the systematic uncertainty i s estimated to be small. The t o t a l uncertainty i n a measurement value can therefore be estimated to be well below 20 %. The s e n s i t i v i t y of the dosemeters and the design of the instruments result i n a greater uncertainty than that given above for radon concentrations below 10 Bq/m according to s t a t i s t i c a l calculations. In the studies performed by Hultqvist during the 1950s instantaneous sampling techniques were used. Equipment was designed by Rolf Sievert for determining radon concentration i n housing and was described by Hultqvist (Hultqvist, 1956). The instrument consisted of a portable measuring box and s i x ion chambers. The a i r was sampled by pumping the a i r into the ion chamber, which was then closed. When radioactive equilibrium was obtained between the radon and radon daughters i n the chamber, a known potential difference was applied between the outer surface of the ion chamber and the central electrode. The charge obtained due to the i o n i z a t i o n was c o l l e c t e d on the central electrode and was measured as a change i n the potential difference with an electrometer tube. This charge was d i r e c t l y proportional to the radon equivalent content when the exposure time was known. The a i r sampled i n the ion chambers contained both Rn-222 and Rn-220 (Tn) together with their decay products. The r e s u l t s , therefore, depend on the actual combination of these two gases with decay products and was c a l l e d radon equivalent content. The variations i n the background, the s e n s i t i v i t y to moisture, the alpha a c t i v i t y of the chamber i t s e l f and the influence of recombination were discussed by Hultqvist. The standard deviation due to counting s t a t i s t i c s was estimated to be about 3 % ( i n a few measurements 6 % ) . The c a l i b r a t i o n was made by counting each alpha p a r t i c l e by a proportional counter s p e c i a l l y designed at the Department for t h i s purpose. The s t a t i s t i c a l uncertainty of the c a l i b r a t i o n of the equivalent radon concentration was estimated to be 12 %. The a c t i v i t y originating from the thoron series was seldom found to be s u f f i c i e n t to allow evaluation by analysis of a d i s i n tegration curve recorded with the ion chamber. The possible thoron contribution was investigated with a corona f i l t e r and was found to be small compared with the radon concentration.


86

RADON AND ITS DECAY PRODUCTS

For the two studies compared i n t h i s paper, d i f f e r e n t measuring methods have been used. However, the systematic difference between the results from the two types of equipment can be estimated to be less than 20 %.


Sampling of the dwellings The selection of the 1980-82 measurements (Swedjemark and Mjônes, 1984) was made on dwellings b u i l t before 1976 and with the aim of determining dose d i s t r i b u t i o n s and the c o l l e c t i v e dose to the Swedish population from the exposure of the short-lived radon decay products. This was done by using the s t a t i s t i c a l selection made by the National Institute for Building Research intended for an energy study of the Swedish stock of houses. From a selection of 3 100 houses i n 103 municipalities, 2 900 were inspected. The data was found to be i n substantial conformity with data from the land r e g i s ter and the population census of 1975. For the study of the radon concentration 752 dwellings were selected at random. The dwellings i n the 1955-56 study selected for gamma measurements were b u i l t before 1946 and were located i n 13 urban areas i n central Sweden. The radon measurements were made i n four of these areas, Stockholm, Norrkôping, Katrineholm and Orebro. Most of the dwellings were erected during the period 1925-1945. No d i s t i n c t i o n was made between multi-family and detached houses. The v e n t i l a t i o n systems were of natural draught type i n a l l houses as opposed to the 1980-82 investigation when many types of v e n t i l a t i o n systems were i n use. Sampling i n the homes At the 1980-82 study two instruments were placed i n each dwelling for two weeks, one i n the l i v i n g room and one i n the bedroom. The dwellings were inspected, the gamma radiation was measured and the participants were asked to answer a questionnaire. The inhabitants were asked to l i v e as usual. In the 1955-56 study the radon measurements were usually taken i n the l i v i n g rooms. The gamma measurements were made i n several rooms. A l l samples were taken before noon. With regard to the v e n t i l a t i o n , the houses were divided into two groups; one termed "ventilated", where the dwelling had been aired one or two hours before the measurements. The dwellings i n the group named "unventilated" had not been aired since the day prior to the measurements. In this comparison both v e n t i l a t i o n groups have been used. That seems to be correct because applying correction factors for weathering, 0.9, (Swedjemark, 1978 and 1985) on the "unventilated" dwellings gave the same average for this group as for the uncorrected average for the whole group of studied dwellings according to Table I.


7.

SWEDJEMARK E T A L .

87


Table I. Measurements of radon concentration i n a i r i n four towns i n Central Sweden at the beginning of the 1950s i n houses b u i l t before 1946 (Hultqvist, 1956)

Radon concentration, UnairecL Aired , c) houses houses λ


Wood Brick Aerated concrete based on alum shale 1 Bq/m

15.2 47.4 133

15.5 26.0 67.0

Bq/m' Weighted average 15.2 40.0 116

« 0.027 p C i / l i t e r .

No thorough a i r i n g was carried out since the day before the measurement. c) Dwellings were aired the morning before the

sampling.

Results Frequency d i s t r i b u t i o n s Fig 1 shows the frequency d i s t r i b u t i o n s of the radon concentrations weighted for the proportion of houses of different types i n order to give the whole number of dwellings i n the country. The results from the 1980-82 study are shown i n Fig 1 a and from the 1955-56 study i n Fig 1 b. When comparing these d i s t r i b u t i o n s the higher average of the 1980-82 study i s obvious. Possible reasons and the magnitude of the increase w i l l be discussed i n the following. Low - intermediate - high radon levels The material from the 1980-82 study has been divided i n three i n t e r vals of the radon concentrations i n order to see differences and s i m i l a r i t i e s between the l e v e l s . (i)

The majority of dwellings are^found i n the group with radon concentrations below 150 Bq/m . In this group new multifamily houses and detached houses with mechanical v e n t i l a t i o n are found. Also detached houses b u i l t during the period 192050 are found i n t h i s group.

(ii)

The intermediate group with radon concentrations of 150 600 Bq/m was dominated by detached houses b u i l t between 1950 and 1975 and multi-family houses with a considerable content of alum-shale-based aerated concrete.

3 ( i i i ) The group with radon concentrations above 600 Bq/m contains only about ten dwellings and i s very disparate. Most of them, however, are found on ground with exposure rates above 30 yR/h



88

30

m .C

20

ALL T Y P E S OF

Φ

DWELLINGS

υ Φ


ο.

ίο H TliT^-i-iJD= 0

100

200

—ι

300

400

1

1

1

ι

r

500

600

700

800

900

1000

3

Bq/m Figure l a . D i s t r i b u t i o n of radon concentrations i n the Swedish dwelling stock 1975. Indicated i n the figure are also three values exceeding 1000 Bq/m (1312, 1537 and 3306 Bq/m ). 3

3

60

50

H

40 H

c φ

ο

30

φ CL 20

ίο H

τ

Γ

100

200

τ 300

1

400

1

500

1

600

1

700

1

800

r 900

1000

Bq/m Figure l b . D i s t r i b u t i o n of radon concentrations i n the Swedish dwelling stock 1950.


3

7. SWEDJEMARK ET AL.


89


according to maps made from gamma measurements carried out by a i r planes (Wilson, 1984). Furthermore, a couple of these dwellings have walls with gamma exposures above 100 yR/h measured one centimeter from the surface. About half of these houses were b u i l t before 1900. The ground would be the most probable radon source i n these old houses because when they were b u i l t no concrete slabs were used. The basement f l o o r s usually consisted of gravel or l a i d stones, which does not prevent the radon gas inflow from the s o i l . This inflow may be very high i f the a i r pressure indoors i s made low compared to the a i r pressure i n the s o i l a i r and the outdoor a i r . This i s the case when the inflow of fresh a i r to the house i s limited by the weather stripping around the windows etc. Building periods A comparison between the radon concentrations i n houses b u i l t before 1946 i n the 1980-82 study and i n the 1955-56 study i s possible to carry out. About 60 % of the dwellings i n the multi-family houses were b u i l t after 1950. As far as t h i s comparison i s concerned i t i s important to keep i n mind that the samples were different i n the two studies for this period although the houses were b u i l t during the same period. In a large number of these houses improvements had been made before 1980-82. In many of them the windows had been f i t t e d with weather stripping. The energy c r i s i s i n 1973-74 has probably hastened a selective demolishment of older houses. It i s not poss i b l e to d i f f e r between detached and multi-family houses i n the 1955-56 study because i t was not at that time thought to be important. Among the houses b u i l t before 1946 i n the 1980-82 invest i g a t i o n 90 % of the detached buildings are made of wood. The d i s t r i b u t i o n s of the dwellings i n the 1980-82 study of a l l kinds of building materials b u i l t before and from 1946 are shown i n Fig 2 a and 2 b. There i s not only a higher average but also a large number of houses with rather high radon concentrations i n the houses b u i l t before 1946. A comparison between detached houses b u i l t before and after 1946 i n the 1980-82 study shows a larger spread of the concentrations during the l a t e r building period. There are many reasons for t h i s (i)

Higher amounts of more active building materials have been used since the 1940s. More stone-based materials have been used i n a l l parts of the country, p a r t i c u l a r l y i n multi-family houses but also i n detached houses. Alum-shale-based aerated concrete constituted about 50 % of the building materials used during a part of the period. Its a c t i v i t y concentration weighted for the production rate of the different factories has been shown to have increased (Swedjemark, 1978).

(ii)

The a i r change rates were reduced to save energy resulting i n higher radon concentrations indoors (Swedjemark, 1978). The energy saving techniques were improved during the 1970s.

( i i i ) Holes around tubes for water and drainage i n the bottom slab were required by many l o c a l building authorities during the 1960s. The reason for t h i s was that p l a s t i c s entered the scene


90


40 .

30 J ALL T Y P E S O F DWELLINGS


20 J

10

r 100

Γ-Π Ί — — r — ~ r 200 300 400 500 Bq/m

600

700 800

900

1000

3

Figure 2a. D i s t r i b u t i o n of radon concentrations i n houses b u i l t before 1946. Indicated i n the figure i s the value exceeding 1000 Bq/m , 1537 Bq/m . 3

3

30

20 -J

10 H

ALL T Y P E S OF DWELLINGS

h

0 ιοο

200

300

400 500 Bq/m

600

•π700 800 900 1000

3

Figure 2b. D i s t r i b u t i o n of radon concentrations i n houses b u i l t after 1945. Indicated i n the figure are also two values exceeding 1000 Bq/m , 1312 and 3306 Bq/m . 3

3




91

as a building material i n the water and drain i n s t a l l a t i o n s and also the large temperature gradients which could occur between the bottom slab and the tubes during the different seasons. I n s t a l l a t i o n of drainage systems are since 1970 controlled by central regulations and the type of i n s t a l l a t i o n mentioned above i s not made today.


(iv)

The low a i r pressure indoors compared with the pressure i n the s o i l a i r and outdoors probably increased due to the decreasing of outdoor a i r supply i n order to save energy resulting i n a higher inflow of s o i l a i r .

Another difference between the 1950s and the 1980s i s that people now l i v e i n other types of houses. 70 % of the population l i v e d i n wooden houses i n 1950 compared with 40 % today. 3 % l i v e d i n houses b u i l t of aerated concrete based on alum-shale, i n 1950. In 1980 the figure was about 10 %. These differences are also an i n d i c a t i o n on an increased c o l l e c t i v e radon concentration during the period. The ground and the building materials During the late 1970s there was a growing awareness of the importance of the ground as regards to radon concentrations i n buildings. A number of municipalities were found to be situated i n areas where problems could occur. Surveys of dwellings with high radon levels were performed by the l o c a l boards of health. Today i t i s assumed that approximately 10 % of the Swedish ground necessitate special arrangements for protection against penetrating radon. A l l towns where radon measurements took place i n the 1950s have such areas. Therefore i t i s reasonable to compare the 1955-56 study not only to the country-wide 1980-82 study but also to those buildings i n the 1980-82 study which were situated i n the same areas as i n the 195556 study. (i)

Wood has been the most common building material i n the largest part of Sweden for many centuries. A study of the radon concentrations i n these buildings may give a good understanding of how building techniques have influenced the concentrations i n general. The frequency d i s t r i b u t i o n results both for the 1955-56 study and for detached houses i n the corresponding municipalities i n the measurements from 1980-82 show many low but also many rather high levels with higher values for the l a t e r study. The enhanced l e v e l s are mostly caused by inflow from the ground.

(ii)

For detached houses b u i l t of concrete and of brick the tendencies i n the comparison are the same as for the wooden houses. Often some parts of these houses have been b u i l t of aerated concrete based on alum shale. About 10 % of the detached houses of today have parts of t h i s material. For the multifamily dwellings the corresponding value i s 35 %. In the 198082 study i t was not possible to divide between sand-based and alum-shale-based aerated concrete at sampling. A l l aerated concrete for which the gamma exposure one cm from the wall was above 40 yR/h was c l a s s i f i e d as alum-shale-based aerated


92



concrete. The highest value measured i n the 1980-82 study i n a house of alum-shale-based aerated concrete b u i l t on ground with high potential radon exhalation.

was

( i i i ) A good i l l u s t r a t i o n of how the a c t i v i t y concentrations i n the building materials have increased since the 1950s i s the results of the country-wide study of gamma radiation i n the Swedish dwellings (Mjônes, 1982). The average exposure was found to have increased by a factor of two since the 1950s (Mjônes, 1986). The gamma radiation from the building mater i a l s r e f l e c t s also other radioactive nuclides than the U-238 chain, but i n Swedish building materials high concentrations of the Th-232 chain i s not common (More, 1985). The increase of the gamma radiation i s therefore an indication also of an increase i n radon concentration. Comparison of averages Table II shows that the average radon concentrations have increased by several factors compared with the 1955-56 measurements for a l l the building materials. The average, weighted for the population, was found to have increased by a factor of four to s i x . Here we must keep i n mind that the ground i n the four municipalities investigated i n 1955-56 has a potential for high radon exhalation compared to the whole country. No dwellings with extremely high radon concentration were found however. A contributing reason for this may be the différencies of the buildings and the habits of the inhabitants now and then. Doors and openings to c e l l a r s were by that time always closed thereby preventing direct i n l e t of radon from the ground. 3 Table I I . Arithmetic means of the radon concentration i n

Homes i n houses b u i l t before 1946 four municipalities Wood Concrete-brick Aerated concrete based on alum shale Average

15.2 40 \ 24

;

Bq/m

Building stock 1975 The whole The same four country municipalities as i n the 1955r56 investigation 100 70 237 101

131 178

158

;

The borders of the municipalities have been changed since so that the regions are not quite comparable.

1956

These averages are weighted for the proportion of houses b u i l t of different building materials i n the whole country, not for the four municipalities. There are differences i n these proportions between parts of the country.



93


Discussion Radon increase i n Swedish homes


The p r o b a b i l i t i e s for an increase i n radon concentration i n Swedish homes between the 1950s and the 1980s are based on (i)

a comparison between the studies 1980-82 for the whole country and 1955-56 for the four municipalities i n both studies.

(ii)

a comparison between the radon concentration i n t e r v a l s i n the 1980-82 study.

( i i i ) a comparison between the houses b u i l t before and after 1946 i n the 1980-82 study and between the houses i n the 1955-56 study. (iv)

the development of the building materials and s o i l conditions.

A l l these comparisons support the hypothesis of an increase. The arithmetic means and thereby the c o l l e c t i v e doses seem to have increased by about a factor of four to six. If the aerated concrete based on alum-shale had not been used, the country-wide average has been estimated to be 30 % lower (Swedjemark 1985). Here we have only discussed the concentration of the radon gas. This i s because the measurements have been made of this nuclide. However, the health effects are referred to the short-lived decay products. The equilibrium factor depends on the v e n t i l a t i o n rate and the p a r t i c l e concentrations. The p r o b a b i l i t y that the F-factors for Swedish dwellings within the i n t e r v a l s given below are 95 % calculated from the t d i s t r i b u t i o n based on the measurements of the day-time radon and radon daughter l e v e l s and the a i r exchange rates (λ ) i n 225 dwel lings (Swedjemark 1983) during the 1970s. Night-time measurements may give a higher F-factor. Low a i r exchange rate (λ < 0.30 h ν Average a i r exchange rate (0.30 < λ < 0.60 h ν High a i r exchange rate (λ > 0.60 h ) ν A l l a i r exchange rates

0.28

< F < 0.74

F =

0.51

0.21

< F < 0.66

F =

0.43

0.21

< F < 0.47

F =

0.33

0.19

< F < 0.69

F =

0.44

The v e n t i l a t i o n rate has decreased since the 1950s indicating a higher equilibrium factor and thereby a higher radon daughter i n crease since the 1950s than the increase of the radon gas concentra t i o n . How the p a r t i c l e concentrations have changed i s not known. Consequencies involving detriment to health are not i n this paper.

discussed


94



Comparison with other countries In no other countries measurements of radon or radon daughter concentrations have been carried out i n the f i f t i e s and the s i x t i e s . There are, however, indications that radon concentrations generally have increased i n houses i n temperate regions during the last decades. To save energy the a i r change rates have been decreased and for houses i n which the major radon source i s the building materials t h i s gives an approximate inverse proportional enhancement of the radon concentration. Tightening of windows and closing of inflow ducts reduce the a i r pressure indoors i n comparison with the a i r pressure outdoors and i n the s o i l a i r with an increased suction of s o i l a i r into the house as a r e s u l t . This increase of the radon concentration may be very high especially for houses with a bad barr i e r or without a barrier between house and s o i l , which i s especiall y common i n old houses. Energy-efficient houses have been shown to have higher radon concentrations than others. In the Netherlands, for example, rooms with double-glazed windows were found to have radon concentrations twice as high as those i n rooms with single-pane windows (Wolfs et a l , 1984). New York energy-efficient houses with heat storage masses had radon concentrations 1.6 times those for conventional homes (Fleischer and Turner, 1984) and i n Sweden the same enhancement i n radon concentration i s found (Nyblom, 1980). Trends In Sweden, requirements on future building (Swedjemark, 1986) and the l i m i t s for existing dwellings w i l l result i n lower radon daughter concentrations i n homes and to some part i t already has. For the c o l l e c t i v e dose, however, the decrease w i l l be very slow because of the low building rate compared with the housing stock. It has been estimated tha^ the average for the Swedish radon daughter concentration, 50 Bq/m EER, may be decreased to half that value after about 100 years, i f the requirements mentioned above are fulfilled. In most other countries regulations or recommendations for a decrease of the radon daughter concentrations i n homes have not been established. In USA and Canada l i m i t s have been given only for special cases, for example building on waste from uranium and phosphate industries (Atomic Energy Control Board, 1977; EPA, 1979; EPA, 1980). In Finland, there are general recommendations for homes (Finnish Radon Commission, 1982). A number of organizations are now working with or have submitted recommendations regarding radon daughter concentrations i n homes. Examples are the International Commission on Radiological Protection (ICRP Publication 39, 1984), the World Health Organizat i o n and the Nordic countries (The National Radiation Protection Authorities i n the Nordic Countries, 1986). In several countries regulations are being discussed. Therefore, the radon daughter concentrations i n homes may decrease i n the future.


7. SWEDJEMARK ET AL. Radon Levels in Swedish Homes

95

References Atomic Energy Control Board, Criteria for radioactive clean-up in Canada, Information bulletin 77-2 (1977). Breslin, A.J. and George, A.C., An Improved Time-Integrating Radon Monitor, Proceedings of the ΝΕΑ specialist meeting, Paris Nov 1978, pp. 133-137, OECD/NEA (1979).


Burén, Α., Hàkansson, Β., Mjönes, L., Möre, H., Nyblom, L. and Wennberg, P., An integrating radon monitor, Report a 82-06, National Institute of Radiation Protection, Stockholm (1982). EPA 79 Federal Register, 44: 128, pp. 38665, Recommendations to State of Florida regarding Florida phosphate lands. Environmental Protection Agency, Washington DC, US (July 2, 1979). EPA 80 Federal Register, 45: 79, pp. 87374-5. Environmental Protec tion Agency, Washington DC, US (April 22, 1980). Falk, R., Hagberg, N., Håkansson, Β., Nyblom, L. and Swedjemark, G.A., Calibration, Radon and Radon Daughters in Air, Report a 82-22, National Institute of Radiation Protection, Stockholm (1982) (in Swedish). Finnish Radon Commission, Final Report 1982, Finnish Centre for Radiation and Nuclear Safety, Helsinki (in Finnish). Fleischer, R.L. and Turner, L.G., Indoor radon measurements in the New York capital district, Health Phys, 46: 5, pp. 999-1011, (1984). Hultqvist, B., Studies on naturally occurring ionizing radiations with special reference to radiation doses in Swedish houses of various types, Kungl. Svenska Vetenskapsakademins handlingar, Ser 4, Band 6, Nr 3, Stockholm, Sweden (1956). ICRP Publication 39: Principles for limiting exposure of the public to natural sources of radiation. Annals of the ICRP, 14: 1, Pergamon Press, (1984). Mjönes, L., Gamma radiation in Dwellings, Report a 81-18, National Institute of Radiation Protection, Stockholm, (1981) (in Swedish). Mjönes, L., Gamma radiation in Swedish Dwellings (to be published in Radiation Protection Dosimetry) (1986). Möre, H.: Radionuclides in building materials, a summary of activity concentrations in samples investigated at the National Institute of Radiation Protection, Report 85-08, National Institute of Radiation Protection, Stockholm (1985) (in Swedish). Nyblom, L., Radon in the air in buildings, where heat accumulating is made in a stone mass, Report Ri 1980-03, Department of Radiation Physics, Karolinska Institute, Stockholm, Sweden (1980) (in Swedish). Hopke; Radon and Its Decay Products ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

96


Swedjemark, G.A., Radon in dwellings in Sweden, in Proceedings from Natural Radiation Environment III, Texas 1978 (Ed Gesell, T.F. and Lowder, W.M.), CONF-780422, pp. 1237-1259 (1980). Swedjemark, G.A., The equilibrium factor F, Health Physics, 45: 2, pp. 453-462 (1983).


Swedjemark, G.A. andMjönes,L.: Exposure of the Swedish population to radon daughters, in Proceedings of the 3rd International Confe rence on Indoor Air Quality and Climate, Stockholm, 1984, 2, pp. 37-43, Swedish Council for Building Research, Stockholm (1984). Swedjemark, G.A., Radon and its decay products in housing - estima tion of the radon daughter exposure to the Swedish population and methods for evaluation of the uncertainties in annual averages, Thesis, Department of Radiation Physics, University of Stockholm (1985). Swedjemark, G.A., Limitation schemes to decrease the radon daughters in indoor air, Report 86-01, National Institute of Radiation Protec tion, Stockholm (1986). Wilson, C., Mapping the radon risk of our environment, in Procee dings of the 3rd International Conference on Indoor Air Quality and Climate, Stockholm, 1984, 2, pp. 85-89, Swedish Council for Building Research, Stockholm (1984). Wolfs, F., Hofstede, H., deMeijer, R.J. and Put, L.W., Measurements of radon-daughter concentrations in and around dwellings in the northern part of the Netherlands: a search for the influences of building materials costruction and ventilation. Health Phys, 47: 2, pp. 271-279 (1984). RECEIVED

August 4, 1986


Radon Levels in Swedish Homes: A Comparison of the 1980s with the

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