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Review of seasonal variation in residential indoor radon concentrations
612
Review of seasonal variation in residential indoor radon concentrations H. Arvela Radiation and Safety Authority – STUK, Box 14, 00881 Helsinki, Finland
The study explored the theoretical basis of radon entry into buildings and processes affecting the seasonal variation in indoor radon concentration. The radon source of the model comprises two components, diffusive and convective entry rate. The force driving the convective flow is the pressure differential caused by the indoor–outdoor temperature difference. The physics of diffusive radon entry into buildings, of airflow from soil, driven by the pressure differences, and of air infiltration explain to a considerable extent the observed seasonal variation in indoor radon concentration. The seasonal variation in houses with natural ventilation has been estimated for both typical Northern European climate conditions and for warmer conditions in Central and Southern Europe. In houses with soil gas radon as the dominant radon source a summer minimum is typically 50% of the winter maximum indoor radon concentration. Ventilation through open windows in summertime may result in significantly lower concentration relative to theoretical calculations. The cold climate in North Europe increases radon concentrations by 50% compared to countries in South Europe. The annual average to winter radon concentration ratio was typically 0.7–0.8. This is in agreement with the measured seasonal variations.
1. Introduction Building materials and radon-bearing soil gas are the dominant sources of indoor radon. The main removal process decreasing indoor radon concentration is the air-exchange of indoor spaces. Both indoor–outdoor temperature difference and wind contribute to the pressure difference in the envelopes of residential buildings which is the driving force for air leakage providing ventilation to those buildings. It is noteworthy that the same pressure difference is the driving force also for airflow from soil into indoor spaces. Modelling these phenomena provides a tool for understanding the seasonal variation in indoor radon concentration. The aim of this study is to review observations on seasonal variation as well as to compare the measurements with simple model predictions. RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07074-3
© 2005 Elsevier Ltd. All rights reserved.
Review of seasonal variation in residential indoor radon concentrations
613
2. Literature review Studies of seasonal and diurnal variation in indoor radon concentration have been carried out, first, in order to reach a general comprehension of the radon problem, entry mechanisms and reasons for elevated radon concentrations; and, second, in order to evaluate the validity of short-term measurements and to estimate the annual average radon concentration. Seasonal variations are affected by many factors, which vary from country to country. House characteristics, building soil and geology, ventilation practices and climatic conditions are the most important factors. Generally in conditions of a temperate or cold climate, winter concentrations are higher than summer concentrations. Patterns of air movement on karstic limestone [1] and on eskers [2] may result in anomalous seasonal variations. An annual average radon concentration is the main quantity used in estimates of the radiological effects of radon in homes. It reflects an average of the radon concentrations in all living spaces in a home. However, measurements taken over the course of an entire year have seldom been possible in practice. Short-term measurements are simpler to take and provide faster results. The expectations of the home-owners as well as the measurement methods used and recommendations by the authorities have led to national practices of using a measurement period of 2 days–3 months in most countries. Correction factors for annual average radon concentration are needed to adjust measurements taken over periods other than twelve months. Majborn has reported highly significant variation in slab-on-grade houses [3]. The mean annual average/winter concentration ratio in all 67 houses with varying types of foundation was 0.77. The ratio was lowest in houses with slab-on-grade. Statistical studies made using radon measurements from the radon-affected area of southwest England were used to estimate seasonal correction factors [4,5]. The factors obtained result in a change, in the estimated annual average, of up to 35% for measurements taken over a six-month period and 56% for measurements taken over a three-month period.
3. Materials and methods The radon source of the model [6] comprises two components, diffusive and convective entry rates, Sd and Ssoil . The force driving the convective flow is the pressure differential p caused by the indoor–outdoor temperature difference T , this phenomenon designated as the stackeffect. Ssoil is proportional to the soil gas radon concentration Asoil and flow rate of soil gas into the house Qsoil , which in turn is the quotient of the stack pressure difference ps at floor level and the flow resistance Rt . Wind and the resulting pressure difference also affect the radon entry rate. Compared to the stack effect the contribution of this steady-state wind effect is of minor importance and has been discounted in this study. The Lawrence Berkeley Laboratory model [7] was used in air infiltration predictions. It adds in quadrature the infiltration rates due to indoor–outdoor temperature differences (stack effect) and wind speed (v), as follows: 2 1/2 Q = ELA fsr T 0.65 + (fwr v)2 (1) , N = QV −1 ,
(2)
614
H. Arvela
where Q is the total infiltration, ELA is the effective leakage area, T is the indoor–outdoor temperature difference, fsr is the stack parameter and fwr is the wind parameter. N is the air exchange rate and V the house interior volume. The fraction of leakage in the floor and ceiling affects both the stack and wind parameters. The detailed choice of the parameters has been presented in Ref. [6]. Equation (3) gives the radon concentration, ARn , used in model calculations. ARn = Asoil Fsoil T (Tin Tout )−1 + Sd (λ + N )−1 V −1 + Aout . (3) The factor Asoil Fsoil (Tin Tout )−1 demonstrates the total convective radon entry rate per unit temperature difference, Asoil being the radon concentration of the leakage air from soil. λ is the radioactive decay constant of 222 Rn, 0.0076 h−1 . Aout is the outdoor air radon concentration, the annual average being about 5 Bq m−3 . The effective leakage area, ELA, is a standard measure of building tightness, which is measured by pressurising a building with a fan. ELA is defined assuming that in the pressure range characteristic of natural infiltration (−10 to +10 Pa) the flow versus pressure behaviour of a building more closely resembles the square root (turbulent rather than viscous flow). The model calculations were made using the parameters estimated for a typical Finnish house [6]. The diffusive radon entry for the house is 1500 Bq h−1 . The parameters for the convective radon entry have been set to a value, which results in the Finnish climate in an annual average radon concentration of 90 Bq m−3 , where the contribution of the diffusive and convective radon entry is 20 and 70 Bq m−3 . Model calculations were carried out also for this house located in different European areas. In this simplified study indoor temperature has been set at a constant level of 20 ◦ C, irrespective of the location and outdoor temperature. The monthly average temperatures were used. A wind speed of 3 m s−1 has been used for locations throughout the year.
4. Results Figure 1 shows the calculated seasonal variation for the Finnish model house located in different European areas. Due to the dominance of convective radon entry, radon concentrations are higher the lower outdoor temperature is. Figure 2 shows the diffusive and convective contributions to indoor radon concentrations for the model house in Helsinki and Athens. In Athens outdoor temperature exceeds the supposed constant indoor temperature of 20 ◦ C in summertime. Therefore, the convective entry is reduced to zero level. The air exchange is in this model house proportional to the absolute value of the indoor–outdoor temperature difference. In the case of a pure diffusion source the decreasing outdoor temperature decreases the indoor radon concentration. In the case of a flow driven by pressure difference, the decreasing outdoor temperature increases the indoor radon concentration. It is noteworthy that in the case of pressure difference driven flow the indoor radon concentration increases simultaneously when air infiltration increases. Soil airflow is proportional approximately to the indoor–outdoor temperature difference, T , whereas the ventilation rate is proportional to the envelope leakage exponent of T , which has a range of 0.5–0.7; a value of 0.65 has been used in this study. Table 1 shows the annual average indoor radon concentrations for the climatic conditions of several European cities. The annual average in southern Europe is 30–40% lower than in
Review of seasonal variation in residential indoor radon concentrations
615
Fig. 1. Seasonal variation in indoor radon concentration of a Finnish model house in different European climates.
Fig. 2. Diffusive and convective contribution to indoor radon concentration for a Finnish model house in Helsinki and in Athens.
Helsinki. Table 1 also gives the quotient of the annual average radon concentration to the December–March average radon concentration. This correction factor is needed to obtain the annual average radon concentration from wintertime results. Although the annual average radon concentrations vary significantly, the seasonal correction factor varies only in the range of 0.68 to 0.81, being lowest in the warm areas. For Central and North Europe the factor is approximately 0.80. Similar experimental results on seasonal variation have been observed in Norway [8], in Denmark [3] and in the radon-affected areas of England [4,5]. In Norway an average correc-
616
H. Arvela Table 1 Annual average indoor radon concentrations and correction factors for the annual average for a Finnish model house located in different European climates City
Annual average indoor radon concentration (Bq m−3 )
Annual average/December–March, radon concentration ratio
Athens Madrid Milan Paris Frankfurt Hamburg Stockholm Helsinki
54 67 71 79 81 86 93 96
0.68 0.72 0.71 0.79 0.77 0.81 0.80 0.81
tion factor of 0.75 has been applied for winter measurements [8]. In Finland a factor of 0.8 has been used as a representative average [6].
5. Conclusions The results demonstrate the theoretical seasonal variation for a model house with a dominating convective radon entry, located in different European areas. The annual average to winter radon concentration ratio was typically 0.7–0.8. This is in agreement with the measured seasonal variations. The summer minimum is typically 40–50% of the winter maximum. Ventilation through open windows in summertime further lowers summertime radon concentrations, which has not been taken into account in the model. The simplified calculations were made using a constant wind speed of 3 m s−1 in all areas although local wind conditions may vary significantly. The cold climate in North Europe increases radon concentration by 50% compared to countries in South Europe. Generally the physics of diffusive radon entry into buildings, of airflow from soil, driven by the pressure differences, and of air infiltration explain to a considerable extent the observed seasonal variation in indoor radon concentration.
References [1] D.L. Wilson, R.B. Gammage, C.S. Dudney, R.J. Saultz, Summertime elevation of Radon-222 levels in Huntsville, Alabama, Health Phys. 60 (2) (1991) 189–197. [2] H. Arvela, Voutilainen, T. Honkamaa, A. Rosenberg, High indoor radon variations and the thermal behaviour of eskers, Health Phys. 67 (3) (1994) 254–260. [3] R. Majborn, Seasonal variation of radon concentrations in single family houses with different sub-structures, Radiat. Prot. Dosim. 45 (1) (1992) 443–447. [4] J.C.H. Miles, B.M.R. Green, P.R. Lomas, Radon affected areas: Derbyshire, Northamptonshire and Somerset, Docs NRPB 3 (4) (1992) 18–28. [5] J. Pinel, T. Fearn, S.C. Darby, J.C.H. Miles, Seasonal correction factors for indoor radon measurements in the United Kingdom, Radiat. Prot. Dosim. 58 (2) (1995) 127–132.
Review of seasonal variation in residential indoor radon concentrations
617
[6] H. Arvela, Seasonal variation in radon concentration of 3000 dwellings with model comparisons, Radiat. Prot. Dosim. 59 (1) (1995) 33–42. [7] M. Sherman, M. Modera, Comparison of measured and predicted infiltration using the LBL infiltration model, LBL-17001, Lawrence Berkeley Laboratory, California, 1984. [8] NRPA, Radon in dwellings. Recommendations for measurements indoors and recommendations for investigation on building site, NRPA Radiation Protection Series No. 2, Norwegian Radiation Protection Authority, Osteraas, 1992.
Review of seasonal variation in residential indoor radon concentrations H. Arvela Radiation and Safety Authority – STUK, Box 14, 00881 Helsinki, Finland
The study explored the theoretical basis of radon entry into buildings and processes affecting the seasonal variation in indoor radon concentration. The radon source of the model comprises two components, diffusive and convective entry rate. The force driving the convective flow is the pressure differential caused by the indoor–outdoor temperature difference. The physics of diffusive radon entry into buildings, of airflow from soil, driven by the pressure differences, and of air infiltration explain to a considerable extent the observed seasonal variation in indoor radon concentration. The seasonal variation in houses with natural ventilation has been estimated for both typical Northern European climate conditions and for warmer conditions in Central and Southern Europe. In houses with soil gas radon as the dominant radon source a summer minimum is typically 50% of the winter maximum indoor radon concentration. Ventilation through open windows in summertime may result in significantly lower concentration relative to theoretical calculations. The cold climate in North Europe increases radon concentrations by 50% compared to countries in South Europe. The annual average to winter radon concentration ratio was typically 0.7–0.8. This is in agreement with the measured seasonal variations.
1. Introduction Building materials and radon-bearing soil gas are the dominant sources of indoor radon. The main removal process decreasing indoor radon concentration is the air-exchange of indoor spaces. Both indoor–outdoor temperature difference and wind contribute to the pressure difference in the envelopes of residential buildings which is the driving force for air leakage providing ventilation to those buildings. It is noteworthy that the same pressure difference is the driving force also for airflow from soil into indoor spaces. Modelling these phenomena provides a tool for understanding the seasonal variation in indoor radon concentration. The aim of this study is to review observations on seasonal variation as well as to compare the measurements with simple model predictions. RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07074-3
© 2005 Elsevier Ltd. All rights reserved.
Review of seasonal variation in residential indoor radon concentrations
613
2. Literature review Studies of seasonal and diurnal variation in indoor radon concentration have been carried out, first, in order to reach a general comprehension of the radon problem, entry mechanisms and reasons for elevated radon concentrations; and, second, in order to evaluate the validity of short-term measurements and to estimate the annual average radon concentration. Seasonal variations are affected by many factors, which vary from country to country. House characteristics, building soil and geology, ventilation practices and climatic conditions are the most important factors. Generally in conditions of a temperate or cold climate, winter concentrations are higher than summer concentrations. Patterns of air movement on karstic limestone [1] and on eskers [2] may result in anomalous seasonal variations. An annual average radon concentration is the main quantity used in estimates of the radiological effects of radon in homes. It reflects an average of the radon concentrations in all living spaces in a home. However, measurements taken over the course of an entire year have seldom been possible in practice. Short-term measurements are simpler to take and provide faster results. The expectations of the home-owners as well as the measurement methods used and recommendations by the authorities have led to national practices of using a measurement period of 2 days–3 months in most countries. Correction factors for annual average radon concentration are needed to adjust measurements taken over periods other than twelve months. Majborn has reported highly significant variation in slab-on-grade houses [3]. The mean annual average/winter concentration ratio in all 67 houses with varying types of foundation was 0.77. The ratio was lowest in houses with slab-on-grade. Statistical studies made using radon measurements from the radon-affected area of southwest England were used to estimate seasonal correction factors [4,5]. The factors obtained result in a change, in the estimated annual average, of up to 35% for measurements taken over a six-month period and 56% for measurements taken over a three-month period.
3. Materials and methods The radon source of the model [6] comprises two components, diffusive and convective entry rates, Sd and Ssoil . The force driving the convective flow is the pressure differential p caused by the indoor–outdoor temperature difference T , this phenomenon designated as the stackeffect. Ssoil is proportional to the soil gas radon concentration Asoil and flow rate of soil gas into the house Qsoil , which in turn is the quotient of the stack pressure difference ps at floor level and the flow resistance Rt . Wind and the resulting pressure difference also affect the radon entry rate. Compared to the stack effect the contribution of this steady-state wind effect is of minor importance and has been discounted in this study. The Lawrence Berkeley Laboratory model [7] was used in air infiltration predictions. It adds in quadrature the infiltration rates due to indoor–outdoor temperature differences (stack effect) and wind speed (v), as follows: 2 1/2 Q = ELA fsr T 0.65 + (fwr v)2 (1) , N = QV −1 ,
(2)
614
H. Arvela
where Q is the total infiltration, ELA is the effective leakage area, T is the indoor–outdoor temperature difference, fsr is the stack parameter and fwr is the wind parameter. N is the air exchange rate and V the house interior volume. The fraction of leakage in the floor and ceiling affects both the stack and wind parameters. The detailed choice of the parameters has been presented in Ref. [6]. Equation (3) gives the radon concentration, ARn , used in model calculations. ARn = Asoil Fsoil T (Tin Tout )−1 + Sd (λ + N )−1 V −1 + Aout . (3) The factor Asoil Fsoil (Tin Tout )−1 demonstrates the total convective radon entry rate per unit temperature difference, Asoil being the radon concentration of the leakage air from soil. λ is the radioactive decay constant of 222 Rn, 0.0076 h−1 . Aout is the outdoor air radon concentration, the annual average being about 5 Bq m−3 . The effective leakage area, ELA, is a standard measure of building tightness, which is measured by pressurising a building with a fan. ELA is defined assuming that in the pressure range characteristic of natural infiltration (−10 to +10 Pa) the flow versus pressure behaviour of a building more closely resembles the square root (turbulent rather than viscous flow). The model calculations were made using the parameters estimated for a typical Finnish house [6]. The diffusive radon entry for the house is 1500 Bq h−1 . The parameters for the convective radon entry have been set to a value, which results in the Finnish climate in an annual average radon concentration of 90 Bq m−3 , where the contribution of the diffusive and convective radon entry is 20 and 70 Bq m−3 . Model calculations were carried out also for this house located in different European areas. In this simplified study indoor temperature has been set at a constant level of 20 ◦ C, irrespective of the location and outdoor temperature. The monthly average temperatures were used. A wind speed of 3 m s−1 has been used for locations throughout the year.
4. Results Figure 1 shows the calculated seasonal variation for the Finnish model house located in different European areas. Due to the dominance of convective radon entry, radon concentrations are higher the lower outdoor temperature is. Figure 2 shows the diffusive and convective contributions to indoor radon concentrations for the model house in Helsinki and Athens. In Athens outdoor temperature exceeds the supposed constant indoor temperature of 20 ◦ C in summertime. Therefore, the convective entry is reduced to zero level. The air exchange is in this model house proportional to the absolute value of the indoor–outdoor temperature difference. In the case of a pure diffusion source the decreasing outdoor temperature decreases the indoor radon concentration. In the case of a flow driven by pressure difference, the decreasing outdoor temperature increases the indoor radon concentration. It is noteworthy that in the case of pressure difference driven flow the indoor radon concentration increases simultaneously when air infiltration increases. Soil airflow is proportional approximately to the indoor–outdoor temperature difference, T , whereas the ventilation rate is proportional to the envelope leakage exponent of T , which has a range of 0.5–0.7; a value of 0.65 has been used in this study. Table 1 shows the annual average indoor radon concentrations for the climatic conditions of several European cities. The annual average in southern Europe is 30–40% lower than in
Review of seasonal variation in residential indoor radon concentrations
615
Fig. 1. Seasonal variation in indoor radon concentration of a Finnish model house in different European climates.
Fig. 2. Diffusive and convective contribution to indoor radon concentration for a Finnish model house in Helsinki and in Athens.
Helsinki. Table 1 also gives the quotient of the annual average radon concentration to the December–March average radon concentration. This correction factor is needed to obtain the annual average radon concentration from wintertime results. Although the annual average radon concentrations vary significantly, the seasonal correction factor varies only in the range of 0.68 to 0.81, being lowest in the warm areas. For Central and North Europe the factor is approximately 0.80. Similar experimental results on seasonal variation have been observed in Norway [8], in Denmark [3] and in the radon-affected areas of England [4,5]. In Norway an average correc-
616
H. Arvela Table 1 Annual average indoor radon concentrations and correction factors for the annual average for a Finnish model house located in different European climates City
Annual average indoor radon concentration (Bq m−3 )
Annual average/December–March, radon concentration ratio
Athens Madrid Milan Paris Frankfurt Hamburg Stockholm Helsinki
54 67 71 79 81 86 93 96
0.68 0.72 0.71 0.79 0.77 0.81 0.80 0.81
tion factor of 0.75 has been applied for winter measurements [8]. In Finland a factor of 0.8 has been used as a representative average [6].
5. Conclusions The results demonstrate the theoretical seasonal variation for a model house with a dominating convective radon entry, located in different European areas. The annual average to winter radon concentration ratio was typically 0.7–0.8. This is in agreement with the measured seasonal variations. The summer minimum is typically 40–50% of the winter maximum. Ventilation through open windows in summertime further lowers summertime radon concentrations, which has not been taken into account in the model. The simplified calculations were made using a constant wind speed of 3 m s−1 in all areas although local wind conditions may vary significantly. The cold climate in North Europe increases radon concentration by 50% compared to countries in South Europe. Generally the physics of diffusive radon entry into buildings, of airflow from soil, driven by the pressure differences, and of air infiltration explain to a considerable extent the observed seasonal variation in indoor radon concentration.
References [1] D.L. Wilson, R.B. Gammage, C.S. Dudney, R.J. Saultz, Summertime elevation of Radon-222 levels in Huntsville, Alabama, Health Phys. 60 (2) (1991) 189–197. [2] H. Arvela, Voutilainen, T. Honkamaa, A. Rosenberg, High indoor radon variations and the thermal behaviour of eskers, Health Phys. 67 (3) (1994) 254–260. [3] R. Majborn, Seasonal variation of radon concentrations in single family houses with different sub-structures, Radiat. Prot. Dosim. 45 (1) (1992) 443–447. [4] J.C.H. Miles, B.M.R. Green, P.R. Lomas, Radon affected areas: Derbyshire, Northamptonshire and Somerset, Docs NRPB 3 (4) (1992) 18–28. [5] J. Pinel, T. Fearn, S.C. Darby, J.C.H. Miles, Seasonal correction factors for indoor radon measurements in the United Kingdom, Radiat. Prot. Dosim. 58 (2) (1995) 127–132.
Review of seasonal variation in residential indoor radon concentrations
617
[6] H. Arvela, Seasonal variation in radon concentration of 3000 dwellings with model comparisons, Radiat. Prot. Dosim. 59 (1) (1995) 33–42. [7] M. Sherman, M. Modera, Comparison of measured and predicted infiltration using the LBL infiltration model, LBL-17001, Lawrence Berkeley Laboratory, California, 1984. [8] NRPA, Radon in dwellings. Recommendations for measurements indoors and recommendations for investigation on building site, NRPA Radiation Protection Series No. 2, Norwegian Radiation Protection Authority, Osteraas, 1992.