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An empirical radon emanation model for residential premises
ARTICLE IN PRESS
Building and Environment 40 (2005) 1566–1571 www.elsevier.com/locate/buildenv
An empirical radon emanation model for residential premises Thomas C.W. Tung, Daniel W.T. Chan, John Burnett Department of Building Services Engineering, The Hong Kong Polytechnic University, Hunghom, Hong Kong Received 12 June 2003; accepted 7 December 2004
Abstract Based on an averaging technique, a methodology has been established to estimate an effective radon emanation factor M for residential premises. The model shows that the new term M and the ventilation rate are the essential parameters in estimating the level of indoor radon. M includes two components: the radon emanation rates of internal surface materials and the ratio of surface areas of applicable materials to premises volume. The value of M can be determined from on-site measurements. Different ventilation modes of a sampled residential unit during daytime and nighttime, with air conditioner on, window-open, and windowclosed were included in site measurements. Each ventilation mode was measured twice during daytime and twice at night. During the investigation, air exchange rate, and indoor and outdoor radon levels were monitored simultaneously. The results of measurements were then used to verify the model. The value of M was found to be 31.7 Bq m3 h1. The model is valid if the air exchange rate is larger than 0.2 h1. r 2005 Elsevier Ltd. All rights reserved. Keywords: Air exchange rate; Indoor radon; Effective radon emanation factor; Radon reduction; Residential premises
1. Introduction Simply increasing the ventilation rate is a common and effective means to reduce radon concentration inside buildings. However, over ventilation introduces other problems. Energy cost is increased if mechanical ventilation is used, and in some densely populated areas, such as these found in Hong Kong, increased ventilation encourages outdoor pollutants to be transmitted indoors, since outdoor air is often polluted by traffic emissions, industrial processes, and construction site activities. However, if the ventilation of premises is insufficient, indoor radon concentrations can accumulate to levels that exceed health standards. Usually, when it is required to predict the indoor radon level, the radon emanation rate of each building material, each material exposing area, ventilation rate, outdoor radon concentration, and the effective volume of the premise are required in detail. However, such detailed informaCorresponding author. Tel.: +852 2766 7956; fax: +852 2765 7198.
E-mail address: [email protected] (T.C.W. Tung). 0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2004.12.012
tion are often difficult to determine, so a surrogate method is useful to determine the relationship between indoor radon concentration and ventilation rate. Radon is a radioactive gas released from the decay of radium which is often present in rock, soil, and building materials. The United States Environmental Protection Agency (US-EPA) has reported that inhalation of radon is the second killer from cancer. Soil is considered as a main indoor radon contributor and there is very common in certain areas of many countries. It can seep into the basement of buildings and accumulate indoors. The rate of radon diffusion into premises is associated with the permeability of the foundation soil, the pressure difference, and the design of the building structure [1]. The situation is somewhat different in Hong Kong where high-rise residential buildings predominate. Concrete and granites, which are the common building materials used, are the main sources of indoor radon. The radon emanation rates of these materials are a concern. Small radon chambers have been used to study radon emanation rates from building material. It was found that radon emanation rates of granites ranged
ARTICLE IN PRESS T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571
Nomenclature
Ei
Cp
M
indoor radon concentration at time t (Bq m3) [Cp]N indoor quasi state radon concentration (Bq m3) /CoS outdoor radon averaging concentration (Bq m3) /[Cp]NS indoor radon averaging concentration (Bq m3) Ai surface area of building material-i (m2)
from 5.2 to 13.6 Bq m2 h1 [2]. Concrete with pulverized fuel ash was found with radon emanation rates in the range from 9.2 103 to 11.3 103 Bq m2 s1 [3]. Besides the studies on radon emanation rates of materials, government departments and researchers have also conducted a series of radon surveys in buildings. The Hong Kong Environmental Protection Department (HK-EPD) carried out territory-wide radon surveys [4,5] using Alpha track detectors. The measurements were carried out over three periods, December 1992–August 1993, July 1994–January 1995, and January 1995–June 1995, respectively. Based on their surveys, the HK-EPD concluded that 5% of residential premises and 10% of non-residential premises had radon levels above the recommended level of 200 Bq m3. Another radon survey was conducted in 1774 sites using alpha track detectors [6] showing that mean radon levels in residential buildings and nonresidential buildings were 62 and 111 Bq m3, respectively. The authors concluded that ventilation was the key factor in the consideration of indoor radon concentration. Detailed measurements of indoor radon levels at five residential premises were carried out by Chao et al. [7]. Daytime and nighttime radon levels and air exchange rates of the premises were monitored. The authors observed that the highest radon levels in the premises usually occurred at nighttime. Influence of ventilation on indoor radon was also studied by Chao et al. [8] involving 12 residential buildings. Air exchange rates, and indoor and outdoor radon levels were the target parameters. It was concluded that indoor radon levels approached the outdoor radon level if the air exchange rate of a premise was larger than 3 h1. Whilst the local research and surveys have paid attention to radon emanation rate and indoor radon levels, no quantitative and detailed investigation on the ventilation rate coupled to the radon emanation rate has been reported. Hence, an investigation into indoor radon concentration, ventilation rate, and the effect of radon emanation rate in premises was targeted in this study.
V Y F l
1567
radon emanation rate with respect to building material-i (Bq m2 h1) effective indoor radon emanation factor of the sampled premises (Bq m3 h1) effective volume of the sampled premises (m3) difference of sample averaging of indoor and outdoor radon levels (Bq m3) an air exchange rate of the sampled premise (h1) radon-222 decay constant, 7.5536 103 h1
2. Measurement setup 2.1. Measurement setup Two solid-state-real time radon monitors, Rad-7, were used in the measurements. The working principle of the monitor is based on alpha spectrometry. An internal pump draws sampled air through a detection chamber inside the monitor. During the decay process of radon-222 (Rn-222), alpha particle will appear simultaneously. The alpha particle energy, which has a characteristic energy level, is transformed as an electronic signal. Rn-222 concentrations are indicated when the signal is analyzed by a microprocessor. Since ambient radon progenies exist as fine particles indoors and confound radon measurements, a fine filter with pore size 0.45 mm is attached at the inlet of monitor to remove the radon progenies and particulates. A column of desiccant, composed of CaSO4 with 3% CoCl2 as indicator, is linked in series with the filter to maintain the relative humidity in the detection chamber below 10%. Fig. 1 is a schematic diagram of the equipment setup. In the measurements, the sampling inlet of the indoor radon monitor was located within the breathing zone (1–1.5 m above the ground). A second monitor was used to measure outdoor concentration via a flexible sampling tube attached to the inlet of the monitor and extended to the outdoors. The logging intervals of both monitors defaulted as 30 min per datum. In the duration of measurements no occupant was inside the premises. Each measurement of ventilation mode was carried over two days. Day and night ventilation rates were measured. Radon concentrations during the daytime (06:00–18:00) and nighttime (18:00–06:00) were used for analysis. Twelve data sets of indoor–outdoor radon Filter (0.45µm)
Outlet
Drying agent (CaSO4, with 3% CoCl2)
Radon Monitor
Fig. 1. Schematic diagram of equipment setup.
Inlet
ARTICLE IN PRESS 1568
T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571
concentrations against ventilation rates were used in the analysis. For the purpose of testing the performance of the model in conditions of low ventilation, an additional sealed-mode test was used for further verification. The windows and door were tightly closed for the test. A special case of high indoor radon concentration with low-ventilation rate was achieved in this test. 2.2. Determination of air exchange rate The ventilation rate of the premises is one of the essential parameters influencing indoor radon levels. It is often defined in term of air exchange rate, which can be determined by a tracer gas decay method. A nondisperse infrared carbon dioxide monitor, TSI Q-Trak Model 8551, was used in the measurement, with carbon dioxide (CO2) as the tracer gas. The gas was injected inside the sampled premises and attained the level around 1800 ppm. The logging interval of the CO2 monitor defaulted to one datum per minute. Each ventilation test was conducted with simultaneous monitoring of radon. By considering the ambient CO2 level and the free decay profile of the tracer gas, the air exchange rates of the sampled premise could be determined.
In practice, the air exchange rate of residential premises is often larger than the radon decay constant. For example, even a very low-air exchange rate of 0.2 h1 is 26.5 times greater than the Rn-222 decay constant (7.5536 103 h1), and in practice, the air exchange rate is generally much greater than 0.2 h1. Except in the sealed-mode verification test, the air exchange rates of the sampled premise were found to range from 0.19 to 2.84 h1 in this study. Accordingly, the radon decay constant can be neglected from Eq. (2), which then simplifies to P q E i Ai q . (3) ½C p 1 ffi Co þ i V V V Rearranging and employing averaging on Eq. (3), a simplified expression is yielded as P E i Ai h½C p 1 i hC o i ¼ i V ðq=V Þ or 1 , (4) F where h½C p 1i is the averaging of indoor radon concentration and ½C o is the averaging of outdoor radon concentration for the test mode of interest,
Y ¼M
Y ¼ ðh½C p 1 i h½C o i, 3. Theoretical approach M¼ 3.1. Relationship between indoor radon, ventilation rate, and the effective radon emanation factor of the sampled premises Indoor radon level is associated to the ventilation rate, building material radon emanation rate, outdoor radon level, dimensions of the premises, and occupant activities. The influence of occupants was eliminated since no one was inside the premises. Indoor radon concentration Cp, can be expressed in terms of radon emanation rate E, area of applied material A, outdoor radon concentration Co, radon decay constant l, and the effective volume of sampled premise V, to give P dC p qðC o C p Þ E i Ai þ i ¼ lC p . (1) V dt V The first term of the right-hand side of Eq. (1) presents the indoor and outdoor radon exchange rate of the sampled premises. The second term is the indoor radon sources and depends on the kinds of building materials inside the premises. The last term is the radon decay rate inside the sampled premises. In quasi-state, indoor radon concentration can be denoted by [Cp]N. Eq. (1) can be simplified and rearranged to be expressed as P q q E i Ai . (2) ½C p 1 l þ ¼ Co þ i V V V
X
E A =V i i i
and F ¼ ðq=V Þ. The averaging on a physical quantity C over a sampling duration is defined as 1X Cj ; N j¼0 j¼N
hCi ¼
where C can be [Cp]N, or Co. By employing linear regression on Y and the inverse of F; a straight line is obtained. The value M is represented by the slope of the regression line shown in Fig. 2, which is the constant for the selected premises. 3.2. Characters of the profile Y and ventilation rate If the region of extreme small ventilation is not included, Y can be a smooth, continuous and defined curve. The left-hand side of the curve is a small ventilation region, where the indoor air concentrations are rapidly in changing. The right-hand side is the large ventilation region, on which indoor concentration becomes gentle in variation. The profile of Y and air exchange rate is shown in Fig. 3. The rate of change of Y can be obtained by taking the partial derivative on
ARTICLE IN PRESS T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571 70
<[Cp] ∞> - (Bq m-3)
60 y = 31.727x
50
R2 = 0.9491
40 30 20 10 0 0
0.2
0.4
0.8
0.6
1
1.2
1.4
1.6
2
1.8
Inverse of air exchange rate (hr)
Fig. 2. Difference between mean indoor radon and outdoor radon level against inverse of air exchange rate.
250 By sealed-mode test
Indoor radon emanation factor = 31.7 Bq m-3 hr-1 Mean outdoor radon level = 6.3 Bq m-3
<[Cp]∞>- (Bq m-3)
200
150
100
By measurement
By prediction
50
0 0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
Air exchange rate (hr-1)
Fig. 3. Y against air exchange rate of the sampled premises.
Eq. (4) with respect to F. that is qY 1 ¼ M 2 . qF F
(5)
The parameter Y is only dependent on the air exchange rate while M is associated to the building material radon emissions, and surface to volume ratio. As a result, the rate of change of indoor radon level of the sampled premises is proportional to the inverse of the square of air exchange rate, as shown by Eq. (5). This expression shows that the variation of indoor radon concentration is very sensitive to circumstances of low ventilation but not so in higher air exchange rate environments.
4. Results and discussion The sampled premises are on the 14th floor of a residential building on Hong Kong Island. The floor of the premises is covered with varnished hardwood blocks, and the concrete walls are covered with wall-
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paper. A window-type air conditioner is used to cool the premises. Results of the measurements are presented in Table 1. From the table, the mean outdoor and indoor radon levels are found to range from 3.6 to 8.5 Bq m3, and 11.7 to 66.6 Bq m3, respectively. The highest indoor radon concentration of 188.9 Bq m3 is found during the sealed-mode verification test with an air change rate of 0.15 h1. In the window-closed and window-open mode measurements, it can be seen that mean indoor radon levels are higher than mean outdoor radon levels by 7.8–8.8, and 2.6–4.0 times, respectively. As shown in Fig. 3, the profile of Y against air exchange rate varies strongly when the air exchange rate of the premises is less than 1 h1. It moderates with increased air exchange rate. It is seen that the mean indoor radon level cannot match the mean outdoor radon level, even for air exchange rate greater than 3 h1. For example, indoor and outdoor radon levels are shown to be 14.1 and 5.2 Bq m3, respectively, at an air exchange rate of 2.84 h1. The experimental indoor to outdoor radon ratio is 2.7. Using Eq. (4), the theoretical value is 2.8. It indicates that indoor radon levels are not likely to reach outdoor radon levels. The difference is probably due to the consideration of the effective radon emanation factor M in the current study. The value of M is found to be 31.7 Bq m3 h1 by regression technique and the resultant surface to volume ratio of 2 m1 is used to predict the lump sum of radon emanation rates of the sampled premises. The total radon emanation rate of the sampled premise is given as 15.9 Bq m2 h1. This value is below the ranges of bare concrete emanation rates (33.2–40.7 Bq m2 h1) reported by Yu [3]. The small value of emanation rate found in this study is no doubt due to the sealing effects of the wallpaper and the wood flooring, because radon emission can be suppressed by well-sealed covering materials. Besides the technique of sealing bare surfaces, indoor radon concentration can be reduced by ventilation strategy. For example, the ratio of the rate of changes of Y at 0.2 and 1 h1, qY 1 0:2 h qF , qY qF
1 h1
is of 25 times but the ratio of the rate of changes of Y at 2.2 and 3.0 h1 is only of 1.86 times, for the same increment of air exchange rate. The profile of Y shows that increase of a small amount of ventilation rate in a small ventilation environment can significantly reduce indoor radon concentration. Practically, indoor radon concentration coupled with air exchange rate exhibits a hyperbolic character. The sealed-mode test is used to further verify the model. The mean indoor radon concentration and air
ARTICLE IN PRESS T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571
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Table 1 Radon levels at daytime and nighttime with air exchange rates of the sampled premise /[Cp]NS /[Cp]NS /CoS (Bq m3) (Bq m3)
/CoS (Bq m3)
h½C p 1 i=hC o i R2 (air (Error)R exchange rate) (%)
Ventilation mode
Time section
Ach (h1)
1/Ach (h)
Window-closed Window-closed Window-closed Window-closed
Daytime Daytime Nighttime Nighttime
0.58 0.59 0.61 0.62
1.71 1.70 1.64 1.62
58.3 56.6 54.1 56.6
66.6 64.9 61.2 63.7
8.3 8.3 7.2 7.2
8.0 7.8 8.5 8.8
0.997 0.996 0.998 0.997
10.71 10.69 10.66 10.64
AC-on AC-on AC-on AC-on
Daytime Daytime Nighttime Nighttime
1.36 1.36 0.87 0.88
0.74 0.74 1.15 1.13
23.6 22.8 28.5 30.8
32.1 31.3 33.7 36.0
8.5 8.5 5.1 5.1
3.8 3.7 6.6 7.1
0.998 0.999 0.998 0.997
10.05 10.05 10.33 10.32
Window-open Window-open Window-open Window-open
Daytime Daytime Nighttime Nighttime
1.99 2.10 2.73 2.84
0.50 0.48 0.37 0.35
10.7 8.2 8.4 8.9
14.3 11.7 13.6 14.1
3.6 3.6 5.2 5.2
4.0 3.3 2.6 2.7
0.966 0.991 0.991 0.953
9.89 9.88 9.80 9.79
Sealed-mode verification
a
0.15
6.67
180.4
188.9
8.5
22.2
0.998
13.89
Not applicable.
exchange rate are demonstrated to be 188.9 Bq m3 and 0.15 h1, shown in Table 1, respectively. This deviates from the predicated value of 13.2%, but is still within the error estimation. In fact, the error is relied on the air exchange rate as discussed below.
5. Error analysis
50 45 40
(Error)R (%)
a
35 30 25
φ = 0.2 hr-1 error = 12.8%
20 15 10
Errors can arise during the measurements and when simplifying the theoretical approach. Based on the error analysis suggested by Chao and Tung [9], the error can be estimated. The resultant error, (Error)R, is made up of three components; the omission of radon decay constant (Error)l, regression on effect radon emanation estimation (Error)RnE, and regression on air exchange rate measurement (Error)Vent. The resultant error can be expressed as follows:
ðErrorÞR ¼ 1 ½1 ðErrorÞl ½1 ðErrorÞRnE ½1 ðErrorÞVent 100%, ð6Þ where
Aystompt = 9.5% 5 0
0
0.2 0.4 0.6 0.8
(7)
ðErrorÞRnE ¼ ½1 ðR2 ÞRnE ;
(8)
ðErrorÞVent ¼ ½1 ðR2 ÞVent
(9)
Here, the coefficients of determination of the effective radon emanation factor and the air exchange rate are denoted by ðR2 ÞRnE and ðR2 ÞVent ; respectively.
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
-1
Fig. 4. Error due to experimental errors and omission of radon decay constant.
Substituting Eqs. (7)–(9) into Eq. (6) gives
l 2 2 ðErrorÞR ¼ 1 1 ðR ÞRnE ðR ÞVent ðl þ FÞ 100%.
l , ðErrorÞl ¼ ðl þ FÞ
1
Air exchange rate (hr )
ð10Þ
Eq. (10) shows that the resultant error is the function of air exchange rate. It drops rapidly when air exchange rate increases. The error becomes significant if the air exchange rate of the premises is extremely small, although this does not often occur. By employing the minimum values of air exchange rate and ðR2 ÞVent found in Table 1 and ðR2 ÞRnE in Fig. 2, the resultant error at 0.2 h1 is obtained to be 12.8%. Based on Eq. (10), the resultant error cannot be less than 9.5% for the whole
ARTICLE IN PRESS T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571
range of air exchange rates, as shown in Fig. 4. When the air exchange rate is greater than 0.2 h1 the indoor radon level estimation is acceptable.
6. Conclusion The relationship between the effective radon emanation factor M and the ventilation rate has been investigated in this study. The value of M for the sampled premises has been found to be 31.7 Bq m3 h1. The ventilation rate and the factor M of the sampled premises play a major role in determining indoor radon level. If the value of M is known, the minimum ventilation rate for a targeted radon level can be determined, provided there are no other potential pollutants inside the premises. Indoor radon is significantly reduced by increase of air change rates. Due to the measurement error and the neglect of radon decay constant, the error of estimation cannot be less than 9.5%, even at air exchange rates greater than 1 h1. The indoor and outdoor radon levels cannot approach each other due to the existence of radon emanation factor M. The further verification in the sealed-mode suggested that the methodology is valid only if the air exchange rate is greater than 0.2 h1. The lump sum of radon emanation from the premises is obtained, but is less than the reported values of Yu [3]. However, this can be explained by effects of surface coverings. It also indirectly supports the practical use of the model.
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Acknowledgements This research is funded by the account G-YW73 of Postdoctoral Fellowship of The Hong Kong Polytechnic University. The authors would like to thank Mr. Hung Kit for his assistance in part of the site measurements. References [1] Nazaroff WW, Nero AV, Transport of radon from soil into residences. Proceedings of the third international conference on indoor air quality and climate, vol. 2. 1984, p. 15–20. [2] Chao YH, Tung CW, Chan WT, Burnett J. Determination of radon emanation and back diffusion characteristics of building materials in small chamber tests. Building and Environment 1997;32(4):355–62. [3] Yu KN. Radon emanation from concrete with pulverized fuel ash (PFA). Building and Environment 1994;29(4):545–7. [4] Pang SW, and Pun WM. Territory-wide indoor radon survey 1992/ 93. Report No. EPD TP/3/94, The Hong Kong Environmental Department, 1994. [5] Lee YM, Ho TS. Radon survey on newly constructed residential estates in Hong Kong. Report No. EPD/TP5/96, The Hong Kong Environmental Department, 1996. [6] Leung JKL, Tso MYW, Ho CW. Behavior of Rn-222 and its progeny in high-rise building. Health Physics 1998;75(3):303–12. [7] Chao YH, Tung CW, Chan WT, Burnett J. Detailed measurements of indoor radon levels in five residential premises and the effect of ventilation. Indoor Built Environment 1997(6):277–81. [8] Chao YH, Tung CW, Burnett J. Influence of ventilation on indoor radon level. Building and Environment 1997;32(6):527–34. [9] Chao YH, Tung CW. An empirical model for outdoor contaminant transmission into residential building and experimental verification. Atmospheric Environment 2001;35:1585–96.
Building and Environment 40 (2005) 1566–1571 www.elsevier.com/locate/buildenv
An empirical radon emanation model for residential premises Thomas C.W. Tung, Daniel W.T. Chan, John Burnett Department of Building Services Engineering, The Hong Kong Polytechnic University, Hunghom, Hong Kong Received 12 June 2003; accepted 7 December 2004
Abstract Based on an averaging technique, a methodology has been established to estimate an effective radon emanation factor M for residential premises. The model shows that the new term M and the ventilation rate are the essential parameters in estimating the level of indoor radon. M includes two components: the radon emanation rates of internal surface materials and the ratio of surface areas of applicable materials to premises volume. The value of M can be determined from on-site measurements. Different ventilation modes of a sampled residential unit during daytime and nighttime, with air conditioner on, window-open, and windowclosed were included in site measurements. Each ventilation mode was measured twice during daytime and twice at night. During the investigation, air exchange rate, and indoor and outdoor radon levels were monitored simultaneously. The results of measurements were then used to verify the model. The value of M was found to be 31.7 Bq m3 h1. The model is valid if the air exchange rate is larger than 0.2 h1. r 2005 Elsevier Ltd. All rights reserved. Keywords: Air exchange rate; Indoor radon; Effective radon emanation factor; Radon reduction; Residential premises
1. Introduction Simply increasing the ventilation rate is a common and effective means to reduce radon concentration inside buildings. However, over ventilation introduces other problems. Energy cost is increased if mechanical ventilation is used, and in some densely populated areas, such as these found in Hong Kong, increased ventilation encourages outdoor pollutants to be transmitted indoors, since outdoor air is often polluted by traffic emissions, industrial processes, and construction site activities. However, if the ventilation of premises is insufficient, indoor radon concentrations can accumulate to levels that exceed health standards. Usually, when it is required to predict the indoor radon level, the radon emanation rate of each building material, each material exposing area, ventilation rate, outdoor radon concentration, and the effective volume of the premise are required in detail. However, such detailed informaCorresponding author. Tel.: +852 2766 7956; fax: +852 2765 7198.
E-mail address: [email protected] (T.C.W. Tung). 0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2004.12.012
tion are often difficult to determine, so a surrogate method is useful to determine the relationship between indoor radon concentration and ventilation rate. Radon is a radioactive gas released from the decay of radium which is often present in rock, soil, and building materials. The United States Environmental Protection Agency (US-EPA) has reported that inhalation of radon is the second killer from cancer. Soil is considered as a main indoor radon contributor and there is very common in certain areas of many countries. It can seep into the basement of buildings and accumulate indoors. The rate of radon diffusion into premises is associated with the permeability of the foundation soil, the pressure difference, and the design of the building structure [1]. The situation is somewhat different in Hong Kong where high-rise residential buildings predominate. Concrete and granites, which are the common building materials used, are the main sources of indoor radon. The radon emanation rates of these materials are a concern. Small radon chambers have been used to study radon emanation rates from building material. It was found that radon emanation rates of granites ranged
ARTICLE IN PRESS T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571
Nomenclature
Ei
Cp
M
indoor radon concentration at time t (Bq m3) [Cp]N indoor quasi state radon concentration (Bq m3) /CoS outdoor radon averaging concentration (Bq m3) /[Cp]NS indoor radon averaging concentration (Bq m3) Ai surface area of building material-i (m2)
from 5.2 to 13.6 Bq m2 h1 [2]. Concrete with pulverized fuel ash was found with radon emanation rates in the range from 9.2 103 to 11.3 103 Bq m2 s1 [3]. Besides the studies on radon emanation rates of materials, government departments and researchers have also conducted a series of radon surveys in buildings. The Hong Kong Environmental Protection Department (HK-EPD) carried out territory-wide radon surveys [4,5] using Alpha track detectors. The measurements were carried out over three periods, December 1992–August 1993, July 1994–January 1995, and January 1995–June 1995, respectively. Based on their surveys, the HK-EPD concluded that 5% of residential premises and 10% of non-residential premises had radon levels above the recommended level of 200 Bq m3. Another radon survey was conducted in 1774 sites using alpha track detectors [6] showing that mean radon levels in residential buildings and nonresidential buildings were 62 and 111 Bq m3, respectively. The authors concluded that ventilation was the key factor in the consideration of indoor radon concentration. Detailed measurements of indoor radon levels at five residential premises were carried out by Chao et al. [7]. Daytime and nighttime radon levels and air exchange rates of the premises were monitored. The authors observed that the highest radon levels in the premises usually occurred at nighttime. Influence of ventilation on indoor radon was also studied by Chao et al. [8] involving 12 residential buildings. Air exchange rates, and indoor and outdoor radon levels were the target parameters. It was concluded that indoor radon levels approached the outdoor radon level if the air exchange rate of a premise was larger than 3 h1. Whilst the local research and surveys have paid attention to radon emanation rate and indoor radon levels, no quantitative and detailed investigation on the ventilation rate coupled to the radon emanation rate has been reported. Hence, an investigation into indoor radon concentration, ventilation rate, and the effect of radon emanation rate in premises was targeted in this study.
V Y F l
1567
radon emanation rate with respect to building material-i (Bq m2 h1) effective indoor radon emanation factor of the sampled premises (Bq m3 h1) effective volume of the sampled premises (m3) difference of sample averaging of indoor and outdoor radon levels (Bq m3) an air exchange rate of the sampled premise (h1) radon-222 decay constant, 7.5536 103 h1
2. Measurement setup 2.1. Measurement setup Two solid-state-real time radon monitors, Rad-7, were used in the measurements. The working principle of the monitor is based on alpha spectrometry. An internal pump draws sampled air through a detection chamber inside the monitor. During the decay process of radon-222 (Rn-222), alpha particle will appear simultaneously. The alpha particle energy, which has a characteristic energy level, is transformed as an electronic signal. Rn-222 concentrations are indicated when the signal is analyzed by a microprocessor. Since ambient radon progenies exist as fine particles indoors and confound radon measurements, a fine filter with pore size 0.45 mm is attached at the inlet of monitor to remove the radon progenies and particulates. A column of desiccant, composed of CaSO4 with 3% CoCl2 as indicator, is linked in series with the filter to maintain the relative humidity in the detection chamber below 10%. Fig. 1 is a schematic diagram of the equipment setup. In the measurements, the sampling inlet of the indoor radon monitor was located within the breathing zone (1–1.5 m above the ground). A second monitor was used to measure outdoor concentration via a flexible sampling tube attached to the inlet of the monitor and extended to the outdoors. The logging intervals of both monitors defaulted as 30 min per datum. In the duration of measurements no occupant was inside the premises. Each measurement of ventilation mode was carried over two days. Day and night ventilation rates were measured. Radon concentrations during the daytime (06:00–18:00) and nighttime (18:00–06:00) were used for analysis. Twelve data sets of indoor–outdoor radon Filter (0.45µm)
Outlet
Drying agent (CaSO4, with 3% CoCl2)
Radon Monitor
Fig. 1. Schematic diagram of equipment setup.
Inlet
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concentrations against ventilation rates were used in the analysis. For the purpose of testing the performance of the model in conditions of low ventilation, an additional sealed-mode test was used for further verification. The windows and door were tightly closed for the test. A special case of high indoor radon concentration with low-ventilation rate was achieved in this test. 2.2. Determination of air exchange rate The ventilation rate of the premises is one of the essential parameters influencing indoor radon levels. It is often defined in term of air exchange rate, which can be determined by a tracer gas decay method. A nondisperse infrared carbon dioxide monitor, TSI Q-Trak Model 8551, was used in the measurement, with carbon dioxide (CO2) as the tracer gas. The gas was injected inside the sampled premises and attained the level around 1800 ppm. The logging interval of the CO2 monitor defaulted to one datum per minute. Each ventilation test was conducted with simultaneous monitoring of radon. By considering the ambient CO2 level and the free decay profile of the tracer gas, the air exchange rates of the sampled premise could be determined.
In practice, the air exchange rate of residential premises is often larger than the radon decay constant. For example, even a very low-air exchange rate of 0.2 h1 is 26.5 times greater than the Rn-222 decay constant (7.5536 103 h1), and in practice, the air exchange rate is generally much greater than 0.2 h1. Except in the sealed-mode verification test, the air exchange rates of the sampled premise were found to range from 0.19 to 2.84 h1 in this study. Accordingly, the radon decay constant can be neglected from Eq. (2), which then simplifies to P q E i Ai q . (3) ½C p 1 ffi Co þ i V V V Rearranging and employing averaging on Eq. (3), a simplified expression is yielded as P E i Ai h½C p 1 i hC o i ¼ i V ðq=V Þ or 1 , (4) F where h½C p 1i is the averaging of indoor radon concentration and ½C o is the averaging of outdoor radon concentration for the test mode of interest,
Y ¼M
Y ¼ ðh½C p 1 i h½C o i, 3. Theoretical approach M¼ 3.1. Relationship between indoor radon, ventilation rate, and the effective radon emanation factor of the sampled premises Indoor radon level is associated to the ventilation rate, building material radon emanation rate, outdoor radon level, dimensions of the premises, and occupant activities. The influence of occupants was eliminated since no one was inside the premises. Indoor radon concentration Cp, can be expressed in terms of radon emanation rate E, area of applied material A, outdoor radon concentration Co, radon decay constant l, and the effective volume of sampled premise V, to give P dC p qðC o C p Þ E i Ai þ i ¼ lC p . (1) V dt V The first term of the right-hand side of Eq. (1) presents the indoor and outdoor radon exchange rate of the sampled premises. The second term is the indoor radon sources and depends on the kinds of building materials inside the premises. The last term is the radon decay rate inside the sampled premises. In quasi-state, indoor radon concentration can be denoted by [Cp]N. Eq. (1) can be simplified and rearranged to be expressed as P q q E i Ai . (2) ½C p 1 l þ ¼ Co þ i V V V
X
E A =V i i i
and F ¼ ðq=V Þ. The averaging on a physical quantity C over a sampling duration is defined as 1X Cj ; N j¼0 j¼N
hCi ¼
where C can be [Cp]N, or Co. By employing linear regression on Y and the inverse of F; a straight line is obtained. The value M is represented by the slope of the regression line shown in Fig. 2, which is the constant for the selected premises. 3.2. Characters of the profile Y and ventilation rate If the region of extreme small ventilation is not included, Y can be a smooth, continuous and defined curve. The left-hand side of the curve is a small ventilation region, where the indoor air concentrations are rapidly in changing. The right-hand side is the large ventilation region, on which indoor concentration becomes gentle in variation. The profile of Y and air exchange rate is shown in Fig. 3. The rate of change of Y can be obtained by taking the partial derivative on
ARTICLE IN PRESS T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571 70
<[Cp] ∞> -
60 y = 31.727x
50
R2 = 0.9491
40 30 20 10 0 0
0.2
0.4
0.8
0.6
1
1.2
1.4
1.6
2
1.8
Inverse of air exchange rate (hr)
Fig. 2. Difference between mean indoor radon and outdoor radon level against inverse of air exchange rate.
250 By sealed-mode test
Indoor radon emanation factor = 31.7 Bq m-3 hr-1 Mean outdoor radon level = 6.3 Bq m-3
<[Cp]∞>-
200
150
100
By measurement
By prediction
50
0 0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
Air exchange rate (hr-1)
Fig. 3. Y against air exchange rate of the sampled premises.
Eq. (4) with respect to F. that is qY 1 ¼ M 2 . qF F
(5)
The parameter Y is only dependent on the air exchange rate while M is associated to the building material radon emissions, and surface to volume ratio. As a result, the rate of change of indoor radon level of the sampled premises is proportional to the inverse of the square of air exchange rate, as shown by Eq. (5). This expression shows that the variation of indoor radon concentration is very sensitive to circumstances of low ventilation but not so in higher air exchange rate environments.
4. Results and discussion The sampled premises are on the 14th floor of a residential building on Hong Kong Island. The floor of the premises is covered with varnished hardwood blocks, and the concrete walls are covered with wall-
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paper. A window-type air conditioner is used to cool the premises. Results of the measurements are presented in Table 1. From the table, the mean outdoor and indoor radon levels are found to range from 3.6 to 8.5 Bq m3, and 11.7 to 66.6 Bq m3, respectively. The highest indoor radon concentration of 188.9 Bq m3 is found during the sealed-mode verification test with an air change rate of 0.15 h1. In the window-closed and window-open mode measurements, it can be seen that mean indoor radon levels are higher than mean outdoor radon levels by 7.8–8.8, and 2.6–4.0 times, respectively. As shown in Fig. 3, the profile of Y against air exchange rate varies strongly when the air exchange rate of the premises is less than 1 h1. It moderates with increased air exchange rate. It is seen that the mean indoor radon level cannot match the mean outdoor radon level, even for air exchange rate greater than 3 h1. For example, indoor and outdoor radon levels are shown to be 14.1 and 5.2 Bq m3, respectively, at an air exchange rate of 2.84 h1. The experimental indoor to outdoor radon ratio is 2.7. Using Eq. (4), the theoretical value is 2.8. It indicates that indoor radon levels are not likely to reach outdoor radon levels. The difference is probably due to the consideration of the effective radon emanation factor M in the current study. The value of M is found to be 31.7 Bq m3 h1 by regression technique and the resultant surface to volume ratio of 2 m1 is used to predict the lump sum of radon emanation rates of the sampled premises. The total radon emanation rate of the sampled premise is given as 15.9 Bq m2 h1. This value is below the ranges of bare concrete emanation rates (33.2–40.7 Bq m2 h1) reported by Yu [3]. The small value of emanation rate found in this study is no doubt due to the sealing effects of the wallpaper and the wood flooring, because radon emission can be suppressed by well-sealed covering materials. Besides the technique of sealing bare surfaces, indoor radon concentration can be reduced by ventilation strategy. For example, the ratio of the rate of changes of Y at 0.2 and 1 h1, qY 1 0:2 h qF , qY qF
1 h1
is of 25 times but the ratio of the rate of changes of Y at 2.2 and 3.0 h1 is only of 1.86 times, for the same increment of air exchange rate. The profile of Y shows that increase of a small amount of ventilation rate in a small ventilation environment can significantly reduce indoor radon concentration. Practically, indoor radon concentration coupled with air exchange rate exhibits a hyperbolic character. The sealed-mode test is used to further verify the model. The mean indoor radon concentration and air
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Table 1 Radon levels at daytime and nighttime with air exchange rates of the sampled premise /[Cp]NS /[Cp]NS /CoS (Bq m3) (Bq m3)
/CoS (Bq m3)
h½C p 1 i=hC o i R2 (air (Error)R exchange rate) (%)
Ventilation mode
Time section
Ach (h1)
1/Ach (h)
Window-closed Window-closed Window-closed Window-closed
Daytime Daytime Nighttime Nighttime
0.58 0.59 0.61 0.62
1.71 1.70 1.64 1.62
58.3 56.6 54.1 56.6
66.6 64.9 61.2 63.7
8.3 8.3 7.2 7.2
8.0 7.8 8.5 8.8
0.997 0.996 0.998 0.997
10.71 10.69 10.66 10.64
AC-on AC-on AC-on AC-on
Daytime Daytime Nighttime Nighttime
1.36 1.36 0.87 0.88
0.74 0.74 1.15 1.13
23.6 22.8 28.5 30.8
32.1 31.3 33.7 36.0
8.5 8.5 5.1 5.1
3.8 3.7 6.6 7.1
0.998 0.999 0.998 0.997
10.05 10.05 10.33 10.32
Window-open Window-open Window-open Window-open
Daytime Daytime Nighttime Nighttime
1.99 2.10 2.73 2.84
0.50 0.48 0.37 0.35
10.7 8.2 8.4 8.9
14.3 11.7 13.6 14.1
3.6 3.6 5.2 5.2
4.0 3.3 2.6 2.7
0.966 0.991 0.991 0.953
9.89 9.88 9.80 9.79
Sealed-mode verification
a
0.15
6.67
180.4
188.9
8.5
22.2
0.998
13.89
Not applicable.
exchange rate are demonstrated to be 188.9 Bq m3 and 0.15 h1, shown in Table 1, respectively. This deviates from the predicated value of 13.2%, but is still within the error estimation. In fact, the error is relied on the air exchange rate as discussed below.
5. Error analysis
50 45 40
(Error)R (%)
a
35 30 25
φ = 0.2 hr-1 error = 12.8%
20 15 10
Errors can arise during the measurements and when simplifying the theoretical approach. Based on the error analysis suggested by Chao and Tung [9], the error can be estimated. The resultant error, (Error)R, is made up of three components; the omission of radon decay constant (Error)l, regression on effect radon emanation estimation (Error)RnE, and regression on air exchange rate measurement (Error)Vent. The resultant error can be expressed as follows:
ðErrorÞR ¼ 1 ½1 ðErrorÞl ½1 ðErrorÞRnE ½1 ðErrorÞVent 100%, ð6Þ where
Aystompt = 9.5% 5 0
0
0.2 0.4 0.6 0.8
(7)
ðErrorÞRnE ¼ ½1 ðR2 ÞRnE ;
(8)
ðErrorÞVent ¼ ½1 ðR2 ÞVent
(9)
Here, the coefficients of determination of the effective radon emanation factor and the air exchange rate are denoted by ðR2 ÞRnE and ðR2 ÞVent ; respectively.
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
-1
Fig. 4. Error due to experimental errors and omission of radon decay constant.
Substituting Eqs. (7)–(9) into Eq. (6) gives
l 2 2 ðErrorÞR ¼ 1 1 ðR ÞRnE ðR ÞVent ðl þ FÞ 100%.
l , ðErrorÞl ¼ ðl þ FÞ
1
Air exchange rate (hr )
ð10Þ
Eq. (10) shows that the resultant error is the function of air exchange rate. It drops rapidly when air exchange rate increases. The error becomes significant if the air exchange rate of the premises is extremely small, although this does not often occur. By employing the minimum values of air exchange rate and ðR2 ÞVent found in Table 1 and ðR2 ÞRnE in Fig. 2, the resultant error at 0.2 h1 is obtained to be 12.8%. Based on Eq. (10), the resultant error cannot be less than 9.5% for the whole
ARTICLE IN PRESS T.C.W. Tung et al. / Building and Environment 40 (2005) 1566–1571
range of air exchange rates, as shown in Fig. 4. When the air exchange rate is greater than 0.2 h1 the indoor radon level estimation is acceptable.
6. Conclusion The relationship between the effective radon emanation factor M and the ventilation rate has been investigated in this study. The value of M for the sampled premises has been found to be 31.7 Bq m3 h1. The ventilation rate and the factor M of the sampled premises play a major role in determining indoor radon level. If the value of M is known, the minimum ventilation rate for a targeted radon level can be determined, provided there are no other potential pollutants inside the premises. Indoor radon is significantly reduced by increase of air change rates. Due to the measurement error and the neglect of radon decay constant, the error of estimation cannot be less than 9.5%, even at air exchange rates greater than 1 h1. The indoor and outdoor radon levels cannot approach each other due to the existence of radon emanation factor M. The further verification in the sealed-mode suggested that the methodology is valid only if the air exchange rate is greater than 0.2 h1. The lump sum of radon emanation from the premises is obtained, but is less than the reported values of Yu [3]. However, this can be explained by effects of surface coverings. It also indirectly supports the practical use of the model.
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Acknowledgements This research is funded by the account G-YW73 of Postdoctoral Fellowship of The Hong Kong Polytechnic University. The authors would like to thank Mr. Hung Kit for his assistance in part of the site measurements. References [1] Nazaroff WW, Nero AV, Transport of radon from soil into residences. Proceedings of the third international conference on indoor air quality and climate, vol. 2. 1984, p. 15–20. [2] Chao YH, Tung CW, Chan WT, Burnett J. Determination of radon emanation and back diffusion characteristics of building materials in small chamber tests. Building and Environment 1997;32(4):355–62. [3] Yu KN. Radon emanation from concrete with pulverized fuel ash (PFA). Building and Environment 1994;29(4):545–7. [4] Pang SW, and Pun WM. Territory-wide indoor radon survey 1992/ 93. Report No. EPD TP/3/94, The Hong Kong Environmental Department, 1994. [5] Lee YM, Ho TS. Radon survey on newly constructed residential estates in Hong Kong. Report No. EPD/TP5/96, The Hong Kong Environmental Department, 1996. [6] Leung JKL, Tso MYW, Ho CW. Behavior of Rn-222 and its progeny in high-rise building. Health Physics 1998;75(3):303–12. [7] Chao YH, Tung CW, Chan WT, Burnett J. Detailed measurements of indoor radon levels in five residential premises and the effect of ventilation. Indoor Built Environment 1997(6):277–81. [8] Chao YH, Tung CW, Burnett J. Influence of ventilation on indoor radon level. Building and Environment 1997;32(6):527–34. [9] Chao YH, Tung CW. An empirical model for outdoor contaminant transmission into residential building and experimental verification. Atmospheric Environment 2001;35:1585–96.