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Associations between grab sample and integrated radon measurements in dwellings in Maine and Texas
Environment International, Vol. 8, pp. 83-87, 1982 Printed in the USA. All rights reserved.
0160-4120/82/070083-05503.00/0 Copyright © 1982 Pergamon Press Ltd.
ASSOCIATIONS BETWEEN GRAB SAMPLE AND INTEGRATED RADON MEASUREMENTS IN DWELLINGS IN MAINE AND TEXAS Howard M. Prichard and Thomas F. Gesell* The Universityof Texas School of Public Health, P.O. Box 20186, Houston, Texas 77025, USA
Charles T. Hess and Conrad V. Weiffenbach The Universityof Maine at Orono, Orono, Maine 04469, USA
Philip Nyberg Office of Radiation Programs, EnvironmentalProtection Agency, Las Vegas, Nevada89114, USA
Radon concentrations were measured in several locations in each of approximately 100 dwellings in central Maine and in Houston, TX. Integrated samples were taken during the heating (or cooling) seasons with commerciallyavailable passive alpha track devices, while grab samples were taken at the time of integrated sampler deployment. It was found that both indoor and outdoor measurements in both areas were distributed log normally, and that the geometric mean of indoor measurements in Maine was three times higher than that of corresponding measurements in the Houston area. It was also noted that the mean of the indoor grab sample measurements was not significantly different from the mean of the indoor integrated measurements, and that the degree of correlation between the grab samples and a given indoor integrated sample was nearly as good as between integrated samples taken at different living area locations.
1982). Approximately 100 dwellings in and around Houston, TX, were surveyed during the summer and fall of 1980 when most of the structures were using air conditioning. A similar number of dwellings in central Maine were surveyed the following winter during the heating season. The H o u s t o n dwellings were predominantly single-story slab-on-grade ranch houses or units in modern two- or three-story apartment complexes, while the Maine structures were for the most part twostory wood-frame houses with full or partial basements. Each house was equipped with three passive integrating radon detectors (Alter and Fleischer, 1981) in the living area and one detector in a sheltered outdoor location. A fifth detector was placed in the basement in the Maine group. At the time of dosimeter deployment, grab samples of indoor air and household water were taken in the Maine dwellings and, to a lesser extent, in the Houston dwellings. The grab samples were returned to the laboratory and analyzed with liquid scintillation techniques (Prichard, 1982; Prichard and Gesell, 1977). After an exposure period of 3-5 months, the integrating radon detectors were recovered and shipped to the vendor for analysis.
Introduction
The design of environmental sampling programs frequently involves tradeoffs between the quantity and the quality of the data obtained. A continuous record of real-time measurements is considered superior to an integrated measurement over a comparable period, which is in turn superior to a small number of instantaneous grab samples taken during the period. Grab samples, on the other hand, are generally the least expensive o f the options mentioned, and offer the additional advantage of prompt availability o f data. Some relationships noted in a sampling program involving the latter two measurement methods are presented here, and possible implications for indoor sampling strategy are discussed. Methods
The methods and primary data summarized here are described at greater length elsewhere (Prichard et al.,
* Current address: Radiological and Environmental Sciences Laboratory, U.S. Department of Energy, Idaho Falls, ID 83401. 83
84
H . M . Prichard, T. F. Gesell, C. T. Hess, C. V. Weiffenbach, and Philip Nyberg
Results
e
Figure 1 shows histograms of the average living area and outdoor integrating detector data for the Maine and H o u s t o n locations. It can be seen that indoor measurements are generally higher than outdoor measurements, and that both the indoor and outdoor values in Maine are considerably higher than those in Houston. It must be noted that the " o u t d o o r " Maine values were taken in sheltered locations such as sheds and porches, and might not be fully representative of ambient outdoor readings. Another feature apparent in Fig. 1 is the appearance of a normal distribution when the data are plotted on a logarithmic scale. The appearance of log normality is verified in Fig. 2, which shows the Maine and H o u s t o n living area average data plotted on a probability scale. In both cases, the logarithms of the data fit normal distributions at the p = 0.01 level. Because the data are m o r e log-normally than normally distributed, the geometric means and standard deviations were considered most appropriate for describing the results. Table 1 shows these values for the measurement sites and measurement methods for both the Maine and H o u s t o n data sets. The site labeled "optional" was generally in the first floor living area in the Maine group, and was in an out-of-the-way closet or pantry in the H o u s t o n group. One set of data f r o m a solar house in Maine was excluded from the collective analyses. The radon measurements ranged f r o m 53 pCi/1 in the b a t h r o o m to 133 pCi/1 in the ballast r o o m adjacent to the master bedr o o m . The exclusion o f this structure was justified on the grounds that it was not representative of the general housing stock in the area and because the inclusion of values from it would unduly influence averages and correlations. Tables 2 and 3 present the results of t-tests and correlations between the logarithms o f the data from various
indoor ........... outdoor
Houston:
(•)
t
GM = 0.47
,
•
|•
]
GSD =1.94
S•
t
$
':,:"' , ' ',
1P
,WY
o
,f
o a.
mu,n,:(,>
eI -1
GM
•
~
=1.46
GSD :2.30
I -2
•
• #
#
I
0.1
1.0 Indoor
Radon
1
Concentration,
pCI/I
Fig. 2. Probability plots of average living-room radon concentrations in central Maine and Houston.
measuring sites in Maine and Houston, respectively. The p-values obtained from a paired t-test for the equivalence of the means of the indicated measurement categories are shown in the upper right-hand portion of the matrix. For example, it is seen in the Maine group that the bathr o o m values are significantly (p < 0.001) higher than the b e d r o o m values and are also higher than the firstfloor measurements at the p = 0.01 level of significance. The first-floor and the b e d r o o m measurements (p = 0.72) are not significantly different, nor are the grab samples different f r o m the living-room measurements. The basement radon levels are unambiguously higher than the living-room levels, and the outdoor values are clearly lower. Values o f the correlation coefficient r are tabulated in a similar fashion in the lower left half of the matrix. Significance at t h e p = 0.05 level is indicated by a single asterisk, while significance at the p -- 0.01 level is indicated by a double asterisk. In the Maine set it is seen that all indoor air and water measurements are correlated at the p = 0.01 level, with the exception o f the water and basement air measurements, which are correlated at the p = 0.05 level. In contrast, the outdoor measurements are not significantly correlated with anything else at the p = 0.05 level.
HouIt on, TX
++ ....Ji "'l'" :....:
Table 1. Geometric sample m e a n s and standard deviations of radon measurements taken at various sites in the two study areas. Central Maine
i~
'"'
......
o,1
i ~,o ~........... .....:
1o
1
Radon Con©entrlUon, pCi/I
Fig. 1. Histograms o f indoor living area and outdoor integrated radon m e a s u r e m e n t s from central Maine a n d H o u s t o n .
Bedroom* Bathroom* Optional* Basement* Outside* Air grab Water grab
N
(pCi/1)
81 81 82 77 67 57 83
1.12 1.62 1.40 2.46 0.46 1.67 5,860
Houston
S
N
(pCi/l)
2.40 2.41 2.43 2.40 2.42 3.01 5.60
103 103 101 -81 23 14
0.39 0.58 0.47 -0.22 0.49 1,084
* Taken with passive integrating radon detector (A181).
S 2.46 2.47 2.67 -2.55 2.26 1.66
Grab sample and integrated radon measurements
85
Table 2. Matrix of correlation coefficients and t-test p-values for Maine radon measurements. Correlation coefficients are on the lower left, t-test p-values are on the upper right. Bedroom Bedroom Bathroom First floor Air grab Basement Outside Water
Bathroom
First Floor
0.000 0.83** 0.85** 0.73** 0.71"* 0.08 0.51"*
0.82** 0.74** 0.66** 0.07 0.54**
0.72 0.01
Air Grab 0.82 0.22 0.90
0.71"* 0.60** 0.12 0.50**
0.46** 0.15 0.55**
Basement 0.000 0.000 0.000 0.011 0.18 0.33**
Outside 0.000 0.000 0.000 0.000 0.000 0.04
* Correlation significant at the p = 0.05 level. **Correlation significant at the p = 0.01 level.
First, the indoor values in both locations are markedly higher than the outdoor values. This is quite consistent with the general literature on i n d o o r / o u t d o o r relationships and is plausibly explained by the dwellings' retention of radon given o f f by the underlying soil, building materials, household water, and other sources. As ventilation is reduced in order to conserve energy, the indoor radon levels will be increased correspondingly. A second notable feature of the data is the strong tendency toward log normality displayed by the distribution of indoor measurements in both locations. The log normality of indoor radon data has been noted by several observers in many locations (Scott, 1981; Sachs et al., 1982; Alter, 1982; Gesell, 1982). The geometric means vary markedly from area to area, as is the case with the data reported here, but the log normal trend in individual data sets is pronounced. While certain anomalies due to construction materials and building styles are to be expected, it appears reasonable to hypothesize that indoor radon measurements from conventional housing in a given locale will fit a single log normal distribution. The definition of "locale" depends on several geological and other factors which can not be expected to coincide with political boundaries. Nevertheless, the available data suggests that in most cases a city or township is small enough to be considered a single "locale." Given the working assumption o f underlying log normality, a great deal of preliminary information can be obtained with a limited number of well-chosen grab samples, as long as due consideration is given to meteorological variables. Integrated or continuous data is
The contrasts in the Houston group are in general less striking than those in the Maine data, due in part to the generally lower radon levels and the reduced number of air and water grab samples. In lieu of basement data, P E R M (Passive Environmental Radon Monitor) measurements are included in Table 3. The difference between all indoor and outdoor means is still highly significant, but the outdoor air measurements are correlated with the indoor measurements from at least the p = 0.05 level, which is in sharp contrast to the Maine data. Table 4 was assembled to demonstrate the usefulness of the various measurement sets as predictors of the long-term integrated bedroom values. The intercept, slope, and the square of the correlation coefficient obtained from linear regression are ranked in descending order of r 2. The best predictors are the other two integrated living area sets, as might well be expected. The relation between the bedroom and first-floor integrated samples is shown graphically in Fig. 3. That plot can be compared with Fig. 4, which shows the air grab samples plotted against the integrated bedroom values. This comparison, along with the regression parameters from Table 4 and the t-test levels from Table 2, demonstrates the ability of grab sample data to characterize a communitywide exposure situation nearly as well as the more time-consuming methods.
Discussion A number o f characteristics of the Maine and Houston data sets will be emphasized at this point.
Table 3. Matrix of correlation coefficients and t-test p-values for Houston radon measurements. Correlation coefficients are on the lower left, t-test p-values are on the upper right. Bedroom Bedroom Bathroom Closet Air grab PERM Outside Water
Bathroom 0.53
0.047 0.060 0.51"* 0.52* 0.39** 0.62*
0.66** 0.76** 0.64** 0.23* 0.45
Closet 0.17 0.03 0.54** 0.48** 0.27* 0.41
* Correlation significant at the p = 0.05 level. ** Correlation significant at the p = 0.01 level.
Air Grab 0.74 0.28 0.25 0.56** 0.084 0.059
PERM
Outside
0.07 0.71 0.06 0.20
0.000 0.000 0.000 0.16 0.000
0.24* 0.034
0.30
86
H . M . Prichard, T. F. Gesell, C. T. Hess, C. V. Weiffenbach, and Philip Nyberg
Table 4. Regression parameters of measurements used as predictors of bedroom radon concentrations (Maine data).
Intercept
Maine'80
~
Site
d.f.
(pCi/l)
Slope
r2
First floor Bathroom Air grab sample Basement Water grab sample Outdoor
75 76 44 69 70 60
0.22 0.13 0.71 0.66 1.46 1•78
0.84 0.75 0•59 0.33 0.04* -0•12
0.84 0.79 0.66 0•50 0.11 0.001
lo
o • •
•
•
e.
•
a m
1.0
.o-
. ' . , ' :
I.o-e o •
.
•
r.- 0 . 7 7
m 0
* pCi/l (air) per nCi/l (water)•
clearly preferable if the object of a measurement program is to assess the dose commitment associated with a given structure. But if the primary objective of a survey is to identify areas in which radon might prove to be a limiting factor for infiltration reduction efforts, then it seems advisable to consider a grab sampling strategy. In general, more grab than integrated samples can be obtained within a given set of resource constraints, particularly at times when human resources are more available than funds for outside services. Perhaps more important, grab sampling offers the advantage of a quick turnaround time, which is important when sample placement is to be guided by previous sampling results. The Maine and Houston grab sample data summarized in Table 1 may be compared with the composite living area integrated data shown in Fig. 2. The geometric means and standard deviations of the grab sample data determine theoretical distributions quite similar to the observed distributions seen in the probability plot. The degree of similarity between radon levels predicted by the two sampling methods is shown in Table 5, in which the living-area radon concentrations are shown at several percentiles of a log normal distribution. Distributions constructed from the integrated data were based on a 3-5 month record from each of three detectors in the living area of each house, while the grab sample dis-
Maine,
10
"80
¢L •
o "o 10
•
80
E
E o
•ILl 00 os,f • • e •e
el
1.0
• •
•
•
•
o • 00el
el
• e•
'o
."
o
.e
o•
•
•
0.1 1.1
I 1.0 Radon
Grab
1~) Samples,
pCl/I
Fig. 4. Scatter diagram of integrated bedroom and grab sample radon measurements made in dwellings in central Maine.
tributions were based on a single instantaneous sample from some of the sites equipped with integrating detectors. Grab samples were not obtained from all of the sites visited, particularly in the Houston case. Nevertheless, in each geographic area the grab and integrated sample sets describe quite similar situations. If, for example, one were to assume a working level ratio of 0.5 and an indoor standard of 0.015 working levels, the radon concentration of interest would be 3 pCi/l. By either the grab or integrated sampling model, it is readily seen from Table 5 that less than 2°7o of the Houston area dwellings would be over that limit, whereas approximately one-quarter of the conventional dwellings in the central Maine study area would be considered to exceed the standard. Aside from considerations of the relative merits of grab and integrated sampling, there are a number of other features of the data that relate to radon survey design• The general agreement of the means of the living-area data in both Maine and Houston suggests that one detector per dwelling would be adequate for most purposes, providing that radon source areas are avoided. The Maine data strongly implicates the basement as a radon source, and, to a lesser extent, the bathroom. The high levels of radon in the Maine water supplies, together with the significantly elevated levels of radon in the bathrooms, demonstrate that the water supply can be an important factor affecting indoor radon concentrations. The slope obtained from the regression of the
m
r = 0.85
I
Table 5. Indoor radon concentrations (pCi/1) at several percentiles as predicted by integrated and grab data. 0.1 0.1
110 First
Floor
Radon,
1~0 pCi/I
Fig. 3. Scatter diagram of integrated bedroom and integrated firstfloor radon measurements made in dwellings in central Maine.
Predictor
Samples
500/0
75°70
90o70
98o/0
Houston integrated Houston grab samples Maine integrated Maine grab samples
303 23 243 57
0.47 0.49 1.5 1.7
0.74 0.85 2.6 3.5
1.1 1.4 4.2 6.9
1.8 2.6 8.1 16.1
Grab sample and integrated radon measurements
bedroom radon concentration on the concentration of radon in the water (c.f. Table 4) is 0.04 pCi/1 (air) per nCi/l (water), in rough agreement with a ratio of 0.1 pCi/1 (air) per nCi/1 (water) projected for a theoretical dwelling (Gesell and Prichard, 1980). Although the value of r 2 obtained from this regression is modest, the link between radon in groundwater and radon in the dwelling is plausible, since elevated concentrations of radon in the soil gas could be expected to affect both the groundwater and structures built on the surface. Conclusion The data presented and referenced here demonstrate a pronounced trend toward log normality in indoor radon measurements taken in a given area. It was seen that logarithmic distributions estimated from grab sample data were similar to those estimated from more extensive 3-5 month integrated data in both the Maine and Houston study areas. The association between water grab samples and integrated indoor radon concentrations, while far weaker than that of the air grab samples, suggests that water sampling programs might also have some role in locating geographic areas in which there might be a potential for elevated indoor radon concentrations.
87
Acknowledgements-This work was supported in part by NIEHS Grant #SR23ES01742 and by NIH-BRSG Grant #S07R05828.
References Alter, H. W. (1982) Indoor radon levels-Field experience using the track etch method, Health Phys. (in press). Alter, H. W. and Fleischer, R. L. (1981) Passive integrating radon monitor for environmental monitoring, Health Phys, 40, 693-702. Gesell, T. F. (1982) Background atmospheric radon-222 concentrations outdoors and indoors: A Review, Health Phys. (in press). Gesell, T. F. and Prichard, H. M. (1980) The contribution of radon in tap water to indoor radon concentrations, in Natural Radiation Environment. III, T. F. Gesell and W. M. Lowder, eds., pp. 1347-1364. Prichard, H. M. and Gesell, T. F. (1977) Rapid measurements of radon concentrations in water using a commercial liquid scintillation counter, Health Phys. 33, 577-581. Prichard, H. M. (1982) A solvent extraction technique for the measurement of radon-222 at ambient air concentrations, Health Phys. (in press). Prichard, H. M., Hess, C. T., Nyberg, P., Weiffenbach, C. V., and Gesell, T. F. (1982) Integrating radon detector data from dwellings in Maine and Texas, Health Phys. (in press). Sachs, H. M., Hernandez, T. L., and Ring, J. W. (1982) Regional geology and radon variability in buildings, Environ. Int. 8, 97-103. Scott, A. (1981) Applications of single grab sample measurements to the decision-making process involved in choosing houses for remedial action. Presented at the International Meeting on Radon and Radon Progeny Measurement, August, 1981, Montgomery, AL.
0160-4120/82/070083-05503.00/0 Copyright © 1982 Pergamon Press Ltd.
ASSOCIATIONS BETWEEN GRAB SAMPLE AND INTEGRATED RADON MEASUREMENTS IN DWELLINGS IN MAINE AND TEXAS Howard M. Prichard and Thomas F. Gesell* The Universityof Texas School of Public Health, P.O. Box 20186, Houston, Texas 77025, USA
Charles T. Hess and Conrad V. Weiffenbach The Universityof Maine at Orono, Orono, Maine 04469, USA
Philip Nyberg Office of Radiation Programs, EnvironmentalProtection Agency, Las Vegas, Nevada89114, USA
Radon concentrations were measured in several locations in each of approximately 100 dwellings in central Maine and in Houston, TX. Integrated samples were taken during the heating (or cooling) seasons with commerciallyavailable passive alpha track devices, while grab samples were taken at the time of integrated sampler deployment. It was found that both indoor and outdoor measurements in both areas were distributed log normally, and that the geometric mean of indoor measurements in Maine was three times higher than that of corresponding measurements in the Houston area. It was also noted that the mean of the indoor grab sample measurements was not significantly different from the mean of the indoor integrated measurements, and that the degree of correlation between the grab samples and a given indoor integrated sample was nearly as good as between integrated samples taken at different living area locations.
1982). Approximately 100 dwellings in and around Houston, TX, were surveyed during the summer and fall of 1980 when most of the structures were using air conditioning. A similar number of dwellings in central Maine were surveyed the following winter during the heating season. The H o u s t o n dwellings were predominantly single-story slab-on-grade ranch houses or units in modern two- or three-story apartment complexes, while the Maine structures were for the most part twostory wood-frame houses with full or partial basements. Each house was equipped with three passive integrating radon detectors (Alter and Fleischer, 1981) in the living area and one detector in a sheltered outdoor location. A fifth detector was placed in the basement in the Maine group. At the time of dosimeter deployment, grab samples of indoor air and household water were taken in the Maine dwellings and, to a lesser extent, in the Houston dwellings. The grab samples were returned to the laboratory and analyzed with liquid scintillation techniques (Prichard, 1982; Prichard and Gesell, 1977). After an exposure period of 3-5 months, the integrating radon detectors were recovered and shipped to the vendor for analysis.
Introduction
The design of environmental sampling programs frequently involves tradeoffs between the quantity and the quality of the data obtained. A continuous record of real-time measurements is considered superior to an integrated measurement over a comparable period, which is in turn superior to a small number of instantaneous grab samples taken during the period. Grab samples, on the other hand, are generally the least expensive o f the options mentioned, and offer the additional advantage of prompt availability o f data. Some relationships noted in a sampling program involving the latter two measurement methods are presented here, and possible implications for indoor sampling strategy are discussed. Methods
The methods and primary data summarized here are described at greater length elsewhere (Prichard et al.,
* Current address: Radiological and Environmental Sciences Laboratory, U.S. Department of Energy, Idaho Falls, ID 83401. 83
84
H . M . Prichard, T. F. Gesell, C. T. Hess, C. V. Weiffenbach, and Philip Nyberg
Results
e
Figure 1 shows histograms of the average living area and outdoor integrating detector data for the Maine and H o u s t o n locations. It can be seen that indoor measurements are generally higher than outdoor measurements, and that both the indoor and outdoor values in Maine are considerably higher than those in Houston. It must be noted that the " o u t d o o r " Maine values were taken in sheltered locations such as sheds and porches, and might not be fully representative of ambient outdoor readings. Another feature apparent in Fig. 1 is the appearance of a normal distribution when the data are plotted on a logarithmic scale. The appearance of log normality is verified in Fig. 2, which shows the Maine and H o u s t o n living area average data plotted on a probability scale. In both cases, the logarithms of the data fit normal distributions at the p = 0.01 level. Because the data are m o r e log-normally than normally distributed, the geometric means and standard deviations were considered most appropriate for describing the results. Table 1 shows these values for the measurement sites and measurement methods for both the Maine and H o u s t o n data sets. The site labeled "optional" was generally in the first floor living area in the Maine group, and was in an out-of-the-way closet or pantry in the H o u s t o n group. One set of data f r o m a solar house in Maine was excluded from the collective analyses. The radon measurements ranged f r o m 53 pCi/1 in the b a t h r o o m to 133 pCi/1 in the ballast r o o m adjacent to the master bedr o o m . The exclusion o f this structure was justified on the grounds that it was not representative of the general housing stock in the area and because the inclusion of values from it would unduly influence averages and correlations. Tables 2 and 3 present the results of t-tests and correlations between the logarithms o f the data from various
indoor ........... outdoor
Houston:
(•)
t
GM = 0.47
,
•
|•
]
GSD =1.94
S•
t
$
':,:"' , ' ',
1P
,WY
o
,f
o a.
mu,n,:(,>
eI -1
GM
•
~
=1.46
GSD :2.30
I -2
•
• #
#
I
0.1
1.0 Indoor
Radon
1
Concentration,
pCI/I
Fig. 2. Probability plots of average living-room radon concentrations in central Maine and Houston.
measuring sites in Maine and Houston, respectively. The p-values obtained from a paired t-test for the equivalence of the means of the indicated measurement categories are shown in the upper right-hand portion of the matrix. For example, it is seen in the Maine group that the bathr o o m values are significantly (p < 0.001) higher than the b e d r o o m values and are also higher than the firstfloor measurements at the p = 0.01 level of significance. The first-floor and the b e d r o o m measurements (p = 0.72) are not significantly different, nor are the grab samples different f r o m the living-room measurements. The basement radon levels are unambiguously higher than the living-room levels, and the outdoor values are clearly lower. Values o f the correlation coefficient r are tabulated in a similar fashion in the lower left half of the matrix. Significance at t h e p = 0.05 level is indicated by a single asterisk, while significance at the p -- 0.01 level is indicated by a double asterisk. In the Maine set it is seen that all indoor air and water measurements are correlated at the p = 0.01 level, with the exception o f the water and basement air measurements, which are correlated at the p = 0.05 level. In contrast, the outdoor measurements are not significantly correlated with anything else at the p = 0.05 level.
HouIt on, TX
++ ....Ji "'l'" :....:
Table 1. Geometric sample m e a n s and standard deviations of radon measurements taken at various sites in the two study areas. Central Maine
i~
'"'
......
o,1
i ~,o ~........... .....:
1o
1
Radon Con©entrlUon, pCi/I
Fig. 1. Histograms o f indoor living area and outdoor integrated radon m e a s u r e m e n t s from central Maine a n d H o u s t o n .
Bedroom* Bathroom* Optional* Basement* Outside* Air grab Water grab
N
(pCi/1)
81 81 82 77 67 57 83
1.12 1.62 1.40 2.46 0.46 1.67 5,860
Houston
S
N
(pCi/l)
2.40 2.41 2.43 2.40 2.42 3.01 5.60
103 103 101 -81 23 14
0.39 0.58 0.47 -0.22 0.49 1,084
* Taken with passive integrating radon detector (A181).
S 2.46 2.47 2.67 -2.55 2.26 1.66
Grab sample and integrated radon measurements
85
Table 2. Matrix of correlation coefficients and t-test p-values for Maine radon measurements. Correlation coefficients are on the lower left, t-test p-values are on the upper right. Bedroom Bedroom Bathroom First floor Air grab Basement Outside Water
Bathroom
First Floor
0.000 0.83** 0.85** 0.73** 0.71"* 0.08 0.51"*
0.82** 0.74** 0.66** 0.07 0.54**
0.72 0.01
Air Grab 0.82 0.22 0.90
0.71"* 0.60** 0.12 0.50**
0.46** 0.15 0.55**
Basement 0.000 0.000 0.000 0.011 0.18 0.33**
Outside 0.000 0.000 0.000 0.000 0.000 0.04
* Correlation significant at the p = 0.05 level. **Correlation significant at the p = 0.01 level.
First, the indoor values in both locations are markedly higher than the outdoor values. This is quite consistent with the general literature on i n d o o r / o u t d o o r relationships and is plausibly explained by the dwellings' retention of radon given o f f by the underlying soil, building materials, household water, and other sources. As ventilation is reduced in order to conserve energy, the indoor radon levels will be increased correspondingly. A second notable feature of the data is the strong tendency toward log normality displayed by the distribution of indoor measurements in both locations. The log normality of indoor radon data has been noted by several observers in many locations (Scott, 1981; Sachs et al., 1982; Alter, 1982; Gesell, 1982). The geometric means vary markedly from area to area, as is the case with the data reported here, but the log normal trend in individual data sets is pronounced. While certain anomalies due to construction materials and building styles are to be expected, it appears reasonable to hypothesize that indoor radon measurements from conventional housing in a given locale will fit a single log normal distribution. The definition of "locale" depends on several geological and other factors which can not be expected to coincide with political boundaries. Nevertheless, the available data suggests that in most cases a city or township is small enough to be considered a single "locale." Given the working assumption o f underlying log normality, a great deal of preliminary information can be obtained with a limited number of well-chosen grab samples, as long as due consideration is given to meteorological variables. Integrated or continuous data is
The contrasts in the Houston group are in general less striking than those in the Maine data, due in part to the generally lower radon levels and the reduced number of air and water grab samples. In lieu of basement data, P E R M (Passive Environmental Radon Monitor) measurements are included in Table 3. The difference between all indoor and outdoor means is still highly significant, but the outdoor air measurements are correlated with the indoor measurements from at least the p = 0.05 level, which is in sharp contrast to the Maine data. Table 4 was assembled to demonstrate the usefulness of the various measurement sets as predictors of the long-term integrated bedroom values. The intercept, slope, and the square of the correlation coefficient obtained from linear regression are ranked in descending order of r 2. The best predictors are the other two integrated living area sets, as might well be expected. The relation between the bedroom and first-floor integrated samples is shown graphically in Fig. 3. That plot can be compared with Fig. 4, which shows the air grab samples plotted against the integrated bedroom values. This comparison, along with the regression parameters from Table 4 and the t-test levels from Table 2, demonstrates the ability of grab sample data to characterize a communitywide exposure situation nearly as well as the more time-consuming methods.
Discussion A number o f characteristics of the Maine and Houston data sets will be emphasized at this point.
Table 3. Matrix of correlation coefficients and t-test p-values for Houston radon measurements. Correlation coefficients are on the lower left, t-test p-values are on the upper right. Bedroom Bedroom Bathroom Closet Air grab PERM Outside Water
Bathroom 0.53
0.047 0.060 0.51"* 0.52* 0.39** 0.62*
0.66** 0.76** 0.64** 0.23* 0.45
Closet 0.17 0.03 0.54** 0.48** 0.27* 0.41
* Correlation significant at the p = 0.05 level. ** Correlation significant at the p = 0.01 level.
Air Grab 0.74 0.28 0.25 0.56** 0.084 0.059
PERM
Outside
0.07 0.71 0.06 0.20
0.000 0.000 0.000 0.16 0.000
0.24* 0.034
0.30
86
H . M . Prichard, T. F. Gesell, C. T. Hess, C. V. Weiffenbach, and Philip Nyberg
Table 4. Regression parameters of measurements used as predictors of bedroom radon concentrations (Maine data).
Intercept
Maine'80
~
Site
d.f.
(pCi/l)
Slope
r2
First floor Bathroom Air grab sample Basement Water grab sample Outdoor
75 76 44 69 70 60
0.22 0.13 0.71 0.66 1.46 1•78
0.84 0.75 0•59 0.33 0.04* -0•12
0.84 0.79 0.66 0•50 0.11 0.001
lo
o • •
•
•
e.
•
a m
1.0
.o-
. ' . , ' :
I.o-e o •
.
•
r.- 0 . 7 7
m 0
* pCi/l (air) per nCi/l (water)•
clearly preferable if the object of a measurement program is to assess the dose commitment associated with a given structure. But if the primary objective of a survey is to identify areas in which radon might prove to be a limiting factor for infiltration reduction efforts, then it seems advisable to consider a grab sampling strategy. In general, more grab than integrated samples can be obtained within a given set of resource constraints, particularly at times when human resources are more available than funds for outside services. Perhaps more important, grab sampling offers the advantage of a quick turnaround time, which is important when sample placement is to be guided by previous sampling results. The Maine and Houston grab sample data summarized in Table 1 may be compared with the composite living area integrated data shown in Fig. 2. The geometric means and standard deviations of the grab sample data determine theoretical distributions quite similar to the observed distributions seen in the probability plot. The degree of similarity between radon levels predicted by the two sampling methods is shown in Table 5, in which the living-area radon concentrations are shown at several percentiles of a log normal distribution. Distributions constructed from the integrated data were based on a 3-5 month record from each of three detectors in the living area of each house, while the grab sample dis-
Maine,
10
"80
¢L •
o "o 10
•
80
E
E o
•ILl 00 os,f • • e •e
el
1.0
• •
•
•
•
o • 00el
el
• e•
'o
."
o
.e
o•
•
•
0.1 1.1
I 1.0 Radon
Grab
1~) Samples,
pCl/I
Fig. 4. Scatter diagram of integrated bedroom and grab sample radon measurements made in dwellings in central Maine.
tributions were based on a single instantaneous sample from some of the sites equipped with integrating detectors. Grab samples were not obtained from all of the sites visited, particularly in the Houston case. Nevertheless, in each geographic area the grab and integrated sample sets describe quite similar situations. If, for example, one were to assume a working level ratio of 0.5 and an indoor standard of 0.015 working levels, the radon concentration of interest would be 3 pCi/l. By either the grab or integrated sampling model, it is readily seen from Table 5 that less than 2°7o of the Houston area dwellings would be over that limit, whereas approximately one-quarter of the conventional dwellings in the central Maine study area would be considered to exceed the standard. Aside from considerations of the relative merits of grab and integrated sampling, there are a number of other features of the data that relate to radon survey design• The general agreement of the means of the living-area data in both Maine and Houston suggests that one detector per dwelling would be adequate for most purposes, providing that radon source areas are avoided. The Maine data strongly implicates the basement as a radon source, and, to a lesser extent, the bathroom. The high levels of radon in the Maine water supplies, together with the significantly elevated levels of radon in the bathrooms, demonstrate that the water supply can be an important factor affecting indoor radon concentrations. The slope obtained from the regression of the
m
r = 0.85
I
Table 5. Indoor radon concentrations (pCi/1) at several percentiles as predicted by integrated and grab data. 0.1 0.1
110 First
Floor
Radon,
1~0 pCi/I
Fig. 3. Scatter diagram of integrated bedroom and integrated firstfloor radon measurements made in dwellings in central Maine.
Predictor
Samples
500/0
75°70
90o70
98o/0
Houston integrated Houston grab samples Maine integrated Maine grab samples
303 23 243 57
0.47 0.49 1.5 1.7
0.74 0.85 2.6 3.5
1.1 1.4 4.2 6.9
1.8 2.6 8.1 16.1
Grab sample and integrated radon measurements
bedroom radon concentration on the concentration of radon in the water (c.f. Table 4) is 0.04 pCi/1 (air) per nCi/l (water), in rough agreement with a ratio of 0.1 pCi/1 (air) per nCi/1 (water) projected for a theoretical dwelling (Gesell and Prichard, 1980). Although the value of r 2 obtained from this regression is modest, the link between radon in groundwater and radon in the dwelling is plausible, since elevated concentrations of radon in the soil gas could be expected to affect both the groundwater and structures built on the surface. Conclusion The data presented and referenced here demonstrate a pronounced trend toward log normality in indoor radon measurements taken in a given area. It was seen that logarithmic distributions estimated from grab sample data were similar to those estimated from more extensive 3-5 month integrated data in both the Maine and Houston study areas. The association between water grab samples and integrated indoor radon concentrations, while far weaker than that of the air grab samples, suggests that water sampling programs might also have some role in locating geographic areas in which there might be a potential for elevated indoor radon concentrations.
87
Acknowledgements-This work was supported in part by NIEHS Grant #SR23ES01742 and by NIH-BRSG Grant #S07R05828.
References Alter, H. W. (1982) Indoor radon levels-Field experience using the track etch method, Health Phys. (in press). Alter, H. W. and Fleischer, R. L. (1981) Passive integrating radon monitor for environmental monitoring, Health Phys, 40, 693-702. Gesell, T. F. (1982) Background atmospheric radon-222 concentrations outdoors and indoors: A Review, Health Phys. (in press). Gesell, T. F. and Prichard, H. M. (1980) The contribution of radon in tap water to indoor radon concentrations, in Natural Radiation Environment. III, T. F. Gesell and W. M. Lowder, eds., pp. 1347-1364. Prichard, H. M. and Gesell, T. F. (1977) Rapid measurements of radon concentrations in water using a commercial liquid scintillation counter, Health Phys. 33, 577-581. Prichard, H. M. (1982) A solvent extraction technique for the measurement of radon-222 at ambient air concentrations, Health Phys. (in press). Prichard, H. M., Hess, C. T., Nyberg, P., Weiffenbach, C. V., and Gesell, T. F. (1982) Integrating radon detector data from dwellings in Maine and Texas, Health Phys. (in press). Sachs, H. M., Hernandez, T. L., and Ring, J. W. (1982) Regional geology and radon variability in buildings, Environ. Int. 8, 97-103. Scott, A. (1981) Applications of single grab sample measurements to the decision-making process involved in choosing houses for remedial action. Presented at the International Meeting on Radon and Radon Progeny Measurement, August, 1981, Montgomery, AL.