United States        Office of          EPA 520/1-86-015
               Environmental Protection     Radiation Programs      August 1986
               Agency          Washingto
               Radiation
v>EPA         Seasonal Variations Of Radon
               And Radon Decay  Product
               Concentrations In Single
               Family Homes

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SEASONAL VARIATIONS OF RADON AND RADON DECAY
PRODUCT CONCENTRATIONS IN SINGLE FAMILY HOMES
by
Joseph M. Hans, Jr.
Robert J. Lyon
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U.S. Environmental Protection Agency
Office of Radiation Programs-Las Vegas Facility
P.O. Box 18416, Las Vegas, Nevada 89114

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DISCLAIMER
The mention of trade names or commercial products in this report does
not constitute a recolTlTlendation or endorsement for their use by the U.S.
Environmental Protection Agency.
i i

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FOREWORD
The Office of Radiation Programs of the U.S. Environmental Protection
Agency carries out a national program designed to evaluate population
exposure to ionizing and nonionizing radiation and to promote development of
controls necessary to protect the public health and safety.
Within the Office of Radiation Programs, the Las Vegas Facility
conducts in-depth field studies of various radiation sources (e.g., nuclear
facilities, uranium mill tailings, and phosphate mills) to provide technical
data for environmental impact assessments as well as needed information on
source characteristics, environmental transport. critical pathways for
population exposure, and dose model validation. The Office of Radiation
Programs-Las Vegas Facility also provides. upon request, technical
assistance to Western States and to other Federal agencies. The Las Vegas
Facility participated in a radiation survey in Butte, Montana. The primary
purpose of the survey was to determine if radioactive materials distributed
in the community affected radon concentration in the homes. This report
presents observations and discussions concerning the seasonal variation of
indoor radon and radon decay products.
The readers of th is report are encouraged to send the authors any
comments. Requests for further information are also invited.
",!~~
Sheldon Meyers, Director
Office of Radiation Programs
i i i

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ABSTRACT
Radon and radon decay product concentrations were measured weekly for a
period of 1 year in 20 homes. A seasonal cycle was observed for the radon
concentrations; with a low during the warmer months and a high during the
cooler months of the year. Radon decay product concentrations were found to
generally follow the radon cycle, as expected. The equilibrium ratio was
observed, however, to vary inversely with the radon and radon decay product
cycles. The indoor radon cycle for the 20 homes was also found to follow
the inverse of the outdoor radon concentrations. Some speculation and
supporting data are presented to explain the inverse indoor and outdoor
relationships.
;v

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Disclaimer
Foreword
Abstract
List of
Figures
and Conclusions
SUl11l1ary
Acknowledgement
Introduction
Method
Results
and Discussion
Radon Concentration Cycle
Working Level Cycle
Equilibrium Ratio Cycle
References
Appendix
CONTENTS
PAGE
i i
i i i
iv

vi
vii
ix
1
2
3
5
7
10
12
14
v

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Number
2
3
4
5
LIST OF FIGURES
Normalized Radon Concentrations, Working Levels and
Equilibrium Ratios for Twenty Houses. . . . . . . . . . .
Average Monthly Outdoor Radon Concentrations
and Temperatures. . . . . . . . . . . . . .
. . . . . . .
Comparison of Normalized Radon Concentrations for
Houses With and Without Yearly Cycles and Normalized
Inverted Outdoor Radon Concentrations. . . . . . . . . .
Comparison
Normalized
Subtracted
of the Normalized Working Level and the
Working Level With Radon Variations
.......
. . . . . . . . . . .
......
.Normalized Working Levels for Houses With Different
Heating Systems Adjusted for Radon Variations. . . . . .
vi
Page
4
6
8
9
11

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SUMMARY AND CONCLUSIONS
Indoor radon concentrations were observed to follow a seasonal cycle in
20 ~utte, Montana, houses. A cycle low occurs during the warmer months of
the year. Th is low is bel i eved to be due to increased vent i 1 at i on of the
houses duri ng the warmer months and, in part, to changes in the radon
source. Increased ventilation results from opening doors and windows for
coo 1 i ng. Changes in the radon source are bel i eved to be caused by ground
freezing and, possibly, snow-cover. This causes an increase in radon soil
gas concentrations, thereby increasing the quantity of radon gas available
for entering houses.
The indoor working-level cycle, as expected, generally follows the
yearly radon cycle; however, it departs from the radon cycle during the cold
months of the year. This departure is generally believed to be caused by
the heating systems, which tend to either remove or precipitate airborne
particulate material or to enhance the removal of the first progeny of radon
or both. A yearly cycle of the working level was observed that, when
adjusted for radon variations, augments this assumption. Little differences
are noted when comparing convection and forced-air heating systems, but
there are significantly lower working levels during the heating season.
The foregoing differences in the radon and working-level cycles result
in the cycling of the equilibrium ratio. The equilibrium ratio is lowest in
its cycle during the cooler months and highest in its cycle during the
warmer months. Its form is approximately the inverse of the radon cycle.
These apparent complex interactions of the outdoor
environments as well as lifestyle influences on the indoor radon
and
indoor
vii

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concentrations and working levels, indicate the need for further work and
studies to better understand their relationships. These relationships, when
better understood, wi 11 afford an opportun ity for better appra i sa 1 s of the
annual average working level in houses and, possibly, measures to better
control both indoor radon concentrations and working levels.
viii

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ACKNOWLEDGEMENT
Well over 100,000 measurements of indoor and outdoor radon
concentrations and indoor working levels were made over a period of 18
months during the Butte Study. The authors express their appreciation to
the staff of the Montana Department of Health and Environmental Sciences for
their commitment to excellence in conducting the field portion of this study
and for their prompt evaluation of the radon source terms in Butte, Montana,
which involved additional hundreds of measurements. We acknowledge the
efforts of Dr. Miron Israeli, visiting scientist from the Israel Atomic
Energy Commission, for his outstanding work in editing and organizing the
data base into comprehensive working files. Our appreciation is extended to
Mr. Allen Sparks, Computer Sciences Corporation, for his tireless efforts in
extracting and presenting information from the files for preparation of this
report.
ix

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INTRODUCTION
Gesell (Ge83) reported in his 1983 review that little or no data was
available concerning seasonal variations of indoor radon background
concentrat ions. Si nce then, severa 1 invest i gators have reported season a 1
variations. George (Ge083) noted seasonal variations in structures around
former Manhattan Engineering D1strict sites in Lewiston, New York;
Cannonsburg, Pennsylvania; and Middlesex, New Jersey. Fleischer (F183)
noted seasona 1 differences in energy effic ient homes in northeastern New
York state, and Swedjemark (Sw83) noted seasonal differences in Sweden.
Abu-Jarad (Ab84) conducted a survey to specifically address the existence of
seasonal variations in England, and George (Ge084) adjusted an ongoing study
in Pennsylvania homes to observe seasonal variations.
In general, the investigators noted that seasonal variations of indoor
radon concentrations appear to be commonplace, with higher concentrations in
the cooler months and lower concentrations in the warmer months. The
wi nter-to-summer rat ios of radon decay product concentrat ions observed by
Jonsson (Jo?4) appear to contradict the seasonal variations of radon
concentrations about 50 percent of the time. This paper confirms the
aforementioned authors. work and, where possible, extends it further.
The data presented and discussed in this report are from the Butte
Study. This study developed as a result of an investigation by the Montana
Department of Health and Environmental Sciences (DHES) staff concerning the
use of phosphate slag for construction purposes in Butte, Montana.
Structures conta in i ng phosphate slag products were samp 1 ed to determi ne if
the products affected indoor working levels. Other structures, sampled for

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comparison, were found to have working levels well above those structures
containing phosphate slag products.
The Environmental Protection Agency.s Office of Radiation Programs (ORP)
supported DHES by contract to expand the structure samp 1 ing program and to
investigate and identify the sources of radon causing elevated working
levels. The sources were evaluated by Lloyd (Ll83). New radon and radon
decay product measurement equ i pment that showed some promi se of reduc i ng
measurement costs was appeari ng in the marketp 1 ace. The contract wi th the
DHES was amended to conduct a study to evaluate this equipment in home
environments.
Radon and working-level measurements were made in 68 houses, which were
divided into three groups. One group contained four houses designated as
Super Intensive Level of Measurement (SILOM) houses. Another group
contained 16 houses designated as Intensive Level of Measurement (ILOM)
houses. The third group was designated Normal Level of Measurement (NLOM)
houses. The principal difference between the house groups was the frequency
of measurements. The SILOM houses had additional instrumentation to measure
hourly radon concentrations and working levels. Houses selected for the
study were not random samples. The SILOM houses were selected to provide a
broad range of radon concentrations and working levels. The ILOM and NLOM
houses were selected from those that had prior working-level measurements or
upon the willingness of the occupants to participate in the study.
There are about 112,000 measurements in the data base, exclusive of an
18-month file of hourly meteorological parameters. Quality assurance for
the radon and radon decay product measurements is addressed by Nyberg (Ny85).
About 11 ,000 measurements of radon and radon decay product concentrat ions
from the data base and 9,000 measurements from the meteorological file were
used for the preparation of this report.
2

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METHOD
Twenty of the 68 homes measured in the Butte Study were se lected to
observe seasonal variations. Measurements of radon concentrations and
working levels were made continuously in these houses for at least 1 year
and with a resolution adequate to determine seasonal variations. The
distribution of their average annual radon concentrations covers a wide
range of values from 3 to 70 pCi/l.
Data used in thi s report are derived from three types of measurement
devices; the Passive Environmental Radon Monitor (PERM); the Radon Progeny
Integrating Sampling Unit (RPISU); and the Radon Gas Monitor (RGM). The
PERM's and RPISU's were used to measure, respectively, indoor concentrations
of radon and radon decay products (e.g., working level) respectively, on a
weekly basis. The RGM was used to measure outdoor radon concentrations
continuously on an hourly basis.
Weekly measurements from PERMls and RPISU's for the 20 houses are
presented ~r~phically in the appendix. Radon concentrations and working
1 eve 1 s are plotted for each house on the same graph for each study week.
Study week 1 began during the week of October 5, 1981.
The yearly data, by month, for each house is normalized to present the
data for all 20 houses as a group. Normalization was done by dividing the
average monthly concentration in each house by the average yearly
concentration for that house. The outdoor radon concentrations were
normalized using the same method.
3

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RESULTS AND DISCUSSION
Figure 1 contains plots of the normalized radon concentrations, working
levels, and equilibrium ratios for the 20 houses. Each point on the plot
represents the mean of the normalized values for 20 houses for each calendar
month.
The plots for both normalized radon concentrations and working levels
indicate a yearly cycle. The equilibrium ratio, derived from the foregoing
measurements, also indicates a yearly cycle. Each plot is discussed
separately.
RADON CONCENTRATION CYCLE
This plot exhibits a strong seasonal cycle having minimum values in the
warmer months and maximum values during the cooler months. The maximum
occurs in December gradually falling until May, and begins a steeper drop to
its lowest. level in August. A steep rise occurs from August to October,
followed by a gradual rise to December. The cycle is believed to be caused
by alteration of the houses' ventilation rates and changes in the source
term. House cooling during the warmer months in Butte is usually
accomplished by opening doors and windows during the daylight hours. This
increases the vent il at i on rate and subsequent ly reduces the indoor radon
concentrations. The effect of the increased ventilation rate on the daily
radon cycle in Butte houses has been studied (Ha8S). The period of
increased vent i 1 at i on is presumed to occur from June through September and
to be proport i ona 1 to the outdoor temperature (i.e., warmer days resu 1 tin
longer ventilation periods).
4

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--r
Sep.
I
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Jan.
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Month
Normalized radon concentrations, working levels, and equlibrium ratios for 20 homes.

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Alteration of the radon source terms is believed to be caused by ground
freezing. Ground freezing lowers the radon exhalation rate. This causes
increased radon concentrat ions below the ground surf ace and subsequent ly
higher radon concentrations available for transport and diffusion into the
houses. This assumption is based on summer and winter measurements of radon
soi 1 gas concentrations and the behavior of outdoor radon concentrations.
Radon concentrations in soil were measured by Lloyd (L183) over deep alluvium
in the southern part of Butte, Montana, at a depth of 30 inches at 30
locations on a half-mile grid. A warm weather measurement and a cold weather
measurement were made at each location. The mean radon concentration for the
warm weather measurements was 300 pCi /1, and the mean of the cold weather
measurements was 500 pCi /1. The warm-to-co ld weather rat i 0 was about 0.6,
indicating increased below-grade concentrations during colder months.
Figure 2 contains plots of monthly averages for outdoor concentrations radon
and temperature. Outdoor radon concentrations exhibit a yearly cycle similar
to the temperature cycle. The concentrations are lower in the cooler months
and higher in the warmer months, possibly due, in large part, to the
alteration of the soil's exhalation rate (i.e., from freezing and thawing of
the ground).
Figure 3 contains a plot of the means of the normalized radon
concentrations in 17 houses, a plot of the means of the normalized radon
concentrations in three houses, and a plot of the normalized outdoor radon
concentratiof.ls. The three houses do not exhibit a clear seasonal cycle of
indoor radon concentrations (appendix figures A-4, A-S, and A-12). Their
plot indicates below normal radon concentrations during the winter and late
summer, with above normal concentrations in the late spring and fall. The
cause of this cycle is presented as pure speculation. House 4 was the office
for the DHES staff and was unoccupied about 14 hours per day and on weekends.
It was kept closed most of the time. It is suspected that houses Sand 12
were kept closed during the summer months because of the low number and high
ages of the occupants. The low amplitude cycle is probably due to lower
ventilation rates.
6

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'-I
x - Temperature
o - Radon Concentration
1-
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Mar.
May.
Ju 1.
Month
Jan.
Sep.
Nov.
Figure 2.
Average outdoor radon concentrations and temperatures.
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The plot of the normalized outdoor radon concentrations is inverted to
show its relationship to the plot for the normalized radon concentrations
for the remaining 17 houses. Both cycles generally follow each other in
time and magnitude, indicating a possible relationship between indoor and
outdoor radon. These observations and relationships are currently under
study at our facility.
WORKING-LEVEL CYCLE
The norma 1 i zed work i ng 1 eve 1 s genera lly fo 11 ow the norma 1 i zed radon
cycle, except for colder months of December through March (figure 1). It is
speculated that the lower values (Le., below the norm) of the working
levels during this time period is due to air cleaning by the heating
systems. Windham (Wi78) and Rundo (Ru78) noted the effect of air
condit ioning and a centra 1 fan on the reduct ion of radon concentrat ions,
working levels, and equilibrium ratios.
Figure 4 shows the observed normalized working level and the working
level without the effect of the radon cycle. The effect of the radon cycle
is removed by adjusting the observed working level up or down opposite the
displacement of the radon cy'cle from its norm. If the normalized radon
value is 10 percent above its norm for a particular month, the observed
work i ng 1 eve.l is reduced by 10 percent for that month. The resu It i ng
working-level plot essentially represents the yearly variation of the
working level with radon variations minimized. The cycle is generally a
smooth curve, gradually peaking in the sUlTlller months and reaching lows in
the winter months. This augments the premise that this cycling is due, in
large part, to the air cleaning action of the heating systems.
Figure 5 compares the effects of two types of heating systems on the
working level. Four houses were selected that have forced air heating with
fi lters in the ductwork, and four houses were selected having convective
heat ing systems. The p lots for each group of four houses are the mean
8

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Comparison of normalized radon concentrations for houses with and without yearly cycles and
normalized inverted outdoor radon concentrations.

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variations subtracted.

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monthly normalized working levels with radon variations subtracted. They
represent a seasonal working level cycle independent of the seasonal radon
cycle. There are no apparent gross differences between the two types of
heating systems. Although small differences occur between the two plots,
they follow a general yearly cycle with lows in the cooler months and highs
during the warmer months.
EQUILIBRIUM RATIO CYCLE
The normalized equilibrium ratio plotted on figure 1 also indicates a
yearly cycle, with a high occurring during the warmer months and lows
occurri ng duri ng the cooler months. Its form is approx imate ly the inverse
of the normalized radon cycle. The cycling is probably caused by a
di sproport i onate reduct i on of radon concentrat ions and/or work i ng 1 eve 1 s by
the heating systems.
11

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Normalized working levels for houses with different heating systems adjusted for radon
variations.

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Ab84
F183
F184
Geo83
Geo84
Ge83
REFERENCES
Abu-Jarad, F. and J.H. Fremlin, 1984, "Seasonal Variation of Radon
Concentration in Dwellings," Health Physics, Vol. 46, No.5, pp.
1126-1129.
Fleischer, R.L., A. Mogro-Campero, and L.G. Turner, 1983, "Indoor
Radon Levels in the Northeastern U.S.: Effects of Energy Efficiency
in Homes," Health Physics, Vol. 45, No.2, pp. 407-412.
Fleischer, R.L., and L.G. Turner, 1984, "Indoor Radon Measurements
in the New York Capital Districts" Health Physics, Vol. 46 No.5,
pp. 999-1011.
George, A.C., and J. Eng, 1983, "Indoor Radon Measurements in
New Jersey, New York, and Pennsylvania," Health Physics, Vol. 45,
No.2, pp. 397-400.
George, A.C., M. Duncan, and H. Franklin, 1984, "Measurements of
Radon in Residential Buildings in Maryland and Pennsylvania, U.S.A."
Radiation Protection Dosimetry, Vol. 7, No. 1-4, pp. 291-294.
Gesell, T.F., 1983, "Background Atmospheric 222Rn Concentrations
Outdoors and Indoors: A Rev i ew, " Hea 1 th Phys i cs Vo 1. 45, No.2,
pp. 289-302.
13

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Ha85
Jo84
Ll83
Ny85
Ru78
Sw83
Wi 78
Hans, J.M., Jr., R.J. Lyon, and M. Israeli, 1985, "Temporal
Variations of Indoor Radon and Radon Decay Product Concentrations in
Single Family Homes, II Proceedings of the Eighteenth Midyear Topical
Symposium of the Health Physics Society, Colorado Springs, co.
Jonsson, G., 1984, "Radon Measurements
Proceedings of the 3rd International
Quality, and Climate, Stockholm, Sweden.
in Sweden Some Results"
Conference on Indoor Air,
Lloyd, L.L., 1983, "Eva luat ion of Radon Sources and Phosphate
Sl~g in Butte, Montana," EPA Contract No. 68-01-6100, Montana
Department of Health and Environmental Sciences, Helena, MT.
Nyberg, P.C., 1985, 'ICalibration and Quality Assurance Techniques
for a Major Radon Measurement Comparison Study, II Proceedings of the
Eighteenth Midyear Topical Symposium of the Health Physics Society,
Colorado Springs, CO.
Rundo, J., F. Markun, H.A. May, N.J. Plondke, and D.J. Keefe, 1978,
"Some Measurements of Radon and Its Daughters in Houses," Presented,
in part, at the 24th Annual Meeting of the Health Physics Society,
Philadelphia, PA, July 8-13, 1979.
Sw~djemark, G.A., 1983, IITemporal Variations of the Radon
Concentrations Indoors," Radiation Protection Dosimetry, Vol. 7, No.
1-4, pp. 255-258.
Windham, S.T., E.D. Savage, and C.R. Phillips, 1978, "Effects of
Home Ventilation Systems on Indoor Radon - Radon Daughter Levels, II
U.S. Environmental Protection Agency, Office of Radiation Programs,
Waterside Mall East, 401 "M" Street, Washington, D.C.
14

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APPENDIX
RADON CONCENTRATIONS AND WORKING LEVEL PLOTS FOR 20 HOUSES.
15

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17
40
50
70

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Study Week
Radon and working level vs. time.
18

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Week
Figure A-4.
Radon and working level vs. time.
19

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Heek
Figure A-6.
Radon and working level vs. time.
21

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Radon and working level vs. time.
22

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Radon and working level vs. time.
23
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Radon and working level vs. time.
24

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Heek
Figure A-10.
Radon and working level vs. time.
25

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Radon and working level vs. time.
26

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Neek
Figure A-12.
Radon and working level vs. time.
27

-------
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Figure A-l3.
Radon and wor~ing level vs. time.
28

-------
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Radon and working level vs. time.
29
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Heek
Figure A-15.
Radon and working level vs. time.
30

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lAleek
Figure A-16.
Radon and working level vs. time.
31

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Heek
Figure A-17.
Radon and working level vs. time.
32

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PERM and RPISU - House lB
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PERM and RPISU - House 19
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Study
Neek
Radon and working level vs. time.
34

-------
PERM and RPISU - House 20
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Study
Neek
Figure A-20.
Radon and working level vs. time.
35

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