Radiation Environment and Medicine 2022 Vol.11, No.1 16–20

NoteHigh Indoor Radon Concentration Observed in Yomitan-son,
Okinawa Prefecture, Southwestern Part of Japan
Masahide Furukawa1*, Yasutaka Omori2**, Nagi Masuda1, Yuki Tamakuma3***, Takahito Suzuki3,
Mika Obata1, Reina Shingaki1, Syunya Nakasone1, Akinobu Ishimine1****, Kaori Nakamura1,
Yoshitaka Shiroma1, Masahiro Hosoda3, Naofumi Akata3 and Shinji Tokonami3

2

1
University of the Ryukyus, 903-0213 Okinawa, Japan
Fukushima Medical University, 960-1295 Fukushima, Japan
3
Hirosaki University, 036-8560 Aomori, Japan

Received 14 September 2020; revised 3 November 2021; accepted 9 November 2021

In Yomitan-son, a village of Okinawa prefecture located in the subtropical region of Japan, thehighest annual average of indoor radon (222Rn) concentration, 220 Bq m-3, has been observed in a
private residence by a nationwide survey. In this study, to estimate the distribution and origin of
the high concentration, measurements for atmospheric radon were conducted on eight dwellings
intermittently from 2005 to 2013. And in situ measurements of gamma radiation energy spectrum
on the outdoor ground were performed at 26 points in 2018 to estimate the origin of the highindoor radon concentration. As the result, the highest indoor radon concentration, 289 Bq m-3,
was observed in a dwelling. For the seasonal variation, indoor radon concentration in winter isobviously higher than that in summer was observed in several dwellings. From the results for the
analyses of gamma radiation data, useful information about the origin of the high indoor radon
concentration was not provided in this study.
Key words: indoor radon, gamma radiation, 238U series, Yomitan village, Okinawa prefecture, Japan

1. Introduction
Assessment of the radiation and radioactive levels due
to natural sources is very important to estimate thecollective and personnel doses to the public. For example,
based on the results for several nationwide surveys, theJapanese population dose from natural radiation wasgiven as 2.2 mSv y-1 1). In this case, the arithmetic mean
*Masahide Furukawa: University of the Ryukyus, 903-0213 Okinawa, Japan
E-mail: m_furu@sci.u-ryukyu.ac.jp**Present Address: Hirosaki University, 036-8564 Aomori, Japan***Present Address: National Institute for Quantum Science and Technology, 2638555 Chiba, Japan****Present Address: Institute for Environmental Sciences, 039-3212 Aomori, Japan
Copyright © 2022 by Hirosaki University. All rights reserved.

of annual effective doses from cosmic rays, terrestrial
radiation, radon (222Rn, 220Rn and these progenies), andfoodstuffs were estimated as 0.29, 0.33, 0.59, and 0.99
mSv y-1, respectively, and the total of these values wasused as the population dose. On the other hand, it is also
important to understand the regional variation of thedoses. Particularly for the high background radiationarea, detailed study is needed to estimate the exact
effective dose including the causes.
In this study, we introduce the preliminary results fora case of Yomitan-son, a village of Okinawa prefecture
located in the subtropical region of Japan (Fig.1). By
a nationwide survey with passive radon monitors at
the end of 20th century, the highest annual mean ofindoor radon ( 222Rn) concentration, 220 Bq m -3, has

Masahide Furukawa et al./ Radiation Environment and Medicine 2022 Vol.11, No.1 16–20

Fig. 1. Location of study area.

been observed in a private residence in this village2).
After that, in a residence that is different from one forthe nationwide survey, anomalously high indoor radon
concentrations for a short time in a day, 400-1000 Bq m-3,
were measured with an active radon monitor3). Indoor
radon is considered to be an important source ofenvironmental radiation exposure among the publicthroughout the world4), and World Health Organization(WHO) issued a reference level of 100 Bq m-3 for indoor
radon concentration5). Therefore, to get an overview
for the distribution of high indoor radon concentrationin Yomitan-son, annual or short-term measurements of
the atmospheric radon were conducted in this study.
In addition, to study the origin of high indoor radon
concentration, activity of the source nuclides (238U series)of the soils distributed in the village were estimated by insitu measurement of terrestrial gamma radiation energy
spectra.
2. Locality and measurements
Yomitan-son is located in the central part of Okinawa
Island, the main island of Okinawa prefecture, and it
faces East China Sea (Fig.1). The area, population andnumber of dwelling units of the village are 35.28 km2,
41,774 and 15,340, respectively6, 7). By the way, Yomitanson is the largest village in Japan in terms of population.
In addition, the areas of residential, farmland and the U.S.
military base (Fig. 2) are 14%, 16% and 36% of the whole,respectively6, 7). In other words, the area occupancy of the17

Fig. 2. Location of measurement site and points.
☆ A-H: Measurement site of indoor/outdoor radon concentration.
● 1-26: Measurement point of gamma radiation.

military base to the whole is very large. The basement
geology is mainly Pleistocene coral reef-origin limestone8),
and the limestone is widely covered with red soil that iscalled “Shimajiri- mahji” in Okinawan language9).
Measurements of atmospheric radon concentrations
were performed with passive monitors (RADUET,Radosys Co. Ltd.)10) in a total of eight stand-alone concrete
houses, about 0.05% of the total number of dwelling
units in the village (Fig. 2, Sites A-H). Two monitors
were installed indoors in Sites A-G. For Site H, sixmonitors were installed indoors. One to three monitors
were also installed outdoors in Sites B, D, E, F, G and H.
The all monitors were hung on the inner or outer wallof the houses. The measurements were taken in five
periods from December 2005 to March 2013, except for
February 2007 to November 2012. The radon and thoron
concentrations were calculated using the track densities
for each of the two pieces of CR-39 and the conversion
factors. The conversion factors from track densities toradon and thoron concentrations were evaluated by thecalibration experiments using radon and thoron exposuresystems at National Institute of Radiological Sciences,
Japan11).
In addition to the measurement of atmospheric radon
concentration, to consider the radon potential in the study
area, in situ measurements of gamma radiation energy
spectrum were performed at 1m in height from theoutdoor soil ground with a 3”φ×3” NaI(Tl) scintillation
spectrometer (EMF211, EMF Japan Co. Ltd.) at total of 26
points in the village (Fig. 2) during January 2018. Based18

Masahide Furukawa et al./ Radiation Environment and Medicine 2022 Vol.11, No.1 16–20

Table 1. Indoor radon concentration at the A to H sites
Place of
Period 1
Monitor
Dec. 2005 - Mar. 2006
Installation
4 months
bedloom
23±3
A
living room30±3dining kitchen
21±3
B
bedloom
16±3living room 1
104±6
C
living room 2
122±6
living room 1
21±3
D
living room 2
18±3
living room 1
30±3
E closet31±3
living room 2
44±4
living room 1
67±6F
living room 2
73±6
closetGliving room
① bedroom② bedroom③ entranceH*④ dining kitchen⑤ dining kitchen⑥ living room
*Location is shown in Fig. 3.
Site ID

Period 2
Apr. 2006 - Jun. 2006
4 months
8±2
5±2
8±2
8±2
25±3
35±4
14±3
11±2
8±2
16±3
42±5
32±4
25±3
7±2
5±2
-

Period 3
JuI. 2006 - Sep. 2006
3 months
14±4
4±3
35±6
25±5
14±4
14±4
4±2
14±2
32±4
25±3
26±3
3±1
2±1
-

Bathroom
Toilet①②

3. Results and discussion
The results for measurements of indoor and outdoor
radon concentrations are summarized in Tables 1 and 2,
respectively. The estimated gamma radiation dose rate in
air and concentrations of natural radionuclides at/aroundthe in situ measuring points are shown in Table 3.
The range of indoor radon concentration was estimatedto be 2-289 Bq m-3. In Sites C, F and H, indoor radon

Living Room④
Dining Kitchen⑤

Bedroom③
Entrance

Storageon the energy spectrum data, the concentration of 238U
series, parent nuclides of radon, contained in the soils andthe gamma radiation dose rate in air were calculated by a
response matrixes method12).

Period 5
Dec. 2012 - Mar. 2013
4 months
211±15200± 9138±18122± 6128± 6160±17⑧ (outdoors: under the eaves)

Table 2. Outdoor radon concentration at the sites
Place of
Period 4
Period 5
Site ID
Monitor
Oct. 2006 - Jan. 2007 Dec. 2012 - Mar. 2013
Installation
4 months
4 monthsB
under the eaves
10±3D
under the eaves
10±3
under the eavesl±2E
under the eaves
6±2F
under the eaves
14±3G
under the eaves
9±3
⑨
30±6
H*
⑦
l±3
⑧
5±2
* Location is shown in Fig. 3.

Period 4
Oct. 2006 - Jan. 2007
4 months
13± 3
16± 3
131± 8128± 8
28± 4
20± 4
22± 4
46± 5
76± 7
101± 8
94± 7
12± 3
5± 2
289±17143±12144±12⑥
Living Room

Wooden Deck(outdoors)↑ ⑦ (under the wooden deck)← ⑨ (under the eaves)Fig. 3. Simplified floor plan of a dwelling, radon measuring Site H(Fig. 2). ① – ⑨ show measuring points where radon monitors wereplaced.

concentration exceeded 100 Bq m -3 regularly fromlate autumn to early spring. Though the fluctuation
range is small, similar seasonal trend of indoor radon
concentration, rise in winter and fall in summer, was also
observed in other sites. The concentrations of 200 Bq m-3
or more observed in a bedroom of Site H (Fig. 3), 200-289
Bq m-3, are roughly comparable to the maximum valueof the nationwide survey, 220 Bq m-3 2). The arithmetic
mean±standard deviation of all data for indoor radon

Masahide Furukawa et al./ Radiation Environment and Medicine 2022 Vol.11, No.1 16–20
Table 3. Gamma radiation dose rate in air and concentrations of
natural radionuclides at/around measuring points (Fig. 2)
Measuring
Point No.
1
2
3
4
5
6
8
9
10
11
12
13
14
15
16
17
18
20
21
22
23
24
25
26

Dose
Concentration (Bq/kg)
Rate
K-40
(nGy/h) U-238 series Th-232 series
56.2
47.8
36.6
214.5
60.8
41.7
48.0
246.8
64.2
63.2
38.7
244.3
60.1
49.6
39.4
286.8
39.4
34.7
25.8
200.8
48.5
43.3
29.9
226.3
67.2
56.2
47.2
261.5
60.8
52.0
35.8
269.0
50.9
42.6
29.6
283.3
50.4
36.2
31.5
317.1
58.1
53.2
34.9
257.0
57.0
51.4
34.2
256.0
57.3
53.5
35.1
231.8
60.0
52.0
35.5
243.5
57.9
50.7
39.4
252.0
57.2
49.4
38.0
242.3
27.9
22.4
18.4
171.6
47.1
34.9
26.7
287.0
44.9
40.9
28.5
193.6
38.4
16.1
20.2
376.3
62.4
54.6
40.0
271.5
42.8
41.8
22.9
173.0
46.0
34.6
26.9
267.3
44.9
28.7
28.3
251.9

concentration was calculated to be 52.2±60.1 Bq m-3. The
annual means of indoor radon concentrations in Japan
were estimated to be 15.5 Bq m-3 from 1994 to 19962) and15.2 Bq m-3 from 2007 to 201013). Based on these results forindoor radon concentration, about 25% of concrete housesin the village may have concentrations above 100 Bq m-3
especially in winter, and the village could be a relatively
high concentration area for the indoor radon in Japan.
The number of data is limited, but the range andarithmetic mean±standard deviation of outdoor radon
concentration were estimated to be 1-30 Bq m-3 and 9.5±
8.8 Bq m-3, respectively. The nationwide annual mean ofoutdoor radon concentration in Japan was estimated to be
6.1 Bq m-3 14). The minimum and maximum mean values
ranged from 3.3 Bq m-3 in Okinawa prefecture to 9.8 Bq m-3
of Chugoku region (Fig. 1) because of geological andpedological characteristics in the nationwide survey14). In
other words, the outdoor radon concentration observed inthis study may be relatively high in Okinawa prefecture,but it is not much dif ferent from the nationwide
perspective in Japan.
Incidentally, the very low indoor and outdoor radon
concentrations, >5 Bq m-3 that is lower than the limit ofdetection for the passive monitor using in this study, wereobserved in Sites B, E, G, and H (Tables 1 and 2).
Based on the results for in situ measurements of19

gamma radiation energy spectrum, the arithmetic mean±
standard deviation and range of the dose rate in air at/around the measuring points were estimated to be 52.5 ±10.2 nGy h-1 and 27.9-67.2 nGy h-1, respectively. These
results roughly correspond to the reported values for
Yomitan-son, 51.4±19.2 nGy h-1 and 23.0-112.0 nGy h-1 15).
These means are almost same as the nationwide one,about 50 nGy h-1 16-18), and as the preliminary means of
Okinawa Island and Ryukyu Islands (Fig. 1) that are about
47 nGy h-1 and 50 nGy h-1, respectively19).
Also based on the analysis of the gamma radiation data,
the arithmetic mean±standard deviation and range ofthe 238U series concentration were estimated to be 44.2 ±11.6 Bq kg -1 and 16.1-63.1 Bq kg -1, respectively. These
values are relatively lower than that of reported ones for
Yomitan-son, 67.1±43.8 Bq kg-3 and 21.0-168.8 Bq kg-3 15).
On the other hand, these are estimated to be higher thanthe values for the main land of Japan. That are, the mean
is about 16 Bq kg-1 and the range is 3-26 Bq kg-1 20).
In brief, it is considered that these results forthe gamma radiation dose rate and the 238U series
concentration are insufficient to explain the relatively
high indoor radon concentrations observed in Yomitanson. For further understanding of this matter for theindoor radon concentration, it is necessary not only to
expand the measurement but also to try other researchmethod, such as the analysis of radionuclides for building
materials and the measurement of radon emanation/
exhalation process for the soils.
By the way, the result for a preliminary study on theradon emanation process suggests that Shimajiri-mahjiwith 95 Bq kg-1 of 226Ra from Yomitan-son has significant
high emanation coefficient, 0.25-0.3921). Furthermore,
recent studies for Shinajiri-mahji suggest that the basematerial originated from eolian dust blown across theEast China Sea mainly from the southeastern part ofChina, a high background radiation area22, 23), during thelast glacial period or after the previous one24-27). In Miyako
and Minami-daitho islands of Okinawa prefecture (Fig.1)
where the shimajiri-mahji soils with high 238U series
concentration (Max. 220 Bq kg-1) are widely distributed,
comparatively high atmospheric radon concentrations
were observed 25, 26, 28, 29). However, there is no clear
evidence to define the relationship between relatively
high indoor radon concentration and the shimajiri-mahjisoils at this time.
4. Summary
In Yomitan-son, a village of Okinawa prefecture, higher
indoor radon concentrations, >100 Bq m-3, were observedin some ordinary concrete dwellings compared to theJapanese annual average, 15.5 Bq m-3 2). The indoor radon
concentration showed that the increasing tendency in20

Masahide Furukawa et al./ Radiation Environment and Medicine 2022 Vol.11, No.1 16–20

winter season, and the maximum value reached to 289
Bq m-3. The origin of the relatively high concentrationis unknown at present, but solving this issue is oneof important challenge in the fields of environmental
radiation and radiation protection. Therefore, continuous
study for the environmental behavior of radon is required.

14.

Oikawa S, Kanno N, Sanada T, Ohashi N, Uesugi M, Sato K, et al.
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Masuda N, Mukai M, Hokama T, Motomura D, Jahana T,
Tokonami S, et al. Relationship between terrestrial gamma
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16.

Minato S. Distribution of terrestrial γ ray dose rates in Japan. J
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Furukawa M, Shingaki R. Terrestrial gamma radiation dose ratein Japan estimated before the 2011 Great East Japan Earthquake.
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Hosoda M, Tokonami S, Furukawa M. (2012) Dose assessment on
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Furukawa M, Kina S, Shiroma M, Shiroma Y, Masuda N,
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Megumi K. Natural radioactive nuclides in soil. Jpn. J Health Phys.
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Furukawa M. Distribution of terrestrial gamma radiation levelin Asia. Radioisotopes. 1996;45:711–6. (in Japanese with English
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Conflict of Interest Disclosure
The authors declare that they have no conflict of interest.
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