Radiation Environment and Medicine 2024 Vol.13, No.2 52–59

Regular Article

Radon in Soil Gas and Ambient Dose Equivalent Rate
Measurements in Yaoundé, Cameroon
Eric Gourda Hebsia1, Oumar Bobbo Modibo1, 2*, Ayoba Ndimantchi1,
Joseph Emmanuel Ndjana Nkoulou II2, Eka Djatnika Nugraha3, Chutima Kranrod4,
Yasutaka Omori4, Masahiro Hosoda4, Saïdou1, 2 and Shinji Tokonami42

1
Nuclear Physics Laboratory, Faculty of Science, University of Yaoundé I, P.O. Box 812, Yaoundé, Cameroon
Research Centre for Nuclear Science and Technology, Institute of Geological and Mining Research, P.O. Box 4110, Yaoundé, Cameroon
3
Research Centre for Technology of Radiation Safety and Metrology, The National Researchand Innovation Agency of Indonesia (ORTN-BRIN), Jakarta 12440, Indonesia
4
Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki-shi, Aomori 036-8564, Japan

Received 23 December, 2022; revised 2 August, 2023; accepted 12 June, 2024

This study measured radon (222Rn) concentrations in soil gas and ambient dose equivalent rates invarious neighborhoods of Yaoundé, Cameroon. MARKUS 10 detector was used to measure radonin soil gas at 50 cm of depth. The ambient dose equivalent rate measurements were performedat the height of 1 m above ground, using a pocket survey meter (RadEye PRD-ER, Thermo
Scientific). The results obtained from the radon concentration in soil gas measurements ranged
from 8.4 to 50.7 kBq m-3 with a geometric mean of 25.4 kBq m-3. Ambient dose equivalent rates
ranged from 0.02 to 0.11 µSv h-1 with an average value of 0.05 µSv h-1. These measurements were
followed by calculating radon surface exhalation rates, and the annual outdoor external effective
dose. Radon surface exhalation rates ranged from 41.5 to 250.5 mBq m-2 s-1 with an average valueof 139.8 mBq m-2 s-1. The annual outdoor external effective dose varied between 0.07 and 0.39
mSv with an average value of 0.17 mSv. The correlation between radon concentration in soil gas
and ambient dose equivalent rates was analyzed. A Pearson correlation coefficient r = 0.81 was
obtained, indicating the strong correlation between them, and one can serve as the indicator of theother.
Key words: Radon, soil gas, ambient dose equivalent rate, exhalation rate, external effective dose.

1. Introduction
Humans are constantly exposed to radiation from naturaland man-made radioactive sources that are unevenly
distributed around the Earth. The natural component of*Oumar Bobbo Modibo: Research Centre for Nuclear Science and Technology,
Institute of Geological and Mining Research, P.O. Box 4110, Yaoundé, Cameroon
E-mail: bobbomodibo@gmail.com
https://doi.org/10.51083/radiatenvironmed.13.2_52
Copyright © 2024 by Hirosaki University. All rights reserved.

this radioactivity is due to radionuclides from the naturally
occurring families of 238U, 235U, and 232Th, and from 40K
as well as cosmic radiation. According to ICRP Publ. 651),
radon gas is the most important source (approximately55%) of human internal radiation exposure.
Radon (222Rn) is a naturally occurring radioactive gasthat comes from the decay of radium (226Ra) in the 238U
decay chain present in the earthʼs crust. It is a colorlessodorless, tasteless, inert gas that is generally present inall types of soil and rock, and its concentration varies

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53

Fig. 1. Location of Yaoundé city with a highlight on the sampling points.

with the geological material and specific site2). Radon
concentration level and radon risk will be increased withhigh uranium and radium content in soil3). So, radonis present everywhere on the earthʼs crust. The 222Rn
originates from geological substratum and diffuses
through pore space in soil and rocksʼ fractures and alongother weak zones as faults, and then it is transported
by advection (described by Darcyʼs law) and diffusion(governed by Fickʼs law) through this space to exhale into
the atmosphere4, 5).
Uranium and radium content of the soil, the typeof the soil, the permeability of the soil, and physicalparameters of the soil such as humidity, pressure, andtemperature are factors that greatly affect the radon
exhalation into the atmosphere and in indoor air. There
are two main sources of radon in the indoor air: the soil
and the water supply. Between the two sources, the soil
is generally considered to be the higher source of radonin the indoor air. So, radon in the soil is the main causeof the radon problem. Radon has been classified as alung carcinogen and is listed as the second risk factor
for lung cancer, behind tobacco6). Extensive radon in soil
gas measurements were being conducted worldwide
for radiological risk assessment purpose7-13). Therefore,
measuring the radon concentration in soil gas, its surface
exhalation rate, and radiological parameters such as
ambient dose equivalent rate is essential. Furthermore,
the measurements of the ambient dose equivalent rate
can serve as a good indicator for the estimation of radon
concentration in soil gas. The research conducted byTchorz-Trzeciakiewicz et al. in Poland showed that theambient gamma dose rate can be used as a good indicatorof the geogenic radon potential14). Several studies inGermany, Switzerland, Hungary, Scotland, and Finland,
used the gamma dose rate for predicting the radon flux
on different rocks15).
The current work aimed to measure radon in soil gas
and ambient dose equivalent rate in Yaoundé, Cameroon.
From the concentrations of radon in soil gas, and theambient dose equivalent rate measured, radon exhalation
rates, and annual outdoor external effective dose were
calculated, respectively. The correlation between theambient dose equivalent rate and radon in soil gas was
also analyzed.
2. Materials and methods
2.1. Geography and geology of the study area
The city of Yaoundé is located in the Region of Centreof Cameroon and it is the capital of the Country. It is
located between 3°
52′
00″
North latitude and 11°
31′
00″
East
longitude. Figure 1 presents the layout of the cityof Yaoundé as well as the location of radon in soil
gas sampling points. According to the Cameroon
population census of 200516), Yaoundé has a population of
approximately 2.8 million inhabitants living in an area of
304 km2. Annual average precipitation and temperature in
the city of Yaoundé are 1600 mm and 24 °C, respectively17).
The climate of Yaoundé is equatorial type marked by two
rainy seasons (March-June and September-November)and two dry seasons (December-February and JulyAugust). Vegetation varies from degraded forest (south)
to the post-forest savannah (north) and Mfoundi is themain river of the city of Yaounde17). The geomorphology

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Eric Gourda Hebsia et al. / Radiation Environment and Medicine 2024 Vol.13, No.2 52–59

Fig. 2. Schematic diagram of the Markus 10.

is characterized by various landforms constituting
mountainous massifs ( > 800 m), plateaus (750 m), andswampy valleys ( < 700 m).
Geologically, the Neoproterozoic basement of
Yaoundé is composed of insoluble impervious andinsoluble migmatites, schists, and gneisses which aretranspierced by diaclases and faults18, 19). This basement
gives permeability to the heterogeneous, anisotropic, andhighly weathered formations and is responsible for the
production of mostly well-drained Ferralitic soils. Mvondo
et al. and Nzenti et al. reported that the most abundant
outcrop along the beds of some rivers in Yaoundé city
is metasedimentary rocks20, 21). The Yaoundé bedrockis composed of fractured embrechites and is generally
covered by alluvial hydromorphic clay and sand in thevalleys and laterites (feralsols) on the hillsides22). The
hydraulic investigations reported that the hydraulicconductivity values of Yaoundé varied between 8.17 x10-7
and 2.26 x10-5 m s-1 23).
2.2. Measurements of radon concentration in soil gas
The measurements of radon concentration in soil
gas were performed using the MARKUS 10 detector,
which is an electronic radon detector manufactured byGammadata in Uppsala, Sweden. The MARKUS 10 radon
detector is constituted with an air pump and a solid-statealpha detector which consists of a semiconductor material(Ortec Ultra Silicon) that converts alpha radiation directly
to an electrical signal. The MARKUS 10 can differentiate
immediately radon from thoron by the energy of thealpha particles released. The equipment is portableand battery-operated, and the measurement is fast. A
stainless-steel probe supplied by Gammadata, immersedin the soil, which was then connected to the MARKUS
10 detector with a water seal (special accessory for the
purpose) was used to collect radon soil-gas samples ofeach measurement point. The probe was penetrated in
the soil with a rotating handle or immersed with gentle
strokes of a sledgehammer. For this study, the depth of
the sampling points was set at 50 cm. The measurements
were performed where the soil was generally free ofrocks. The operation principle of the instrument ispresented in Figure 2.
The MARKUS 10 operates by pumping out gas fromthe ground through a probe for 30 seconds. After thepumping phase, the Radon gas is driven and stored for
10 min in a measuring chamber, time during which it
decays into its progeny (218Po and 214Po) through alpharadiation detected with an ultra-silicon surface barrier
detector operating under a high electric field (~600
VDC)24, 25). The detector pulses are amplified and filteredin an analyzer, which transmits only the pulses from theshort-lived radon decay product, 218Po. This eliminates
the low background variations from 214Po. The pulses
are counted and converted by the device into volumeconcentrations in kBq m-3. All these measurements werecarried out during the daytime from 10 AM to 2 PM andin the absence of rainfall during the rainy season in May
2021. Period during which the average temperature andrelative humidity were 24°
C and 84%, respectively.
2.3. Calculation of radon surface exhalation rate
In the absence of disturbance, radon migration in thesoil towards the atmosphere is ensured only by diffusion,
described by the law of Fick. In the one-dimensional
system, the diffusion flux density J of radon at the surface

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55

of a porous medium assumed homogeneous and isotropic
is defined by Fickʼs law expressed in Equation (1) 26-29):

Finally, the radon surface exhalation rate on the soil
surface is obtained using Equation (9):dCs
J = - D ―― ,dZ

E = ηCs λ D .

(1)

where D is the effective radon diffusion coefficientfor natural soil (4.62 × 10-6 m2 s-1)29), and Cs is the radon
concentration in the medium (Bq m-3). Cs is the solutionof the one-dimensional time-independent equation ofradon diffusion through a porous medium described in
Equation (2) 26-29):
d2Cs
D ――
+ α - λ Cs = 0 ,
d Z2

(2)

where α is the radon gas production rate in the mediumʼspore space (Bq m-3 s-1), λ is the radon decay constant (s-1).
Thus, the solution Cs of the Equation 2 is presented in
Equation (3):Cs = Aeλz- λα＋Be D z ＋―
λ (-< z < 0 ).

(3)

When applying boundary conditions, when z- , Csz
should be finite, and when0, Cs = 0. It supposes theradon concentration at the soil surface from atmosphericapport is negligible. Thus, Equation 3 is substituted by
Equation (4):αCs = ―λ ( 1−e

λ

).

(4)

From Equations (1) and (4), the radon diffusion flux
density at the soil surface (z = 0) is expressed in Equation(5):
J0 = αD/λ(5)Radon surface exhalation rate E (Bq m-2 s-1) at the soil
surface is expressed using Equation (6)28, 29):E = ηJ 0 ,(6)whereηis the soil porosity for natural soil (50%)29). Thus,
the Equation (6) can be substituted with the Equation (5)to express the radon surface exhalation rate in Equation(7):
E = ηα D/λ.
(7)
Radon in soil gas concentration Cs remains in equilibriumin the medium, which means the radon decayed isreplaced by the radon that is being produced. Thus, theradon production rateαis expressed in Equation (8)30):

α = λ CS .

(8)

(9)

2.4. Ambient dose equivalent rate measurements
Firstly, the outdoor ambient dose equivalent ratemeasurements were performed at a height of 1 m above
ground, using a portable instrument (NucScout monitor
manufactured by Sarad). It integrates a sensitivity NaI(Tl)scintillation detector with an integrated photo multiplierand high voltage supply31). The sampling interval for thisstudy was adjusted to 12 min. Total activity is calculated
by instrument software integrated from direct 226Ra
detection, 214Bi direct detection, 40K detection, and 208Tl
detection. The measurements with this instrument werecarried out in the same places and at the same times
during the measurement of radon concentrations in soilgas. Thus, thirteen points in Yaoundé were measured
to study the correlation between the ambient dose
equivalent rate and the concentration of radon in soil gas.
Secondly, another ambient dose equivalent ratemeasurements were performed at 220 points in Yaoundé
at the height of 1 m above ground, using a pocket
survey meter (RadEye PRD-ER, Thermo Scientific).
The instrument incorporates a high-sensitivity NaI(Tl)scintillation detector with a miniature photo-multiplier
allowing the detection of very low radiation levels with
an emphasis on gamma emissions below 400 keV. The
results obtained from these measurements serve to assess
the annual outdoor external effective dose.
2.5. Annual outdoor external ef fective dose assessment
The annual outdoor external effective dose was assessedusing the formula expressed in Equation (10)32):
Eext = Ḣ×T out(10)where Ḣ is the arithmetic mean of ambient dose
equivalent rate measured by the above-mentioned
RadEye PRD-ER pocket survey meter, and T out is the time
passing outdoors during a year32).
3. Results and discussion
3.1. Radon concentration in soil gas
The results of radon measurements in soil gas in
Yaoundé are given in Table 1. The lowest value ofradon concentration in soil gas was recorded in theneighborhood of Nsam (8.4 kBq m-3), while the highest
value was recorded in the neighborhood of Ahala (50.7
kBq m -3). The arithmetic mean and geometric meanof radon concentration in soil gas were 28.25 and 25.43
kBq m-3, respectively. The highest radon concentration

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Eric Gourda Hebsia et al. / Radiation Environment and Medicine 2024 Vol.13, No.2 52–59
Table 1. Radon concentration in soil gas, and radon surface exhalation rate
Statistical parameters
Range
AM
GM
SD

Radon concentration in soil
(kBq m-3)
8.4-50.7
28.3
25.4
12.3

Radon exhalation rate
(mBq m-2 s-1)
41.5-250.5
139.8
125.5
60.7

AM: arithmetic mean, GM: geometric mean, SD: standard deviation

in soil gas observed in the neighborhood of Ahala could
be explained by the presence of much higher relative
humidity than at other measurement points. The
measurement points at the neighborhoods of Nsam and
Mvan show lowest levels of radon concentration in soilgas. In general, the variations of radon concentration insoil gas in the present study area may be due to porosityand density of soil, radium content and distribution,
underlying bedrock and meteorological parameters ofeach measurement point (barometric pressure, rain, etc.),
since that these parameters play important role in therelease of radon in soil-gas33).
The radon concentrations in soil gas obtained in thisstudy were compared with those reported elsewhere.
Petersell et al.7) reported radon concentrations in soil gas
from Estonia in the range of 1-2112 kBq m-3, in Islamabad,
Pakistan radon concentrations in soil gas ranged from 3
to 97 kBq m-3. Gondji et al. reported radon concentrations
in soil gas ranging from 3.6 to 63.2 kBq m-3 at the CobaltNickel Bearing Area of Lomié, in Eastern Cameroon30).
Al-bakhat et al. reported radon concentrations in soil gas
between 0.87-16 kBq m-3 in Al-Tuwaitha Nuclear Site andthe Surrounding Areas, Iraq27). The radon concentration
in soil gas from Franklin Mountain in El Paso, Texas in
the United States of America ranged between 1.35-7.75
kBq m-3 12). The radon concentrations in soil gas at 1 mof depth in Northern Rajasthan, India ranged between
0.72-10.40 kBq m-3 9). In Abeokuta, Southwest Nigeria,radon concentrations in soil gas studies reported aradon concentration in the range of 1-19.25 kBq m-3 13).
Around Castle Island, Co. Kerry in Ireland the radon
concentrations in soil gas lie in the range of 10-1,433
kBq m-3 8). Baiwa et al.34) reported radon concentrations
in soil gas from the Tusham ring complex of Haryana in
the range of 42.80 to 71.50 kBq m-3. Radon concentrations
in soil gas measured in the present investigation are in
the range of the values reported in these cited studies
except those in Estonia7), Islamabad district, Pakistan,
CastleIsland, Co. Kerry, Ireland8), and Tusham ring
complex of Haryana9).
3.2. Radon surface exhalation rate
The radon surface exhalation rate was calculated fromthe measured values of radon concentrations in soilgas. The calculated radon surface exhalation rates arepresented in Table 1. Radon surface exhalation rates
ranged from 41.5 to 250.5 mBq m-2 s-1 with a geometricmean value of 125.5 mBq m-2 s-1. It was seen from theresults that, the radon surface exhalation rates varied
significantly from one sampling point to another. Thisvariation may be due to the differences in the large
radium and uranium contents and porosity of each soil
measurement point although it also depends on manyother factors, such as permeability and density of soil, soil
cover, texture, and grain size. The values of radon surface
exhalation rates for the city of Yaoundé are higher than
the reported worldwide mean of 16 mBq m-2 s-1, and thosefound in Kobe City (2 ± 2 - 30 ± 8 mBq m-2 s-1), in Gifu
Prefecture (4.5±0.3 mBq m-2 s-1), Japan35-37). This could be
explained by the geological and environmental factorsof the study area and the methodology used for radon
surface exhalation rate measurements. According to
Hosoda et al.36), the calculated radon surface exhalation
rate values are usually higher than the directly measured
values.
3.3 Ambient dose equivalent rate
The results of the ambient dose equivalent rates
measurements obtained for this study are presented in
Figure 3.
A total of 220 measurement points were recorded using
a RadEye PRD-ER pocket survey meter. The ambient
dose equivalent rate values varied from 0.02 to 0.11 µSv h-1,
with a mean value of 0.05 µSv h-1. According to Gulan
and Spasović38) it is known that the outdoor ambient
dose equivalent rate varies from place to place and alsowith meteorological parameters39), relief, vegetation,and diversity soil composition. When compared withother studies, it can be stated that the range of resultsfrom the present study are lower than those in Celleno
municipality in central Italy 40) which varies from 0.13 to0.42 µSv h-1 with an average of 0.22 µSv h-1. However, therange values obtained in the present study are similar
to the range values measured at Tuzla City, Bosnia andHerzegovina41). Furthermore, the mean value obtained inthis study is higher than the mean value obtained at the
Bunkyo Campus of Hirosaki University, Japan by Hosoda
et al.42).

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57

90
80
70

Frequency

60
50
40
30
20
10
0

[0 . 0 2 , 0 . 0 3 ]( 0. 0 3 , 0. 0 4 ]( 0. 0 4 , 0.0 5 ]( 0 .0 5 , 0 . 0 6 ]( 0 .0 6 , 0 .07 ]( 0.0 7 , 0 .08 ]( 0 . 08 , 0 . 09 ]( 0 .0 9 , 0 . 1 ]( 0 .1, 0 .11 ]Ambient dose equivalent rate ( µSv h－1)

Fig. 3. Distribution of ambient dose equivalent rate in the city of Yaoundé.

3.4. Annual outdoor external ef fective dose
The annual outdoor external effective dose received by amember of the public living in the surrounding area wascalculated from the results of the ambient dose equivalent
rates measured using RadEye PRD-ER. It ranges from0.07 mSv to 0.39 mSv with an average value of 0.17 mSv.
These results indicate that the outdoor exposure of amember of the public to the external radiation in Yaoundé
is not critical when compared to the values reported bySaïdou et al., and Oumar Bobbo et al. in different regionsof Cameroon. The authors found an average of annual
outdoor external effective dose values of 0.7 mSv, and 1.2
mSv in Southern Adamawa, and Kitongo, respectively 43, 44).
These average values are higher than the ones obtainedin the present study. However, the results of the present
study are similar to those obtained by Koyang et al., in
the Far North Region of Cameroon45).
3.5 Correlation between radon concentration in soil gas
and ambient dose equivalent rate
The relationship between ambient dose equivalent rate
and radon concentration in soil gas was studied. A Pearson
correlation coefficient r = 0.81 was obtained and indicatesambient dose equivalent rate and radon concentration insoil gas in Yaoundé City are well correlated.
This correlation reveals a good dependence between
radon concentration in soil gas and ambient dose
equivalent rate. This is because radon decay products
being gamma emitters contribute to external gamma
radiation. These results indicate that it is possible topredict the radon in soil gas from the ambient dose
equivalent rate4). The results obtained in the present
study are in good agreement with those carried out inother countries. Also, a positive correlation between radon
concentration in soil gas and ambient dose equivalent
rate was reported in Kumaon Himalaya, India, andPoland by Ramola et al. and Tchorz-Trzeciakiewicz et al.,respectively 4, 14). Furthermore, the research conducted byTchorz-Trzeciakiewicz et al. indicates that the ambient
gamma dose rate can be a good indicator of the geogenic
radon potential14).
4. Conclusion
Radon concentrations in soil gas measured in this study
were found to vary between 8.4–50.7 kBq m-3, with ageometric mean of 25.4 kBq m-3 which is above the theglobal average range (0.4 to 40) kBq m-3. The ambient
dose equivalent rate values varied from 0.02 to 0.11 µSv h-1,
with a mean value of 0.05 µSv h -1. Radon surface
exhalation rates ranged from 41.5 to 250.5 mBq m-2 s-1. The
annual outdoor external effective dose varied between
0.07 and 0.39 mSv with an average value of 0.17 mSv. A
strong correlation (r = 0.81), was obtained between radon
concentration in soil gas and ambient dose equivalent
rates. That indicates the ambient dose equivalent rate
can be a good indicator of the radon concentrations in soil
gas radon potential. Additional measurement points of222
Rn concentration in soil gas for the other major cities of
Cameroon is required, to establish a complete database of
the Country for radon risk mapping.
Acknowledgment
The International Atomic Energy Agency (IAEA) is
thanked for its funding and its technical support through58

Eric Gourda Hebsia et al. / Radiation Environment and Medicine 2024 Vol.13, No.2 52–59

Fig. 4. Correlation between radon in soil gas and ambient dose equivalent rate.

the CMR9009 project. The Ministry of the Scientific
Research and Innovation, of Cameroon is acknowledged
for the support of the field work through the public
investment budget 2021. This work was supported by
ERAN Y-23-18.

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Conflict of Interest
The authors declare that they have no conflict of interest.
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