Radon/Thoron and Progeny Concentrations in Dwellings: Influencing Factors and Lung Cancer Risk in the Rutile Bearing Area of Akonolinga, Cameroon

Abstract

This paper reports the levels of radon (Rn), thoron (Tn), and their progeny (TnP) concentrations in dwellings; studies factors influencing these concentrations; and assesses the associated lung cancer risk in Akonolinga’s area in Cameroon, where rutile deposits have been identified but are not yet industrially exploited. Indoor Rn and Tn were determined using CR39-based detectors. Additionally, Rn in soil gas, ²²⁶Ra, and ²³²Th concentrations in soil were measured using Markus 10, high purity germanium detector (HPGe), respectively. On average, indoor Rn, Tn concentration, and the equilibrium equivalent Thoron concentration (EETC) or TnP were 39.5, 68.1, and 5.0 Bq m⁻³, respectively. Average concentrations of Rn in soil gas, ²²⁶Ra, and ²³²Th in soil were 24.3 kBq m⁻³, 17 Bq kg⁻¹, and 27 Bq kg⁻¹, respectively. Correlation analysis indicates that indoor radon and thoron levels were tightly linked with factors such as their precursor concentrations in soil, the building materials, dwelling architecture, and inhabitant living habits. Furthermore, it was observed that Rn and TnP were the major contributors to the inhalation effective dose, accounting for 39.6% and 56.7% of the total, respectively. The estimated excess lifetime cancer risk (ELCR) from the exposition to Rn and TnP was found to be 2.93 × 10⁻³ and 4.36 × 10⁻³, respectively, exceeding the global average, raising health concerns.

1. Introduction

Mining activities often cause major environmental disruption, which has a lasting impact on local ecosystems, with direct consequences on the health of humans, living beings, and the environment [1,2,3,4,5]. Among the factors that are subject to an upheaval following a mining activity is the state of radiological exposure of the public living in the vicinity of the mining sites [6], especially from Naturally Occurring Radioactive Materials (NORM). Radon (Rn) and thoron (Tn) associated with their progeny are the main concern in terms of radiological exposure in humans [7]. Rn and Tn are radioactive gases produced by the disintegration of the isotopes of radium (²²⁶Ra and ²²⁴Ra), respectively, naturally present in soil and rocks. Due to their physical properties and their ability to move easily through soil and construction materials, these gases can accumulate in homes and enclosed spaces (buildings, basements, mines, tunnels) through cracks in foundations and building joints [8,9], posing risks to human health [10]. The World Health Organization (WHO) [11] stated that exposure to Rn, Tn, and their decay products could be the second most common cause of lung cancer after smoking. These discoveries have sparked a renewed interest in radon research, resulting in a large-scale radon mapping program aimed at defining public exposure and assessing the importance of risk due to radon.

As cancerogenic elements, Rn, Tn, and their progeny monitoring in the living environment is important for annual effective dose calculation and the assessment of the risk for developing lung cancer [12,13,14]. Despite the recognized importance of this monitoring, research has primarily focused on radon and its effects, while thoron has received relatively limited attention. Tn has often been neglected due to its low concentration in households and shorter radioactive half-life than Rn. However, in recent decades, interest in Tn has also increased globally [15,16,17,18,19], and in Cameroon, in particular, numerous studies have been undertaken to pinpoint the contribution of Rn and Tn to the total effective dose by inhalation [20,21,22,23,24,25]. In countries where the soil contains a high concentration of ²³²Th, Tn often contributes more to the total effective dose by inhalation than Rn. This is notably the case in Cameroon, in Lolodorf, Fongo-Tongo, and Ngaoundere, as well as in Chad, where Ziebno conducted research [20,26,27,28]. Although Tn has a very short half-life, its progenies have considerably longer half-lives. Therefore, it is essential to consider Tn levels because it is present in all environments with Rn and can sometimes be found in much higher quantities, significantly impacting the total inhalation dose. Additionally, it is important to note that thoron and radon do not originate from the same parent [29,30].

In Cameroon, the Akonolinga region hosts significant rutile deposits, a mineral rich in titanium. Although these deposits are not yet being exploited industrially, the area is currently undergoing an advanced phase of mining exploration, with exploitation scheduled to begin in 2025. This context justifies a preventive scientific approach aimed at establishing baseline data on radiological exposure before the start of extractive activities. These data will be essential for later assessing the potential impact of mining operations on the population’s exposure to Rn, Tn, and their progeny. Furthermore, the region presents geological characteristics favorable to the release of naturally occurring radionuclides, such as radium (²²⁶Ra) and thorium (²³²Th), the respective precursors of radon and thoron. Therefore, the prior implementation of an environmental radiological assessment is necessary to better understand the natural levels of exposure and to anticipate the potential effects of future mining activities.

This study is part of a broader investigation into the environmental radiological status of the study area, including the presence of trace elements [31]. The aim of this study is to examine the exposure of the population to indoor Rn, Tn, and progeny. Therefore, this study analyzes variations in their concentrations based on several potentially influential factors, including the proximity of detectors to the wall, the building materials, seasonal fluctuations in environmental conditions, and the specific geological characteristics of the region. This research also aims to evaluate the lifetime cancer risk (ELCR) associated with chronic exposure to these radioactive gases.

2. Materials and Methods

2.1. Study Area

The rare earth deposit region of interest in this study is located in the city of Akonolinga, in the Center Region of Cameroon. The city is geographically situated at 3°45′ North latitude and 12°15′ East longitude with an altitude of 643 m (Figure 1). It covers an area of 1420.9 km² and has a population of 80,361 people.

Geologically, the Akonolinga region belongs to the Yaoundé Group, a major Proterozoic metamorphic unit mainly composed of high-grade metamorphic rocks such as gneisses, schists, and quartzites [32]. These metamorphic formations often contain titaniferous minerals such as rutile (TiO₂), an important indicator of mineralization in the region. Although rutile itself is not radioactive, the host rocks frequently contain accessory minerals rich in radium (²²⁶Ra) and thorium (²³²Th), which are the parent elements of Rn and Tn [33,34].

Above these metamorphic rocks lie superficial formations mainly composed of lateritic soils, clayey sands, and weathered materials. These superficial layers vary in thickness from approximately 1 to 5 m, depending on local topography and erosion processes [35]. These formations can accumulate radionuclides through weathering, particularly in poorly drained or clay-rich zones, which promote the retention of ²²⁶Ra and ²³²Th [36]. Studies conducted in similar geological contexts have shown that these surface deposits can contain significant concentrations of ²³²Th and ²²⁶Ra, thus contributing to radon and thoron exhalation at the surface and into dwellings [37].

The region is also intersected by several minor faults and fractures inherited from ancient tectonic episodes. These discontinuities may facilitate the upward migration of radon and thoron gases from depth to the surface by increasing rock permeability and serving as preferential migration pathways. Although detailed mapping of these faults remains limited, preliminary geophysical observations and remote sensing data indicate NE-SW and NW-SE structural trends associated with the Pan-African orogeny [38].

Indoor Rn and Tn measurement points were selected among houses constructed either with adobe bricks or with cement and sand blocks. These building materials can contribute to indoor concentrations of natural radionuclides, depending on their geological origin. Several of these houses are located in quarters near the study area.

Figure 1. Map of the study area locating the sample points [39].

Figure 1. Map of the study area locating the sample points [39].

2.2. Measurement of Radon in Soil Gas

The radon in the soil gas was measured using the namely Markus 10 detector (Radonova, Uppsala, Sweden). The is made from ORTEC Ultra silicone, with an energy resolution greater than 16 keV, with a flow rate capacity during sampling the soil gas of 1.8 L min⁻¹ and an active surface area of 100 mm². The battery capacity allows for approximately 70 measurements. The detector was calibrated at the ISO 17025-certified RADONOVA laboratory in Sweden [40].

The probe was inserted into the soil using a rotating handle or gently immersed with 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 of rocks. This depth was selected based on its relevance for estimating soil radon exhalation rates, which are closely correlated with radon concentrations at this level. Although radon concentrations in soil gas are known to be highly variable depending on depth and environmental factors, several studies conducted in Israel, China, and Russia [41,42,43] have shown that the gradient of radon concentration is usually highest up to a depth of 1 m and then tends to decline. Long-term measurements in boreholes have also demonstrated strong variability with meteorological conditions and soil granulometry. Despite this variability, the 50 cm depth provides a robust compromise between accessibility and representativity, making it particularly suitable for estimating radon exhalation, in line with the epidemiological aims of the study.

The measurement process is as follows: During the initial measurement phase, air from the soil is drawn through a sampling tube into the measurement cell for approximately 30 s, ensuring that all fresh air is integrated into the system. A pressure sensor stops the pump if the pressure drops too low, restarting it when the pressure rises. This process ensures that a minimum volume of air is always delivered before starting the measurement. If pumping exceeds 2 to 3 min, the soil is likely too compact, and the measurement should be resumed elsewhere [44,45].

Subsequently, the measurement phase begins. The detector is activated, and voltage is applied to the measurement chamber. Radon progeny-charged descendants are directed toward the detector by an electric field, which then captures the emitted alpha radiation. Pulses are amplified and filtered by a single-channel analyzer that only allows those from ²¹⁸Po pass, thus eliminating slow variations due to ²¹⁴Po [44,45]. Pulses are counted, and the result, indicating radon activity in kBq m⁻³, is displayed on the device screen. The display flashes during measurement and stabilizes at the end. A new measurement can be started after 18 min, allowing sufficient decay of previous activity. A schematic diagram of the sampling of soil gas using Markus 10 is illustrated in Figure 2.

2.3. Gamma Spectrometry of Soil Samples

Twenty-five soil samples were collected from the study area for the analysis of primordial radionuclides ²²⁶Ra and ²³²Th, using gamma spectrometry with a High-Purity Germanium (HPGe) detector (GEM40190, AMETEK ORTEC, Oak Ridge, TN, USA). The samples were dried at 110 °C for 24 h, ground to remove organic matter, and then sieved to obtain a homogeneous particle size. Approximately 100 g of each sample were placed in hermetically sealed U-8 containers (PURATUBO 3–20 type 100 cm³ U8, AS ONE, Osaka, Japon) and stored for a minimum of 30 days to allow secular equilibrium to be established between the radionuclides and their gamma-emitting progeny. A similar methodology was applied by Nguelem et al. [47] in the evaluation of bauxite deposits in western Cameroon, as well as by Ngachin et al. [48] during the study of radioactivity levels and radon exhalation in a volcanic area of the country.

The HPGe characteristics are as follows: GEM40190 (AMETEK ORTEC, Oak Ridge, TN, USA), which has a relative efficiency of 30% and an energy resolution of 1.85 keV Full Width at Half Maximum (FWHM) at 1.33 MeV for ⁶⁰Co. The detector was housed in a 10 cm thick lead shielding, covered with 5 mm copper and 5 mm plexiglass layers to maintain a low background noise. For gamma spectral analysis, the detector was coupled with a multichannel analyzer (MCA-7, Seiko EG & G, Tokyo, Japan). The energy and efficiency calibrations were performed using a set of gamma calibration sources containing various radionuclides ¹⁰⁹Cd, ⁵⁷Co, ⁶⁰Co,¹³⁹Cs, ¹³⁷Cs, ⁵¹Cr, ⁸⁵Sr, ⁵⁴Mn, and ⁸⁸Y, provided by the Japanese Radioisotope Association. These sources emit gamma rays with energies ranging from 88 to 1836 keV, with an overall uncertainty of less than 10%. Each soil sample was measured for 86,400 s to optimize counting precision.

The activity concentration of ²²⁶Ra was calculated by summing the counts in the photoelectric peak channels corresponding to the gamma-ray energies of 609.3 keV from ²¹⁴Bi and 352 keV from ²¹⁴Pb, while for ²³²Th, the counts were taken in the photoelectric peak channel at the gamma-ray energy of 911.2 keV from ²²⁸Ac [49,50].

2.4. Measurement of Indoor Rn, Tn, and Progeny Concentrations

Integrated passive type Rn-Tn discrimination detectors (RADUET, Radosys Ltd., Budapest, Hungary) developed at the National Institute of Radiological Sciences (NIRS) in Japan have been used to measure indoor Rn and Tn concentrations in the present study. For Tn progeny (TnP) measurements, Thoron progeny monitors were used and installed in 50 homes together with RADUET. This type of measurement, which allows for accurate distinction between radon and thoron isotopes, is essential for reliable evaluation of internal exposure, as demonstrated by Sanada [51] in his work. Each detector was suspended at a height of 1 to 2 m above the ground and placed at varying distances of a minimum of 20 cm to 50 cm from the walls of the living room or bedroom, depending on where the residents spent most of their time at home. The RADUET detectors were calibrated using the IREM calibration system [52].

Detectors were deployed in homes over a period of 110 days, from December 2021 to March 2022. During this period, the detectors were left under the supervision of the residents. The reliability of the measures has been optimized by the cooperation of the inhabitants based on common sense. However, it is important to emphasize that the activation of the detector by a resident in the absence of the practitioner, for example, by exposing the sensor to a source of heat or moisture, can potentially influence the measurement results.

The detectors were collected after 110 days of exposure and transferred for analysis at the University of Hirosaki in Japan. Additional details on the analysis of CR-39 are provided in previous works [20,22]. The minimum detection limits (MDLs) for RADUET were 3 Bq m⁻³ for Rn and 4 Bq m⁻³ for Tn. Furthermore, the track density analysis on CR-39 detectors was carried out using standardized etching protocols and image analysis systems to ensure reliable differentiation between alpha tracks from Rn and Tn sources [53,54]. Using the alpha track densities of low and high diffusion chambers of the detector, the average activity concentrations of Rn and Tn can be obtained using Equations (1) and (2) [55].

$$\underset{\_}{C_{Rn}}=\left(d_L-\underset{\_}{b}\right){\displaystyle \frac{f_{Tn2}}{t.\left(f_{Rn1}{.f}_{Tn2}-f_{Rn2}{f}_{Tn1}\right)}}-\left(d_H-\underset{\_}{b}\right){\displaystyle \frac{f_{Tn1}}{t.\left(f_{Rn1}{.f}_{Tn2}-f_{Rn2}{f}_{Tn1}\right)}}$$

(1)

$$\underset{\_}{C_{Tn}}=\left(d_H-\underset{\_}{b}\right){\displaystyle \frac{f_{Rn1}}{t.\left(f_{Rn1}{.f}_{Tn2}-f_{Rn2}{f}_{Tn1}\right)}}-\left(d_L-\underset{\_}{b}\right){\displaystyle \frac{f_{Rn2}}{t.\left(f_{Rn1}{.f}_{Tn2}-f_{Rn2}{f}_{Tn1}\right)}}$$

(2)

where $d_L$ and $d_H$ are the alpha tracks densities for the low and high air rate chamber, respectively, in traces per square centimeter (track cm⁻²); b is the track density due to background noise in (tracket cm⁻²); t is the sampling time (h); $f_{Rn1}$ and $f_{Tn1}$ are the calibration factors for Rn and Tn in a low air exchange rate chamber, respectively, in (Track cm⁻² h⁻¹); and $f_{Rn2}$ and $f_{Tn2}$ are the establishment coefficients for Rn and Tn in a high-rate chamber (track cm⁻² h⁻¹).

The equilibrium factor is a parameter used to describe the relationship between radon (Rn), thoron (Tn), and their respective progeny (RnP and TnP). It corresponds to the ratio of the progeny concentration (EETC/EERC) to the parent gas concentration (C_Tn/C_Rn) under equilibrium conditions. UNSCEAR reported global average values for the Rn/RnP and Tn/TnP equilibrium factors are 0.4 and 0.02, respectively [56]. However, the factor is strongly influenced by environmental conditions such as climate, ventilation, geology, and building materials. With the direct and simultaneous measurements of Rn, Tn, and TnP concentrations, the equilibrium factor of Tn is determined in situ in this study by using Equation (3). However, the concentrations of Rn progeny were not measured directly and were calculated by using Equation (4).

$$F_{Tn}={\displaystyle \frac{EETC}{C_{Tn}}}$$

(3)

$$F_{Rn}={\displaystyle \frac{EERC}{C_{Rn}}}$$

(4)

2.5. Total Annual Inhalation Effective Dose

The total annual inhalation effective dose due to Rn, Tn, RnP, and TnP is estimated using Equations (5), (6), (7) and (8), respectively [37].

$$E_{Rn}\left(mSv\right)=C_{Rn}\times 0.17\times 8760\times {10}^{-6}$$

(5)

$$E_{Tn}\left(mSv\right)=C_{Tn}\times 0.11\times 8760\times {10}^{-6}$$

(6)

$$E_{RnP}\left(mSv\right)=C_{Rn}\times 0.4\times 9\times 8760\times {10}^{-6}$$

(7)

$$E_{TnP}\left(mSv\right)=EETC{\times 40\times 8760\times 10}^{-6}$$

(8)

where $E_{Rn}$, $E_{Tn}$, $E_{RnP}$ and $E_{TnP}$, represent the total inhalation effective dose due to Rn, Tn, and their progeny, respectively. The dose conversion factors for Rn and Tn are 0.17 and 0.11 nSv Bq⁻¹ m⁻³ h⁻¹, 9 and 40, respectively, representing the dose converter for the offspring of radon or thoron, and 0.6 represents indoor occupation; 8760 (24 h × 365 d) is a period of the year expressed in hours. The time people spend inside their homes is estimated at 60% of the day [23].

2.6. Excess Lifetime Cancer Risk

The excess lifetime cancer risk (ELCR) is the probability of developing cancer during a life with a given exposure. The ELCR due to Rn, Tn, their progenies, and gamma exposure rates were calculated using Equation (9) [37].

ELCR = AED × DL × RF

(9)

where DL is the typical lifetime (70 years), RF is the risk factor (Sv⁻¹) or the deadly cancer risk per sievert, and AED is the annual effective dose. ICRP, in publication 103, recommended a value of 0.057 for public exposure to the stochastic effects of low-dose background radiation [57]. Statistical analyses and graphical representations were conducted using Origin Pro software (V.9.9.0.225 (SR1) × 64, OriginLab corporation, Northampton, MA, USA).

3. Results and Discussion

3.1. Indoor Rn, Tn, and Progeny Concentrations and the Equilibrium Factor

The concentrations of indoor Rn, Tn, and their progeny are presented in

Table 1. Radon (Rn), thoron (Tn), radon progeny (EERC), and thoron progeny (EETC) concentrations in homes.

Statistical Parameters	CRn (Bq m−3)	EERC (Bq m−3)	CTn (Bq m−3)	EETC (Bq m−3)	FTn
Min.	22.5 ± 0.4	9.0	8.2 ± 0.6	0.2 ± 0.4	0.01
Max.	65.7 ± 2.1	26.3	209.9 ± 4.4	23.5 ± 0.7	0.26
Median	38.3	15.3	55.1	4.2	0.07
Mean	39.5 ± 9	15.8	68.1 ± 2.9	5.0 ± 0.5	0.08
S.D.	10.7	4.8	50.7	4.6	0.06

Min.: Minimum; Max.: Maximum; S.D.: Standard Deviation.

.

Indoor Rn concentration in the study area varies from 22.5 to 65.7 Bq m⁻³, with an average of 39.5 Bq m⁻³. Figure 3a illustrates the normal distribution of indoor Rn concentration, showing that 42% of houses have values above the world average of 40 Bq m⁻³ given by UNSCEAR [56]. The present results show that there were no dwellings exceeding the WHO reference level for the action plan set at 100 Bq m⁻³ [11] and far lesser than the WHO and the International Commission on Radiological Protection (ICRP) reference level of 300 Bq m⁻³ [58]. The average value of indoor Rn for the present study is less than 59 Bq m⁻³ found in the city of Yaoundé and higher than 37 Bq m⁻³, 30 Bq m⁻³, 25 Bq m⁻³, and 37 Bqm⁻³, which represent the averages value for Okola, Obala, Monatele, and Mbalmayo cities, respectively, located in the same Center Region [59]. Measured levels of Rn appear to be within acceptable limits according to health standards, but continuous monitoring is crucial, particularly during the incoming mining activities.

Indoor Tn concentration ranges from 8.2 to 209.9 Bq m⁻³, with an average of 68.1 Bq m⁻³. This wide range highlights the significant fluctuation in the concentration of Tn depending on the house types in the region. It is important to note that the indoor concentration of Tn is influenced mainly by the building materials type. Figure 3b shows that a small percentage of the houses surveyed have a concentration of Tn below the global average of 10 Bq m⁻³ [56]. However, the average concentration of Tn obtained in the village of Akongo 53 Bq m⁻³ is lower than the value found in the present study but below the average values obtained in Bikoue, Ngombas, Awanda, and Nkouloungui, respectively, 182, 142, 76 and 153 Bq m⁻³ in the South Region of Cameroon [37].

The indoor concentrations of Rn and Tn measured in the study area can largely be explained by the geological characteristics of the Akonolinga region. The area lies on the Yaoundé Group, a Proterozoic metamorphic unit composed mainly of gneisses, schists, and quartzites [38]. These rocks are known to contain accessory minerals enriched in natural radionuclides such as ²²⁶Ra and ²³²Th, which are the parent elements of Rn and Tn, respectively. Overlying this bedrock are weathered surface formations, primarily laterites and clayey sands that are friable and more permeable. These superficial layers can accumulate radionuclides through chemical weathering and are especially prone to retaining them in poorly drained, clay-rich zones. These shallow formations tend to favor Tn exhalation due to its short half-life and limited diffusion distance, while Rn, which is more stable, may migrate from deeper layers. Additionally, the region is intersected by ancient faults and fractures inherited from the Pan-African orogeny. These structural discontinuities may enhance rock permeability and serve as preferential pathways for the advective transfer of radon and thoron gases from deeper sources to the surface. In summary, the geogenic origin of Rn and Tn in the area is influenced by both the radionuclide-rich nature of the underlying rocks and soils and by geological structures that facilitate their migration, either by diffusion or advection.

The EERC and EETC shown in Table 1 were determined by indirect and direct methods, respectively. In fact, Tn progeny monitors were deployed in houses together with RADUETs to measure the concentrations of Tn progeny (EETC). However, the Rn progeny concentration (EERC) calculated using the 0.4 equilibrium factor ranges from 2.32 to 32.83 Bq m⁻³ with an average of 15.97 Bq m⁻³. The measured EETC ranges from 0.25 to 23.5 Bq m⁻³ with an average of 5 Bq m⁻³. Figure 3c,d indicate that EERC and EETC follow normal distribution.

Additionally, F_Tn was calculated using concentrations of Tn and progeny measured simultaneously in homes at the same time. F_Tn ranges from 0.01 to 0.26, with an average of 0.08. The average F_Tn determined is 4 times higher than that of UNSCEAR, which is 0.02. It is important to note that 95% of houses have an F_Tn greater than 0.02. F_Rn is less than 0.4 in 92% of households in the present study. Studies have shown that the equilibrium factor varies depending on several parameters: house ventilation, season, house type, soil geology, and even lifestyle [20]. This demonstrates that the use of the equilibrium factor would lead to great uncertainty in the assessment of the inhalation effective dose, as the equilibrium factor of a home depends on its own environmental conditions. Table 1 summarizes the equilibrium factor of Tn.

3.2. Influence of Wall Distance

The results presented in

Table 2. Thoron concentration as a function of distance from the wall.

Distance Range from the Wall (cm)	Numbers of Houses	Tn Concentration Range (Bq m−3)	Average Tn Concentration (Bq m−3)
10–20	12	80–220	149
20–40	15	40–80	63
40–70	12	20–40	31
70–80	5	≤15	10

show that Tn concentration decreases sharply with increasing distance from the wall. Detectors placed approximately 20 cm from the wall recorded the highest concentrations (80–220 Bq m⁻³), confirming that, due to its short half-life (~55.6 s), Tn primarily accumulates near building materials containing thorium. Between 20 and 40 cm, the concentration drops significantly (40–80 Bq m⁻³), illustrating the effects of diffusion and ventilation. This trend continues between 40 and 70 cm (20–40 Bq m⁻³), where indoor air circulation further reduces thoron presence. Beyond 70 cm, concentrations become nearly negligible (<15 Bq m⁻³), highlighting the combined effect of distance and ventilation on gas dispersion.

This strong decline underscores the influence of environmental factors such as air circulation, humidity, and ventilation, which contribute to Tn dispersion but remain insufficient to homogenize its distribution in indoor spaces. As a result, exposure to Tn varies significantly depending on detector placement. Indeed, individuals spending prolonged periods near walls while sleeping, sitting, or working against them are likely exposed to higher Tn concentrations than those occupying central areas of a room. These observations emphasize the importance of detector positioning for accurately assessing indoor Tn exposure.

These results are consistent with previous studies [20,60]. The spatial distribution of Tn concentrations has been shown to exhibit significant heterogeneity, with steep concentration gradients near emission sources such as walls and floors, due to the short half-life and limited diffusion range of the gas [61]. Similarly, Frutos-Puerto et al. demonstrated that certain building materials used in the Iberian Peninsula, due to their high ²³²Th content, exhibit elevated Tn exhalation rates, which contributes to the accumulation of the gas near indoor surfaces such as walls and floors [62]. However, this study expands on prior research by integrating an analysis of additional environmental parameters, such as ventilation efficiency and humidity levels, which may influence the spatial distribution of Tn and its persistence in indoor environments. A more comprehensive understanding of these factors is essential for improving exposure assessment models and implementing effective indoor air quality management strategies.

3.3. Influence of Building Materials

As shown in

Table 3. Rn and Tn concentrations, EETC, and equilibrium factors of Tn (F_Tn) in the two types of houses.

Building Material	Statistical Parameter	CRn (Bq m−3)	CTn (Bq m−3)	EETC (Bq m−3)	FTn
Cement	Min	22.5	8.4	0.3	0.003
	Max	65.7	73.4	10.3	0.14
	AM ± SD	39.9 ± 12.9	36.1 ± 19.7	2.8 ± 2	0.08 ± 0.05
	GM	38	30.3	1.6	0.05
Earthen	Min	24.2	8.2	0.5	0.06
	Max	56.5	209.9	23.5	0.11
	AM ± SD	39.2 ± 8.6	90.8 ± 54	6.6 ± 4.9	0.07 ± 0.05
	GM	38.2	73	4.8	0.07

AM: arithmetic mean; GM: geometric mean.

, Rn concentration in dwellings with earthen bricks ranges from 24.2 to 56.5 Bq m⁻³, with an arithmetic average of 39.2 Bq m⁻³ and a geometric mean of 38.2 Bq m⁻³. In contrast, the concentration of Tn varies from 8.2 to 209.9 Bq m⁻³, with arithmetic and geometric averages of 90.8 Bq m⁻³ and 73 Bq m⁻³, respectively. However, in dwellings made of blocks, cement, and sand (cement houses), the thoron concentration ranges from 8.4 to 73.4 Bq m⁻³, with arithmetic and geometric averages of 36.1 Bq m⁻³ and 30.3 Bq m⁻³, respectively. As for TnP, their concentrations range from 0.3 to 10.3 Bq m⁻³ in cement houses and from 0.5 to 23.5 Bq m⁻³ in earthen houses.

It is clear from the results that the concentration of Rn in earthen houses is slightly higher than in cement houses (Figure 4). This result can be explained by the fact that the building material of earthen houses contains ²²⁶Ra as opposed to cement houses that contain less than ²²⁶Ra [63,64]. On the other hand, the average concentration of Tn is 2.5 times higher in earthen houses than in cement houses.

People of this locality are absent for most of the day due to their activities (agriculture, bakery, small business for parents, and school for children), so the heat is kept in the house and contributes to the accumulation of concentrations of Rn and Tn. The openings could have provided potential paths for Rn and Tn exhalations from the basement to spread more easily into the open air. Furthermore, during the exposure time of the detectors, the climate was cold, and thus, the amount of water in the air was high. As Rn, Tn, and their progenies are moderately soluble in water, they remain trapped by water vapor and are more difficult to remove from the indoor environment than free gases.

3.4. Analysis of Correlation Between Rn in Soil Gas and Indoor Rn Concentration, ²²⁶Ra-Rn, and ²³²Th-Tn

The measurement of Rn in soil was carried out in the immediate vicinity, where indoor Rn was also measured. The Rn concentrations in soil gas range from 5.4 to 75.5 kBq m⁻³, with an average of 24.3 kBq m⁻³. One of the main sources of Rn in homes is its exhalation from the soil, and it enters homes through cracks in foundations, pipes, and other openings. The correlation between soil gas and indoor Rn concentrations is shown in Figure 5, with a correlation coefficient of r = 0.51, indicating a moderate correlation between these two parameters.

Interestingly, at certain measurement points, when the Rn concentration in the soil was high, the indoor Rn concentration was also high, demonstrating that the increase in indoor Rn largely depends on soil Rn. However, at other points, high soil Rn concentration did not correspond to high indoor Rn levels, likely due to factors such as house ventilation conditions, construction materials, or building tightness. These parameters can strongly influence indoor Rn accumulation.

Several studies indicate varying values regarding the correlation between soil and indoor Rn concentrations: 0.32 in Lomié [65], 0.54 in India [66], and 0.4 in China [67]. These results indicate that there is no universal correlation between the two concentrations. While indoor Rn concentration does depend on soil Rn, it is also influenced by factors such as climate, soil characteristics, atmospheric pressure, building materials, ventilation, and occupants’ behavior. For instance, cold weather conditions can reduce natural ventilation in homes, leading to Rn accumulation, while clayey or water-saturated soils can limit Rn migration from the ground to the surface.

Analysis of soil samples taken from the immediate neighborhood of the dwelling revealed that the content of ²²⁶Ra ranged from 5 to 56 Bq kg⁻¹, with an average of 17 Bq kg⁻¹. The content of ²³²Th varied from 9 to 52 Bq kg⁻¹, with an average of 27 Bq kg⁻¹. Radon (Rn) and thoron (Tn) gases are, respectively, the descendants of the radionuclides ²²⁶Ra and ²³²Th and constitute an important source of these gases in homes. Correlations have been established between primordial radionuclides and gases, as illustrated in Figure 6 The correlation coefficient between ²²⁶Ra and indoor Rn is 0.71, while the correlation between ²³²Th and indoor Tn is 0.88.

The strong correlation between ²³²Th and indoor Tn (r = 0.88) demonstrates a close linear relationship between these two variables. In fact, Tn is a direct descendant of ²³²Th, and this correlation indicates that the thoron present in homes greatly depends on the content of ²³²Th in the surrounding soil. This result suggests that the local geology, rich in ²³²Th, plays a key role in the presence of thoron in homes, particularly in environments where thorium is naturally abundant in the top layers of soil.

Similarly, the correlation between indoor Rn concentration and soil ²²⁶Ra content is considerable (r = 0.85). This indicates a significant relationship between the presence of ²²⁶Ra in the soil and the radon levels inside homes. This result shows that, in areas with high concentrations of ²²⁶Ra, one can expect to find high radon concentrations as well, although other factors, such as the structure of the house and its ventilation systems, may also influence this relationship.

Moreover, this correlation could be reinforced by the geological characteristics of the terrain and the nature of locally sourced construction materials, as highlighted by Michael et al., who demonstrated that the radioactivity levels in building materials vary significantly depending on the geological origin of the raw materials used [68]. In Akonolinga, many houses are built with earth bricks that naturally contain ²²⁶Ra, thus contributing to the continuous release of radon inside homes. This factor, combined with cracks in foundations or poor ventilation, may exacerbate radon concentration in indoor air. On the other hand, the strong correlations between ²²⁶Ra, ²³²Th concentrations with those of radon and thoron, respectively, confirm the geogenic origin of the studied gases.

3.5. Evaluation of Total Inhalation Effective Dose Due to Rn, Tn, RnP and TnP

The total effective dose due to the inhalation of indoor Rn, Tn, RnP and TnP was evaluated, and the summary of the results is presented in

Table 4. Total annual effective dose by inhalation due to Rn, Tn, RnP, and TnP using direct and indirect methods.

Statistical Parameter	ERn (mSv)	ERnP (mSv)	ETn (mSv)	ETnP (mSv)	ET (mSv)
Min	0.03	0.43	0.01	0.09	0.57
Max	0.06	1.24	0.12	4.94	6.03
Average	0.03	0.69	0.04	1.09	1.89

.

For the entire study area, the average contributions of Rn and Tn to the total inhalation effective dose were 0.03 mSv (1,6%) and 0.04 mSv (2.1%), respectively. For RnP and TnP, the contributions are 0.69 mSv (39.6%) and 1.09 mSv (56.7%), respectively (Figure 7).

The contribution of Rn and RnP is, therefore, 0.72 mSv, while that of Tn and TnP was 1.13 mSv. These results show that the contributions of Rn and Tn are negligible to those of their progenies. In fact, once inhaled, radon and thoron gases are almost entirely exhaled due to their inert chemical nature and short residence time in the lungs. However, their progenies’ solid radioactive decay products, such as ²¹⁴Pb (for Rn), ²¹²Bi, and ²¹²Pb (for Tn), tend to attach to airborne particles or aerosols. When inhaled, these particles can deposit along the respiratory tract, particularly in the bronchial and alveolar regions, where they irradiate sensitive tissues through alpha decay. This deposition is primarily responsible for the internal effective dose received by individuals, highlighting the critical role of progeny in the health risk associated with radon and thoron exposure. They can thus cause irritation of bronchial and pulmonary tissue cells capable of inducing cancer. Thus, the health risk from radon and thoron is not related to the gas itself but to its progenies [69]. In this case, the total effective dose ranges from 0.57 to 6.03 mSv with an average of 1.89 mSv. This average is 1.47 times higher than the 1.26 mSv global average of total inhaled effective dose.

The results obtained allow for the characterization of the current radiological situation in the study area through the assessment of the effective inhalation dose due to Rn, Tn, RnP, and TnP within a context of undisturbed natural radioactivity. This snapshot of the baseline state provides an essential reference for any future monitoring efforts. However, this situation is likely to change significantly with the development of the rare earth deposit planned in Akonolinga. Indeed, numerous studies conducted in regions with intense mining activity, particularly in Central Asia, Southern Africa, and North America, have demonstrated that extraction operations, whether open-pit or underground, result in a significant increase in environmental radon concentrations. For instance, a recent study conducted in a uranium mining area in Kazakhstan revealed radon levels not only within the extraction zones but also inside nearby dwellings, illustrating the possible transfer of radioactive gas into living environments [70]. In South Africa’s West Rand region, which is dominated by gold mine tailings, radon concentrations measured around abandoned tailings dams were found to be twice as high as those in control areas, thus exposing local populations to increased health risks [71]. These increases are generally attributed to several mining-related factors: direct exposure to uranium-rich materials, mechanical rock fragmentation, and changes in soil physical properties that enhance the exhalation of radioactive gases. Moreover, recent research has introduced new on-site evaluation methods capable of efficiently detecting radon fluxes in mining environments, highlighting the need for tailored monitoring in these high-risk contexts [6]. In this context, an increase in inhalation exposure dose is reasonably anticipated for nearby populations and workers at the Akonolinga site. Therefore, the implementation of a rigorous radiological monitoring system, including regular measurements of radon and thoron in ambient air, becomes a top priority starting from the advanced exploration phases in order to assess and manage medium- and long-term health risks.

3.6. Correlations Between Radon, Thoron, EETC, and Annual Inhalation Effective Dose

The analysis of correlations between Rn, Tn, EETC concentrations, and the annual inhalation effective dose (E_T) highlights complex and crucial relationships for assessing radiological risk. The weak correlation between Rn and Tn (r = 0.18) demonstrates that these two gases, although emitted from the same natural sources, do not necessarily evolve synchronously in the studied environment, likely due to differences in their transport and dispersion mechanisms. In contrast, the moderate to strong correlation between Tn and EETC (r = 0.56) suggests a more significant influence of Tn on exposure parameters, reinforcing the idea that this often-underestimated gas could play a substantial role in the inhalation effective dose received by exposed populations.

The most striking element of this analysis lies in the extremely strong relationship between EETC and annual inhalation effective dose (r = 0.91), revealing a direct and undeniable link between EETC and the increase in radiological risk. This high correlation underscores the importance of EETC as a reliable indicator of total exposure and, by extension, the associated health risk. Moreover, the distribution of values highlights distinct trends, with greater variability observed in thoron concentrations, possibly due to its short half-life and its strong dependence on indoor ventilation conditions. Figure 7 displays the correlation graphs between the concentrations of Rn, Tn, and TnP and the total inhalation effective dose. They also emphasize the need for an in-depth study of the factors influencing the weaker relationship between Rn and EETC, particularly through better characterization of building materials, atmospheric conditions, and gas accumulation mechanisms. Ultimately, this analysis highlights the urgency of adopting a more comprehensive approach to radiological risk assessment, fully integrating thoron’s contribution into exposure dose models to refine prevention and protection strategies for affected populations.

3.7. Assessment of Excess Lifetime Cancer Risk (ELCR)

The excess lifetime cancer risk (ELCR) is an estimate of the additional risk of developing cancer over a lifetime due to prolonged exposure to a carcinogenic agent, such as ionizing radiation. According to the methodology of the United States Environmental Protection Agency (U.S. EPA), ELCR values corresponding to exposures of 1, 10, 100, and 1000 mSv are estimated at 0.004%, 0.04%, 0.4%, and 4%, respectively [72]. These values help to assess the additional risk of developing fatal cancer due to prolonged exposure to increased levels of radiation. The internal and external ELCR values resulting from the gamma-absorbed dose rate range from 0.02 × 10⁻⁴ to 0.05 × 10⁻⁴ for internal ELCR, with an average of 0.03 × 10⁻⁴ and from 0.01 × 10⁻⁴ to 0.50 × 10⁻⁴ for external ELCR. These results indicate that exposure to gamma radioactivity in the study region is significantly lower than the global average of 2.90 × 10⁻⁴, as well as the values reported in countries such as India (1.70 × 10⁻³) and Turkey (0.50 × 10⁻³) [73,74,75].

However, these values align with those reported by Serge et al. [37] in a study conducted in southern Cameroon, suggesting a consistent trend in natural radioactivity levels across these regions. The estimated ELCR values for Rn, Tn, RnP, and TnP range from 0.06 × 10⁻⁴ to 1.50 × 10 ⁻⁴ with an average of 0.48 × 10⁻⁴, 0.20 × 10⁻⁴ to 4.80 × 10⁻⁴ and an average value of 1.60 × 10⁻⁴, 4.40 × 10⁻⁴ to 62.0 × 10⁻³ with an average of 29.3 × 10⁻⁴ and 3.60 × 10⁻⁴ to 197 × 10⁻⁴ with a mean of 43.6 × 10⁻⁴, respectively. ELCR associated with RnP and TnP significantly exceeds the global average, highlighting increased exposure to highly radiotoxic alpha-emitting particles. In contrast, ELCR values for radon alone remain below the global average, while 12% of dwellings in the study area show ELCR values due to thoron above the global benchmark [75]. The results are presented in

Table 5. Excess lifetime cancer risk (ELCR) due to indoor radon, thoron, radon, and thoron progeny.

	ELCR (×10−4)
	Rn	Tn	RnP	TnP
Min	0.06	0.20	4.40	3.60
Max	0.15	4.80	62.0	197
Mean	0.48	1.60	29.3	43.6

.

The study highlights significant potential carcinogenic effects for the population in the study area. However, the estimated cancer risk does not necessarily reflect the actual individual risk, as it depends on various factors, including genetic predisposition, lifestyle, and exposure to other carcinogens. Epidemiological and experimental studies confirm that prolonged exposure to Rn and Tn increases the risk of lung cancer. According to the French National Cancer Institute, living in a home with Rn concentrations between 200 and 400 Bq m⁻³ presents a risk comparable to that of a non-smoker exposed to passive smoke. In France, 5% to 12% of lung cancer deaths are attributed to indoor Rn exposure [76].

In the study area, tobacco and alcohol consumption further aggravate this risk. Tobacco smoke interacts with radioactive particles in the air, facilitating their deposition in the respiratory tract and significantly increasing the likelihood of lung cancer. Studies indicate that lung cancer risk rises by approximately 16% for every 100 Bq m⁻³ increase in Rn concentration [76,77].

Although Rn is an inert gas, its real danger comes from its solid progeny, particularly ²¹⁴Pb and ²¹⁸Po, which emit highly radiotoxic alpha particles when inhaled. Exposure levels vary depending on factors such as building materials, home ventilation, residents’ lifestyles, and the geological characteristics of the soil.

The study reveals substantial contamination by Rn, Tn, and their progeny in the Akonolinga region, particularly concerning ELCR values for Rn and Tn progeny. The prevalence of smoking and other risk factors further exacerbates the health threat, increasing the likelihood of lung disease, including cancer.

Given these findings, rigorous and continuous monitoring of radon and thoron levels in homes is essential to reduce exposure and prevent a rise in radiation-induced cancers. Preventive measures tailored to the region’s environmental and social context are crucial to effectively mitigating risks and protecting public health in the long term.

4. Conclusions

This study assessed the indoor concentrations of radon (Rn), thoron (Tn), and their progeny (RnP, TnP), highlighting the radiological risks associated with indoor exposure to radon (Rn), thoron (Tn), and their progeny in the rutile-bearing area of Akonolinga, Cameroon. The findings indicate a strong dependence of indoor Rn, Tn, and their progeny with the influencing factors such as Rn in soil gas concentration, ²²⁶Ra and ²³²Th concentrations in soil, building materials, and inhabitants’ lifestyles. Also, the results further underscore the dominant contribution of RnP and TnP to the total effective inhalation dose, at 39.6% and 56.7%, respectively, compared to the lower contributions of Rn (1.6%) and Tn (2.1%) themselves. The estimated excess lifetime cancer risk (ELCR) values for RnP (2.93 × 10⁻³) and TnP (4.36 × 10⁻³) significantly exceed the global average, raising concerns about the health risks associated with prolonged exposure in the study area. Thus, risk assessment should be expanded to include other potential exposure pathways, particularly water and food. Groundwater and locally grown crops, influenced by radon- and thoron-rich soils, could constitute additional sources of exposure, necessitating further studies for a more comprehensive estimation of health risks.