Assessment of Radiological Risks due to Indoor Radon, Thoron and Progeny, and Soil Gas Radon in Thorium-Bearing Areas of the Centre and South Regions of Cameroon

: Indoor radon, thoron and thoron progeny concentrations, along with the equilibrium factor for thoron progeny and soil gas radon concentrations, have been measured to assess radiological risks in the centre and south regions of Cameroon. Indoor radon and thoron concentrations were estimated using radon–thoron discriminative detectors (RADUET), while thoron progeny monitors measured the equilibrium equivalent thoron concentration (EETC). Radon concentrations in the soil were determined using a MARKUS 10 detector. It was found that radon, thoron and thoron progeny concentrations range between 19 and 62 Bq m − 3 , 10 and 394 Bq m − 3 and 0.05 and 21.8 Bq m − 3 , with geometric means of 32 Bq m − 3 , 98 Bq m − 3 and 4.9 Bq m − 3 , respectively. The thoron equilibrium factor ranges between 0.007 and 0.24, with an arithmetic mean of 0.06 ± 0.03; this is higher than the world average value of 0.02 provided by the United Nations Scientiﬁc Commission on the Effects of Atomic Radiation(UNSCEAR, New York, USA). The level of the soil radon concentration ranges from 4.8 to 57.3 kBq m − 3 , with a geometric mean of 12.1 kBq m − 3 at a depth of 0.7 m. Of the sampling points, 66% fall within normal radon risk areas, and 3% of the sampling areas are high radon risk areas exceeding 50 kBq m − 3 . The annual effective dose was found to be 0.03 ± 0.01 mSv for radon, 0.08 ± 0.05 mSv for thoron, 0.63 ± 0.12 mSv for radon progeny and 1.40 ± 0.84 mSv for thoron progeny. The total dose is estimated to be 2.14 mSv y − 1 . The mean estimated indoor excess lifetime cancer risk values due to radon, thoron, radon progeny and thoron progeny are 0.12 × 10 − 3 , 0.31 × 10 − 3 , 2.51 × 10 − 3 and 5.58 × 10 − 3 , respectively. Thoron progeny contributed 60% to the effective dose. Thus, thoron progeny cannot be neglected in dose assessments, in order to avoid biased results in radio-epidemiological studies.


Introduction
Natural and man-made radionuclides can enter the human body, causing damaging effects and leading to health risks.On the other hand, radionuclides can cure cancer and other non-cancer diseases by irradiating the malignant cells and tissues [1].In nature, uranium and radium radioisotopes are radioactive materials that are widely present, particularly in soils and rocks.The most common isotopes of radon are radon gas ( 222 Rn), which is a product of the natural decay of 238 U, and thoron gas ( 220 Rn), which is a product of the decay of 232 Th.Both 238 U and 232 Th are found as trace elements in most rocks and soils in different concentrations across Cameroon [2][3][4].These radionuclides are the primary contributors to human ionizing radiation exposure from natural sources.The inhalation of radon, thoron and their decay products contributes to more than 50% of the total annual dose.Furthermore, the inhalation doses attributed to radon and thoron in indoor atmospheres are mostly contributed by their decay agent concentrations in indoor environments [5].The fluctuations in radon concentrations in dwellings are mostly caused by changes in ventilation conditions and air exchange between interior and outdoor spaces.Radon may enter dwellings by diffusion or pressure-driven flow, if appropriate pathways between the earth and living areas exist.As a result, soil type properties such as permeability and porosity may have an effect on the indoor accumulation of radon [6].The health effects due to exposure to these radionuclides are caused primarily by damage due to alpha particles.The main organ at risk following exposure to radon and its decay products are the lungs.The dose to the lungs is delivered predominantly by the decay products, rather than by the radon gas itself, and >95% of the effective dose is contributed by doses to the lungs.The lack of knowledge about the physical and chemical forms of the radionuclide can introduce very large errors in internal dose assessment [7].Since people spend a large fraction of their time indoors, account must be taken of the indoor occupancy and of the filtration effect to estimate the time-integrated air concentration. 222Rn, 220 Rn and their progeny are inhaled through the nose or mouth, and pass through the larynx and down the trachea, which branches (bifurcates) into a series of airways called bronchi.In turn, the bronchi bifurcate into smaller and smaller airways called bronchioles.The bronchi and bronchioles are lined with cilia and a surface layer of mucus.Synchronized beating of the cilia serves to move the mucous layer proximally toward the throat, where mucus is swallowed.Foreign particles, including deposited radon progeny, are trapped in this mucus, and are thus continually cleared from the bronchi and bronchioles by ciliary action.In the smallest (most distal) bronchioles, mucus is cleared slowly.The rate of clearance increases as the airways become progressively larger (toward the trachea), resulting in a roughly constant thickness of mucus throughout the bronchi and bronchioles [5].The daughter's progeny of radon and thoron accumulates in the lungs for the three modes (unattached, nucleation and accumulation) during respiration and irradiates the tissue, thereby destroying the cells and causing lung cancer [8].Radon has been identified as the second-leading cause of lung cancer after tobacco smoking [9,10].In the past, Bineng et al. [3] conducted a study on the importance of direct progeny measurements for accurate estimation of the effective dose due to radon and thoron, in seven inhabited areas of the uranium-and thorium-bearing region of Lolodorf, located in southwestern Cameroon; this study showed the importance of directly measuring the radon and thoron progenies for correct, effective dose estimation.
The aim of this study is to assess radiological risks due to soil gas radon, indoor radon, thoron and thoron progeny in dwellings of the central and southern regions of Cameroon.The levels of radon, thoron and equilibrium equivalent thoron concentration (EETC) were measured in 50 dwellings with RADUET detectors (Radosys, Ltd., Budapest, Hungary).The average value of each dwelling was used for analysis according to the type of construction materials.The radon in soil gas was measured in 29 sampling points at a depth of 0.7 m, according to the soil radon index (SRI), which is an indicator used to estimate the potential risk of a given area.It consists of a dimensionless value calculated from measurements of radon concentrations in soil gas at a depth of 0.7 m and the permeability of this soil [11].The results of the activity concentrations of radon, thoron and their associated progenies, as well as the absorbed dose rates, will be helpful to determine the radiological risks to the public in indoor environments in central and southern regions of Cameroon.

Study Areas
This study was conducted in dwellings of the central region of Cameroon, in the villages of Ngombas (E11 • 06 , N3 • 25 ), Akongo (E11 • 03 , N3 • 14 ), Nkouloungui (E11 • 57 , N3 • 22 ) and in the southern region of Cameroon in the villages of Awanda (E10 • 59 , N3 • 22 ), Bikoué (E10 • 51 , N3 • 19 ), all located in the uranium-and thorium-bearing areas of Lolodorf.These villages are located between 70 km and 340 km of the capital city, Yaounde.The study areas consist of rocks such as syenite, granite, granulites, rhyolites and plutonic rock, which can have high uranium and thorium contents [3].For the present investigation, different types of dwellings made from earth, mud and bricks are the main building materials of houses.The houses selected for the study were built from earth and cement, with building areas ranging from 40 m 2 to 100 m 2 .

Radon in Soil Gas Measurements
The Markus 10 (RADONOVA, Uppsala, Sweden) detector was used to measure radon in soil gas.It is designed to be as simple as possible to operate.Such detectors are made of ORTEC Ultra silicon; the pump capacity is 1.8 L/min, with an active area of 100 mm 2 .The energy resolution is above 16 keV (in vacuum), and its battery capacity is about 70 measurements.Calibration was performed by the ISO 17025-accredited RADONOVA Laboratories AB. Figure 1 shows the methodology and working principles for using the Markus 10 detector.To measure the radon concentration, the sounding tube was inserted into the soil at a depth of 0.7 m.In the initial measuring phase, air from the soil is pumped up through a sounding tube into a measuring cell.The pumping time (about 30 s) was chosen to ensure that all fresh air in the system was pumped out.The pressure sensor stops the pump if the pressure in the tube drops below 0.95 Atm.When the pressure rises, the pump starts again.The pump's effective running time is always the same, which guarantees a certain minimum volume of air (0.9 L) to be measured.The detection limit of 0.1 kBq m −3 can be offered on request.After the pumping phase, the measuring phase begins.The detector is activated, and the voltage to the measuring chamber is switched on.The charged radon daughters formed by the decaying radon gas are driven towards the detector by an electric field in the chamber.The detector registers the alpha radiation coming from the radon daughters.The detector pulses are amplified and filtered in a single-channel analyzer that only ac- After the pumping phase, the measuring phase begins.The detector is activated, and the voltage to the measuring chamber is switched on.The charged radon daughters formed by the decaying radon gas are driven towards the detector by an electric field in the chamber.The detector registers the alpha radiation coming from the radon daughters.The detector pulses are amplified and filtered in a single-channel analyzer that only accepts pulses from the short-lived radon progeny 218 Po (T 1/2 = 3.07 min, E α = 6.11MeV).This eliminates the slow variations in the background from 214 Po (T 1/2 = 162 µs, E α = 7.83 MeV).The display flashes during the measuring phase, and becomes steady when the measurement is completed.As the instrument only counts pulses from the short-lived nuclide 218 Po, a new measurement can be started after just 18 min.In that time, activity from the previous measurement will have decayed sufficiently.The accuracy of measurement is about 10% at 50 kBq m −3 .

Radon, Thoron and Thoron Progeny Measurements
To determine the radon and thoron concentrations in the dwellings, passive integrated radon/thoron discriminative detectors developed at the National Institute of Radiological Sciences (NIRS) in Japan (commercially known as RADUET) were used during the dry season (December 2021 to March 2022).The RADUET detectors were calibrated before their use.The calibration was carried out by randomly choosing groups of RADUET and exposing them to three different known radon and thoron concentrations at NIRS and Hirosaki University in Japan.These detectors consist of two diffusion chambers with different ventilation rates, and each chamber contains a CR-39 chip for detecting the alpha particles emitted from radon and thoron, as well as their progenies [13][14][15].The RADUET detector contains paired detection chambers: a low-diffusion chamber and a high-diffusion chamber.The low-diffusion chamber limits the diffusion of thoron into the chamber, while the high-diffusion chamber is designed such that both radon and thoron can diffuse into the chamber easily.All of the detectors were hung at a height of 1 to 2 m above the ground, and positioned at least 50 cm away from any of the wall surfaces in living rooms, bedrooms and kitchens according to the place where inhabitants spent the major part of their time.A total of 50 RADUET detectors and 50 thoron progeny monitors (Figure 2) [16] were deployed in 50 dwellings.After the exposure time, the detectors were collected and sent back to Hirosaki University in Japan for tracks evaluation.The lower detection limits (DLs) of the RADUESTs were determined to be 3 Bq m −3 for radon and 4 Bq m −3 for thoron [17].
Using alpha track densities of low-and high-diffusion chambers, the average radon and thoron concentrations can be obtained by solving following equations [18]: where, d L and d H are alpha track densities (track cm −2 ) for the low and high air-exchange rate chambers, respectively.f Rn1 and f Tn1 are the respective conversion factors from the alpha track densities to radon and thoron activity concentrations for the low-exchange air chamber [(tracks cm −2 h −1 )/(Bq m −3 )].f Rn2 and f Tn2 are the respective conversion factors from the alpha track densities to radon and thoron activity concentrations in the high air-exchange rate chamber [(tracks cm −2 h −1 )/(Bq m −3 )].t is the exposure time (h), and b are the backgrounds of the alpha track density (tracks cm −2 ) on the CR-39 detector.The lower detection limits were 10 Bq m −3 for radon and 20 Bq m −3 for thoron.
The activity concentration of thoron progeny, collectively expressed in terms of the equilibrium equivalent thoron concentration (EETC [Bq m −3 ]), can be obtained by solving the following equation: where N TnP is the background track density of CR-39 in the thoron progeny deposition detectors, T is the exposure time and F TnP is the conversion factor for the thoron progeny deposition detector.From the results of a field survey and the chemical etching conditions [19], an F TnP value of 6.9 × 10 −2 tracks cm −2 (Bq m −3 h) −1 was obtained.The detection limit of EETC was less than 0.01 Bq m −3 for a measurement period of about six months.
Atmosphere 2023, 14, x FOR PEER REVIEW 6 of 23 Radon and thoron gas contribute very little of the dose to the lungs.It is the inhalation of the short-lived, solid decay products and their subsequent deposition on the walls of the airway epithelia of the bronchial tree that delivers most of the radiation dose to human lungs [20].Currently, the direct measurement of activity concentrations of all short-lived radon and thoron decay products are limited according to the number of studies carried out in the world.The equilibrium factors between radon, thoron and their progeny determine the level of radioactive equilibrium.In the present study, only the equilibrium factor for thoron ( ) was calculated from the field data, and is given by the following equation: where  n (Bq m −3 ) is the thoron gas concentration and EETC refers to the equilibrium equivalent thoron concentration.Radon and thoron gas contribute very little of the dose to the lungs.It is the inhalation of the short-lived, solid decay products and their subsequent deposition on the walls of the airway epithelia of the bronchial tree that delivers most of the radiation dose to human lungs [20].Currently, the direct measurement of activity concentrations of all short-lived radon and thoron decay products are limited according to the number of studies carried out in the world.The equilibrium factors between radon, thoron and their progeny determine the level of radioactive equilibrium.In the present study, only the equilibrium factor for thoron (F Tn ) was calculated from the field data, and is given by the following equation: where C Tn is the thoron gas concentration and EETC refers to equilibrium equivalent thoron concentration.

Annual Effective Dose
To assess the total annual effective doses due to indoor radon, thoron and thoron progeny exposure for each area, the following formulas given by UNSCEAR were used [5]: E Tn mS y −1 = 0.11 × C Tn × 8766 × 0.6 × 10 −6 (6) where 0.17, 0.11 and 40 nSvBq −1 h −1 m 3 are the conversion factors for radon concentration (C Rn ), thoron concentration (C Tn ) and thoron progeny (EETC), respectively.The time spent in dwelling in one year is 8766 h; 0.6 represents the indoor occupancy factor, 0.4 is the equilibrium factor for radon [21] and 10 −6 is the multiplication factor used to convert the nSv to mSv.The ambient gamma radiation levels were measured indoors and outdoors from different locations of the study area.The indoor and outdoor effective doses were calculated using the following equations: where D in and D out are the indoor and outdoor absorbed gamma dose rates in air (nGy h −1 ).

Excess Lifetime Cancer Risk
The excess lifetime cancer risk (ELCR) is the potential carcinogenic effect, which is characterized by evaluating the probability of cancer incidence in a population of individuals for a specific lifetime from exposures.The ELCR due to radon, thoron, radon progeny, thoron progeny and gamma exposure rate was calculated using the following equation [22]: where AED is the annual effective dose, DL is the average lifespan (70 y) and RF is the risk factor (Sv −1 ), which is the fatal cancer risk per sievert.For the stochastic effects from lowdose background radiation, ICRP 103 suggested a value of 0.057 for public exposure [23].

Geogenic Radon Potential
The geogenic radon potential (GRP) determines the potential of an area to produce geogenic radon.It is determined by factors such as the concentration of radium in the soil, the porosity of the soil and the permeability of the soil.Geogenic radon potential estimation at any point is significant for assisting in human health risk assessment and risk reduction.Soil gas permeability is considered a significant factor that determines the room entry rate of radon, which increases with increasing soil permeability [24,25].If the numerical value of the permeability is not available (as in our case), it is sufficient to estimate it as low, medium or high; the radon index of the building is assessed using the classification reported by Essan et al. [26].A high GRP indicates that an area is likely to have higher levels of geogenic radon, while a low GRP indicates that an area is likely to have lower levels of geogenic radon.The equation proposed by Neznal et al. [27] was used to evaluate and categorize the geogenic radon potential data.This equation is presented as follows [27]: where C is the equilibrium 222 Rn concentration in the soil gas in kBq m −3 , and k is the soil gas permeability measured in m 2 , which we assumed for bare earth varying from 10 −11 to 10 −13 [28].
The radon index (RI) is a measure of the potential for indoor radon concentration in a specific area.It takes into account both the geogenic radon potential and the buildingspecific factors that can affect indoor radon levels, such as the ventilation rate and the construction materials used.The radon index is used to identify areas where indoor radon levels may be elevated, and to prioritize radon mitigation efforts.The RI has been defined in this way in several countries such as the USA [29], the Czech Republic [30] and Germany [31].Several studies have investigated the use of soil gas radon and soil permeability measurements to assess radon risk at the local scale [32][33][34].

Inhalation Dose from Outdoor Radon Concentration
The annual effective dose due to the exposure to radon gas from the surface of the soils received by the public due to inhalation was calculated using the UNSCEAR approach [5], as follows: where D inh is the inhalation dose rate, C Rn is the amount of radon concentrated in the soil gas of the ground, F Rn is the equilibrium factor between radon and its progeny (0.6), T is the exposure time, 0.4 is the average outdoor occupancy time per person and DCF is the dose conversion factor for radon exposure (9 nSv/h/Bq m −3 ).The radon concentration on the surface of soil (C Rn ) is calculated using the relation given by Karthik et al. [35] as follows: where C SG is the activity of radon in soil, d is the exhalation diffusion constant, i.e., 0.05 cm 2 /s, and D is the eddy diffusion coefficient, i.e., 5 × 10 4 cm 2 /s.The exhalation diffusion constant (d) is a parameter that describes the rate at which radon gas moves from the soil into the air.It represents the diffusion process by which radon is released from the soil and transported to the surface, and the eddy diffusion coefficient (D) is a parameter that characterizes the mixing and dispersion of radon gas in the air.It represents the turbulent diffusion process by which radon is dispersed into the atmosphere after it is released from the soil.

Statistical Analysis
The statistical analysis was carried out using Origin Pro to illustrate the data distribution, measures of central tendency and variability of the measured parameters (indoor radon/thoron, radon and thoron progeny and radon in soil gas) in histogram and box plots.The box plot graphically depicts numerical data through their quantiles.The line inside each box represents the median (Q2 or second quartile), while the lower and upper edges of the box are the Q1 and Q3 (first and third quartile), respectively.The Shapiro-Wilk test was applied to test the normality of the soil gas radon measurements.

Indoor Radon, Thoron and Thoron Progeny Concentrations
A total of 50 dwellings from five villages in central and southern regions of Cameroon was chosen for radon, thoron and thoron progeny concentrations measurements during the dry season (December 2021 to March 2022).Three months of indoor data were collected, and the results are summarized in Table 1.
The indoor radon concentration ranges from 19 to 62 Bq m −3 , with an arithmetic mean of 33 ± 6 Bq m −3 and a geometric mean of 32 Bq m −3 .Only 9% of the dwellings have higher values than the world average of 40 Bq m −3 specified by the UNSCEAR [5].The present indoor radon concentration results indicate that no dwelling sites had higher values than the action level (100 Bq m −3 ) recommended by the World Health Organization (WHO) to minimize health hazards due to indoor radon exposure [9].These values are much lower than the recommended reference level of the International Commission on Radiological Protection (ICRP) of 100 Bq m −3 [36].The highest radon concentration was observed in Akongo village, with a value of 62 Bq m −3 , with a geometric mean of 37 Bq m −3 and a standard deviation of 1.This value is lower than the world geometric mean of 45 Bq m −3 [21].The high values of radon concentration in this area could be explained by the fact that buildings were made out of earthen bricks, and they were poorly ventilated (only one door and one window on the same side).The frequency distribution graph shows (Figure 3b) that the radon concentrations in 94% of the dwellings were between 19 and 45 Bq m −3 , and 6% of the dwellings were between 46 and 62 Bq m −3 .The frequency distribution graph shows that the thoron concentration in 11% of the dwellings were between 10 and 45 Bq m −3 ; 39% of the dwellings were between 45 and 100 Bq m −3 , 25% of the dwellings were between 100-200 Bq m −3 and 200-400 Bq m −3 (Figure 3c).These radon, thoron and thoron progeny concentrations are lower than those obtained by Saïdou et al. [37] and Bineng et al. [3] in the Lolodorf locality.This can be explained by the fact that the recommendations made during the last measurement campaign were applied by the local population: increasing the air flow in the house by opening windows and using fans and vents to circulate air, and sealing cracks in floors and walls with plaster caulking or other materials designed for this purpose.The frequency distributions of the radon, thoron and thoron progeny concentrations in the five villages of the study area are shown in Figure 3.The variation characteristics of indoor thoron are similar to those of indoor radon.As in the case of indoor radon, the indoor thoron concentration can also vary widely, and depends not only on local geology, but also on building construction type and inhabitant living style.As shown in Figure 3c and summarized in Table 1, the thoron concentration in dwellings ranges from 10 to 394 Bq m −3 , with an average value of 134 ± 82 Bq m −3 and a geometric mean of 98 Bq m −3 .In practically all of the dwellings (Figure 4a), the thoron concentration is much higher than the world average value of 10 Bq m −3 [5].The lower and the higher concentrations of thoron with values of 10 and 394 Bq m −3 were obtained in Awanda and Ngombas villages (Figure 3a), respectively.The distributions of the thoron and thoron progeny follow the lognormal law, as presented in Figure 3c,d, while the indoor radon concentration follow no particular distribution (Figure 3b).The data presented in Table 2 are based on the measurements performed in earthen and cement houses.The variation characteristics of indoor thoron are similar to those of indoor radon.As in the case of indoor radon, the indoor thoron concentration can also vary widely, and depends not only on local geology, but also on building construction type and inhabitant living style.As shown in Figure 4c and summarized in Table 1, the thoron concentration in dwellings ranges from 10 to 394 Bq m −3 , with an average value of 134 ± 82 Bq m −3 and a geometric mean of 98 Bq m −3 .In practically all of the dwellings (Figure 4a), the thoron concentration is much higher than the world average value of 10 Bq m −3 [5].The lower and the higher concentrations of thoron with values of 10 and 394 Bqm −3 were obtained in Awanda and Ngombas villages (Figure 4a), respectively.The distributions of the thoron and thoron progeny follow the lognormal law, as presented in Figure 4c,d, while the indoor radon concentration follow no particular distribution (Figure 4b).The data presented in Table 2 are based on the measurements performed in earthen and cement houses.
As shown in Table 2, the concentration of thoron was found to range between 11 and 394 Bq m −3 , with an arithmetic mean of 152 ± 67 Bq m −3 and a geometric mean of 119(2) Bq m −3 for earthen dwellings.As shown in Table 2, the concentration of thoron was found to range between 11 and 394 Bq m −3 , with an arithmetic mean of 152 ± 67 Bq m −3 and a geometric mean of 119(2) Bq m −3 for earthen dwellings.
For cement houses, the concentration of thoron ranges from 10 to 271 Bq m −3 , with an arithmetic mean of 81 ± 53 Bq m −3 and a geometric mean of 55(3) Bq m −3 .In most types of dwellings, the indoor thoron concentration is higher than the indoor radon concentration.This can be explained by the geology of the soil, according to a study by Bineng et al. [3] Geological maps of the study areas show that the soil and the bedrocks of the uraniumand thorium-bearing region of Lolodorf consists of rocks and minerals.Similarly, the thoron progeny concentration varies from 0.5 to 21.8 Bq m −3 , with an arithmetic mean of 6.7 ± 4.0 Bq m −3 and a geometric mean of 4.0 Bq m −3 , as shown in Table 2.These values are much higher than the range values of 0.04-2 Bq m −3 prescribed by the ICRP in its Publication 126 for an estimated expected concentration in buildings for thoron progeny [37].However, the EETC of earthen and cement dwellings have arithmetic mean values of 7.3 ± 4.1 and 5.1 ± 3.2 Bq m −3 , respectively, as shown in Table 2.These values are much higher than the limit recommended by the ICRP.
The equilibrium factor for thoron has been found to vary from 0.01 to 0.24, with a mean value of 0.06 ± 0.03 and a geometric mean of 0.05 for earthen houses.For cement houses, it was found to vary from 0.04 to 0.18, with an arithmetic mean of 0.07 ± 0.03 and a geometric mean of 0.06.These results are shown as a box-whisker plot in Figure 4 for the thoron equilibrium factor, and are summarized in Table 2.
These mean values are higher than the value of 0.02 given by the UNSCEAR [21].However, recently in the uranium-and thorium-bearing region of Lolodorf, Cameroon, Bineng et al. [3] reported an equilibrium factor of 0.09 ± 0.01, and in their study, the authors estimated the radiation dose derived from inhalation of radon and thoron using direct measurements and indirect measurements by considering different thoron equilibrium factors, and compared this with the UNSCEAR value.They concluded that direct measurement of thoron progeny is important for dose assessment.In Douala city, Cameroon, Takoukam et al. [38] reported an average value of the equilibrium factor for thoron that is 0.1 ± 0.1 higher than the UNSCEAR value.As a result of the large dependence of the equilibrium factor on environmental conditions, the authors concluded that this value can be closer with the UNSCEAR world average value.

Annual Effective Dose Assessment
The main pathways of human exposure leading to internal irradiation are ingestion and inhalation of radionuclides.Radionuclides move through the human body following various modes of the amount of radioactive material that enters the three-dimensional confines of the human body.Krewski et al. [39] reported on 20 case-control epidemiologic studies of residential radon and lung cancer that were completed, including seven studies in North America, eleven in Europe and two in China.Meanwhile, some epidemiologic studies reported by Smith et al. [40] investigated the potential relationships between residential radon and leukemia.Thus, it is strongly important to evaluate the inhalation doses of radon, thoron and thoron progeny.
In the current study, the indoor radon, thoron and thoron progeny effective doses were determined directly using experimental measurements.That of the radon progeny was indirectly calculated using the radon equilibrium factor (F Rn = 0.4) given by the UNSCEAR [5].For the entire study area, the data are presented in Table 3 for the different localities (Figure 5), and in Table 4 for the two types of dwellings.The arithmetic/geometric mean of the annual effective dose due to radon in the five villages is the same and equal to 0.03 mSv y −1 .This value of the radon dose is less than 1.15 mSv y −1 , which is the world average value [5].The highest effective dose due to indoor thoron (0.23 mSv.y −1 ) and thoron progeny (4.59 mSv y −1 ) was observed in Ngombas locality.However, the highest effective dose due to indoor radon progeny was observed in Akongo, with a mean value of 0.73 ± 0.22 mSv y −1 .
These mean values are higher than the value of 0.02 given by the UNSCEAR [21].However, recently in the uranium-and thorium-bearing region of Lolodorf, Cameroon, Bineng et al. [3] reported an equilibrium factor of 0.09 ± 0.01, and in their study, the authors estimated the radiation dose derived from inhalation of radon and thoron using direct measurements and indirect measurements by considering different thoron equilibrium factors, and compared this with the UNSCEAR value.They concluded that direct measurement of thoron progeny is important for dose assessment.In Douala city, Cameroon, Takoukam et al. [38] reported an average value of the equilibrium factor for thoron that is 0.1 ± 0.1 higher than the UNSCEAR value.As a result of the large dependence of the equilibrium factor on environmental conditions, the authors concluded that this value can be closer with the UNSCEAR world average value.

Annual Effective Dose Assessment
The main pathways of human exposure leading to internal irradiation are ingestion and inhalation of radionuclides.Radionuclides move through the human body following various modes of the amount of radioactive material that enters the three-dimensional confines of the human body.Krewski et al. [39] reported on 20 case-control epidemiologic studies of residential radon and lung cancer that were completed, including seven studies in North America, eleven in Europe and two in China.Meanwhile, some epidemiologic studies reported by Smith et al. [40] investigated the potential relationships between residential radon and leukemia.Thus, it is strongly important to evaluate the inhalation doses of radon, thoron and thoron progeny.
In the current study, the indoor radon, thoron and thoron progeny effective doses were determined directly using experimental measurements.That of the radon progeny was indirectly calculated using the radon equilibrium factor (FRn = 0.4) given by the UN-SCEAR [5].For the entire study area, the data are presented in Table 3 for the different localities (Figure 5), and in Table 4 for the two types of dwellings.The arithmetic/geometric mean of the annual effective dose due to radon in the five villages is the same and equal to 0.03 mSv y −1 .This value of the radon dose is less than 1.15 mSv y −1 , which is the world average value [5].The highest effective dose due to indoor thoron (0.23 mSv.y −1 ) and thoron progeny (4.59 mSv y −1 ) was observed in Ngombas locality.However, the highest effective dose due to indoor radon progeny was observed in Akongo, with a mean value of 0.73 ± 0.22 mSv y −1 .According to Table 4, for earthen dwellings, the effective dose was found to vary from 0.02 to 0.04 with an arithmetic mean of 0.03 ± 0.01 mSv y −1 for radon, 0.01 to 0.23 with an arithmetic mean of 0.09 ± 0.05 mSv y -1 for thoron, 3.0 to 4.6 with an arithmetic mean of 1.5 ± 0.9 mSv y −1 for thoron progeny and 0.4 to 0.8 with an arithmetic mean of 0.6± 0.1 mSv y −1 for radon progeny.For cement dwellings, the effective dose ranged between 0.02 and 0.06 mSv y -1 with a geometric mean of 0.03(1.4)mSv y −1 for radon, 0.01-0.16mSv y −1 with a geometric mean of 0.03(2.5)mSv y −1 for thoron, 0.13-3.36mSvy −1 with a geometric mean of 0.8(2.3)mSv y −1 for thoron progeny and 0.44-1.17mSv y −1 with a geometric mean of 0.6(1.4) for radon progeny (Figure 5). Figure 6 shows a correlation between the inhalation doses due to radon and thoron.There is not a strong correlation between the two distributions.It is not easy to distinguish the inhalation doses due to thoron from that of radon; therefore, radon and thoron concentrations should be separately measured.
the inhalation doses due to thoron from that of radon; therefore, radon and thoron concentrations should be separately measured.The total average effective dose for inhalation due to radon, thoron radon progeny and thoron progeny is estimated to be 2.14 mSv y −1 , as shown in Table 5.The radon, thoron, radon progeny and thoron progeny contributions to the total inhalation dose vary from 0.5 to 3% (0.02-0.06 mSvy −1 ), 0.2 to 13% (0.01-0.23 mSvy −1 ), 11 to 71% (0.36-1.17) and 12 to 87% (0.12-4.60 mSvy −1 ), respectively.We found that the highest contribution to the inhalation dose of 60% stems from thoron progeny, and the corresponding lowest contribution of 2% belongs to radon.However, thoron and thoron progeny contribute a significant fraction of 64% to the total inhalation dose.Thoron and thoron progeny are no longer negligible from the viewpoint of health risk assessment.The values of the annual effective dose obtained for the study areas are within the safe range of 3 to 10 mSv y −1 recommended by the ICRP [41], and below the reference level of 10 mSv y −1 recommended by the WHO [9].A higher clear tendency of the annual effective dose due to the progeny of indoor radon and thoron is visible in the results: TnP > RnP > Tn > Rn.The total average effective dose for inhalation due to radon, thoron radon progeny and thoron progeny is estimated to be 2.14 mSv y −1 , as shown in Table 5.The radon, thoron, radon progeny and thoron progeny contributions to the total inhalation dose vary from 0.5 to 3% (0.02-0.06 mSvy −1 ), 0.2 to 13% (0.01-0.23 mSvy −1 ), 11 to 71% (0.36-1.17) and 12 to 87% (0.12-4.60 mSvy −1 ), respectively.We found that the highest contribution to the inhalation dose of 60% stems from thoron progeny, and the corresponding lowest contribution of 2% belongs to radon.However, thoron and thoron progeny contribute a significant fraction of 64% to the total inhalation dose.Thoron and thoron progeny are no longer negligible from the viewpoint of health risk assessment.The values of the annual effective dose obtained for the study areas are within the safe range of 3 to 10 mSv y −1 recommended by the ICRP [41], and below the reference level of 10 mSv y −1 recommended by the WHO [9].A higher clear tendency of the annual effective dose due to the progeny of indoor radon and thoron is visible in the results: TnP > RnP > Tn > Rn.

Radon in Soil Gas Concentration, Geogenic Radon Potential and Annual Effective Dose
The activity concentrations of radon in soil gas at different locations of southern and central regions of Cameroun, at a depth of 0.7 m, are shown in Table 6 and plotted in Figure 7 for the 29 sampling points.According to Table 6, the soil gas radon concentration values obtained across all the sampling locations ranged between 4.8 kBq m −3 and 57.3 kBq m −3 , with a mean of 14.1± 6.1kBq m −3 ; this mean value was within the allowed level, which ranges from 0.4 to 40 kBq m −3 [42].Only 3% of the data among those surveyed had a soil gas concentration higher than the allowed level prescribed by the UNSCEAR [42].Radon in soil gas concentration can be influenced by many environmental factors, the most important among these factors being air temperature, barometric pressure, rainfall and snowfall on the radon signal acquired to possibly filter them out [40,[43][44][45].In the other hand, radon release from soils and rocks is complex because in rock, uranium fixed in minerals along grain boundaries and in defects is susceptible to leaching, thus releasing radon trapped therein.In soil, radon release is affected by moisture content, permeability and porosity [46].The higher radon soil gas concentration (57.3 kBq m −3 ) reported in Bikoue (P6) comes from an area known as having uranium and thorium anomalies at some specific places [3].Of the data, 83% are below 20 kBq m −3 , with few values (14%) of the data having high activity concentrations (above 20 to 50 kBq m −3 ), and 3% with very high values (above 50 kBq m −3 ).

Radon in Soil Gas Concentration, Geogenic Radon Potential and Annual Effective Dose
The activity concentrations of radon in soil gas at different locations of southern and central regions of Cameroun, at a depth of 0.7 m, are shown in Table 6 and plotted in Figure 7 for the 29 sampling points.Table 7 compares the radon soil gas activity concentrations of the present study with the radon gas concentration values obtained from different parts of the world.The radon activity concentrations in soil gas in Saudi Arabia, in Garhwal Himalaya, in Tumkur and in the United Kingdom are lower than the reported values in the present investigation.However, in Malaysia, the far-north region of Cameroon, in Nigeria, Italy and the Czech Republic, the radon soil gas concentration is much higher than the present study values.Based on the assumed permeability (10 -11 , 10 -12 and 10 -13 ) of bare earth, the highest set of values of the GRP (57.3, 28.7 and 19.1) were found at Bikoué, while the lowest set of values of the GRP (4.8, 2.4 and 1.6) were found at Nkouloungui.Neznal et al. [27] recommended three categories of GRP: low (GRP < 10), medium (10 < GRP < 35) and high (35 < GRP) for the geogenic radon potential.In the present study, all of the sampling points were classified as sites under low and medium GRP, except Bikoué (P6), which was classified as a high GRP site with a GRP of 57.3 under permeability 10 -11 .Bikoue is located at a place where anomalies of uranium and thorium have been detected [3].The annual effective dose due to radon inhalation was determined for all of the studied locations, and it was found to vary between 0.09 to 1.08 mSv y −1 , with a mean value of 0.27 ± 0.12mSv y −1 , less than the reference level proposed by the ICRP of 1 mSv y −1 [56].The annual effective dose of 22 (76%) over 29 sampling points is higher than the value of 0.1 mSv y −1 recommended by the World Health Organization [57].
Figure 7 displays the histogram with statistical indicators of the soil gas radon concentration in the study area.The skewness and kurtosis values show that the data are asymmetric and non-normally distributed.These results did not reject the null hypothesis at a 5% level of significance that the data distribution complies with a log-normal distribution, having a p-value of 0.69.

Indoor, Outdoor Gamma Exposure Rate and Corresponding External Effective Doses
The indoor and outdoor gamma ray dose rates were measured using a dosimeter in 48 locations of the study area.The variations in absorbed gamma dose rates and effective doses due to gamma rays from the study area are shown in Table 8.The indoor and outdoor absorbed gamma dose rates in air varied from 0.08 to 0.4 nGy h −1 and 0.08 to 0.64 nGy h -1 , respectively, with the same arithmetic mean of 0.19 ± 0.06 nGy h −1 .According to the UNSCEAR report findings [5], the average (range) of the outdoor absorbed dose rates in 25 countries was 59 nGy h −1 (18-93 nGy h −1 ).The results in the present study for all of the areas were 300-400 times lower compared to the worldwide average.It should be noted that direct measurements of the absorbed dose rate in air generally showed reasonable agreement with the calculated absorbed dose rate in air from the soil concentration results.In most of the previous studies, considerable discrepancies were observed for some countries such as Luxembourg, Sweden, Syria and Albania, depending on the activity concentrations of natural radionuclides in the soil; furthermore, the UNSCEAR report noted that a discrepancy of 30% or more indicates that a single survey should not be considered representative for a country [5].The annual effective doses due to indoor and outdoor gamma are shown in Table 8; it varies from 0.3 × 10 −3 to 1.5 × 10 −3 mSv y −1 and 0.2 × 10 −3 to 1.6 × 10 −3 mSv y −1 , with mean values of 0.7 × 10 −3 and 0.5 × 10 −3 mSv y −1 , respectively, which are lower than the allowable averages of 0.40 mSv y −1 (indoor) and 0.06 mSv y −1 (outdoor).According to the external effective dose due to gamma rays, the median values are higher indoors.This means that long-term low-level exposure has negative health repercussions such as for tissue degeneration, DNA in genes, cancer or cardiovascular disease [58].

Assessment of Excess Lifetime Cancer Risk
Regarding the excessive lifetime cancer risk (ELCR), i.e., the probability of developing cancer over a lifetime at a given exposure, U.S. EPA methodology shows that values of ELCR equal to 1, 10, 100 and 1000 mSv y −1 will cause an additional risk to develop a mortal cancer of 0.004, 0.04, 0.4 and 4%, respectively [59].The indoor and outdoor ELCR values due to gamma-absorbed dose rate range from 0.001 × 10 −3 to 0.006 × 10 −3 with a mean value of 0.003 × 10 −3 , and from 0.001 × 10 −3 to 0.006 × 10 −3 with a mean value of 0.002 × 10 −3 , respectively (Table 9).These values are lower than the values obtained by the studies conducted by Ramasany et al. in Kerala, India, in 2013 (1.70 × 10 −3 ), by Aytas et al. in Kirklareli, Turkey, in 2012 (0.50 × 10 −3 ), and also lower than the world's average value (0.29 × 10 −3 ) in both study areas [22,60,61].The estimated indoor ELCR values due to radon, thoron, radon progeny and thoron progeny vary from 0.07 × 10 −3 to 0.22 × 10 −3 with a mean value of 0.12 × 10 −3 , from 0.02 × 10 −3 to 0.91 × 10 −3 with a mean value of 0.31 × 10 −3 , from 1.45 × 10 −3 to 4.65 × 10 −3 with a mean value of 2.51 × 10 −3 and from 0.46 × 10 −3 to 18.33 × 10 −3 with a mean value of 5.58 × 10 −3 , respectively.In 39% of the dwellings, the ELCR due to thoron is higher than the recommended value of 0.29 × 10 −3 [22].The overall values of ELCR due to progeny of radon and thoron are higher in all dwellings.From the study, potential carcinogenic effects of the population in the study area are significant.However, potential lifetime excess cancer risk does not estimate the actual risk for any one individual, as individual risk is influenced by a range of genetic and lifestyle factors.For environmental concentrations, these levels may change at any one place over time due to changes in individual activity, new technology or changes in residents' eating habits [62].The results of some experimental studies carried out on animals, and of epidemiological studies carried out in the workplace among uranium miners and on the general population show that long-term exposure to radon can induce lung cancer.In its 2011 publications, the French National Cancer Institute revealed that the risk incurred by a person living in a dwelling with radon concentrations of between 200 and 400 Bq m −3 is close to that of a non-smoker living in an atmosphere of passive smoking [63].A similar study carried out in France showed that the number of deaths from lung cancer attributable to domestic radon is estimated at between 5% and 12% per year [64].
In addition, a number of studies found a link between increased concentrations of radon in a dwelling, smoking (active or passive) and the risk of developing lung cancer.It has been established that this risk is proportional to the radon concentration in air breathed and the duration of exposure [63,[65][66][67][68].Over a lifetime, this risk increases linearly with radon exposure, by around 16% per 100 Bq m −3 [64,66].
However, in this study area, there are many smokers among the population.In many of the dwellings surveyed, the people living there are exposed to tobacco smoke on a daily basis, either as smokers or as casual observers.
Taking into account all sources of exposure (radon, thoron and their associated progeny), the various data obtained in this study show that the radiological risk is fairly high for the public living permanently in the study area.
In the current study, the data show that the public living permanently in the localities of Akongo, Awanda, Bikoué, Nkouloungui and Ngombas is exposed to low and medium levels of radon and thoron.For this type of exposure, some studies have shown that there is no linear human response [69].In other terms, there is no correlation between radon/thoron concentrations and the risk of developing a radio-induced cancer.Many studies show that low-level radioactive radon progeny are naturally protective against lung cancer, including smoking-related lung cancer.Nevertheless, this benefit diminishes as concentrations approach the Environmental Protection Agency's action level [69,70].In short, radon concentrations at this level are much more beneficial to humans from a therapeutic point of view.In some European countries, such as Germany and Austria, the positive medical effects for certain diseases far exceed anything seen in the past [69,70].
It is also well known that radon is an inert gas that has no affinity with the surrounding matter.Therefore, the real danger comes not from the gas itself, but from its solid progeny, such as lead and polonium, which are recognized as highly radiotoxic elements [3,5,8,9].In practice, each member of the public is individually exposed to natural radioactivity.This exposure depends on their lifestyle, environment and workplace.This means that the average dose does not provide information on the dose received by each individual living permanently in the region studied.This concentration is simply a tool that informs public opinion about changes in the population's exposure over time.
The results of this study reveal high concentrations of thoron and radon progeny in certain homes.In view of all the above, it is clear that people living in these dwellings are more exposed to radon and thoron progeny than to the gases themselves.

Radon Index
In Sweden, the Swedish Radiation Protection Authority conducted an extensive program of determining the indoor radon concentrations in dwellings, particularly those built on aluminous schist.Thus, the local risk maps were based on geological criteria, like soils rich in uranium and thorium, and very permeable soils.Thus, criteria for risk assessment known as the "Sweden Criteria" were established, creating a classification based on radon concentrations in soil.According to the Swedish [26] classification, the results of the risk criteria are presented in Table 10.Soils exhibiting radon concentrations below 10 kBq m −3 are classified as low risk, while those having concentrations within 10 kBq m −3 to 50 kBq m −3 are classified as normal risk.However, those with concentrations exceeding 50 kBq m −3 are classified as high risk.Of the sampling locations in this study, 31% fall within the low radon risk areas.Also, 66% of the sampling locations fall within normal radon risk areas.However, 3% of the sampling areas are high radon risk areas, since their soil gas radon concentrations exceed 50 kBq m −3 .In the study conducted by Bachirou et al. [71], the area in Adamawa region, Cameroon, is mostly classified as high risk (80%) according to the Swedish classification, and 20% as medium risk.

Conclusions
Indoor radon, thoron and thoron progeny concentrations, along with the equilibrium factor for thoron, were measured in order to assess radiological risks in central and southern regions of Cameroon.Two types of nuclear track detectors were used for three-month measurements.Soil radon gas was also measured using the Markus 10 device in about 29 locations.For this study, one can conclude that the radon concentration varies from 19 to 62 Bq m −3 with a geometric mean of 32 Bq m −3 , while the thoron concentration varies from 10 to 394 Bq m −3 with a geometric mean of 98 Bq m −3 , and the EETC varies from 0.05 to 21.8 Bq m −3 with a geometric mean of 4.9 Bq m −3 .The thoron equilibrium factor ranges between 0.007 and 0.24 with an arithmetic mean of 0.06 ± 0.03, which is higher than the default value of 0.02 outlined by the UNSCEAR.The mean annual effective dose from the concentration measurement of radon, thoron and thoron progeny in various types of dwelling are estimated as 0.03 ± 0.01 mSv, 0.08 ± 0.05 mSv and 1.40 ± 0.84 mSv, respectively.The total dose is estimated to be 1.51 mSvy −1 .The radon soil gas concentrations in the five villages also ranges from 4.8 to 57.3 kBq m −3 with a geometric mean of 12.08 kBq m −3 for a 0.7m depth at the sampling points.This soil gas radon concentration within a few meters of the ground surface is important in determining radon rates of entry into pore spaces and subsequently into the atmosphere.The estimated indoor ELCR values due to radon, thoron, radon progeny and thoron progeny vary from 0.07 × 10 −3 to 0.22 × 10 −3 with a mean value of 0.12 × 10 −3 , from 0.02 × 10 −3 to 0.91 × 10 −3 with a mean value of 0.31 × 10 −3 , from 1.45 × 10 −3 to 4.65 × 10 −3 with a mean value of 2.51 × 10 −3 and from 0.46 × 10 −3 to 18.33 × 10 −3 with a mean value of 5.58 × 10 −3 .The health hazards related to radon exposure are not negligible in the studied area, while the resulting average ELCR values demonstrate the need for further studies.The results show that the radiological risks due to indoor and outdoor environments in the study area are low for public exposure indoors and outdoors, except for three locations.

Atmosphere 2023 , 23 Figure 1 .
Figure 1.The set up for measuring radon concentration in soil using the Markus 10.Units are in kBq m −3 [12].

Figure 1 .
Figure 1.The set up for measuring radon concentration in soil using the Markus 10.Units are in kBq m −3 [12].

Figure 4 .
Figure 4. Variations in radon, thoron and thoron progeny concentrations (left); box-whisker plot of equilibrium factor of thoron with different dwelling types in the study area (right).

Figure 5 .
Figure 5. Annual frequency distribution for indoor radon, thoron, thoron progeny and radon progeny concentrations in five villages (left); box-plot of annual effective doses with different dwelling types (right).

Figure 5 .
Figure 5. Annual frequency distribution for indoor radon, thoron, thoron progeny and radon progeny concentrations in five villages (left); box-plot of annual effective doses with different dwelling types (right).

Figure 6 .
Figure 6.Correlation between inhalation doses due to radon and thoron.

Figure 6 .
Figure 6.Correlation between inhalation doses due to radon and thoron.

Figure 7 .
Figure 7. Distribution of soil gas radon and statistical indicators.

Table 6 .
Soil gas radon concentrations, annual effective doses and geogenic radon potentials in different localities.

Figure 7 .
Figure 7. Distribution of soil gas radon and statistical indicators.

Table 1 .
The ranges, arithmetic means, geometric means and medians of the indoor radon, thoron and progeny concentrations in different localities.Origin Pro software (OriginLab Corporation, Northampton, MA, USA) was used for statistical analysis.

Table 2 .
The ranges, arithmetic means, geometric means and medians of the indoor radon, thoron, thoron progeny and equilibrium factors in different dwelling types.

Table 3 .
The ranges, arithmetic means, geometric means and medians of the inhalation doses for radon, thoron and progeny in different localities.

Table 4 .
The ranges, arithmetic means, geometric means and medians of the inhalation doses for radon, thoron and progeny in different dwelling types.

Table 5 .
Ranges, means and contributions of indoor radon, thoron and thoron progeny to the total inhalation dose received by the public.

Table 5 .
Ranges, means and contributions of indoor radon, thoron and thoron progeny to the total inhalation dose received by the public.

Table 6 .
Soil gas radon concentrations, annual effective doses and geogenic radon potentials in different localities.

Table 7 .
Comparison of soil gas radon concentrations (kBq m −3 ) obtained in the present study with those obtained in other countries.

Table 8 .
Absorbed dose rates, external effective doses due to gamma rays.