Introduction
Human beings are continuously exposed to ionizing radiation exuded by naturally occurring radiative material (NORM) and radon gas found in the environment [1]. Exposure to such background radiation is a contributing factor to the occurrence of serious health problems such as lung cancer. An increased level of radiation exposure compared to natural exposure levels may occur owing to naturally occurring (e.g., earthquakes) or human-induced (e.g., mining activities) phenomena [2, 3]. Mining is typically performed using surface or underground techniques. These mining activities thus make it possible to bring to the surface of the earth not only desired minerals but also NORM-containing waste material that may seriously pollute the environment and affect human health [4]. The main components of NORM are typically terrestrial radionuclides: 40K, 238U, and 232Th. In particular, the radon isotopes 222Rn (radon) and 220Rn (thoron), which are members of the 238U and 232Th decay chains, respectively, and their decay products are often deemed the main contributors to the general population’s exposure to radiation. From the perspective of protection from radiation, determining the level of public exposure to radiation before mining is essential to accurately assess the radiological impact of said mining on the local population.
Cameroon has one of the richest subsurfaces in Central Africa, endowed with abundant mineral resources of international value such as gold, diamond, bauxite, rutile, uranium, tin, cobalt, nickel, manganese, and iron [5]. To date, the exploitation of these natural resources has operated on an artisanal and semimechanized basis. However, according to the government, the mining sector will play a major role in Cameroon’s “Vision 2035” strategy to establish the country as an emerging market. Thus, multiple actions such as the adoption of a new mining code in 2017 [6] have been taken by the government to better organize and promote this sector. In addition, the establishment of a modernized mining cadastre, which began in 2011, has revealed more than 300 new mining deposits as of 2019 [5].
Gold mining in Cameroon dates back to 1934 under colonial administration. Most of the 140 sites identified with gold deposits in Cameroon are located in the Eastern region of the country, in particular in the cities of Yokadouma, Garoua-Boulai, Batouri, and Betare-Oya. For most of the 20th century, the gold mining performed here was purely artisanal, but transitioned into a semi-industrial activity in the early 2000s, and over the past decade it has yielded an annual production of around 20,000 kg. Public exposure to radiation from such mining activity in Eastern Cameroon has been the subject of few studies [7, 8]. The present study aims to not only reevaluate public exposure to natural radiation, based on measurements obtained from previous studies, but also to assess for the first time the lung cancer risk due to this exposure.
Materials and Methods
1. Survey Area and Route
The absorbed dose rates in air (nGy/hr) from natural radionuclides (40K, 238U series, and 232Th series) and indoor radon isotopes and their progeny (radon and thoron) were measured from July 8th to 10th and from July 9th to September 30th 2016 respectively in Betare-Oya town and its vicinity in Eastern Cameroon. The survey routes (Fig. 1) were selected inside Betare-Oya city and on the main roads linking Betare-Oya to its surrounding localities (Ndokayo and Mali). The main road from Ndokayo to Betare-Oya town is asphalt, while Betare-Oya is connected to Mali village by a dirt road. The route map was drawn using the generic mapping tools created by Wessel and Smith [9] and the weather conditions were sunny or cloudy throughout the survey. Intensive gold mining in this region has caused serious environmental disruption, leaving a landscape strewn with deadly open pits and potentially leading to increased levels of radiation and radioactive contamination.
2. External Exposure to Natural Radiation
(1) Car-borne, in situ and laboratory measurements of outdoor absorbed dose rate in air
A car-borne survey is a convenient method for the evaluation of radiation dose across a wide area over a short period [10–12]. For this study, measurements were recorded using a 7.62 cm×7.62 cm in NaI(Tl) scintillation spectrometer (EMF-211; EMF Japan Co.) and a global positioning system (GPS) setup inside a car (Land Cruiser 79 Double Cabin Pick-Up; Toyota). Measurements of the count rate and GPS data were recorded at 30 seconds intervals while driving along the survey road at a speed of around 40 km/hr (Fig. 1). The car body correction (shielding factor [SF]) was estimated as the ratio of the count rate inside the car to the count rate outside the car. Count rates were measured at consecutive 30 seconds intervals over a total recording period of 2 minutes inside and outside the car at 10 locations. The outdoor absorbed dose rate in air 1 m above the ground surface, can be estimated using Equation (1):
where Nin is the number of counts inside the car obtained during a 30-second measurement. The dose rate conversion factor (DRCF) is the slope of the regression line fitted to a graph of measured dose rates plotted against the total count rates of the NaI(Tl) detector given by Ngoa Engola et al. [7].
In situ measurements were carried out using the same NaI(Tl) detector as that used for the car-borne survey. For that, measurements of gamma-ray pulse height distributions were performed at 1 m above the ground surface outside the car for 15 minutes at 24 measurement points (Fig. 1). These 24 measurement points were selected based on high ambient dose rates observed at specific points along the study route. For the calculation of activity concentrations of 226Ra, 232Th, and 40K, and estimation of outdoor absorbed dose rate in air, the 22×22 response matrix method, developed by Minato [13, 14] was used to unfold the gamma-ray pulse height distributions.
Laboratory measurements of the activity concentrations of radionuclides, used to evaluate the dose rate absorbed in air in the present work, were carried out by Dallou et al. [15]. Details of sampling, conditioning, and activity concentration calculations are well described in their publication [15], with an outline described here. In their study, Dallou et al. [15] collected 32 soil samples from seven gold mining sites in Eastern Cameroon, including five sites located in the gold-bearing area of Betare-Oya. Gamma-ray spectrometry measurements of these soil samples were performed with a Canberra NaI(Tl) detector (Model 802), with a crystal size of 7.6 cm×7.6 cm and a resolution of 7.5% at 667 keV, housed inside a thick lead shield (5 cm) to minimize background radiation. The detector was coupled to a computer-based multichannel analyzer which was used for the acquisition of data and analysis of gamma-ray spectra. Spectra were acquired and analyzed using GENIE 2000 software (Mirion Technologies). Thus, from the activity concentrations of 226Ra, 232Th, and 40K measured in soil samples, the absorbed dose rate in air was evaluated according to Equation (2):
where Au, ATh, and AK are the activity concentrations of 226Ra, 232Th, and 40K respectively in Bq/kg. The coefficients of 0.462, 0.604, and 0.0417 are conversion factors (absorbed dose rate in air per unit activity per unit of soil mass) in units of (nGy/hr)/(Bq/kg) evaluated for 238U-series, 232Th-series, and 40K respectively [16].
2) External annual effective dose recalculation
The external annual effective dose Eext (mSv/yr) due to gamma-rays from primordial radionuclides in the study area was calculated using Equation (3) [7]:
where Fc is the dose conversion factor from the dose rate to the external effective dose for adults (0.7 Sv/yr), D is the absorbed dose rate in air, Iin is the indoor occupancy factor (OF, 0.6), which implies that people spend 40% of their time outdoors. SF is the experimental SF of buildings, which is given by the rate of indoor and outdoor dose [7]. T is the number of hours in 1 year (8,760 hr/yr).
3. Internal Exposure to Natural Radiation
1) Indoor radon, thoron, and thoron progeny measurements
Passive integrated radon-thoron discriminative detectors, namely the RADUET detector developed at the National Institute of Radiological Sciences (NIRS) in Japan, alongside the thoron progeny detector developed by Zhuo and Iida and improved by Tokonami et al. [17, 18] were used to simultaneously measure indoor radon, thoron, and thoron progeny concentrations (Fig. 2). RADUET detectors have two diffusion chambers with different ventilation rates (i.e., one with a low air-exchange-rate and one with a high air-exchange-rate). Each chamber contains a CR-39 plate 10 mm×10 mm in size (RADUET; Radosys Ltd.) for detecting alpha particles emitted from radon and thoron, as well as their progeny. The thoron progeny deposition rate detector is made of stainless steel and consists of CR-39 plates covered with an aluminized Mylar film of 71 mm thickness (Ganapathy Industries), designed for the discrimination of high-energy alpha particles, especially those emitted by 212Po (8.78 MeV). Calibration of the RADUET monitor was performed by placing the detector inside a calibration chamber. Calibration coefficients were then estimated based on the correlation between the recorded alpha particle track density and the time-integrated radon and thoron concentrations [19, 20].
Fifty dwellings studied during the previous survey [8] were selected to estimate indoor exposures to radon isotopes and their progenies. RADUET [17] and thoron progeny [18] detectors were deployed in each house and suspended from the bedroom ceiling at a height of 1.5 to 2.0 m from the floor and 0.5 m from the wall. After a measurement period of 3 months, the detectors were collected and sent to Hirosaki University for the evaluation of alpha particle tracks.
The average radon (
C R n ¯ C T n ¯
dL and dH represent alpha track densities (track/cm2) for the low and high air-exchange-rate chambers, respectively. The conversion factors from alpha track densities to radon and thoron activity concentrations for the high exchange-rate air chamber [(track/cm2·hr)/(Bq/m3)] are given by fRn2 and fTn2, respectively. In Equations (4) and (5), t represents exposure time (hr) and b̄ represents the background alpha track density (track/cm2) recorded by the CR-39 detector. In Equation (6)NB2 is the background alpha track density of the CR-39 plate in the thoron progeny deposition detectors, T is the exposure time, and FTnP the conversion factor for the thoron progeny deposition detector.
2) Inhalation dose assessment
To assess inhalation doses (mSv/yr), Equations (7) and (8), which take into account the dissolution of the radon and thoron gas in blood [16], were used.
CRn, CTn, and CTnP are activity concentrations of radon, thoron, and thoron progeny respectively. Dose coefficients of 0.17 and 0.11 nSv/(Bq·hr/m3) were used for radon and thoron gas dissolved in blood, respectively. DCFRn and DCFTn are the inhalation dose conversion factors (9 and 40 nSv/Bq·hr/m3) for radon and thoron progeny, respectively). An OF of 0.6 was assumed for the studied areas. FRn represents the equilibrium factor (EF) for radon (0.4) [16], 8760 is a factor that adjusts exposure time to 1 year, expressed in hours. The International Commission on Radiological Protection (ICRP) conversion factors for radon and thoron progenies (16.8 and 107 nSv/Bq·hr/m3, respectively) were also used to determine the inhalation dose using a dosimetric approach [22].
4. Excess Lung Cancer Risk Estimates
Several epidemiological studies on underground miners have shown that long term exposure to radon increases the risk of lung cancer. Indoor radon exposure is responsible for 6% to 15% of all lung cancer deaths [23] and, according to International Agency of Research on Cancer, it is the second leading risk factor for lung cancer after tobacco [24]. In the present study, the excess lung cancer risk (ECR) risk model was used for the calculation of lung cancer risk resulting from indoor radon exposure. ECR is defined as the number of additional deaths per million persons per year (MPY) from lung cancer as a result of exposure to radon and its progeny [25]. The ECR was estimated using Equation (9) [25–27]:
where EF is the EF between radon and its progenies, OF is the fraction of time that people spend indoors, and WLM is the Working Level Month. For this study, default values of 0.4 for the radon EF and 0.8 for the OF, as given by United Nations Scientific Committee on the effects of Atomic Radiation (UNSCEAR) [16], were used. The risk factor given by various international committees/agencies (UNSCEAR, ICRP, and U.S. Environmental Protection Agency [US EPA]) describes the lifetime risk of lung cancer mortality due to lifetime exposure to radon and its progeny (Table 1) [1, 26–28]. In other words, it is the number of lung cancer deaths per million persons per WLM. A conversion factor of 73.9 Bq/m3=1 WLM/yr was used to convert radon concentration level into WLM.
Results and Discussion
1. External Exposure to Natural Radiation
1) Car-borne, in situ, and laboratory measurements of activity concentrations and related outdoor absorbed dose rate in air
Table 2 shows activity concentrations of the naturally occurring radionuclides 226Ra (238U), 232Th, and 40K, and the outdoor absorbed dose rate obtained by the car-borne survey, in situ measurements, and the laboratory-based gamma spectrometry measurements of soil samples. Results obtained by the three techniques showed a low amount of deviation, despite the fact that different locations were used for the in situ measurements and the soil sampling. For activity concentrations, the ratios between in situ and laboratory measurements were 0.93, 0.91, and 0.74 for 226Ra (238U), 232Th, and 40K respectively. For the outdoor absorbed dose rate in air, the car-borne survey and the in situ measurements had a similar average value (44 nGy/hr) which was slightly below the average value (47 nGy/hr) of the laboratory measurements.
2) External annual effective dose recalculation
The effective external doses shown in Table 3 were evaluated taking into account the experimental value of the SF (SFexp =1.12) of dwellings towards natural gamma-rays. This value was calculated from the ratio between the internal dose rate and the external dose rate, illustrated in Fig. 3. The external annual effective dose is 42% lower than the worldwide average value (0.5 mSv/yr).
2. Internal Exposure Due to Radon, Thoron, and Thoron Progeny
Indoor radon, thoron, and thoron progeny concentrations from our previous study [8] are summarized in Table 4. For radon, the activity concentrations range from 88 to 282 Bq/m3 with an arithmetic mean of 133±39 Bq/m3 and a geometric mean of 128±1 Bq/m3. For thoron, the concentrations ranged from 4 to 383 Bq/m3 with an arithmetic mean of 93±76 Bq/m3 and a geometric mean of 70±2 Bq/m3. Finally, concentrations of thoron progeny ranged from 1 to 19 Bq/m3 with an arithmetic mean of 6±4 Bq/m3 and a geometric mean of 4±2 Bq/m3. Table 5 displays the inhalation doses received from radon progeny and thoron progeny based on their corresponding measured activity concentrations and Equations (7) and (8), respectively. When using the UNSCEAR 2000 dose conversion factors, the yearly inhalation dose for radon and its progeny ranged from 1.74 to 5.58 mSv/yr with a mean value of 2.63±0.77 mSv/yr, while for thoron and its progeny, the inhalation dose ranged from 0.26 to 4.05 mSv/yr with a mean value of 1.32±0.88 mSv/yr. The total inhalation dose due to radon, thoron, and their progenies ranges from 1.96 to 9.63 mSv/yr with an arithmetic mean of 3.95±1.65 mSv/yr. Different values were obtained when using the ICRP 137 conversion factors. These ranged from 3.18 to 10.21 mSv/yr, with a mean value of 4.82 mSv/yr for radon and its progeny and from 0.62 to 10.74 mSv/yr, with a mean value of 3.43 mSv/yr for thoron and its progeny. The mean total inhalation dose due to radon, thoron, and their progenies was calculated to be 8.22 mSv/yr, which is more than double the value calculated using the UNSCEAR factors. This highlights that is important to follow the future trends of international organizations in what values they use and to carefully consider what values are most appropriate for conducting effective risk assessments. However, regardless of which set of dose conversion factors are used, the inhalation dose calculated in this work is relatively high when compared to the world average value of 1.26 mSv/yr.
3. Excess Lung Cancer Risk Estimates
From the measured indoor radon concentrations (Table 4), the ECR was estimated for the population residing in the study area, using the risk factors (Table 1) recommended by the US EPA, UNSCEAR, and ICRP. The results obtained are presented in Table 6. From these results, the calculated ECR values at all selected houses are well above the world average values of 25 to 87 excess deaths per MPY, 32 to 97 excess deaths per MPY, and 54 excess deaths per MPY reported by the US EPA, UNSCEAR [1] and ICRP [28] respectively.
Conclusion
Public exposure to natural radiation was studied in Betare-Oya, a gold mining area in Eastern Cameroon. By using the activity concentrations of 238U, 232Th, and 40K (Bq/kg) in soil obtained in previous studies, the outdoor absorbed gamma dose rates in air were estimated via a car-borne survey, in situ measurements, and conventional laboratory measurements of soil samples. The outdoor absorbed dose rates in air obtained from direct (i.e., the car-borne and in situ) measurements (44 nGy/hr) agreed well with that obtained via laboratory measurements (47 nGy/hr). The effective external doses were calculated based on the experimentally determined value of the SF (SFexp =1.12) of the dwellings. The resulting external effective dose (0.29±0.09 mSv/yr) is similar to that of our previous study (0.33±0.03 mSv/yr) calculated using a different SF (SF=1.4). Thus, radioactivity levels in the surveyed areas are within normal global levels. On the other hand, inhalation doses due to radon, thoron, and their progenies, and ECRs were determined from the radon and the thoron activity concentrations measured in previous work. From these calculations, the total annual inhalation dose was estimated to be 3.95±1.65 mSv/yr, which is higher than the world average value of 1.26 mSv/yr. Additionally, the ECR for the area studied is about 60% higher than the world average. Therefore, residents in the vicinity of Betare-Oya gold mining areas have a high level of risk from indoor radon. Thus, it would be desirable to educate members of the public about the harmful effects of radon exposure and to consider implementing remedial actions to protect residents against radon and its progenies.
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