What can we learn from high background radiation area (HBRA) studies in three Asian countries: India, China and Indonesia? Radiological aspects in various HBRAs

Abstract

Radiation is a pervasive natural phenomenon that has been present on earth since its inception. However, exposure to high background radiation levels can pose significant health risks to individuals living in affected areas. In recent years, several studies have been conducted in high background radiation areas (HBRAs), including high radon concentration areas, to understand the radiological aspects and the lessons learned of radiation exposure. The purpose of this article is to provide a comprehensive review of radiological hazards and lessons learned from studies in high-background radiation areas in some countries of Asia (India, China and Indonesia). In this article, we will explore the hazards associated with radiation exposure from terrestrial radiation and additionally radon inhalation, the different studies conducted in HBRA and the lessons learned from these studies. Ultimately, this article aims to provide a better understanding of the radiological aspects of HBRAs and to identify the key lessons learned from previous studies to prevent future health risks. Likewise, research conducted in different high-background radiation areas in some countries of Asia has provided valuable insights into the radiological aspects of these areas and their potential impact on human health.

INTRODUCTION

In recent years, studies have been conducted in various parts of around the world to understand the impact of high background radiation areas (HBRAs) on human health. HBRA is characterized by abnormally high levels of background radiation, which can have potential implications for individuals living in these areas. This article will explore the radiological aspects of different HBRA in some countries of Asia such as India, China and Indonesia and the health effects of exposure to high levels of background radiation based on our collaborative research project from 1992–2023. By studying these areas, we can gain valuable insights into the dose assessment and the potential risks associated with radiation exposure and improve our understanding of the long-term effects on human health.

HBRAs are areas with abnormally high levels of background radiation, where the natural radiation levels are significantly higher than the global average of 2.4 mSv y⁻¹ [1, 2]. These areas are found all over the world, and they often occur due to the natural presence of radioactive materials in the soil, rocks, or water, including the presence of radon in the atmosphere. The levels of radiation in these areas can be significantly higher than in other parts of the world. Studies have been conducted in different parts of the world, some of the world’s HBRAs are found in Kerala (India) [3], Orissa (India) [4], Guarapari (Brazil) [5], Yangjiang (China) [6], Ramsar (Iran) [7] and Arkaroola (Australia) [8] to understand the impact of these areas on human health. These studies have provided valuable insights into the radiological aspects of these areas and their potential impact on human health.

Studies have been conducted in various HBRAs in Asia to understand the radiological aspects of these areas. According to a recent study, the cancer incidence rate in a HBRA in Ramsar, Iran, is lower than that in neighboring areas [9]. In India, the state of Kerala has several HBRAs that have been studied extensively, including the areas of Chavara, Puthenthura and Neendakara [10]. In China, a study was conducted in the HBRA of Yangjiang, where the health survey results from 1972 to 1990 were presented [11]. The study found that the incidence of cancer in the HBRA was not significantly higher than in other areas, suggesting that exposure to higher levels of background radiation may not necessarily lead to adverse health outcomes. In Iran, the city of Ramsar has some areas where people receive an annual radiation dose from background radiation that is up to 260 times higher than the global average [12]. Despite the high levels of radiation, studies have shown that cancer incidence rates are not significantly higher in these areas than in other parts of the world. Overall, these studies suggest that exposure to high levels of background radiation may not necessarily lead to adverse health outcomes and more research is needed to understand the long-term effects of radiation exposure. Similar results were reported by Hendry et al. [13], they provided an overview of the studies of populations living in HBRAs (Brazil, India, Iran, China), including radon-prone areas, for low dose risk estimation. The overall review demonstrated that there are no increased risks in the HBRA compared to control/reference populations. However, some studies showed a significant excess of non-cancer mortality, including cerebrovascular diseases, tuberculosis, viral infections and diseases of the digestive system, but these results should be considered with caution due to uncontrolled confounding factors.

The committee of UNSCEAR2017 [14] indicated that studies of high dose exposure from environmental sources have the potential to contribute to understanding radiation-induced cancer risk. Exposure to high levels of background radiation can have potential health effects on individuals living in these areas. The primary health concerns from radiation exposure are genetic effects and cancer. Ionizing radiation damages the genetic material in reproductive cells and results in mutations that are transmitted from generation to generation. Exposure to ionizing radiation can also increase the risk of cancer by damaging DNA and causing mutations that can lead to the development of cancer cells. Studies conducted in different HBRA in Asia have shown that the incidence of cancer is not significantly higher in these areas than in other parts of the world [9, 12]. However, improvements would be needed to overcome the key limitations of these studies including low statistical power, dosimetric uncertainties, imperfections in the control of confounding and any other biases.

Our studies conducted in different high-background radiation areas in Asia has important implications for radiological protection and public health. By studying the risks and benefits of radiation exposure in HBRA, we can improve our understanding of radiological protection and develop strategies to minimize radiation exposure. The collaboration between public health experts and radiology and radiation safety experts can help assess the risks. Public health can also play a role in minimizing exposure to ionizing radiation and ensuring quality in healthcare, particularly in radiology. The findings from these studies can help improve radiological protection and public health policies in areas with high levels of background radiation. Note that this study refers to natural radiation, not accident-induced radiation. In China and India, epidemiological studies on cancer risks associated with low-dose and low-dose-rate radiation exposure have also been reported. Particularly in India, epidemiological studies on non-cancer health effects have also been conducted, therefore included in this review. China and India have long been recognized as high natural radiation areas, where epidemiological studies on cancer risk associated with low-dose and low-dose-rate radiation exposure have been reported. These areas have also been characterized by the fact that many residents have lived in the same area over generations and have long been considered suitable for epidemiological studies. However, even in the same high natural radiation area, there are differences in the types of exposure, such as the difference between external and internal exposure doses, and this review is based on the characteristics of each type of exposure.

In addition, recent surveys in Indonesia have found extremely high levels of radiation exposure. Although many reports on systematic health effects have not yet been published in Indonesia, it is an area of focus for future epidemiological studies and is therefore included in this review.

Overview of high background radiation areas in some Asian countries based on the collaborative research projects between 2004 and 2023

The attention on HRBAs has increased as potential health risk factors due to the presence of natural radioactive elements. Moreover, the radiological aspects of HRBAs need to be fully understood. In particular, the total radiation exposure of inhabitants of HRBAs may be much higher than the recommended safe dose limits, and this exposure is composed of both external and internal radiation doses. In addition, the radon isotopes radon (²²²Rn) and thoron (²²⁰Rn) are critical radionuclide in Uranium- and Thorium-rich areas, and the presence of radon in HRBAs is always detected [10]. In this regard, collaborative research projects were conducted at HRBAs in China, India and Indonesia to assess the external gamma radiation dose and inhalation dose due to radon and thoron.

1. High Background Radiation in China

Several areas of China were reported under the grip of HBRA, such as the areas of Yangjiang, Gansu and Yunnan [15–17]. In these areas, the presence of uranium and thorium were found in soils and rocks [18]. The area is also prone to high levels of radon and thoron gas from soil, which can seep into homes and buildings. Morishima et al. [19] indicated that the natural radioactive nuclides in building materials using such soil were one of the main sources of HBRA. Based on the previous collaborative research projects, the gamma dose rates in HBRAs of China were found to range from 99 to 182 nSv h⁻¹. We also observed high radon and thoron concentrations with a maximum concentration of 1471 Bq m⁻³ and 7900 Bq m⁻³, respectively. The total effective dose was estimated to be (5.3 ± 3.5) mSv per year. Some of the previous collaborative research projects in HBRAs of China were summarized below:

Sun et al. [15] studied an epidemiological on lung cancer in areas of the eastern part of Gansu Province by investigating indoor radon and thoron exposures. Radon was determined with passive radon-thoron discriminative detectors and thoron progeny deposition rate devices. They observed the indoor radon concentration in this area ranged from 17 to 179 Bq m⁻³. The study also revealed that several hundred lung cancer cases might be diagnosed with pathological evidence in 3–5 years.

Tokonami et al. [20] measured natural radiation in cave dwellings located in Shanxi and Shaanxi provinces using grab sampling measurements by the Trembley method with a silicon semiconductor detector and its associated equipment as well as a portable multi-channel analyzer. In addition, gamma-ray dose rates were measured both indoors and outdoors with an electronic pocket dosimeter. Moreover, the long-term measurement of radon and thoron gas was made using a passive integrating radon-thoron discriminative detector. The results showed that indoor radon concentrations ranged from 19 to 195 Bq m⁻³ with a geometric mean (GM) of 57 Bq m⁻³ and indoor thoron concentrations ranged from 10 to 865 Bq m⁻³ with a GM of 153 Bq m⁻³. Arithmetic means of the gamma-ray dose rates were estimated to be 140 nGy h⁻¹ indoors and 110 nGy h⁻¹ outdoors.

Yamada et al. [21] conducted field measurements to determine radon concentrations, using two passive detectors and a special method for thoron decay products, in three provinces of China including Shaanxi, Shanxi and Gansu. The highest radon concentration of 1471 Bq m⁻³ was observed with the mean 240 Bq m⁻³. The mean concentration of thoron decay products was 2.2 Bq m⁻³. The effective dose was estimated to be around 2.4 mSv per year.

Tokonami et al. [22] carried out preliminary radiation measurements of indoor and outdoor gamma doses and radon, thoron and their decay product concentrations in cave dwellings and their surrounding areas in China (Shanxi and Shaanxi provinces). They measured the gamma dose rates using a compact gamma dose meter with a semiconductor detector at all the sites, and they observed mean gamma dose rates indoors and outdoors about 150 and 110 nSv h⁻¹, respectively. They also measured radiation with a NaI scintillation spectrometer. The indoor and outdoor gamma dose rates were found to range from 121 to 182 nSv h⁻¹ indoors and 99 to 142 nSv h⁻¹ outdoors, respectively. For radon measurements, the results showed indoor radon concentrations ranging from 18 to 224 Bq m⁻³, and indoor thoron concentrations ranging from 8 to 1176 Bq m⁻³.

Ishikawa et al. [23] carried out a preliminary survey of indoor radon and thoron concentrations and terrestrial gamma doses in Yunnan, China. The measurements were conducted both short-term and long-term measurements. The long-term measurements were made using two types of passive-type detectors: (i) radon/thoron discriminative monitors; and (ii) deposition rate monitors for thoron decay products. Both types of monitors employ nuclear track detectors (CR-39) to detect alpha particles. For the short-term measurements, a continuous radon monitor (AlphaGUARD), a radon/thoron discriminative monitor based on alpha spectrometry (RAD7) and a spectroscopic device for measuring the concentration of each radon decay product were used for the short-term measurements. They found very high thoron concentrations in some hoses (maximum: 7900 Bq m⁻³). The mean annual dose from thoron decay products was estimated to be around 2.9 mSv which is larger than that from radon decay products (1.6 mSv).

Kudo et al. [16] assessed radiation dose from radon and thoron in HBRAs Yangjiang of China. A passive integrating radon–thoron discriminative monitor (RADUET with CR-39 detector) was used to determine the indoor concentrations of radon and thoron. The results showed that radon, thoron and equilibrium equivalent thoron concentrations were observed as 124 ± 78, 1247 ± 1189 and 7.8 ± 9.1 Bq m-3, respectively. They estimated the annual effective doses to be 3.1 ± 2.0 mSv for radon and 2.2 ± 2.5 mSv for thoron. The total dose was estimated to be 5.3 ± 3.5 mSv per year. The study has revealed that the total dose was about two times higher than the worldwide average.

Sorimachi et al. [24] performed short-term measurements of indoor radon and thoron concentrations in dwellings in Gansu Province, China. The measurements were made using an AlphaGUARD monitor and a RAD7 monitor. They provided the results of measurements during summer and winter, the daily means for indoor radon concentration were observed more than the WHO reference level (100–300 Bq m⁻³). From the results, it was found that there were only very slight clear diurnal and seasonal variations in indoor thoron concentration due possibly to its short half-life, and this differed from the tendency seen for radon.

Health effect

Wang et al. [25] conducted a case-control study of lung cancer risk in Gansu Province. The results showed that the mean radon concentration was 230.4 Bq m⁻³ in cases and 222.2 Bq m⁻³ in controls. Using a linear model, the excess odds ratio (EOR) at 100 Bq m⁻³ was 0.19 (95% confidence interval (CI) 0.05, 0.47) for all subjects.

Lubin et al. [26] pooled data from two case-control studies of residential radon representing two large radon studies conducted in China. One is the Shenyang case-control study, the other is the Gansu study. Based on a linear model, the OR with a 95% CI at 100 Bq m⁻³ was 1.33 (95% Cl: 1.01, 1.36). For subjects resident in their current home for 30 years or more, the OR at 100 Bq m⁻³ was 1.32 (95% Cl: 1.07, 1.91).

Zou et al. [27] examined cancer and non-cancer disease mortality from 1979 to 1998 in the HBRA in Yangjiang, Guangdong, China. This prospective mortality study followed 125 079 individuals and accumulated 1 992 000 between 1979 and 1998.

The average annual effective doses received by residents from natural sources of external and internal radiation exposure in the HBRA were estimated to be 2.10 mSv and 4.27 mSv respectively, and 0.77 mSv and 1.65 mSv in the control area (CA). The relative risk (RR) for overall cancers was 1.00 (95% CI, 0.89, 1.14), indicating that there was no difference in overall cancer mortality between the HBRA and the CA.

The RR for non-cancer diseases excluding external cause mortality in the HBRA was 1.06 (95% CI, 1.01,1.10), and was significantly higher than that in the CA. However, the excess was limited to those aged under 50 and the latter half of the observation period (the period between 1987 and 1998), suggesting that the excess mortality may be due to recent changes in lifestyles of the younger generations.

Tao et al. [28] evaluated the effects of HBRA on mortality in Yangjiang area in Guangdong Province, China. They reported mean cumulative radiation doses during the study period from natural radiation in the HBRA was 84.8 mGy. The excess relative risk (ERR) per 100 mGy for all cancers except leukemia was estimated to be −0.101 (95% CI: −0.253, 0.095). In a site-specific analysis, liver cancer mortality was inversely associated with cumulative dose, with an estimated ERR of −0.338 (95% CI: −0.516, 0.061). However, this was attributed to the difficulty of accurately diagnosing liver cancer. Therefore, the ERR of all solid cancers, excluding liver cancer, was estimated to be 0.019 (95% CI: −0.187, 0.304). They found that the cumulative HBR dose was not related to mortality due to cancer or all non-cancer diseases among residents in Yangjiang HBRA.

Su et al. [17] evaluate the risk and threshold doses of lens opacity among residents aged ≥45 years from an HBRA in Yangjiang City. Life-time cumulative doses were estimated using gender, age, occupancy factors and environmental radiation doses received indoors and outdoors. From the study, the cumulative eye lens dose was estimated to be 189.5 ± 36.5 mGy in the HBRA. The logistic analysis resulted in odds ratios at 100 mGy of 1.26 [95% Cl: 1.0, 1.60], 0.81 [95% Cl: 0.64, 1.01] and 1.73 [95% Cl: 1.05, 2.85] for cortical, nuclear and subcapsular lens opacities (LOPs), respectively. For cortical LOPSs, a logistics analysis with threshold dose gave a threshold estimate of 140 mGy [90% CI: 110–160 mGy]. The results indicated that the population exposed to lifetime, low-dose-rate environmental radiation was at an elevated risk of cortical and eye LOPs.

Kudo et al. [29] reevaluated lung cancer risk among the residents in Gansu, China. They analyzed data from a hospital-based case-control study that included 30 lung cancer cases and 39 controls with special attention to internal exposure assessment based on the discriminative measurement technique of radon isotopes. Logistic analysis adjusted for age, smoking and total income showed that there were 0.35 (95% CI: 0.07, 1.74) and 0.27 (95% CI: 0.04, 1.74) for groups living in residences with indoor radon concentrations of 50–100 Bq m⁻³ and over 100 Bq m⁻³, respectively, compared with those with lower than 50 Bq m⁻³ indoor radon concentrations. Results from the analyses showed non-significant increased lung cancer risks for groups living in residences with indoor radon concentrations of 50–100 Bq m⁻³ and over 100 Bq m⁻³, compared with those with lower than 50 Bq m⁻³ indoor radon concentrations.

1. High Background Radiation in India

Some regions of India such as Odisha and Kerala were characterized as HBRAs. These regions were found high natural sources such as thorium and uranium concentration in the ground of the area as local geology is characterized by the deposition of high concentrations of monazite and zircon radionuclide minerals [30]. These natural sources of radiation contribute to the elevated levels of gamma radiation, radon and thoron in the region. Our previous collaborative research projects found that the gamma dose rates in HBRAs of India ranged from 77 to 1813 nGy h⁻¹. The highest radon and thoron concentration in these areas was observed around 333 Bq m⁻³ and 1004 Bq m⁻³, respectively.

Tokonami et al. [31] carried out preliminarily natural radiation measurements in HBRAs, in India. The radon/thoron concentrations were measured using passive radon-thoron discriminative monitors with the CR-39 detector and the measurements of indoor/outdoor gamma dose rates were made using NaI (Tl) scintillation spectrometer. The results showed the radon and thoron concentration ranged between 2–70, and 6–690 Bq m⁻³, respectively. The indoor and outdoor gamma dose rates ranged from 0.3 to 3.9 and from 0.4 to 6.2 μGy h⁻¹. From the sampled data, the annual effective dose ranged from 4 to 22 mSv with an arithmetic mean of 9.3 mSv and the dose contribution was significantly due to external exposure.

Ramola et al. [30] presents the preliminary results of radon and thoron measurements in the houses of Orissa, one of the HBRAs in India, which consists of monazite sand as the source of thoron. The measurements were made by both active (RAD7 monitor) and passive (RADUET detector) methods. Radon and thoron concentrations in the houses of the study area were found to vary from 8 to 47 Bq m⁻³ and the below detection level to 77 Bq m⁻³, respectively. The preliminary investigation showed that the thoron concentration is higher than the radon concentration.

Gusain et al. [32] measured environmental terrestrial gamma radiation dose rates using a pocket survey dosemeter with a NaI detector around the eastern coastal area of Odisha, India. They found the values of the terrestrial gamma dose rate ranged from 77 to 1651 nGy h⁻¹, with an average of 230 nGy h⁻¹. During the measurement, sand and soil samples were also collected for the assessment of natural radionuclides using gamma-ray spectrometry with a NaI(Tl) detector. The measurements showed the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K ranged from 15.6 to 69 Bq kg⁻¹ with an average of 46.7 Bq kg⁻¹, from 28.9 to 973 Bq kg⁻¹ with an average of 250 Bq kg⁻¹ and from 139 to 952 Bq kg⁻¹ with an average of 429 Bq kg⁻¹, respectively.

Ramola et al. [33] reported a high thoron concentration that was found in some houses of HBRA on the southeastern coast of Odisha, India. The measurement of radon and thoron was performed using the RAD-7 and RADUET with CR-39. The results showed radon and thoron concentrations in this area were found to vary from 2 to 333 Bq m⁻³ with an average value of 91 Bq m⁻³ and below the detection limit to 1004 Bq m⁻³ with an average value of 105 Bq m⁻³, respectively.

Rautela et al. [34] investigated terrestrial radionuclides such as ²²⁶Ra, ²³²Th and ⁴⁰K in soil and sand of the Chattarpur area of the southeastern coast of Odisha, India, using NaI(Tl) gamma-ray detector. They found the ²²⁶Ra, ²³²Th and ⁴⁰K concentrations in the soil samples vary from 18 to 778 Bq kg⁻¹ with an average of 136 Bq kg⁻¹, 50 to 2405 Bq kg⁻¹ with an average of 790 Bq kg⁻¹ and below detection limit (BDL) to 925 Bq kg⁻¹ with an average of 383 Bq kg⁻¹, respectively. Moreover, they estimated the gamma radiation dose originating from the terrestrial radionuclides to be varied from 95 to 1813 nGy h⁻¹ with an average of 700 nGy h⁻¹. The natural radiation level in this area was estimated to be very high in comparison to other normal radiation areas of India.

Hosoda et al. [35] carried out a car-borne survey using a NaI(Tl) scintillation spectrometer in Kerala, India to estimate external dose. Their survey observed the maximum air kerma rate of about 2.1 μGy h⁻¹, on a beach sand surface. They also observed that the ²³²Th activity concentration for the beach sand was higher than that for soil and grass surfaces, and the range of activity concentration was estimated to be 0.7–2.3 kBq kg⁻¹. The contribution of ²³²Th to air kerma rate was over 70% at the measurement points with values larger than 0.34 μGy h⁻¹. The maximum value of the annual effective dose in the study area was estimated to be 13 mSv y⁻¹.

Omori et al. [36] conducted long-term measurements of residential radon, thoron and thoron progeny concentrations around the Chhatrapur placer deposit, an HBRA in Odisha, India. A passive-type radon-thoron discriminative detector (RADUET) was used to measure radon and thoron concentrations. For the measurement of the thoron progeny, a thoron progeny deposition detector was used. The results showed that radon and thoron concentrations differ by one order of magnitude depending on exposure periods, while thoron progeny concentration is nearly constant throughout the year. Based on the measurements, the effective doses due to inhalation of radon and thoron are evaluated as 0.1–1.6 mSv y⁻¹ and 0.2–3.8 mSv y⁻¹, respectively. The total dose is 0.8–4.6 mSv y⁻¹, which is the same order of magnitude as the worldwide value.

Sahoo et al. [37] carried out a comprehensive study to determine the radioactivity concentration of soil samples from an HBRA on the eastern coast of India, Odisha state. The collected samples were measured using a p-type coaxial HPGe detector in the laboratory. Moreover, they also measured in situ the ambient gamma dose rate by a NaI(Tl) scintillation spectrometer. The measurement results showed that the dose rate measured in situ varied from 0.25 to 1.2 mSv h⁻¹. The measurements also indicated that Th series elements were the main contributors to radiation level and the mean level of the activity concentration (±SD) for ²²⁶Ra, ²²⁸Th and ⁴⁰K was estimated to be around 130 ± 97, 1110 ± 890 and 360 ± 140 Bq kg⁻¹, respectively. Moreover, they evaluated the exposure doses from radionuclides ranged from 0.14 ± 0.02 to 2.15 ± 0.26 mSv and was higher than the UNSCEAR annual worldwide average value of 0.07 mSv.

Omori et al. [38] evaluated internal dose exposure from inhalation of radon, thoron and its progeny in HBRAs of Kerala, India. The measurements were conducted using passive-type radon-thoron detectors and thoron progeny detectors. The results showed effective doses due to inhalation were estimated to be 0.10 mSv y⁻¹ for radon, 0.44 mSv y⁻¹ for thoron and 0.54 mSv y⁻¹ in total (geometric mean).

Health effect

Nair et al. [39] A cohort study was conducted in Karunagappally in the 1990s to assess the health effects of HBR. Poisson regression analysis of cohort data stratified by sex, age attained, follow-up interval, socio-demographic factors and bidi smoking showed no excess cancer risk from terrestrial gamma radiation exposure. The ERR for all cancers except leukemia was estimated to be −0.013 per 100 mGy (95% CI: −0.58, 0.46). Site-specific analyses for oropharyngeal, gastrointestinal and lung cancers showed no significant association with cumulative radiation dose for any cancer site. Leukemia also showed no significant association with HBR. The result of this study area was followed up by Jayalekshmi et al. [40], their analysis confirmed that no cancer site was significantly related to cumulative radiation dose. Leukemia was not significantly related to HNBR, either. Poisson regression analysis of cohort data stratified by sex, attained age, follow-up periods and the original/additional sub-cohorts estimated an ERR of cancer excluding leukemia as −0.05 Gy-1 (95% CI: −0.33, 0.29) when adjusted for education, bidi smoking, tobacco chewing and alcohol drinking in a statistical model.

Koya et al. [41] conducted a case-control study on congenital mental retardation and cleft lip and palate in high-dose natural radiation areas (>1 mSv y⁻¹) in the south-west coast of Kerala between 2006 and 2009. Conditional logistic regression did not suggest a statistically significant association between mental retardation (n = 445) or cleft lip and palate (n = 116) and high-dose natural radiation. The odds of mental retardation and cleft lip and palate among those exposed to high doses of natural radiation relative to normal levels of natural radiation (<1 mSv y⁻¹) were 1.26 (95% CI: 0.91–1.73) and 0.56 (95% CI: 0.31–1.02), respectively. The data did not suggest any dose-related trend in the risk of either mental retardation (P = 0.113) or cleft lip/palate (P = 0.908).

Sreekumar et al. [42] report that thyroid nodule prevalence among women in Karunagappally. The prevalence of thyroid nodules was 14.1% (n = 42) in high-dose panchayats and 14.5% (n = 33) in low-dose panchayats. Logistic regression analysis showed that age-adjusted logistic regression analysis showed no linear relationship between the prevalence of solitary thyroid nodules and cumulative childhood dose.

There was no linear relationship between cumulative childhood dose (P for trend = 0.159) and cumulative lifetime dose (P for trend = 0.333). The prevalence of thyroiditis and hypothyroidism was not associated with natural radiation exposure.

Sudheer et al. [43] assessed the effect of chronic low-dose radiation (LDR) exposure on the birth prevalence of congenital heart disease (CHD) in newborns on the coast of Kerala, India.

They identified normal level natural radiation areas (NLNRAs) below 1.50 mGy y⁻¹ and high natural radiation areas (HLNRAs) above 1.50 mGy y⁻¹, which were further stratified into three dose groups: 1.51–3.0 mGy y⁻¹, 3.01–6.00 mGy y⁻¹ and 6.0 mGy y⁻¹ or more. Newborns were monitored for congenital malformations in two hospital units in the study area. Of the 193 634 newborns screened, 289 CHDs were identified with a frequency of 1.49%, accounting for 6.03% of all malformations and 16.29% of major malformations. Multiple logistic regression analysis suggested a significantly lower risk of CHD in neonates of HLNRA mothers in the dose group 1.51–3.0 mGy y⁻¹ compared with NLNRA (OR = 0. 72, 95% CI: 0.57–0.92), while HLNRA in the 3.01–6.00 mGy y⁻¹ dose group (OR = 0.55, 95% CI: 0.31–1.00) and similarly in the >6.0 mGy y⁻¹ (OR = 0.96, 95% CI: 0.50–1.85) CHD frequency showed no radiation dose-related increasing trend. However, the birth prevalence of CHD in HLNRA neonates (1.28%) was significantly reduced (P = 0.005) compared to NLNRA neonates (1.79%). Chronic LDR exposure did not show an increased risk for birth prevalence of CHD from the high-level natural radiation areas of the Kerala coast, India. No linear increasing trend was observed concerning different background dose groups.

1. High Background Radiation in Indonesia

Mamuju and Bangka Island of Indonesia is recently discovered as HBRA [44, 45]. The area has become a subject of scientific interest due to its unique geological features, which contribute to high levels of natural radiation in the area. The high levels of natural radiation in Mamuju and Bangka Island are primarily caused by the presence of thorium and uranium in the soil and rocks. The previous collaborative studies have found that the absorbed dose rates in air in the study area of HBRA of Indonesia vary widely between 50 nGy h⁻¹ and 1109 nGy h⁻¹. The highest radon and thoron concentration was observed at about 1015 Bq m⁻³ and 618 Bq m⁻³, respectively.

Saputra et al. [46] measured radon, thoron and its progeny in HBRAs of Takandeang, Indonesia. The measurement was made using passive radon and thoron discriminative detector and thoron progeny detector. Their results showed the indoor concentrations of radon, thoron and thoron progeny as 42–490 Bq m⁻³, 20–618 Bq m⁻³ and 4–40 Bq m⁻³, respectively, and the concentrations for outdoor were 49–435 Bq m⁻³, 23–457 Bq m⁻³ and 4–37 Bq m⁻³, respectively. Moreover, they calculated the annual effective dose to be around 9.8–28.6 mSv y⁻¹.

Nugraha et al. [47] estimated dose of Radium-226 in drinking water collected from HBRAs in Mamuju, Indonesia. The water samples were measured by Liquid scintillation counting (LSC). Their results showed a concentration range of 14–238 mBq L⁻¹. They determined the annual effective dose caused by ²²⁶Ra through the ingestion of drinking water in the study area to be in the range of 3–49 μSv.

Rosianna et al. [48] analyzed laterite and volcanic rock samples for radioactive mineral exploration in Mamuju, Indonesia. They used a high-purity germanium (HPGe) detector to measure the radioactivity of the sample and used X-ray fluorescence (XRF) to determine the rock mineral composition. Their study indicated that Mamuju is anomalous due to its high content of ²³⁸U and ²³²Th concentrations of 539–128 699 Bq kg⁻¹ (average: 22882 Bq kg⁻¹) and 471–288 639 Bq kg⁻¹ (average: 33549 Bq kg⁻¹), respectively. The major elements are dominant, including Fe₂O₃, SiO₂, Al₂O₃ and Na₂O, with several other major elements such as MnO, TiO₂ and CaO.

Hosoda et al. [44] conducted a car-borne survey to estimate the external doses from terrestrial radiation using a setup of NaI(Tl) scintillation spectrometer together with radon measurements in Mamuju, Indonesia. They found that the absorbed dose rates in air in the study area vary widely between 50 nGy h⁻¹ and 1109 nGy h⁻¹. Indoor radon concentrations ranged from 124 Bq m⁻³ to 1015 Bq m⁻³. They estimated the annual effective dose due to external and internal exposures in the study area to be 27 mSv.

Nugraha et al. [49] performed a comprehensive study of dose exposure assessments from the viewpoint of health in an HBRA of Mamuju, Indonesia. They determined that Mamuju was a unique HBRA with an annual effective dose between 17 and 115 mSv, with an average of 32 mSv. The lifetime cumulative dose calculation suggested that Mamuju residents could receive as much as 2.2 Sv on average which is much higher than the average dose of atomic bomb survivors for which risks of cancer and non-cancer diseases are demonstrated.

Yamaguchi et al. [50] studied the biological effects of chronic LDR exposure on a human population in an HBRA of Mamuju in Indonesia. In this study, they reported the mean total effective dose of approximately 69.6 mSv y⁻¹ (range: 47.1 to 115.2 mSv y⁻¹). They also indicated that the alterations in the expression of specific proteins and the oxidative modification responses of serum albumin found in exposed humans may be important indicators for considering the effects of chronic LDR exposure on living organisms, implying their potential utility as biomarkers of radiation dose estimation.

Pradana et al. [45] conducted a car-borne survey and a measurement of indoor and outdoor ambient dose rates in HBRAs of Bangka Island, Indonesia. The radiation measurements were performed using a NaI(Tl) scintillation detector together with a Global Positioning System (GPS) and automatic data logger. From the survey, they found that the highest ambient dose equivalent rate was 596 nSv h⁻¹, with a mean value of 101 nSv h⁻¹ and a median value of 95 nSv h⁻¹. The annual effective dose received from external radiation in the 146 houses on Bangka Island ranged from 0.44 to 1.30 mSv y⁻¹, with a median value of 0.66 mSv y⁻¹.

Comparison of HBRA in China, India and Indonesia

China, India and Indonesia are three countries in Asia that have HBRA due to different causes and locations. In China, the HBRA is located in Yangjiang, where the high radiation is attributed to the presence of thorium and uranium deposits in the soil. In India, the state of Kerala is known for its high natural background radiation, which is caused by the presence of monazite sands in the soil. In Indonesia, the district of Mamuju and Bangka Island are HBRAs due to the presence of tin mining activities, which have led to the accumulation of radioactive elements in the soil. Despite these differences in location and causes, China, India and Indonesia share the commonality of having areas with elevated gamma levels of natural background radiation in the range of 99–182 nGy h⁻¹, 77–1813 nGy h⁻¹ and 50–1109 nGy h⁻¹, respectively (Table 1). The observed values from the previous collaborative studies are relatively higher than the world average value. According to the report data from UNSCEAR (2000) [2], the world’s absorbed dose rates in air are in the range of 24 to 160 nGy h⁻¹, with an average of 57 nGy h⁻¹.

Table 1.

Observed results of background radiation dose rates, the highest radon and thoron concentrations, and estimated annual effective doses from the previous collaborative research projects in China, India, and Indonesia

	China	India	Indonesia
Background radiation dose rate (nGy h−1)	99–182	77–1813	50–1109
Radon concentration (Bq m−3)	10–1471	2–333	42–1015
Thoron concentration (Bq m−3)	2.2–7900	6–1004	20–618
Estimated annual effective dose (mSv)	1.8–8.8	0.8–13	0.44–28.6

Moreover, the previous collaborative research projects observed high radon and thoron concentrations (Table 1) in some HBRAs of China, India and Indonesia, which are higher than the world’s average radon concentration of estimated at 39 Bq m⁻³ in residential homes [51].

The health effects and risks associated with exposure to high levels of natural background radiation also differ between these countries. For example, a study conducted in Yangjiang, China, found that the population living in the HBRA had higher rates of cancer incidence and mortality compared to the national average [18, 26]. In India, studies have linked exposure to high levels of natural background radiation to increased rates of genetic damage and chromosomal aberrations among the population living in the HBRA [24, 35, 36]. In Indonesia, a comprehensive exposure assessment found that the population living in the HBRA had elevated levels of heavy metal exposure, which could lead to adverse health effects such as cardiovascular disease and kidney damage [50, 52]. Despite these differences, all three countries share the commonality of having populations living in areas with increased health risks due to exposure to high levels of natural background radiation.

Potential benefits of studying HBRAs

The study of HBRAs has provided valuable insights into the effects of long-term radiation exposure on human health. Despite the increased levels of natural background radiation, epidemiological studies have not found any adverse health effects in populations living in these areas. However, the text indicates that the natural high radiation background may not be harmless. The concentrations of radioactive elements in the environment have been used to study the effects and evaluate external and internal doses of long-term radiation on human health using specialized equipment such as passive and active types of radon-thoron discriminative monitors, thoron progeny monitors and gamma survey meters. Additionally, studying HRBAs offers numerous potential benefits, such as increased access to skills training and radiation risk assessment. The information obtained from studying HRBAs can be used to inform the development of adequate protective measures and can contribute to achieving health outcomes and promoting the right to the highest attainable standard of physical and mental health. In addition, the information would be useful as an effective tool for risk communication about radiation among the general public.

The factual profit information on studied in India for HBRA is that the gamma dose rate in Kerala is not uniformly distributed and high dose rates are widely distributed along the coast. Also, there are rare metal mines on the beach, and miners collect beach sands (monazite) manually. The residues are stored on the beach again after refinement; thus, the by-products enhance the gamma radiation level along those areas, which is an important point to clarify from the viewpoint of dose assessment. Due to the heterogeneous environmental doses, accurate dose assessment for individuals is difficult. On the other hand, the radiological aspect in Yangjiang, China, is quite different from India due to the previous study, which revealed that there is no increase between radiation doses and RRs for cancer [28] because only external doses were considered for risk assessment, though the dose was not high. To understand the radiation exposure situation in Yangjiang, China, a dose assessment due to the inhalation of radon and thoron was conducted, and it was found that the thoron dose was comparable to the radon dose, a new scientific finding. Recently, we found a new HBRA in Mamuju, West Sulawesi Province, and carried out a comprehensive dose assessment with external, inhalation and ingestion doses [48, 52] As a result, the annual average gamma dose rate, inhalation dose rate from radon and thoron, and radon ingestion dose due to drinking water were estimated to be 3.8 mSv, 22 mSv and 1.1 mSv, respectively. Most of the radon concentrations in drinking water samples collected from wells exceeded an international reference level. Therefore, the Mamuju area is an important and unique area to conduct a risk assessment on HBRA through epidemiological studies for a better understanding of health effects relating to chronic LDR exposure.

On 11 March 2011, a 9.0-magnitude earthquake struck the Tohoku region of Japan. The subsequent tsunami caused extensive damage to the Fukushima Daiichi Nuclear Power Plant (FDNPP). At the time of the accident, information about radiation exposure was disseminated by the government and various media [53], causing a vague sense of anxiety about radiation exposure among the general public [54]. This is because there was no systematic education or information about radiation for the general public before the FDNPS accident. Under these situations, it would be difficult for the public to select appropriate information and to accurately understand the health effects of radiation. In addition, Japan is the only country in the world exposed to radiation from nuclear weapons. Many health problems due to acute radiation damage have been reported from exposures to the atomic bombs in Hiroshima and Nagasaki, and the perception that ‘exposure to radiation causes cancer’ seems well established. There are two types of exposure, acute exposure to a single large dose of radiation and chronic exposure, each of which is considered to have different health effects, but the general public may be confused with the health effects of chronic exposure due to the strong impression left by the Atomic bomb.

It is thought that low-dose-rate exposure and high-dose-rate exposure produce different health effects. In addition, deterministic effects below 100 mSv are not known for acute exposures. A characteristic of deterministic effects is the existence of a ‘threshold dose’ below which no effects occur and above which effects occur [55]^. For radiation protection, on the other hand, it is assumed that there is no threshold dose for stochastic effects, with the probability of occurrence increasing with increasing dose. However, in the low dose range below 100 mSv, it is considered difficult to detect the stochastic effects of radiation exposure epidemiologically. The International Commission on Radiological Protection (ICRP) has therefore set radiation protection standards on the assumption that dose-dependent effects (linear dose response) exist even in the low dose range [55]. Previous assessments of the risk of cancer from low-level radiation have mainly adopted the results of epidemiological studies of the Hiroshima and Nagasaki Atomic-bomb survivor populations. However, it is not clear whether the correlation between radiation dose and cancer incidence increases linearly with risk at doses lower than 100 mSv [56].

More than 10 years after the accident, public concerns about the risks of radiation exposure have not been reduced or eliminated [57]. UNSCEAR stated that there are no recorded health risks directly attributable to radiation exposure from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. It also reported that radiation health effects from the FDNPP accident are unlikely to be a regular occurrence in the future [58]. However, some 40% of Japanese people are still concerned about the effects on future generations [59, 60]. The health effects on future generations have been also reported to be a cause of great emotional distress for the people of Fukushima Prefecture [61]. People living in high natural radiation areas have been living there for several generations and are chronically exposed to the situation, but so far there have been no prominent health effects.

The health effects of chronic exposure to low doses in the high natural radiation areas of Kerala in India and the Yangjang region in China, for example through epidemiological studies, may help to unravel the strings of health effects due to stochastic effects. As these studies were conducted in a single region, however, individual studies may have low statistical power. HBRA is a term that refers to various factors such as differences in dose between external and internal exposures, lifestyle, smoking habits, etc. Therefore, it will be necessary to conduct studies in many regions and conduct meta-analyses by looking at the relationship between dose and health effects in various regions in the future.

Comprehensive investigations show that long-term radiation exposure does not cause death in those exposed, moreover its carcinogenic effects are still unproven. In particular, the people living in the HBRAs worldwide continue to expose the current and future generations to a growing amount of natural radiation. However, a dose–response curve for health risks that has been shown to be valid at low levels is still needed. With improved methodology, studies on the health concerns related to population radiation exposures in HBRAs may offer the necessary knowledge. Although thorough reviews and proceedings from a series of conferences on the subject are accessible, the knowledge is limited in terms of protective strategies for the impacted residents of those locations. Information on the fact that there is not only exposure to radiation due to the Fukushima nuclear accident, but also to natural radiation even before the accident, and in particular the fact that internal exposure to radon is constantly occurring through the unconscious life activity of breathing. The public should be provided with information on the risk of lung cancer associated with radiation doses in an easy-to-understand manner.

Highly accurate scientific results are therefore necessary to promote a correct understanding of various environmental risks, including radiation risks. Further research is also needed to fully understand the effects of high natural background radiation on human health and to develop effective protective measures and health risk communication protocols. Overall, HRBA studies may have some potential to improve our understanding of radiation exposure and its effects on human health.

CONCLUSION

Comparative studies and research have been conducted to better understand the health effects of exposure to high levels of natural background radiation in China, India and Indonesia. For example, a study conducted in Mamuju, Indonesia, characterized the exposure of the entire region as an HBRA and assessed the heavy metal content in soil samples. In China, an epidemiological study has been conducted in Yangjiang to investigate the health effects of exposure to high levels of natural background radiation. In India, studies have been conducted to assess the genetic damage and chromosomal aberrations among the population living in the HBRA. These comparative studies and research provide valuable insights into the health effects and risks associated with exposure to high levels of natural background radiation in different locations and can inform strategies for mitigating these risks.

In addition, we believe that these findings on the health effects of radiation can be made available to the general public to improve their vague knowledge on radiation exposure and to reduce their anxiety.

In Japan, the accident at the Fukushima nuclear power plant has triggered widespread public interest in radiation exposures. On the other hand, fragmented knowledge has led to some people having incorrect knowledge about the health effects of radiation exposures. Incorrect knowledge may lead to harmful rumors. Therefore, in the future, these scientific results on the health effects of natural radiation exposures should be used as an effective tool for radiation risk communication and widely disseminated to the general public.

CONFLICT OF INTEREST

The author has no conflicts of interest to declare.

Presentation at the 7^th International Symposium of the Network-type Joint Usage/Research Center for Radiation Disaster Medical Science: Radiation Medicine from the Perspective of Radiation Disaster Medical Science Research, 20 February 2023, Koujin Conference Hall on Kasumi Campus, Hiroshima University.

FUNDING

This work was partially supported by the JSPS KAKENHI [JP18KK0261 and JP20H00556].