Review of low-level background radioactivity studies conducted from 2000 to date in people Republic of China

About 87% of the total dose received by the general public is due to naturally existing radioactive nuclides. Long-term exposure to the relatively long half-life radioactive elements (including Uranium, Thorium, and Potassium) and their decay products pose severe health threats to the population including lung cancer. The related available data has been collected from numerous studies conducted in China over the last two decades. Several studies have been conducted to determine the radioactive content of primordial Uranium-238, Thorium232, Radium-226, Potassium-40, and anthropogenic Cesium-137 radionuclides in building materials, soil samples from cultivated lands, fertilizers used for reformation of soil, ash (fly, bottom) from coal fired power plant, water for irrigation and dietary items by means of High Purity Germanium HPGe or Sodium Iodide NaI (Tl) detectors. The radium equivalent activity ðRaeqÞof all the studied materials was found to be less than 370 Bq/kg except in soil from Baoji coal fired power plant (CFPP), building materials from Weinan, Xing, Fly ash from Xiangyang and coal-bearing strata from East china. The glazed tiles from Guandong, Xhaanxi, Shandong, and Zhejiang with Raeq> 370 Bq/kg, Gamma Index ðIγÞ > 6, corresponding to a γdose rate higher than the permissible limit of 1 mSv/yr. The volume of the gathered data is vast and scattered, and it is required to compile all data regarding the natural as well as man-made radioactivity in China. The main goal of this paper is to establish a nationwide baseline data on natural radioactivity levels by reviewing and compiling the outcomes of the conducted studies within the entire country. ARTICLE HISTORY Received 7 September 2019 Accepted 15 March 2020


Introduction
Naturally occurring radioactive materials (NORMS) are an important constituent of the Earth's crusts and soil. Terrestrial or cosmogenic are both regarded as continuous sources of radiation. Radiation emitted from multiple sources is ubiquitous in nature. A relatively small proportion of these is extraterrestrial; others originate from soil, air, and water (Namvaran & Negarestani, 2013). This natural radioactivity contains Uranium/ Thorium decay species as well as Potassium-40 (Hosseini, 2007). When human beings are exposed to these radiations, chemical changes occur within the tissue and as a result this can produce harmful effects in it. Radioactive substances exist all over the world and are encountered during routine every-day activities. It is therefore important to assess radioactivity level in the environment for the protection of general public health, especially if the emitted radioactivity enters the food chain. In 1989 the member states of International Atomic Energy Agency (IAEA) demanded a radio-analysis of environmental samples following several nuclear accidents worldwide (IAEA, 1989). The identification of these radiation sources as well as their dose assessment through accurate measurement in soil, building materials, water, and dietary objects is essential in order to determine their potential impact on human health (Afshari et al., 2009). Different radiological surveys/studies on soil, rocks, water, and air have been conducted/reported in different parts of the globe in order to assess the radiation doses to living organisms (Jibiri & Biere, 2011;Korkmaz et al., 2017;Manigandan, 2009;M Rafique et al., 2011;Usikalu et al., 2015). Soil is one of the important environmental materials which is used for many purposes and it contains natural radionuclides that contribute to both indoor and outdoor exposure (Dabayneh et al., 2008). Therefore, for environmental protection the measurements of natural radioactivity in rock and soil are considered very important (Tufail et al., 2007). The strength and intensity of gamma (γ) radiation are strictly associated with the quantity of these radioactive nuclides in a particular territory (Saghatchi et al., 2010). The territorial, geographical, and geological circumstances are the main and vital measurable factors affecting the quantity of radioactive nuclides in the environment (UNSCEAR, 1988). Much recent research has focused on the assessment of radioactive nuclides, taking into account their significance (Abbasi & Bashiry, 2016;Ćujić et al., 2015;Jibiri & Biere, 2011;Mehra et al., 2007;Parial et al., 2016;Muhammad;Muhammad Rafique et al., 2013;Rahman et al., 2008;Taqi & Battawy, 2018;Turhan et al., 2018;Veiga et al., 2006).
In the same way, various research groups in the People's Republic of China have also carried out the measurement of natural as well as artificial radioactivity in the environment. The volume of the gathered data is vast and scattered, and it is required to compile all data regarding the natural as well as man-made radioactivity in China. The main goal of this paper is to establish a nationwide baseline data on natural radioactivity levels by reviewing and compiling the outcomes of the conducted studies from the entire People's Republic of China. This paper assembles and organizes the scattered data in one article for the first time, which will help the researchers intended to conduct studies of this kind in future.

Methods adopted for the assessments of background radioactivity concentrations in People's Republic of China
This section provides an overview of the different methods adopted by various researchers for the assessment of the background radioactivity in samples of different environmental media. All radionuclides were measured either by γ-spectrometry with HPGe or by Na(Tl) detectors. Tables 1 and 2 summarize the conclusions of the measurements of natural & artificially occurring radioactivity conducted by different researchers since 2000.
These tables include the radioactivity concentrations of primordial radionuclides (Uranium-238,  and anthropogenic (Cesium-137) obtained from samples collected country-wide. Normally, the specific radioactivity concentrations could be used for the assessment of radiological hazards linked with the environmental mineral samples. The samples contained a mixture of radioactive nuclides which were known to significantly contribute to the γ-doses. Therefore, a single quantity, i.e. Radium equivalent activity ðRa eq Þ in Bq/kg could be used to illustrate the γ-output from the mixture of these radionuclides present in the sample. Ra eq can be calculated using equation 1 (Hamilton, 1971) The gamma activity index I γ is an alternative standard used for the analysis of gamma emissions associated with the natural occurring radionuclides inherent in substances used for construction throughout the world particularly in the European Union (EU). The world safe limit for Ra eq of 370 Bq/kg corresponds to a maximum external γ-dose of 1.5mGy/yr. It can be estimated using equation 2 as follows (Alias et al., 2008) Where A Ra , A Th and A K in equation 1&2 are specific activity values of Ra-226, Th-232, and K-40. Values of gamma index, i.e. 2 ≤ I γ ≤ 6 are in consistent with a standard dose rate of 1 mSv/yr (Anjos et al., 2005;Righi & Bruzzi, 2006). Building materials with I γ > 6 corresponds to γ-dose rates higher than the UNSCEAR recommended limit of 1 mSv/yr and use of such materials in infrastructure should be avoided (UNSCEAR, 1988).

Summary of the results
A specific summary of the calculated radioactivity concentrations of Radium, Thorium, and Potassium is presented in this section of the article. Radium equivalent activity (Ra eq ), and I γ were also calculated and presented in Table 1 for the mentioned three nuclides of interest. Xinwei in 2004 studied the natural radioactivity in building materials and the by-products (Fly ash & Slag) from coal fired power plants ( L Xinwei, 2004a). The investigated building materials include cement, cement plaster, cement brick, red-clay brick, sand, gravel aggregate, lime/limestone, and roof asbestos. The samples of Sand and gravel aggregate were obtained from the construction location and other building materials from various agencies supplying raw materials for building construction. Samples of fly ash and slag were collected from Baiji, western suburb of Xi'an and Baqiao power station situated in Shanxi. A report was published in July 2004 showing the radioactivity concentration of Radium, Thorium, and Potassium in samples of glaze and ceramic tiles using gamma ray techniques (L Xinwei, 2004b). The samples were obtained from Guangdong, Shaanxi, Shandong, and Zhejiang Provinces. In 2005 building materials (Cement, Cement brick, Cement plaster, Gravel aggregate, Red-clay brick, Sand, Lime/limestone, and Roof asbestos) were again investigated for the radioactivity concentration by Lu Xinwei and the samples were obtained from Xi'an (Xinwei, 2005a). Lu Xinwei used high purity gamma detector for the radioactive analysis of cements, cement brick, and cement plaster used in Xi'an (Xinwei, 2005b Xinwei and Zhang Xiaolan determined the activity concentration in sand samples obtained from Baoji Weihe Sands Park (Lu Xinwei & Xiaolan, 2006b). Lijun Dai, Haiyan Wei, and Lingqing Wang during 2007 collected soil samples from the area immediately surrounding the Baoji coal-fired power plant (CFPP) for the assessment of radiological health hazards to the public living in this territory (Dai et al., 2007). Lu Xinwei et al. in 2007 studied the radiological hazards associated with lime and cement fabricated in China (Xinwei Lu et al., 2007). Furthermore, low-level background radioactivity was measured in building material including the By-product of a coal fired power plant in Baoji in the west of China by Xinwei Lu and Xiaolan Zhang (X. Lu & Zhang, 2007 (Xinwei Lu & Zhang, 2008a). Soon after that during the same year, i.e. in 2008 Xinwei Lu and Xiaolan Zhang studied beach sand collected from Rizhao bathing beach background radioactivity in the samples (Xinwei Lu & Zhang, 2008b). Low-level background radioactivity was measured by Xinwei Lu, Xiaolan Zhang, and Fengling Wang in sediment collected from the Wei river (Lu et al., 2008). In 2009 Xinwei Lu and Zhang collected sand samples from four (4) big rivers flowing in Xi'an for the determination of natural radioactivity concentration (Xinwei Lu & Zhang, 2009 (Lu, Yang et al., 2012c). Caifeng Zhao et al. published an article in 2012 recorded the activity and concentration of Radium, Thorium, and Potassium in the building materials commonly used in Baotou city of Inner Mongolia (Zhao et al., 2012). Xinwei Lu, Caifeng Zhao, Cancan Chen, and Wen Liu measured radioactivity level naturally occurring in soil around the Baqiao CFPP (Xinwei Lu, Zhao et al., 2012d). Lu et al. investigated coal and ash samples from the Xi'an coal combustion power plant for assessing radiological hazards to workers of this plant and general public living around this power plant (Xinwei Lu, Li, Lu, Li, Wang et al., 2012b contamination (Xinwei Lu, Li et al., 2013). Xiang Ding et al. also analyzed the materials commonly used in Urumqi for the construction of buildings (Ding et al., 2013). Shigang Chao et al. (2014) determined the concentration of radioactivity in the building materials from Xining . In a separate study, these researchers also investigated the radioactivity in building materials from Weinan (Xinwei Lu et al., 2014). They also analyzed building material obtained from Xiangyang during this year (Feng & Lu, 2014). Mu You et al. in 2015 collected Coal, Ash (bottom and fly) and soil samples from the vicinity of coal fired power plant in Huainan and assessed for the radiological characterization by (You et al., 2015). Junyan Ge and Zhang Jianguo estimated the radiological hazards from the building materials mostly used in Anhui Province (Ge & Zhang, 2015). Vanadium mine situated in the central china was chosen for the collection of bone-coal samples to analyze for the gamma radioactivity by Yan-Jun Huang et al. (Huang, Chen et al., 2015). Beach sand samples were collected by Yingnan Huang et al. from Xiamen Island for activity concentration of natural radioactivity (Huang, Lu et al., 2015). In 2015 Guoqing Liu et al. studied samples of soil from the surrounding of Mawan CFPP situated in Shenzhen for the gamma radiation in this area (Liu et al., 2015). Radon exhalation and radioactivity concentration in fly ash used as construction materials in most parts of Xiangyang were studied by Tingting Feng and Xinwei Lu for gamma activity concentration (Feng & Lu, 2016). Hu B et al. made choice of Tongliao and collected some soil samples around the uranium mine of this region for the radiological analysis and heavy metals (Haribala et al., 2016). Xinwei Lu et al. collected sediment samples in the nearby ponds of the industrial park of northwest and analyzed for the gamma activity concentration in it (Lu, Pan et al., 2016). Yuxin Li, Xinwei Lu, and Xiaolan Zhang in 2016 determined the radioactivity concentration of naturally occurring radioactive nuclides in the samples of sand and cements used commonly in the region of Dingxi . X. Lu, S. Chao, and X. Ding found radioactivity concentration in the sediment samples obtained from the Xining sector of the Huangshui River in the northwest of China (X X Lu et al., 2016). Jin Wang et al. in 2016 representative soil samples were collected from the surrounding of granitic uranium deposit situated in Guangdong to measure the radioactivity in the soil of this mine and hazards to the workers working in it (J. J. Wang et al., 2016). During 2017, Haribala Bai et al. studied the radiological health hazards of the heavy metals in collected surface soil samples from the vicinity of Bayanwula prospective uranium mining territory (Bai et al., 2017). High Purity Germanium (HPGe) spectrometry and inductively coupled plasma-mass spectrometry (ICP-MS) were used by them for the natural and anthropogenic radioactive nuclides and heavy metals in the obtained soil samples. Samples of geothermal water from the capital of PR China for comprehensive investigation of radiation concentrations of the radionuclides were collected by Shufang Wang et al. in 2017. Yin et al. (2017) building material including sand samples from Weifang was gotten for the radio analysis (Yin et al., 2017). In 2018 tiles samples were obtained by E. S. Joel to determine the presence of radioisotopes in the collected samples. During the same year, i.e. 2018 stone coal-bearing strata in East China was evaluated by Naizheng Xu et al. for radioactivity concentration (Joel et al., 2018;Xu et al., 2018). Baolu Yang et al. in 2019 collected samples of topsoil from Shangrao Prefecture (eastern region) for the assessment of natural as well as artificial radioactivity concentration (B. B. Yang et al., 2019). Soon after that high concentration of Radon in the region of Shawan Cave in southwest of china were continuously monitored by Yanwei Wang et al. using the RAD7 radon detector (Y. Y. Wang et al., 2019).
Following is the outline of some other studies apart from the summary presented in Tables 1 and 2 regarding the assessment of background level radioactivity in samples of various materials from the human environment in China.
Weihai Zhuo et al. (2001) studied ground water samples for the assessment of natural radioactivity (Rn-222, Ra-226, Ra-228, and Uranium). Samples (total 552) were collected from various parts of Fujian province using a Radon (Rn) bubbler and αscintillation cell method. These investigators also estimated the average lifetime risk of 1:3 Â 10 À3 due to the ingestion of Rn-222 in the ground water of Fujian (Zhuo et al., 2001).  (Bai et al., 2017) In 2009, Gang SONG et al. studied the Pearl River Delta Economic Zone, situated in Guangdong for the assessment of natural low-level background radioactivity in this area. They found outdoor absorbed dose rate in the range of 55.7 to 88.7 nGyh À1 from cosmic radiation with a mean value of 68.2 nGyh À1 and from terrestrial γradiation it was found in the range of 22.6 to 522 nGyh À1 with an average value of 137nGyh À1 . A higher absorbed dose rate of 522 nGyh À1 in Conghua was found. They also estimated the level of radon content, i.e. indoor radon (30.5Bqm À1 ) and outdoor (11.2Bqm À1 ) in PRDEZ.
The average values of U-238, Ra-226, Th-232 and, K-40 in the soil samples from PRDEZ was found as 137.6, 136.7, 175.3, and 700.7 Bqk À1 respectively. The reported result showed that natural radioactivity in Guangdong is higher than in other parts of China due to granite bedrocks (song et al., 2009).
In 2010 an inter-comparison exercise was conducted for the assessment of radioactivity contents in soil and building materials was organized by the National Institute for Radiological Protection (NIRP), CDC China. They used a γ-ray detector for the analysis of soil samples obtained from farms in Jiuquan and bricks from suppliers and factories situated in Beijing (Tuo et al., 2010).
In 2013 Xinwei Lu et al. observed the presence of heavy metals and natural radioactivity in samples of soil around the major coal power plants operated in Xi'an using XRF and γ ray spectrometry (Xinwei Lu, Liu et al., 2013).
Heavy metals (Cu, Pb, Zn, Co, and Cr) were found in higher concentration in the studied soil samples compared to the corresponding values for background in Shaanxi soil showed contamination of soil with heavy metals. Measured values for activity concentration of Ra-226, Th-232, and K-40 were also found higher than the mean of Shaanxi soil. The contamination of soil with heavy metals and higher concentration of primordial radionuclides were mainly due to the combustion of coal for the energy generation. During the same year (G. Yang et al., 2013) Guang Yang, Xinwei Lu, Caifeng Zhao, and Nan Li studied building materials for γ activity using gamma spectrometry techniques and found the radioactivity concentration of K-40, Th-232, and Ra-226 in the range of 96. 1-819.0, 9.9-138.8, and 14.6-131.2, respectively, in the studied building materials. Results of this study were compared with available data for other countries as well as world's average values and found that H ext < 1 for all studied samples, while H int and I γ for gravel aggregate and hollow brick exceeded the world permissible limit, indicated to avoid these as building materials (G. Yang et al., 2013). In 2013 Xiao-Xiang Miao et al. collected water samples from the vicinity of the nuclear facility of seven provinces of PR. China for the assessment of radiation content in it (Miao et al., 2013). They found gross alpha and Beta (α & β) values in various water samples less than the World Health Organization (WHO) permissible limits of 0:5BqL À1 or 1:0BqL À1 respectively.
In 2015 Jerzy Falandysz et al. conducted a study for the comparison of radiocesium and the activity concentration of naturally γ-emitting radionuclide K-40 Boletus mushrooms grown in various regions of Yunnan and Europe in China. This study revealed that samples of mushroom obtained from the European region were found 2 to 4 times more contaminated with Cs-137 compared to mushroom collected from Yunnan province (Falandysz et al., 2015).

Discussion
This section includes a summary of the measurement of radioactivity concentration in the environmental samples conducted by various research groups in People's Republic of China from 2000 to 2019. The results summarized in Tables 1 and 2 show a broad fluctuation in the concentration level of radioactivity of the desired radioactive nuclides in the analyzed minerals environmental samples collected from some parts of the country.
These samples included water, sediment, soil, rocks, ash, bricks, cement, etc. This variation in the concentration of radioactivity of the natural radionuclides is mainly due to the difference in soil, geological positioning, or geographical distinctiveness of the region. The data reported to date has confirmed the level of background radiation of NORMS within the internationally permissible limits. The radioactivity concentration of Radium, Thorium, and Potassium ranges from the lower limits of detection to 1800 Bq/kg, 1220 Bq/kg and 2168 Bq/kg, respectively. Radium equivalent activity (Ra eq ) in most case is less than 370 Bq/kg but it was found to be higher than 370 Bq/kg in soil samples collected from the surrounding areas of Baaoji CFPP, building materials commonly used in Weinan and Xining, fly ash from the Xiangyang and stone coal-bearing strata from east china. The gamma activity index (I γ ) was also found less than 6 corresponding to a gamma dose rate less than 1 mSv/yr. But radium equivalent activity of glazed tiles from Guandong, Xhaanxi, Shandong, and Zhejiang was found to be greater than 370 Bq/kg with gamma index ranges from 1.14 to 10.6 corresponding to γ-dose rate higher than the permissible limit of 1 mSv/yr.
The reported values of activity concentration of Cs-137 and U-238 were also found in the range of 4.23 ± 4.76 to 6 Bq/kg and 3.3 to 442 Bq/kg, respectively.
All the literature reported to date revealed the hazards from NORM in environmental samples including dietary items and building materials. In soil, the concentration level of natural radioactivity was assessed taking into account the soil type and its surface specification. Many studies comprising measurements of the concentration of radioactivity in building materials as well as soil from agriculture land collected from across China are available, but none of the studies reported the measurement of NORMS and other radionuclides in dairy items (milk, butter, cheese, and yogurt), air and drinkable bottled water. Also, there is no availability of data for condensed milk. In addition, insufficient data exists for underground Uranium and some other mineral deposits existing in rocks and soil. The investigation of natural radioactivity in bones & teeth of animals and humans is also needed at the current time.

Conclusion & Future work
The reviewed literature demonstrated the high level of current research activity in the assessment and measurement of natural as well as artificial radioactivity concentrations in soil, water, building materials (sand, cement, and cement plaster, bricks, tiles glazed & ceramic), air, and dietary items in various parts of the country. Most researchers used high purity Germanium detectors along with NA(Tl) for these investigations. A large-scale fluctuation in radioactivity concentration has been observed in the investigated materials. Furthermore, soil around the Baoji CFPP, fly ash from Xiangyang coal fired power plants, some building materials from Weinan, Xining, and stone coal-bearing strata from east china showed radioactivity levels higher than all of the studied materials with Ra eq > 370 Bq/kg and I γ < 6, while the radioactivity concentration in the rest of the studied samples was observed to be well below the internationally permissible limits and poses no health hazards to the public. A higher gamma activity index (1.14-10.06) with Ra eq > 370 Bq/kg was noticed in glazed tiles from Guandong, Xhaanxi, Shandong, and Zhejiang corresponding to γ-dose rate higher than the permissible limit of 1 mSv/yr, which may pose possible health risks for its users and it is therefore suggested to avoid their use as building materials. None of the studies reported the measurement of NORMS and other radionuclides in dairy items (milk, butter cheese yogurt), air, and drinkable bottled water. Also, there is no availability of data for condensed milk. In addition, insufficient data exists for underground Uranium and some other mineral deposits existing in rocks and soil. The investigation of natural radioactivity in bones & teeth of animals and humans is also needed at the current time. Further investigation is proposed to explore natural radioactivity and radon concentration in unexplored regions (Central and Northeast china) of China.

Disclosure statement
No potential conflict of interest was reported by the authors.