Environmental radon mapping in Sudan, orderly review

The goal of this work is to build up a national database for radon in houses, soil, and water and to demonstrate health risk due to radon inhalation and to construct radon map in the country in order to provide the scientific basic information for the establishment of radon national standard level. The radon predictive maps were built employing familiar GIS software (ArcView 3.2). The review outcomes highlight the characteristics and behaviour of radon in the environment as well as the importance of legislation making radon monitoring in the country is compulsory. If the outcome of this review is hoped that will increase regulation awareness and culture to deal with radon level in Sudan.


Background
A program of radon monitoring in Sudan was initiated in 1992 with the aim of environmental radon monitoring. Radon monitoring in Sudan did not undergo any radiation authority commands so far. There is no general awareness and knowledge regarding the radiation hazards and exposure levels to indoor radon, radon in soil and water [1]. There is no public consciousness and perception regarding the health problem of indoor radon, radon in soil and water. Although, radon is considered to be the essential root of natural radiation and the possibility to promote and raised lung cancer [2][3][4][5][6][7][8][9][10]. 222 Rn and their progeny are classified as category one as a human carcinogen by [11]. Scientists around the world were achieved various health studies to evaluate the severity of lung cancer owing to the 222 Rn [12]. In Sudan some few efforts have been made to measure radon in air, soils, building materials, and water [13][14][15][16][17][18][19]. Numerous nationwide radon studies were conducted in European countries [20][21][22][23][24][25][26][27][28][29][30][31], [32]. and in the US [33][34][35]. In China, researchers have accomplished a list of a survey on 222 Rn everywhere [36,37]. UNSCEAR reports provide a synopsis of the radon survey in many countries around the world [2,4]. In African countries, radon was measured in different countries Ethiopia [38], Uganda, Kenya [39], Ghana [40] and Latin American countries [41]. On the other hand, radon measurement was conducted in some Arab countries through under research program supported by the Arab Atomic Energy Agency [42]. Based on the above investigations, an attempt was made in this review to build up a national program for radon level to provide the scientific basic information, to expand the knowledge regarding radon health risk as well as to establish a national standard level in Sudan.

Materials and methods
At first, an examination of the accessible research concerning the estimation of radon concentration was achieved employing an online literature survey using the ISI Web of Knowledge, Pub-Med, Springer, Science Direct, Scopus and Google scholar databases. The keywords "radon, Sudan" or "radon measurement in Sudan", were used in this search. All studies related to the radon measurement until 2017, in both English and Arabic languages, were obtained. In this study, the determination of radon in water, indoor places, soil, and building material was considered. The review study is based on 18 radon studies including indoor radon, radon in water and radon from building material that were screened in this review. The detailed information of the papers is shown in Table 1 and the studied areas are depicted in Figure 1. Radon dose assessment was calculated for health risk using three mathematical models obtained from UNSCEAR to quantify 222 Rn residential dose [4]. The radon predictive maps were built employing familiar GIS software (ArcView 3.2).

Result
Monitoring of radon in the environment provides the data on the national distribution of radon level in houses, soil and water and the factors influence on environmental radon level. The outcome of radon monitoring is the basis for the development of a strategy for environmental radon protection as well as adequate prevention action. This review is directed to create a radon map in the country and to find out the spots of the high radon level in houses, water, and soil in order to control the radon level in new buildings and water sources. This review study is based on a number of investigations carried out in a different region of Sudan as described below:

Khartoum state
The survey of radon in Greater Khartoum was carried out in the dwellings and water in four sites, three studies on radon in dwellings and one in water: Soba, Omdurman and Khartoum. In Soba, it is observed that the radon level varies between (89. 13

South Kordofan state
222 Rn was measured in the dwellings, soil, and water in three areas. The level of samples around Kadugli ranged from 3 to 139 Bq/ l. The measured activity concentrations of radon in Uro houses ranged from 20 to 4482 Bq/m 3 , with a geometric mean of 109.43 Bq/m 3 . 222 Rn in soil gas ranged between 20 and 1359 Bq/m 3 with geomean of 102.80 Bq/m 3 .

Gezira state
In Gezira State, radon level was investigated in seven zones involving radon in soil (three-zones) and dwelling (five zones). In ElHosh town radon in soil were ranged between 2110 and 9500 Bq/m 3 , with a mean of 5500 Bq/m 3 . Um-Turibat exhibit radon level in soil between 7.310 and 12,750 Bq/m 3 , with a mean of 11,050 Soil radon concentration in Medani varied from 8570 to

White Nile state
The study was performed in Rabak city for radon in the soil, the result showed that radon level varied from 5100-10,700 with an average value of 8200 Bq/m 3 .

Kassala state
The radon level in soil was studied in four cites Kassala, Aroma, Khashm Algirba and Halfa Aljadida. The radon level ranged from 452. 10

Sinnar state
Soil radon was studied in Singa city and radon level ranged from 16,100-22,800 with average value of 19,900 Bq/m 3.

Discussion
The review of radon concentration in Sudan involving the evaluation of radon levels, assessment of annual absorbed dose and annual effective as well as examine radon with international guidance.

Radon in houses, health centres and schools
Exposure to 222 Rn and their daughter are allegedly related to the risen risk of promoting lung cancer [47]. From the result of this review, the highest level of indoor radon range was recorded in South Kordofan State (Uro Zone), while the lower average level was observed in Medani Gezira State [18]. This great difference is principally associated with the materials from which each home is built as well as the geological structure of the area. The houses South Kordofan State (Uro Zone) with high radon levels were located close to a mountain surrounded by granite and phosphate. Moreover, the rooms are bad environmental engineering design such as the rooms are cramped, poor airing, and people's lifestyles. Although the mean value of indoor 222 Rn in most areas that have been investigated was lower the ICRP recommended range (200-600 Bq/m 3 ). However, some sites are significantly higher than recommended limit set by United State Environmental Protection Agency limit of 148 Bq/m 3 , World Health Organization 100 Bq/m 3 and, worldwide average radon concentration 40 Bq/m 3 reported by UNSCEAR [4] except Khartoum exhibited radon concentration 25.67 Bq/cm 3 . Figure 2 presents the geographic information system (GIS) predictive map for indoor radon concentration the spatial distribution map explained that there are some regions displayed high indoor 222 [48]. The result showed the radon level is greater than worldwide radon average concentration reported UNSCEAR by level 40 Bq/m3 except for Medani health centre. The reason could be attributed to the ventilations system and public lifestyle. Indoor radon level was done at 82 schools in Sudan located in four cities: Omdurman, Madani, Sinnar, and Al Hosh, and found to be in the range of 37-124, 26-87, 34-88, 37-58 with an average value of 76, 58, 55, and 48 Bq/m 3 respectively. From the result, the highest average value of radon in schools was recorded in Omdurman 76 Bq/m 3 because of the style of life of the people. Although the average value of radon investigated in schools was found to be lower the recommended level set by ICRP (200Bq/m3), EPA United(148 Bq/m 3 ) and WHO (100 Bq/m3), however, the recorded average values obtained were higher than worldwide average radon level 40 Bq/m 3 reported by UNSCEAR [4]. In the past, it was believed that only radon levels above 400 Bq/m 3 could develop a health problem, nevertheless, new epidemiological studies exhibit lung cancer risk from exposure to 100 Bq/m 3 indoor radon level. Radon levels may be high in houses, health centres, and schools as a result of the use of waters, the soil around the buildings and building materials that have high of 226 Ra level. Buildings may have high radon levels depending on the composition of the soil, presence of cracks in the floors and foundation of the building, ventilation habit, lifestyles, meteorological and seasonal parameters, the pressure-driven flow of the gas. This flow occurs owing to the houses are normally small, the pressure under their surroundings. The estimated average absorbed dose of residential radon inhalation falls within a narrow range from 0.51 to 3.01 mSv/y. with corresponding annual effective  dose ranging between 1.22 and 7.22 mSV/year. In general, the mean values of annual effective dose due to radon inhalation in houses were found to be above the UNSCEAR dose limit 1.0 mSv/y While the cities of Khartoum, Medani, Elhosh, El managil, Haj Abd Allah and Wad Elmahi were showed annual effective doses of less than 3-10 mSv/y recommended action level by the ICRP [49]. Soba city is considered as the major contributor to the residential radon absorbed dose in the Sudanese cities 23.54% [50].

Radon in soil
Determination of radon level in the soil is very important regarding the environmental engineering for 222 Rn exhalation studies in building materials [47]. The highest level of indoor radon was observed in Medani city at Gezira State, radon level varied from 8570 to 19.080 Bq/m 3 , with a mean value of 15,100 ± 1470Bq/m 3 . Whereas, the lower level of radon in soil was recorded in the Uro site at South Kordofan State with a range of 20-1359 Bq/m 3 [51]. The radon level in soil differs because of the geographical, physicogeological and geochemical features climatological circumstances of the area [52]. The movement of 222 Rn in the soil and rocks primarily depend on many factors such as porosity, permeability, and cleft [45]. GIS predictive mapping of radon in the soil is depicted in Figure 3. The spatial distribution maps revealed that it increased towards the central region of Sudan, a region that covers the border between Sinnar and Gezira State. Comparing the Sudan data with some countries showed that the values of 222 Rn lower than the India range (0.4-25.78 kBq/ m 3 [53] and Slovenia (0.9-32.9 kBq/m 3 ) [54] and relatively high than Libya (31.17-469 Bq/m 3 ), Portugal (102-2.982 Bq/ m 3 , and Syria (76-3143 Bq/ m 3 [55]).

Radon in water
It is fully documented that 222 Rn released from residential water provides the level of 222Rn less than 2% of total indoor 222 Rn [56]. The high radon in water has been seen in Greater Khartoum 1.58-345.10 Bq/L with a mean of 59.20 Bq/l. Whereas the lowest was determine in Aroma at Kassala state in and found to be in a range of 5.88-6.43-17.14 with an average value of 10.91 Bq/l [57]. Geologically, Greater Khartoum is characterized by basement and sedimentary formation [10]. Comparing the obtained average radon level with WHO it was found full far below the allowable limit 100 Bq/l. Figure 4 illustrates the predictive mapping of radon in water concentration. 222 Rn level in the water depends on numerous factors such as radon emanation properties of the area rock, travel time of the water in the pores and the physicochemical nature of the area [58]. The GIS map revealed that the Greater Khartoum state South Kordofan State at the southwest of Sudan exhibited hot spots radon concentration. The reason referred to the geological setting of the area which is covered by basement complex formations [59].

Radon in building materials
Building materials have been specified as one of the principal sources of radon exposure. Hence, the assessment of radon level and 222 Rn exhalation rate from building materials is necessary to work in terms of environmental protection and radioecology. Recently in Sudan, the utilization of modern kinds of building materials has grown as a result of the architectural revolution. Inevitably some of these materials contain levels of radioactivity, thus many studies have been achieved around the country [60]. The strong correlation between indoor radon exposure and potential health hazard to occupants is well known. The indoor radon concentrations mainly depend on radon the exhalation from surrounding soil as well as on exhalation from building materials. Hence, the rate of radon exhalation rate from building materials should be taken into account to determine the radon concentration from an environmental engineering perspective [61].
The level of radon ranged from 71 to 292 Bq/m3 with an average value of 154 Bq/m3 (Isam et al, 2014). The results showed that the average value was lower than the worldwide data reported by ICRP (200 Bq/m 3 ). The level of natural radiation in building materials varies from materials to others relying on natural geological forms From the result obtained we can report that the domestic construction materials in Sudan contain a low level of 222 Rn. whereas, the imported materials contain a high level of 222 Rn. Of course, such materials can be replaced by other local building materials to avoid and minimalize 222 Rn [62].

Conclusion
To sum up, the current 222 Rn review demonstrated that the average indoor radon concentration in the various region in Sudan less than the ICRP limit (200-600 Bq/m 3 ). However, some sites were exhibited concentration higher than the reference level of EPA (148 Bq/m3), WHO 100 Bq/m 3 and UNSCEAR (40 Bq/m 3 ). From our review, we specified that a need to apply a variety of statistical tests in order to describe the 222 Rn behaviour radon in(homes, soil, and water). in Sudan, building materials contain a low level of 222 Rn. whereas, the imported materials contain a high level of 222 Rn.

Disclosure statement
No potential conflict of interest was reported by the author(s).