First indoor radon mapping and assessment excess lifetime cancer risk in Iran

Graphical abstract


Indoor radon
Protocol name: Indoor radon mapping and health risk assessment Reagents/tools: solid-state nuclear detectors of SSNTDs type CR-39, Radon mapping by ARC GIS (Ver. 10.3) How data were acquired: All available data relevant to indoor radon surveys across country up to 2019 were collected and used in mapping. CR-39 detectors after three-month exposure in dwellings of Shabestar county were analyzed to determine radon concentration according to the U.S. EPA protocol [1]. Trial registration: Not applicable Ethics: Not applicable

Value of the Protocol
We reviewed and summarized all researches conducted on the levels of indoor radon and provided first radon map in Iran based on the published papers. Public exposure database in terms of effective dose, ELCR and risk of lung cancer was prepared.

Description of protocol
Radon is a colorless, odorless and tasteless natural radioactive gas. It is a product of the degradation of uranium and has a radioactive half-life of about four days. Prolonged exposure to elevated radon concentrations has been linked to an increased lung cancer risk [2].
High background radon radiation levels has been reported in Guarapari, Brazil; Kerala, India; Yangjiang, China ; Ramsar, Iran [3][4][5]. However, not all areas have a high radon concentration and there is no way to know the radon level and consequently possible risk in a specific site before testing. Radon concentration varies across the country, so easy, cost-effective and reliable measuring radon levels at dwellings of different areas can be used to develop a national database including maps of residential radon exposure.
In Iran, Radon gas measurement was considered by the Iranian Ministry of Health since 2013 in the framework of the National Radon Measuring Plan. Subsequently, in different provinces and cities of Iran, the program has been implemented sporadically and the results of these monitoring are published in the form of scientific papers. For example, the most recent researches carried out in Northern Iran [4], Central Iran [5], Tehran [6], Tabriz [7], Isfahan [8], Shiraz [9], Mashhad [10], Hamedan [11], Ramsar [12], Yazd [13], Qom [14], Kermanshah [15], Khoram Abad [16], and Minab [17] can be mentioned. Despite some scattered regional indoor radon surveys in Iran, radon mapping has not been carried out yet to increase awareness of the hazards of exposure to radon and to target future radon surveys. So, the main objectives of this study was review and summarize all researches conducted on the levels of indoor radon and also provide map of radon concentration across the country based on the published papers.
Besides, we have tried to measurement of indoor radon levels in Shabestar residential homes in the East Azerbaijan-northwest of Iran as a case survey along with analyzing the factors influencing the concentrations. A little information on indoor radon activity in Azerbaijan district is available in literature. The results of this study could be useful in developing the radon map of Iran.

Development a first trial radon map
All available data relevant to indoor radon surveys across country up to 2019 was collected and used in mapping and producing database. The map includes data from 20 cities displaying the levels of indoor radon activity in 3441 dwellings in Iran. Fig. 1 depicts the map of mean indoor radon concentration values in Iran. Our database including indoor radon levels and associated radiological parameters are given in Table 1 as well. However, only data from 20 cities representing 4.5% of the Iranian cities with population more than 20,000 people was available.
As shown in Table 1 and in indoor radon activity map (Fig. 1), the mean indoor radon concentration levels in most cities are below the WHO action level and no more than 10% of them have radon concentration exceeding 200 Bq/m 3 . The geometric mean of radon concentration was calculated to 72.05 Bq/m 3 . But anyway, many areas of country are still not covered by this map and further surveys should be carefully designed. Table 2 displays the comparison of global indoor radon concentrations in different countries with the results of the present study. The residential radon value in Iran is lower than Romania and Jordan.

Calculation of Annual effective dose, ELCR and LCC associated with radon exposure
The annual effective dose by the indoor air radon was estimated by the following equation: Annual effective dose (DT) = CR Â D ÂH Â F Â T (mSv/yr) [9,18,19].   Annual effective dose (ET) to lungs = DT Â WR Â WT DT = annual absorbed dose (mSv/yr); WR = radiation weighting factor (20 for alpha particles recommended by the ICRP); and WT = tissue weighting factor (0.12 for lung) [20] The Excess life time cancer risk (ELCR) was calculated using the Equation 3: Where DT is the annual effective dose, DL is the average duration of life estimated to a 70 years and RF is the fatal cancer risk per Sievert (5.5 Â 10 À2 Sv À1 ) recommended by ICRP 103.
Finally, the lung cancer cases per year per million person (LCC) is estimated by using the risk factor lung cancer induction 18 Â 10 À6 mSv -1 and can be obtained using the Equation 4: LCC = DTÂ18 Â 10 À6 [17,22].
According to the Table 1 the values of annual effective doses for radon inhalation by the inhabitants were found to vary in the range 0.28 (Kermanshah) to 11.07 (Ramsar) mSv y -1 with a mean of 2.6 AE 2.4 mSvy -1 . It has been observed that majority of the cities monitored for indoor radon concentration were shown annual effective dose within the recommended action level (3-10 mSvy À1 ) [23].
The ELCR and risk of lung cancer estimated from 3441dwellings surveyed are presented in Fig. 2. It is found that LCC ranged between 5.04 and 199.2 per million persons per year with mean value of 46.8 per million persons per year which is lower than the limit range of 170-230 per million persons recommended by ICRP [24]. Case survey: Shabestar County

Study area and data collection
Shabestar County is located in 45 05 0 to 46 09 0 eastern longitudes and 37 42 0 to 38 24 0 northern latitudes is a county in East Azerbaijan province in Iran. It is limited to Tabriz city and Urmia Lake from northwest and northeast respectively. The climate of study area is mostly semiarid and the minimum and maximum of temperature in the area are À14 C in winter and +31 C in summer, respectively [25,26].
This cross-sectional study was carried out during winter of 2016 on 50 residential houses which were randomly selected with an emphasis on coverage of whole investigated area from Shabestar, Khamaneh, Vayqan and Daryan cities. The location of study area and sampling points has been shown in Fig. 3.
A passive sampling using solid-state nuclear detectors of SSNTDs type CR-39 was performed to measure the concentration of radon gas. CR-39 polycarbonate film placed inside a plastic holder. Detectors were numbered and for each building unit, two detectors were installed in the bedroom and living room and left for a period of three months while placed at a distance of 90 cm from the floor and away from sunlight and windows.
After three months of exposure, detectors were wrapped with aluminum foils and transferred to the Reference Radon Lab, Mazandaran University of Medical Sciences. In the laboratory, then detectors placed in a 6.25 N solution of NaOH at 85 C for 3 h due to magnify the alpha tracks. The films were washed with distilled water after the time was spent to read. The automatic counting system with taking microscope images was used for counting alpha particles (tracks) recorded on CR-39 detectors, then using calibration and conversion factors track density was converted to the radon concentration in Bq/m 3 . All the radon determination process was carried out according to the U.S. EPA protocol [1,27].
In order to evaluation of affecting parameters on radon levels, the information about type of building (villa or apartment), floor numbers, the age of the building, the type of skeleton construction materials and cracking on the wall and roof were gathered and recorded.
Statistical analysis of the data was performed by SPSS version 20 and Excel v. 2016. The variables were normalized with the log-transformation and analyzed using parametric tests.
The average concentration of indoor radon in Shabestar, Khamaneh, Vayqan and Daryan buildings were (in order) 43 Table 1. However, the mean value is higher than the global average (40 Bq/m 3 ) [28].The maximum measurement was 520.12 Bq/m 3 (approximately 3.5 times higher than the limit imposed by EPA) in bedroom (1 st floor) of a 5-year age building with granite stones in facade and some artificially fashioned building materials (Patina) in rooms.
The minimum and maximum values in bedrooms amounted to 3.92 and 520.12 Bq/m 3 while the concentration of radon gas in living rooms was ranged from 4.94 to 155.02 Bq/m 3 in the studied area. Despite the large variations in bedrooms than living rooms, there was no statistical significant difference between radon concentration in these environments (p > 0.05(. The results of Pearson correlation analysis between the indoor radon and some affecting variables in radon emission was presented in Table 3 and confirmed a good linear relationship between bedroom and living-room radon concentration. Although, some moderate associations were detected with the factors "building age, cracks on the walls/floor, window type"; nonetheless the pairwise regression analysis confirmed no strong correlation between them and radon concentration in bedroom and living rooms. Also, a significant difference was observed between radon levels in apartment (multi-story homes) and villa dwellings (p < 0.05). According to the Fig. 4, radon content was higher in villa relative to apartments The average radon content was found to be 65.51 Bq/m 3 for the villa and 34.22 Bq/m 3 for the apartments. Because radon is heavier than air, it tends to sink to the lowest possible levels of homes and also its concentrations are appreciably high in isolated houses than in blocks of apartments [29]. As reported in some other studies [14,[34][35][36].

Radiation dose estimation
The mean annual effective doses resulting from the radon gas in different floors of dwellings in Shabestar County are shown in Table 4. The results comparison with data obtained from other parts of the country (See Table 1) suggests that the average mean value for Shabestar County is less than the calculated mean effective dose rates for other parts of Iran (2.6 AE 2.4 mSv.y À1 ).
The mean annual effective dose and effective dose (to lungs) received by the residents of the studied area in Shabestar county were estimated to be 1.4 and 3.5 mSv/y respectively ( Table 5). The mean ELCR for indoor exposure in the area was found to be 0.54/100 people that is small as compared with action level of EPA, the estimated risk of 1.3% corresponding with radon exposure of 148 Bq/m 3 for the entire population [32].
The annual average effective doses received by residents in the villa homes (1.65mSv.y À1 ) are about two times the doses received in the apartments (0.83mSv.y À1 ).

Additional information
Soil, uranium and phosphate mines, and coal combustion can be considered as the main sources of radon release to the environment. Outdoors radon levels are generally low. Indoors radon levels, in buildings and homes especially in basements and lower floors can become much higher because of radon entrance through cracks and openings in the foundation. Health effects of radon are well known [33]. Exposure to radon often occurs primarily from breathing radon in air, resulting in an increase in the incidence of lung cancer due to damage DNA of cells lining the airways. Exposure to radon is the second leading cause of lung cancer. The World Health Organization (WHO) has estimated that in a country, between 3-14% of all lung cancers can be attributed to radon. This percent depends on the national average radon level and smoking prevalence [34].
To the best of our knowledge, no attempts have been made to harmonize data on the distribution of indoor radon concentration and mapping at the country level so far. To assess and compare data measured in different cities, we produced a preliminary map but it is evident the reported data is sketchy and local surveys are still ongoing. The average radon activity concentration less than 100 Bq/ m 3 was found in 64% of studied areas in Iran and no more than 10% of them have radon concentration exceeding 200 Bq/m 3 . Also, the geometric mean of radon concentration was calculated to 72.05 Bq/m 3 .
Considering Shabestar County, the radon data recorded in dwellings (mean: 56.19 AE 45.96 Bq/m 3 ) is less than prescribed levels by WHO, EPA, ICRP and as well values averaged at the level of country (117.4 AE 97.7 Bq/m 3 ). Also, the value of the annual effective dose ranged from 1.12 to 1.66 mSv.y À1 with   a mean value of 1.4 mSv.y À1 , which is below even the lower limit of the recommended action level of 3-10 mSv.y À1 . Due to complexity of influential factors and variability of radon gas levels, conducting further studies especially on radon release sources, local geology and soil-gas radon concentration, radon concentration in drinking water and estimation of annual ingestion and inhalation doses is recommended.

Declaration of Competing Interest
The authors declare that no conflict of interest exists in publishing this article.