A Systematic Review of Radon Investigations Related to Public Exposure in Iran

Background The main sources of radiation exposure of all living organisms including humans are natural. In fact, radon and its decay products are the cause of 50% of the total dose that is derived from natural sources. Because of the significant health hazards of radon gas, its levels are widely monitored throughout the world. Accordingly, considerable researches have also been carried out in Iran. Objectives The aim of this research is a systematic review of the most recent studies associated with evaluation of radon gas levels in Iran. The main emphasis of this study was on public exposure to radon gas. Materials and Methods The most important route of exposure to such radiation is indoor places. In this investigation measurement of radon in water resources, tap water, indoor places and exhalation of radon from building material, the major sources of indoor radon gas emission, were considered. Results Significantly high levels of radon gas were found mostly in water and residenvial buildings. Conclusions It conclusion with regard to the study of building materials, granite stone and adobe coverings cannot be recommended for construction purposes.


Background
Natural radiation is the main source of radiation in the surrounding environment. It is estimated that the contribution of natural sources of radiation exposure to humans is approximately 90% (1). Accordingly, radon and its decay products are the cause of 50% of the total dose originating from natural sources (2).
Radon (Radon-222), a trace element, is colorless, odorless and tasteless. It is a natural radioactive gas that is derived from uranium decay present in rocks and soil (2). Radon is highly soluble in the water, thus the radon gas present in the underlying rocky bed can easily pass through the soil and rocks, inevitably entering underground water sources. Therefore, soil and, various types of rock in the earth's crust and underground water are the main sources of radon gas propagation (3). The concentration of radon in the outdoors is much lower than in the indoor places, where subsequent radioactivity has been found to increase. The significant aspect of radon at high concentrations can be dangerous for humans, mostly leading to lung cancer (2,4). The alpha-emitting particles of radon gas that enter tissues through water, food and inhalation can have negative biological effects on such organs (5)(6)(7)(8).
Because of the significant health hazards associated with radon, its concentrations are widely monitored throughout the world. Considerable research has been carried out in Iran, mainly in the northern region, which has high background radioactivity. As reported by the United Nations Scientific Committee on Atomic Radiation the coastal city of Ramsar has been shown to have the highest levels of radioactivity when compared to other inhabited areas of the world (2). The studies that have been conducted on radon have been ineffective. Hence an investigation and review of all researches that have been carried out so far will be highly valuable to any future studies on radon gas in Iran.

Objectives
This paper presents a review of the studies that have been carried out on the measurement radon in water, indoor places and building materials used in Iran, all of which have a critical and important role in the exposure of the general population to radiation.

Materials and Methods
Initially, an investigation of the available research regarding the measurement of radon was carried out through an online literature search using the Pub-Med, Science Direct, ISI Web of Knowledge Scopus, Medlib, SID, Iranian Research Institute for Information Science and Technology (IRANDOC) and IRANMEDEX databases. The keywords "radon, Iran " or "radon measurement in Iran", used in this search. All studies associated with the mea-surement of radon until August 2012, in both the English and Persian languages, were collected and investigated. The relevance of articles have been screened by two independent reviewer initially. In total number of 1455 investigated papers, 176 titles and abstracts were recognized as potentially appropriate. Papers which containeded other natural radioactivity were excluded. The main purpose of this study was on public exposure to radon gas. The most important route of exposure to such radiation is indoor places.  In this investigation, the measurement of radon in water resources, tap water, indoor places, and exhalation of radon from building material, as the major sources of radon gas emission, was considered. The reviewe articles were included as potentially appropriate. The results of multiple publications were cosidered only once, and the newest reference of it is reported. All of papers were monitored for inclusion by two reviewers separately. Disagreement was resolved during disscusion. Due to inclusion and exclusion criterions,the full text of total 27 papers (12 studies on radon measurement of water resources, 11 studies on indoor radon measurement and 4 studies on radon exhalation from building material) were screened in this systematic review. All investigated articles in this review were cross-sectional studies. Accordingly, information on the methods measurement, sample size and sampling locations and the subsequent results of the selected studies are indicated in Tables 1 and 2. Radon concentration values in the water resources, indoor places and the amount of radon exhaled from construction materials are classified in above mentioned tables. In this paper, the studies involving the mining industry workers and occupational exposure to radon were not reviewed.

Results
A review of researches involving the evaluation of radon levels in Iran indicated that of the studies measured radon in water resources, especially in areas with high radiation levels and dewellings.
All investigate studies in this paper were description and outcome has not been reported as OR or RR. Therefore, the publication bias could not be prove or disprove by the Statistical and graphical methods.
The general overview of research regarding measurement of radon levels in the water resources in Iran (Table  1) showed that the concentration of radon was higher than the values recommended by USA Environmental Protection Agency (8). Thus such, high levels of radon in the water supplies must be reduced before reaching the consumers. In fact, a comparison of radon levels in surface and ground water sources indicated that the concentration of radon in the groundwater sources and hot springs were much higher than those of surface water sources . The highest levels of radon, in the range of 145 ± 37 to 2731 (Bq/L) were reported from the Mahallat hot spring in northern region. Therefore, using certain methods such as mixing groundwater with surface water in large reservoirs can reduce the radon activity to acceptable limits. In regions where only groundwater sources are used, aerating the water before consumption lead to a dramatic reduction of radon levels in drinking water.
A review of households radon studies ( Table 2) also shows that the radon concentrations, especially in areas with high background radiation, and basements during the cold seasons, were not at desirable levels, thus emphasizing the need for improving ventilation. The highest indoor radon concentration level was reported in a house in the Tallesh Mahalleh of Ramsar, that received doses of 640 mSV/y. Studies also indicated that the indoor radon levels in apartments were higher than those in houses, due mainly to the presence of unsuitable ventilation and air conditioning in the former.
A review of radon activity in building materials, demonstrated that the use of local granite and stone in areas with high background radiation areas should not be recommended. Investigation of Iranian compressed granite depicted that the amount of 222 Ra present in this material was 1.605 ± 0. 055 kBq/m 3 , which is 4 times higher than the level recommended by the International commission on radiological protection (ICRP) (200 -600 Bq/m 3) . According to the conclusion drawn by this review, the levels of uranium and radium present in granite are high, and can thus significantly increase radon levels in areas where it has been used. Hence, the use of such stones in buildings can become health hazard necessitating the need for a solution (32). Furthermore, a study on 10 pieces of granite stone used in building construction showed that 226 Ra and 232 Th cause emission of radon from granite stones (33). Other investigations also showed that the rate of radon release from building materials used in Ramsar areas with high background radiation. A linear correlation coefficient between the emission of radon and radium concentration was estimated as 0.90. The results showed that the radon exhalation rate and radium content in a number of local stones used in the basement were at high intensity levels. These were the main sources of radon and gamma emission from uranium (34). A study of different covering materials has indicated that individuals resident in houses covered with materials such as plaster and wallpaper received average annual doses less than those living in houses with walls covered with wood and plastic paint, (P = 0.05). Therefore, the use of wallpaper and plaster to cover the parts of residential buildings is recommended. In addition, a comparison the type of materials used in buildings presented a significant difference (P = 0. 05) in radon gas concentration levels between buildings, that were constructed with adobe and concrete. However, there were no significant differences between buildings constructed with concrete and brick material, and those that were built with adobe and brick buildings (31). In another study, results showed that houses constructed with adobe emitted radon gas more than buildings made of concrete , brick and plaster (P < 0.05), while plaster walls emitted the lowest levels of radon gas (P < 0.01) (25). According to other researchers, adobe has a highly porous structure, thus scattering high levels of radon gas.

Discussion
It should be noted that the most studies in Iran are in high background radiation areas, which it could be lead to overestimation of results presentation. On the other the only published articles were examined in this study also the number of investigated studies were limited, probably many studies are published and it is so far a study was part of gray literature.
In this paper, studies involving the evaluation of radon levels in Iran with an emphasis on public exposure rather than occupational were reviewed. Most of the reported was associated with measurement of radon in water supplies; especially in high background radiation areas, such as Ramsar, and also indoor places with health hazards.
Consequently, the highest values of radon gas in the water resources were reported in the hot springs of Mahallat and a house in the Talesh Mahalleh area of Ramsar. Most of the studies have proposed methods to reduce radon levels in water supplies, prior to its consumption by humans. In many of the studies involving the measurement of radon in residential buildings, the calculated dose received by humans was higher than the recommended amount. As for studies involving assessment of radiation levels in buildings, construction materials such as granite stone and adobe are thus not recommended. However, it should be noted that the evaluation of radon levels carried out in Iran, took place in a period of less than one year, where seasonal changes were not reported.