Estimating excess of lung risk factor of radon gas for some houses in Al-Fallujah city

Instruments for the measurements of radon, thoron and its decayproducts in air are based mostly on the detection of alpha particles.The health hazards of radon on general public are well known. Inorder to understand the level and distribution of 222Rn concentrationsindoor in Al-Fallujah City; new technique was used, this techniquewas three radon–thoron mixed field dosimeters is made up of a twinchamber cylindrical system and three LR-115 type II detectors wereemployed. The aim of this work was to measurement radon gas usingSSNTD technique door in in Al-Fallujah City, and estimation ofexcess in cancer due to increment in radon gas. Results for sampleswhich are collected from January to April 2013 show that the 222Rnconcentration varies from 52.33 to 108.70 Bq.m-3 with an average of84.64 Bq.m-3, according to EPA and ICRP, the average indoor radonlevel should be 148 Bq/m3 and 300 Bq/m3, respectively, whereasapproximately 15 Bq/m3 (ranging from 1 Bq/m3 to 100 Bq/m3 ofradon concentration is normally found in outside air. Also the valuesof annual effective dose and excess lung cancer per million peopleper year (ELC) have also been calculated and found to vary from0.941 to 2.350 mSv.y-1 with an overall average of 1.581 mSv.y-1 and,565 to 1410 with an overall average 948, respectively.


Introduction
Radon is a form of ionizing radiation; therefore exposure to it is the second leading cause of lung cancer after smoking. Radon is an odorless, tasteless and invisible gas produced by the decay of naturally occurring uranium in soil and water. Lung cancer is the only known effect on human health from exposure to radon in air. Radon is produced by the decay of naturally occurring uranium in soil and water, and is found in outdoor air and in the indoor air of buildings of all kinds [1]. Uranium is found in most soils and in granite as its nucleus decays to form stable lead, a process taking many, many years; it changes from one radioactive element to another in a sequence known as the uranium decay cycle. Partway through this cycle, the element radium becomes radon which as a gas moves up through the soil to atmosphere [2]. Radon, Rn-222 (T 1/2 = 3.82 days), is a daughter product of 226 Ra, which in turn is derived from the longer-lived antecedent, 238 U. Thoron, Rn-220 (T 1/2 = 56 seconds) is a daughter of 232 Th, which is present in larger amount in the earth's crust than radon. Because of thoron's short half-life, it is essentially all gone before it leaves the ground, and is of no significant radiobiologic consequence. These radionuclide series are present in slowly decreasing amounts in the environment (geologic time scale), due to radioactive decay of their parents, which has been known and understood since the end of the 20 th century. Widely varying radon levels exist in different regions related to geological circumstances. New concern regarding radon exposures is traceable to the discovery that there are more houses with high radon levels than previously realized and to the use of a new method of expressing and summing doses from partial body exposures, such as the lung dose from radon daughters. This method of expressing dose was promulgated by the ICRP and the NCRP based on defined weighting factors which make it possible to sum partial body doses and thereby estimate a total body dose which would have a quantifiable risk. This quantity is defined as the Effective Dose (ED). Thus, the previously estimated partial body environmental radon dose to the tracheobronchial epithelium (TBE) (2500 mrem/year) was not included in whole body dose calculations because that exposure was limited to a small fraction of the body [3]. There are many articles studied the concentration of radon in several countries such as:  In 2010, Abdullah [7].
In 2003, a new war against Iraq brought an even greater load of depleted uranium to Iraq. In November 2004, the city of Fallujah was targeted by heavy attacks, by the latest occasion of which the city was completely destroyed. Exceptional increases in birth defects and cancer among the survivors led to the expectation that depleted uranium had been used in the heavy fighting between the Americans and the locals. The aim of this work is to measure radon gas using SSNTD technique and estimation of excess in cancer due to increment in radon gas.

The study area
Al-Fallujah is a city in the Iraqi province of Al-Anbar, located at 69 kilometers west of Baghdad on the Euphrates. Al-Fallujah city grew from a small town in 1947 to a population of 326,471 within Iraq, it is known as the "city of mosques" for the more than 200 mosques found in the city and surrounding villages. Fallujah lies in a strategic position at a junction between the Iraqi Capital; Baghdad, and Al-Anbar Governorate. This city stands at an elevation of around 40 meters above sea level. The Twin cup technique was applied in order to prepare the radon dosimeters. These dosimeters were settled in 16 different locations in Al-Fallujah city. In order to measure the outdoor radon concentrations, the locations of the dosimeters were chosen to cover approximately the hall area of Al-Fallujah. However, Table 1 shows the locations of the radon dosimeters.  [8].The WL unit was developed for use in radon occupational exposure assessment since often there was incomplete information on the degree of equilibrium with daughter products. It is the dose delivered in one liter of air that results in the emission of 1.3 x 10 5 MeV of potential alpha energy. The amount of time spent in the mine or in the home determines the number of WLM associated with a particular exposure level, but because most people spend more time at home than at work, the WLM could be higher than from a comparable mine radon daughter concentration. Typical outdoor levels in the U.S.A. are given by NCRP report No. 78 [9] as 0.2 pCi/Liter. The correspondence between WLs and radon concentration in air in pCi/liter depends on the extent to which radon daughters (which impart dose to the tracheobronchial epithelium dose, "TBE") are in equilibrium with the parent radon. At complete equilibrium, one pCi/liter results in an exposure equal to 0.01 working levels.

Radiation hazard
Radiation effects comprise of (1) deterministic effects that occur with certainty after doses high enough to cause major cell killing and (2) stochastic effects that are considered to occur more or less in proportion to the dose at all dose levels. Table 2 shows average dose received due to different radiation sources [10].

Equilibrium factor
The short-lived decay products of radon gas are assumed to achieve a state of radioactive equilibrium with their parent in open air, with an effective half-life of about 30 min. If one considers only radioactive disintegration, the state of equilibrium generally varies as a function of the mean age of the radon atoms. Atmospheric processes such as turbulent mixing or wet and dry deposition however results in a state of disequilibrium between the activity concentration of radon gas in air and its decay products associated with the aerosols. The state of equilibrium can then be described by the equilibrium factor, F, i.e. the ratio of the equilibrium equivalent concentration (EEC) to the radon activity concentration: F= EEC / C Rn (1) EEC = F×C Rn (2) Knowledge of the amount of the shortlived radon progenies in air is necessary to obtain: (1) equilibrium factor (F) and (2) the unattached fraction (fp) of radon progeny. The fraction of indoor radon progeny attached to airborne particles is highly dependent on particle concentration [11]. The particle concentration varies between 10 3 particles/cm 3 and 10 6 particles/cm 3 depending upon the aerosols sources and human activities indoors and outdoors.
The lungs dose due to the unattached radon decay products is much greater than the dose due to the radon progeny attached to the aerosol particles [12]. Unattached fraction can be measured directly or can be calculated from the concentration measurement aerosols. The fp values vary from 0.005 and 0.2 and depend on the concentration number of the aerosols. Freshly generated 218 Po ions are mostly positively charged (88%) and react rapidly with trace gases and vapors in air and become small particles called clusters or "unattached" decay products with a size spectrum between 0.5 and 3 nm diameters. Neutralization of the unattached 218 Po ions can be described by three processes: Recombination with small air ions, electron scavenging by OH radicals formed by radiolysis of water vapor and charge transfer by molecules of lower ionization potential [13].
EPA relates radon levels to the radon decay product levels by assuming that 40% of the radon decay products produced remain in the air for inhalation. However, equilibrium factor can vary between 0.1-0.9.

Lung cancer risk
Lung cancer risk is defined as the excess deaths per million persons per year (MPY) due to lung cancer per unit exposure to radon and its short lived daughters. A risk coefficient, defined as the number of lung cancer cases per MPY per working level month (WLM), is determined from the epidemiological data of the occupationally exposed mine workers. The excess lung cancer risk per MPY was estimated by using the following equation [14]:

Experimental work
The radon-thoron mixed field dosimeter employed for this work measurements is made up of a twin chamber cylindrical system (as mentioned in the previous subsection) using 12   During the exposure time, α-particles emitted by the radon and thoron radionuclides and their progeny have bombarded the SSNTD films. After the irradiation, the exposed films were etched in a NaOH solution with optimum conditions (2.5N at 70°C for 60 minutes for LR-115 II films). After these chemical treatments the track densities on the LR-115II SSNTD were determined by an optical microscope.
To estimate the radon, thoron and their progeny concentrations, the standard calibration method was used. The details of standard calibration methods, which used for Twin cup dosimeter, are described and the concentration of radon and thoron gases were calculated in terms of (Bq.m -3 ) using the following expression [15]. (4) where is the radon or thoron or their progeny concentration, is the tracks registered on SSNTD films, is the exposure time (in this work, 45 days were used), and is the calibration factor, which have the following values for the three films; [16]  for radon measurements only (compartment M) is = 0.023 ± 0.004 tracks.cm -2 d -1 /Bq.m -3 .  for radon and thoron measurements (compartment F) is = 0.018 ± 0.002 tracks.cm -2 d -1 /Bq.m -3 .  for radon and thoron and their progeny measurements (for bare mode) is = 0.020 ± 0.002 tracks.cm -2 d -1 / Bq.m -3 [16].

Results and discussion
The overall results, which contain the radon concentration, the equilibrium equivalent radon concentration, working level month, annual effective dose, and excess lung cancer per million person per year (ELC), of radon, thoron, and radon's progeny illustrated in Tables 3-5. It is clear from Table 3 that the values of radon gas levels in the districts of Al-Fallujah city varied from 52.33 to 108.70 Bq.m -3 with an overall average of 84.64 Bq.m -3 , and 0.291 to 0.605 for WLM with an overall average of 0.471, respectively. The values of annual effective dose and ELC have been found to vary from 1.320 to 2.742 mSv.y -1 with an overall average of 2.135 mSv.y -1 , and 792.22 to 1645.38 with an overall average of 1281.26, respectively. Fig.2 shows the histogram of the radon concentration at the measured location, while Fig.3illustrates the WLM as a function of radon gas. The annual effective dose and ECL was plotted against radon concentration and nice fitting equations were suggested, as shown in Fig. 4. It is evident from Table3 that there is a large variation in the values of concentration of radon. This is due to the reason that there was a wide variation in the construction of the houses in Al-Fallujah city. There are houses made of clay bricks and plastered without any white-wash having higher radon levels. The lower values of radon and its progeny have been observed in the houses which were plastered and painted with good quality distemper and adequately ventilated. It is also seen from Table 3 that all  investigated  locations  have  radon  concentration below the action level 200 Bq.m -3 recommended by ICRP [17].

Fig. 4: The black line represents Excess Lung Cancer per Million Persons per Year (ELC) as a function of radon concentration while the blue line represents the annual effective dose as a function of radon concentration.
For thoron gas only, the results illustrated in However, Fig. 6 shows the radon, thoron, and radon progeny concentrations for the measured locations. It is clear from this figure that radon concentration larger than radon progeny which in turn larger than thoron concentration. These values reflect the increase of radium contain in the Al-Fallujah environment (espial in soil) comparing with thorium contain.  [14].