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Assessment of lung cancer risk due to exposure to radon from coastal sediments

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A. El-Gamal1 and G. Hosny2

تقييم اختطار الإصابة بسرطان الرئة الناجم عن التعرُّض للرادون المنبعث من التـرسُّبات الساحلية

أيمن الجمل، جيهان حسني

الخلاصـة: أجرى الباحثان تقيـيماً لاختطار الإصابة بسرطان الرئة الناجم عن التعرُّض المنزلي لغاز الرادون 222Rn المتواجد بصورة طبيعية، في داخل المباني وخارجها، واستخدما سلسلة من المعادلات لتقدير تركيزات الرادون داخل المباني وخارجها، وما يتـرافق مع ذلك من مخاطر الإصابة بسرطان الرئة، في عدد من المناطق الساحلية في مصر. وقد بلغ متوسط تركيز الرادون 42.98 بيكيريل/م3 داخل المباني (بانحراف معياري قدره 33.12) و8.63 بيكيريل/م3 خارج المباني (بانحراف معياري قدره 6.16) وبلغ وسطي اختطار الإصابة بالسرطان المحرّض بالرادون (مقدَّراً بعدد الوفيات لكل مليون شخص) 83.4 في داخل المباني (بانحراف معياري قدره 64.67)، و25.1 خارج المباني (بانحراف معياري قدره 19.52) وكانت المعدَّلات أعلى في المناطق الغربية من البلاد مقارنة بالمناطق الشرقية، إلا أن أعلى المعدَّلات وجدت في رشيد (دلتا النيل)، وتبين أن التدخين يزيد كثيراً من اختطار الإصابة.

ABSTRACT: We conducted a lung cancer risk assessment for internal exposure to naturally occurring 222Rn gas both indoor and outdoor. A series of equations were used to estimate Rn concentrations indoor and outdoor and the associated lung cancer risk in some coastal regions in Egypt. The mean 222Rn concentrations were 42.98 (SD 33.12) Bq/m3 and 8.63 (SD 6.16) Bq/m3 indoor and outdoor respectively. The mean risk of radon-induced cancer (deaths per million population) was 83.4 (SD 64.67) indoors and 25.1 (SD19.52) outdoors. Levels were higher for western regions of the country compared to eastern ones but the highest levels were in Rashid (Nile delta). Smoking was shown to increase the risk considerably.

Évaluation du risque de cancer du poumon dû à une exposition au radon provenant de sédiments côtiers

RÉSUMÉ: Nous avons mené une évaluation du risque de cancer du poumon lié à l’exposition interne au gaz radon (222Rn) présent à l’état naturel à l’intérieur comme à l’extérieur des habitations. Une série d’équations a été utilisée pour estimer les concentrations en radon en milieu clos et en plein air, ainsi que le risque associé de cancer du poumon dans certaines régions côtières d’Égypte. Les concentrations moyennes de 222Rn étaient respectivement de 42,98 (écart type 33,12) Bq/m3 et de 8,63 (écart type 6,16) Bq/m3 à l’intérieur et à l’extérieur des habitations. Le risque moyen de cancer du poumon provoqué par le radon (nombre de décès pour un million d’habitants) était de 83,4 (écart type 64,67) à l’intérieur des habitations et de 25,1 (écart type 19,52) à l’extérieur. Les niveaux étaient plus élevés dans les régions de l’ouest du pays que dans l’est, mais c’est à Rosette dans le delta du Nil que l’on enregistrait les niveaux maximums. Il a été démontré que le tabagisme augmentait considérablement le risque de cancer.

1Department of Oceanography, Coastal Research Institute, National Water Research Centre, Alexandria, Egypt.
2Environmental Health and Molecular Carcinogenesis Division, Department of Environmental Studies, Institute of Graduate Studies and Research, University of Alexandria, Alexandria, Egypt (Correspondence to G. Hosny: This email address is being protected from spambots. You need JavaScript enabled to view it.)
Received: 05/02/06; accepted: 18/07/06
EMHJ, 2008, 14(6):1257-1269

Introduction

Radon (Rn) is a naturally occurring radioactive, odourless and colourless gas [1]. It is of particular concern because it is ubiquitous, and it is very mobile in the environment [2]. There are 3 isotopes of radon but it is 222Rn that is of particular epidemiological interest [1].

It is well known that inhalation of the short-lived decay products of 222Rn provides the main pathways for radiation exposure of the lungs [3]. When 222Rn gas itself is inhaled, most is exhaled before it decays but 222Rn progeny may be deposited on the cells lining the airways where they can damage the DNA and potentially cause lung cancer.

It is recognised that 222Rn is a health hazard in both mining and non-mining environments [4,5]. 222Rn is the second most important risk factor for lung cancer after smoking, and causes between 6% and 15% of all cases [6]. Exposure to 222Rn in the home and workplace is one of the main risks of exposure to ionizing radiation, causing tens of thousands of deaths from lung cancer each year [7,8].

The concentration of 222Rn and 222Rn daughters in the indoor air depends on the amount of 226Ra in the soil and how easily 222Rn products can move through soil and walls and mix with room air. Because 222Rn is a gas, changes in the atmospheric pressure also affect its emission from the ground and its accumulation in the indoor air [9].

Risk assessment is a method to assess the likelihood that exposure to hazardous agents will harm people or the environment and is conducted to estimate the probability of specific harm to an exposed individual or population [10].

The purpose of our study was to conduct a risk assessment of lung cancer due to inhalation of either indoor or outdoor 222Rn in Egypt. In order to carry out the risk assessment, a series of equations were systematically used to estimate i) the concentrations of 222Rn in air that emanated and was exhaled from sediments containing 226Ra and ii) the risk of death from lung cancer from 222Rn internal exposure.

Methods

The following estimations and calculations were made.

Estimation of 222Rn concentrations in both the indoor and outdoor environment based on its emanation and exhalation from sediments containing 226Ra using a combination of previously published equations.

Calculation of 222Rn internal exposure.

Calculation of annual effective doses from 222Rn exposure.

Calculation of the risk of death from lung cancer from 222Rn exposure based on international risk values and the risk assessment model [11,12].

Estimation of the risk of death from lung cancer from 222Rn exposure taking account of the synergistic effects with smoking.

Principles for the risk assessment model

The original values used in these calculations are taken from published data of coastal sandy sediments [13–15]. The 226Ra values were measured by means of a high resolution, low background gamma spectrometer, using a multichannel analyser and coaxial high-purity germanium detector [13,14]. The geographical locations of the selected regions under consideration are given in Table1, and are located on the Mediterranean coast of Egypt. These stations were selected as large coastal cities based on radiological assessment of the Egyptian Mediterranean coast [15]. Coastal inhabitants of these governorates probably use the marine sediments as building materials. Black sand, which is present in sediments in Rashid, was also assessed for 222Rn emanation and exhalation rates, because it is known to have relatively high amounts of uranium. According to the Egyptian Central Agency for Public Mobilization and Statistics (CAPMAS), the population census estimates (× 103) on 1 January 2006 of Matrouh, Alexandria, Rashid, Damietta, Port Said and North Sinai governorates were 278, 3885, 4777, 1100, 546 and 317 people respectively.

The results of risk due to exposure to 222Rn and its daughters are presented as risk per million inhabitants because the real population number has spatial and temporal variations. The inhalation rate varies with activity level, age, weight and general physical condition but some of these variations were not taken into account in the present work [16]. This calculation was carried out for inhalation rate of an adult man.

In the present study, the estimated risks were modified according to sex and smoking habit. The population unit was theoretically divided equally into 500 000 males and 500 000 females. The categorization of Rogers and Powell-Griner [17] and Maillie et al. [18] was used to classify smokers as: light smokers (LS) < 25 cigarettes/day and heavy smokers (HS) ≥ 25 cigarettes/day. Former smokers (FS) have a reduced survival when compared with never smokers (NS). We assessed the risk for both males and females for all categories of smoker (NS, FS, LS or HS) according to Shopland, Eyre and Pechcek’s relative risk (RR) values [17]. They suggested the values of RR for males for FS, LS or HS as 9.36, 18.8 and 26.9 respectively, and for females as 4.69, 7.3 and 16.3 respectively.

Methods for calculating attributable risk

Calculation of 222Rn emanation and exhalation rates

In order to estimate the amount of indoor and outdoor 222Rn, a series of equations from (1) to (5) was used to calculate the emanation and exhalation rates of 222Rn from its original 226Ra sediment holder. The typical value of the 222Rn emanation coefficient for sand is 0.14. The value of total porosity was changed according to the grain size from 0.43 for fine sand to 0.39 for coarse sand [19]. The concentration of 222Rn emanating from dry sediments gas, CERn (Bq/m3), in the absence of 222Rn transport, can be calculated from equation (1) [3,19].

Where: R is the 226Ra activity in the sediment particles (Bq/kg), E is the 222Rn emanation coefficient, ρ is the dry bulk density of the soil (kg/m3) and ε is the total porosity.

Exhalation rate (flux density) of 222Rn at the surface of dry sediments, JD (Bq/m2s), can be calculated using equation (2) [3].

 

Where: λ is the 222Rn decay constant (2.1 × 10–6/s) and L is the diffusion length and derived from equation (3) [3,20]

Where: De is the effective 222Rn diffusion coefficient (m2/s) and is derived from equation (4) [20].

Where: Do is the 222Rn diffusivity in open air (1.1× 10–5 m2/s) and Rs is the volumetric water saturation (0 in dry sediments).

The rate of 222Rn entry from sediments in cubic metre volume U (Bq/m3h) is given by equation (5) [3].

Where: SB is the surface area of the walls (m2), JD is the flux density (Bq/m2h) and V is the volume of the area (m3).

Calculation of indoor and outdoor 222Rn

The 222Rn concentration in air in a typical room is determined by the equilibrium between the 222Rn gain (exhalation from walls and soil) and loss (ventilation and 222Rn radioactive decay). When the room is open to the outside atmosphere, the 222Rn concentration is low due to the outdoor level. Using the single compartment room model with different ventilation rates, 222Rn concentrations can be estimated either indoor or outdoor according to the ventilation rate. Ventilation rates 1 and 5 h were used to estimate indoor and outdoor 222Rn respectively using equation (6) [21].

Where: CRn is the 222Rn concentration (Bq/m3), λ0 is the 222Rn decay constant (7.58 × 10–3/h) and λυ is the ventilation rate (/h).

Calculation of 222Rn exposure

The equilibrium factor (F) as in equation (7) below is the ratio of the equilibrium equivalent 222Rn concentration (CEEC) to the 222Rn concentration (CRn) [22].

 

A special unit, the working level (WL), is used to describe exposure to short-lived radioactive decay products of 222Rn. The WL is defined as any combination of short-lived 222Rn decay products in 1 litre of air that will result in the ultimate emission of 1.3 × 105 MeV of alpha energy [23].

Exposure to 222Rn (WL) both indoors and outdoors can be calculated from the relation in equation (8) (1 Bq/m3 = 0.27mWL)[3], which can be converted to the form of equation (9).

 equation-7

The working level month (WLM) is the unit of cumulative exposure and defined as the exposure to 1 WL for 170 hours (1 working month) [24] per unit 222Rn concentration (EEC). Exposure to 222Rn (WLM) both indoors and outdoors can be calculated from equation (10) [3,23].

 

Calculation of annual effective doses from 222Rn exposure

The annual effective doses are derived from equation (11) below [22]. The dose conversion factors used in the calculations in this study were based on the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) values for indoor and outdoor 222Rn as 3.6 and 5.4 (nSv/h)/(Bq/m3) respectively [22].

Where: DRn is the annual effective dose from 222Rn exposure (indoor or outdoor) (mSv/year), CRn is the concentration of 222Rn in indoor or outdoor air (Bq/m3), Of is the occupancy factor (7000 hours indoor or 1760 hours outdoor), n is the conversion factor from nano (n) to milli (m) and DfRn is the dose conversion factor for 222Rn, calculated from equation (12).

 

Where: F is the equilibrium factor (0.4 for indoor and 0.6 for outdoor) and Dc is the dose coefficient (9 EEC).

Calculation of risk of death from lung cancer from 222Rn exposure

According to ICRP [11], the population cancer mortality risk per WLM of whole body dose is given as:

Where: R is the number of deaths per 1 000 000 persons due to 222Rn daughter exposure (EWLM) for 1 year.

Results

The calculated 222Rn emanation and exhalation rate values are shown in Table 1. Radon concentrations in indoor and outdoor air were calculated using equation (6) and are also listed in Table 1. The worldwide median value and other national and international values are shown in Table 2 for comparison.

The mean 222Rn concentrations in the areas under investigation were 42.89 (SD 33.12) Bq/m3 and 8.63 (SD 6.66) Bq/m3 for indoor and outdoor air respectively. The 222Rn concentration in indoor air ranged from 17.696 Bq/m3 in Damietta to 129.197 Bq/m3 in Rashid. The detected 222Rn concentration in outdoor air ranged from 3.561 Bq/m3 in Damietta to 25.996 Bq/m3 in Rashid.

The UNSCEAR committee suggests rounded values for the equilibrium factor of 0.4 and 0.6 for the indoor and outdoor environment respectively [22]. The calculated CEEC values for both the indoor or outdoor environment are shown in Table 3. Indoor CEEC ranged from 7.078 to 51.679 Bq/m3 and outdoor CEEC from 2.136 to 15.597 Bq/m3. The minimum and maximum values of both indoor and outdoor air were seen in Damietta and Rashid respectively.

Exposure to 222Rn as WL and WLM both indoor and outdoor were calculated using equations (9) and (10) (Table 3). The 222Rn exposure (WL) in indoor air ranged from 0.0019 WL in Damietta to 0.014 WL in Rashid. The exposure from outdoor air ranged from 0.0006 WL in Damietta and North Sinai-2 to 0.0042 WL in Rashid. Table 4 gives the international and recommended levels of indoor exposure to 222Rn (EWL in) for comparison. 222Rn exposure (WLM) in indoor air ranged from 0.098 WLM/y in Damietta to 0.719 WLM/y in Rashid. The exposure from outdoor air ranged from 0.03 WLM/y in Damietta to 0.217 WLM/y in Rashid (Table 3).

The annual effective doses from 222Rn exposure both indoor and outdoor are shown in Table 5. The maximum indoor and outdoor values were 3.256 and 0.247 mSv/y respectively, detected in Rashid. The minimum indoor and outdoor values were 0.446 and 0.034 mSv/y respectively, detected in Damietta.

As regards the estimation of lung cancer attributable to exposure to 222Rn and its progeny, the maximum indoor and outdoor risks were 252 and 76 per million population respectively, in Rashid. The minimum indoor and outdoor risks were 34 and 10 per million population respectively, in Damietta (Table 5).

As seen in the tables, large differences were observed between the black sand and the other samples for all the values estimated.

Figures 1 and 2 show the variations in the risk of death from lung cancer for males and females according to smoking status and exposure to indoor and outdoor 222Rn progeny in each region. The lowest risk was detected in Damietta for both NS males and females with an estimated 17 and 5 deaths per 500 000 due to indoor and outdoor 222Rn exposure respectively. The risk rose steeply to 464 and 140 deaths per 500 000 for HS males and 281 and 85 deaths per 500 000 for HS females respectively.

Figure 1 Estimated risk of death from lung cancer due to indoor radon exposure per 500 000 by sex for never smokers (NS), former smokers (FS), light smokers (LS) and heavy smokers (HS)

Figure 2 Estimated risk of death from lung cancer due to outdoor radon exposure per 500 000 by sex for never smokers (NS), former smokers (FS), light smokers (LS) and heavy smokers (HS)

Discussion

The estimated results are discussed according to the risk assessment model steps. These steps can be summarized as hazard identification, hazard characterization, exposure assessment, dose assessment and risk characterization [23,24].

Hazard identification and characterization

The investigation of 222Rn emanation and exhalation rates showed different patterns between the regions west of Rashid (Matrouh and Alexandria governorates) from the regions east of Rashid (Damietta, Port Said, North Sinai governorates). 222Rn exhalation rates in western regions were higher than eastern regions. This may be due to the presence of uranium in the mineral structure of the sediments. Rashid had the highest 222Rn exhalation rate. It is located at the end of Rashid estuary, which is characterized by the presence of black sand in its sediments. Our study demonstrated large differences between the black sand and the samples taken from other regions for all factors estimated. The presence of black sand in this region has been previously reviewed [13]. The high 222Rn exhalation rates of some dry sediments indicate that 222Rn could contribute significantly to the lung dose rate in houses built with these sediments. The concentration of indoor 222Rn can decrease rapidly with increasing ventilation rate.

The mean concentration of indoor 222Rn (42.89 Bq/m3) is lower than the worldwide median value (46 Bq/m3) [3]. The international published mean values in randomly selected dwellings in some countries for indoor 222Rn concentrations show wide variations; some of them are relatively higher such as the Czeck Republic (140 Bq/m3), Finland (120 Bq/m3) and Minia City, Egypt (123 Bq/m3) and others are lower such as Cyprus (7 Bq/m3) and Kazakhstan (10 Bq/m3) [3,25]. The arithmetic mean values 39.5, 41, 45, 46 and 48 Bq/m3 for Cairo-Egypt, Poland, Romania, United States and Belgium respectively [3,26] are the most comparable with our value. The estimated indoor 222Rn concentrations were lower than the published upper limits except for Rashid (129.197 Bq/m3) which was higher than the Canadian Nuclear Safety Commission public exposure limit (70 Bq/m3).

The mean indoor 222Rn concentration in the western regions was higher [43.76 (SD 9.35) Bq/m3] than eastern regions [20.22 (SD 2.12) Bq/m3]. The same was true for outdoor 222Rn. Again Rashid had the highest 222Rn concentration both indoor and outdoor. Very high 222Rn concentrations were found for pure black sand (1104.35 and 222.21 Bq/m3 for indoor and outdoor 222Rn respectively).

The US Environmental Protection Agency (US EPA) suggests modifications to homes when 222Rn levels exceed 148 Bq/m3 (EPA action level) [8]. The Canadian Nuclear Safety Commission uses 148 Bq/m3 as the upper limit for annual occupational exposure and 70 Bq/m3 as the annual exposure limit for the general public [9]. On the other hand, the National Council on Radiation Protection and Measurements remedial action level is twice as high as the EPA limit at 296 Bq/m3 [8,27].

According to 222Rn classification of soil reported by Ljungquist [28], the majority of the estimated values of indoor 222Rn in our study are around the normal risk level (10.0–50.0 Bq/m3). Matrouh-1 and Alexandria-1 fell between normal risk and high risk. The only region that could be categorized as a high risk area was Rashid.

Exposure assessment

The main contribution to the exposure of the population to natural radiation comes from the inhalation of short-lived 222Rn decay products. Direct measurements of the concentrations of all short-lived decay products of 222Rn are difficult and limited. They are estimated from considerations of equilibrium between 222Rn and its decay products. Applying the classification of indoor exposure of Walsh and Lowder [23], where an exposure around 0.05 WL is considered high and 0.5 WL extremely high, the estimated exposure levels in our study are all less than this guideline. Walsh and Lowder also noted that the outdoor exposure is generally near 0.001 WL. The outdoor exposure levels in our regions are in line with this guideline except Rashid which has a higher value (0.0042 WL).

The western regions had exposure values slightly higher than the EPA-recommended background value (0.004 WL) [9]; Rashid showed a considerably higher value. On the other hand, the eastern regions had lower values.

Dose assessment

Actual measurement of lung dose is not feasible, and measurement of 1 or more parameters related to the lung deposition of 222Rn daughters has been used as proxy for actual lung dose. In the home it is only feasible to measure 222Rn concentration or 222Rn daughter concentrations [21].

The mean annual effective doses from 222Rn exposure in our study are consistent with the worldwide values 1.0 and 0.1 mSv/y for indoor and outdoor exposures respectively [22]. The same pattern was again observed between the western and eastern regions, the western regions having higher means for both indoor and outdoor air than the eastern regions. Rashid again also had the highest annual effective dose.

The annual effective doses calculated are well below the recommended dose limits of 20 mSv/y (corresponding to 4 WLM or 3000 Bq/m3 222Rn gas concentration) averaged over 5 consecutive years or an effective dose of 50 mSv (corresponding to 10 WLM or 8000 Bq/m3 222Rn gas concentration) in any single year [29]. UNSCEAR gives the annual effective dose equivalent per capita from natural sources in areas of normal radiation background as 900 µSv/y for internal exposure of 222Rn and 222Rn daughters until polonium-214 [30].

Risk characterization

Risk characterization is the final step of risk assessment. The attributable risk is defined as the excess lung cancer rate in a population due to 222Rn exposure as a fraction of the total lung cancer rate [27]. The number of annual lung cancer deaths that can be attributed to residential exposure to 222Rn and 222Rn daughters is 350 per 106 population per WLM/year, based on the risk value recommended by ICRP [11], where 350 is the number of persons that will be expected to die per 1 000 000 persons due to exposure to 1 WLM/year.

We calculated that the overall mean number of radon-induced lung cancer deaths was 83.4 (SD 64.67) and 25.1 (SD 19.52) deaths per million for indoor and outdoor exposures respectively. Compared with a risk estimation value for the Greek population of 65 deaths per million [31], our values are slightly higher for indoor but lower for outdoor exposure. The same pattern was observed between the western and the eastern regions, the western regions having higher means for both indoor and outdoor risks with 85 (SD 18.38) and 25.6 (SD 5.5) deaths per million respectively compared with the eastern regions with 39.25 (SD 4.11) and 11.75 (SD 1.26) deaths per million respectively. Rashid had the highest estimated risks with 252 and 76 deaths per million for indoor and outdoor 222Rn exposure respectively.

The US regulatory agencies assumed in cancer risk assessment that risk is directly proportional to dose and that there is no threshold of carcinogenesis. On the other hand, it has been established in recent years that there is a threshold for lung cancer induction by 222Rn in humans of around 600 to 1000 Bq/m3 in air for permanent intake, in particular at home and at the working place in areas of high natural uranium/radium geological situations [32]. All the estimated values in our study were below this threshold range, except for black sand which was higher.

For smokers the risk of lung cancer is significantly higher due to the synergistic effects of 222Rn and smoking [8]. Our results show an increased risk of 222Rn-induced death from lung cancer among smokers compared with non-smokers, highlighting the importance of the synergistic effect of 222Rn exposure and tobacco exposure.

Conclusion

The majority of our estimates for indoor 222Rn are within the normal risk level. However, Matrouh-1 and Alexandria-1 fell between normal and high risk while Rashid can be classified as a high risk area. The estimated risk of radon-induced lung cancer death in Rashid was 252 and 76 deaths per million for indoor and outdoor 222Rn exposure respectively. Furthermore smoking increased the risk of death from radon-induced cancer.

Our data suggest that increased attention needs to be paid to exposure to radon and the associated health risks in order to preserve public health and reduce the incidence of cancer. The estimated values for lung cancers possibly due to indoor 222Rn should be considered preliminary. Further epidemiological studies should be undertaken to examine the suggested hypothesis.

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