Estimation of unattached and aerosol-attached activities of airborne short-lived radon progeny in indoor environments

https://doi.org/10.1016/j.jenvrad.2021.106665Get rights and content

Highlights

  • Activity concentration of airborne radon progeny were studied in Moroccan houses and workplaces.

  • Room parameters and aerosol physical processes that affect the unattached and attached concentrations were determined.

  • Developed PC-based software was upgraded to solve mathematical equations and calculate required physical quantities.

  • Equilibrium-equivalent concentration (Ceq), unattached fraction (fp), and equilibrium factor (F) were estimated.

  • Dose conversion factors (DCFs) and effective dose were calculated.

Abstract

Inhalation of the airborne short-lived radon progeny is regarded as the most crucial way of internal exposure to the natural radiation dose delivered to the human lung. In this respect, this study aims to determine the unattached and aerosol-attached activities of radon progeny and to estimate some important physical parameters employed to assess the radiological impact of this radiation on humans. For this purpose, radioactive aerosol samples collected on polycarbonate membrane filters to measure total alpha activity by passive alpha dosimetric technique using CR-39 detectors in sixteen different locations including some houses and workplaces in El Jadida city, in Morocco. In addition, the room-specific parameters and aerosol physical processes that affect the unattached and attached activity concentrations were determined. The obtained experimental results by the three-count method and room model parameters were used as input data on a developed PC-based software that we have developed to solve mathematical equations and calculate required physical quantities. Accordingly, the individual activities of radon progeny namely 218Po, 214Pb, and 214Po as well as radon activity concentration were determined. Simultaneously, the unattached and aerosol-attached activity concentrations (Cju and Cja) of radon progeny were calculated based on the room model calculation. Consequently, some radiological quantities used in the calculation of the lung dose were estimated. The results showed that the indoor radon activity concentration in different targeted locations ranges between 38 and 143 Bq. m−3 with an average value of 84.8 ± 9.5 Bq. m−3. The average obtained values of the equilibrium-equivalent concentration (Ceq), unattached fraction (fp), and equilibrium factor (F) at low and good ventilation rates change respectively from (24 Bq. m−3, 0.08, and 0.25) to (34 Bq. m−3, 0.02, and 0.41). Under normal environmental conditions, the average obtained values of Ceq, fp, and F, in houses and at workplaces were (17 Bq. m−3, 0.07, and 0.25) and (32 Bq. m−3, 0.04, and 0.35) respectively. Depending on the different aerosol conditions and obtained values of unattached fraction fp, the calculated average values of dose conversion factors (DCFs) were 8.70 mSv.WLM−1 and 11 mSv.WLM−1 in houses and workplaces respectively. These values were in good agreement with the recommended values by ICRP, which are in the order of 9 mSv.WLM−1 and 12 mSv.WLM−1 for houses and workplaces respectively.

Introduction

The natural exposure to ionizing radiation, especially the internal exposure to the short-lived radon decay products, by inhaling in indoor houses and at workplaces contributes the greatest fraction, up to 50%, of the radiation dose delivered to the human lung of all natural sources of (UNSCEAR, 2000). Therefore, it has been currently recognized that airborne short-lived radon progeny in indoor environments is a potential danger to public health and causes lung cancer (WHO, 2009). Radon gas is infiltrated from rocks, soil, building materials, underground water, and natural energy sources (gas, coal, etc.) by molecular diffusion or by convection; diffuses into the atmosphere (Porstendörfer, 1994; Ivanova et al., 2017). This infiltration is typically the primary source of indoor radon pollution where it is in higher concentrations (about 2–10 times) inside of houses. Besides natural radiation sources, radon and its progeny are released from toxic waste (solid, liquid, or gaseous) of many industrial facilities into the atmosphere (Smerajec and Vaupotič, 2012; Boukhair et al., 2016a).

In general, these natural or anthropogenic airborne contaminants are originally in a molecular or atomic state, and they are undergoing removal by radioactive decay or by elimination processes like deposition and ventilation (Katona et al., 2005; Mishra et al., 2009). The main carrier of these radionuclides is the atmospheric aerosol that becomes available for inhalation and can be deposited in different respiratory tract regions (Smerajec and Vaupotič, 2012; Abdo et al., 2021). Their behavior, hence, is controlled by the physical behavior of the aerosol particles in indoor environments (Pagelkopf and Porstendörfer, 2003; Hammer et al., 2019).

There are about thirty-three known radioactive isotopes of the radon element. Three of them (222Rn, 220Rn and 219Rn) occur naturally in the free atmosphere. They come from the disintegration of radium isotopes in the natural radioactive decay chains of 238U, 232Th, and 235U in the Earth's crust (Shikha et al., 2018; Abdo et al., 2021). The 222Rn isotope is the most abundant in nature due to its relatively long lifetime (3.825 days) that allows it to travel, from its different sources, further in distance and then accumulate into the indoor environment (Vogiannis and Nikolopoulos, 2015; Abdo et al., 2021). 222Rn gas decays into solid radionuclides series of successive short-lived radon progeny (218Po, 214Pb, 214Bi, and 214Po) (Huet et al., 2001; Abdo et al., 2021). After radon's solid progeny generation, radioactive aerosols are formed in two steps. The newly generated radionuclides, mostly positively charged, react very fast with atmospheric trace gases and molecules of water vapor in the air, neutralize and grow up to clusters with a diameter between 0.5 and 5 nm, called unattached fraction (Skubacz and Wołoszczuk, 2019). In addition to the above processes, a large fraction of these clusters is attached to the surface of atmospheric aerosol particles within 1–100 s (by condensation or coagulation processes), forming the radioactive aerosol, called attached fraction (Porstendörfer, 1994). Both attached and unattached radon progeny aerosol in room's air accrue in terms of size and activity; their activity concentrations vary according to the strength of its parent 222Rn sources, and its removal rates (ventilation, deposition, decay, and attachment to aerosol particle). The inhaled attached and unattached radioactive aerosol particles are deposited in different regions of the human respiratory tract by one of three mechanisms: diffusion, impaction, and sedimentation. They carry sufficient energy amounts to interact with biological tissue in the lungs and leading to DNA damage that may cause cancer (Darquenne, 2012; Levin et al., 2017). The indoor distribution of airborne radon progeny is described by the activity size distribution that indicates that the unattached activity constitutes about 10% of the amount of the total activity whereas the aerosol-attached activities are the dominant fraction of all. The activity size distribution of the attached fraction can be described by three distinct modes, called the nucleation, the accumulation, and the coarse modes with the activity median aerodynamic diameters (AMAD) are 50, 250, and 2000 nm respectively. (James et al., 1991; Porstendörfer, 2001; Khalaf et al., 2019). It is worth noting that the largest radon progeny activity fraction is attached to particles in the accumulation mode since the nucleation mode particles have a relatively short lifetime in the atmosphere and often end up in the accumulation mode (Porstendörfer, 2001). The equilibrium-equivalent concentration (Ceq) of airborne radon progeny, unattached fraction (fp), and equilibrium factor (F) are among the most important input parameters in almost all dosimetric models to determine exposure and radiation dose by inhalation (Jílek et al., 2010; Guo et al., 2012).

The objective of this study is to measure the activity concentrations of radon progeny and to estimate the parametersfp, Ceq, and F in houses and at workplaces. Based on the obtained results of fp and Ceq, the dose conversion factor and effective dose are evaluated. For this purpose, about two years period, collecting the aerosol-attached radon progenies on filters and detecting the emitted α-particles (three-count method) using a passive system dosimetric in 16 locations in El Jadida city (Morocco) were performed. Additionally, we have developed a new and reliable method on PC-based software (Maple software) that combines the experimental data and mathematical calculations. This software can assist in solving differential equations and provide a basis for predicting corresponding activities (unattached and attached) based on the experimental data and the determined physical parameters of aerosol and room model. The province of El Jadida was selected for this study because it is positioned among the major industrial centers of Morocco. The industrial landscape of the region develops around three important sectors: chemical-parachemical industry, energy, and metallurgy in two industrial zones, the most important of which is located in the south of the city, in the region of Jorf-Lasfar. Almost all generated wastes by these industrial zones are discharging without any pretreatment. This can contribute to increase the natural radioactivity until reaching some levels that require the setting up of special precautions.

Section snippets

Theoretical considerations

222Rn gas is releases from the walls, floors, and ceilings of the room; it accumulates indoor according to the strength of its emanation sources and dilutes by only ventilation. The airborne solid progeny formed by decay radon (218Po, 214Pb, and 214Bi/214Po) is also diluted by ventilation and other removal processes (decay, deposition, recoil probability … etc.). These airborne progenies, unattached and attached, in room's air are usually expressed by the equilibrium-equivalent concentration Ceq

Results and discussion

One hundred ninety-two measurements were carried out in different seasons at sixteen positions (houses and workplaces) under various environmental conditions of ventilation and aerosol concentrations in El Jadida city from Sep. 2018 to Dec. 2019. In our present work, measured outdoor radon activity was ranging between 11 and 28 Bq. m−3 with an average value of 15 Bq. m−3. Experimental results of alpha track numbers over three-time intervals and physical parameters of the room model were used as

Conclusions

In this study, our essential research interest is to estimate the unattached and aerosol-attached activities of airborne radon progeny and their physical behavior in houses and at workplaces in order to assess the level of exposure to such ionizing radiation. For this purpose, radioactive aerosol was sampled on polycarbonate membrane filters and the total alpha activity was measured using the passive alpha dosimetry CR-39 detectors in sixteen places of El Jadida city, in Morocco. Several

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank Mrs. Rim Jouraiphy for proofreading the manuscript. Special thanks are addressed to Mr. Mhammed Zaimi as well as to Mr. Hamid Mazouz for their prompt help.

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