Assessment of radiation dose hazards caused by radon and its progenies in tap water by the human dosimetric model

Radon, a colorless, odorless inert radioactive gas, has four radioactive isotopes (²¹⁸Rn, ²¹⁹Rn, ²²⁰Rn, and ²²²Rn) in nature. ²²²Rn has received extensive attention because its parent nucleus ²²⁶Ra is abundant in soil and rock and has the longest half-life (3.82 days) (ICRP 2007). The decay of ²²²Rn emits α-particles and produces a series of short-lived progenies such as ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po (UNSCEAR 2000). Considering the soluble readily in water, ²²²Rn can easily diffuse from solid materials into the water and be transported when various types of water resources in nature come into contact with soil and rocks (Zhuo et al. 2001; Yasouka et al. 2008; Jantsikene et al. 2014; Mehra et al. 2016).

Water is an important factor affecting public health (WHO 2009). The major source of domestic water for urban residents is tap water supplied through the urban water supply network. ²²²Rn dissolved in tap water can be transported to residential buildings far away from water supply plants in a short period through the pipe network, and then degassed from water to indoor environment in the process of domestic water use, or be ingested by residents along with drinking water or food (UNSCEAR 2000; Vinson et al. 2008). The ²²²Rn and progenies which enter the respiratory system of the human body through respiration cannot be completely filtered, in breathing, the inhaled ²²²Rn is almost exhaled again, but the progenies of decay can attach to air particles and have a certain probability in the respiratory tract tissue surface adhesion and deposition (Vinson et al. 2008; Oner et al. 2009), α- or β-particles emitted by the decay of these short-lived progenies cause genetic damage to the cells on the surface of the deposited organ or tissue and can also penetrate the mucous membrane on the surface of the tissue or organ, causing damage to stem cells deep inside those tissues or organs (Zhuo et al. 2001; Kendall & Smith 2002; Darby et al. 2005; Alghamdi & Aleissa 2014). Meanwhile, some progenies are transferred to the body fluids through which they are transported to organs other than the respiratory organs, causing radiation damage to those organs. The ingestion of tap water with a higher ²²²Rn concentration was also associated with an increased risk of visceral disease, especially the incidence of gastric cancer and gastrointestinal cancer. Besides causing radiation damage to surface cells in the lining of digestive organs, ²²²Rn can also be absorbed by the gastrointestinal tract into body fluids and cause damage to other radiation-sensitive tissues or organs in the body. Therefore, the measurement and dose assessment of ²²²Rn and its progenies in tap water are of great significance in preventing random biological effects and improving public health (Kulali et al. 2019).

Many previous studies have calculated the effective inhaled or ingested dose of radon exposure due to domestic water (Binesh et al. 2012; Nita et al. 2013; Sharma et al. 2019). Nevertheless, the dose calculation of α- and β-particles to specific organs and tissues in the respiratory tract and alimentary tract is rarely involved, and the effect of temperature on radon concentration in water was not considered. In the present study, ²²²Rn concentrations in different tap water samples were measured, and the variation trend of ²²²Rn concentrations in tap water at different temperatures was analyzed. Based on the measurement results, the effective doses of α- and β-particles to the public were estimated using the human respiratory and alimentary tract model the human digestive tract model provided by ICRP.

Urumqi River is the water source of Urumqi City in Xinjiang. The water supply plants in the city carry out centralized filtration and purification treatment for the river water and then transport it to the residents. In the study, 400 mL glass sampling bottles were used to collect indoor tap water from six residential areas in Urumqi. After the collection, the samples were sealed and brought back to the laboratory for radon concentration measurement under a constant temperature environment. Meanwhile, the collected water samples are heated to 25, 40, 55, 70, 85, and 100 °C in a constant temperature water bath with the same water bath time, and the radon concentration in the samples was measured at different water bath temperatures. All the measurements are completed on the same day of sampling.

The concentration of radon in the sample was measured by FD216, an environmental radon measurement instrument based on the scintillation chamber method, and its principle and structure are shown in Figure 1. Detailed experimental methods and procedures are described in the previous work (Yong et al. 2020).

Figure 2 shows the human respiratory tract model and part of human digestive tract model provided by ICRP publication (ICRP 2015).

Compartment ET₁: retention of material deposited in the anterior nose (region ET₁); compartment ET_seq: long-term retention in airway tissue of a small fraction of particles deposited in the nasal passages (region ET₂); compartment ET'₂: short-term retention of the material deposited in the posterior nasal passage, larynx, and pharynx (ET₂ region); compartment BB': retention of particles in the bronchial (region BB), with particle transport to ET'₂; compartment bb': retention of particles in the bronchiole (region bb), with particle transport to BB'; compartment BBseq: long-term retention in airway walls of a small fraction of the particles deposited in the bronchiole (region bb); compartment bbseq: long-term retention in airway walls of a small fraction of the particles deposited in the bronchiole (region bb); compartment ALV: retention of particles deposited in the alveoli. INT: long-term retention of the particles deposited in the alveoli that penetrate to the interstitium: the particles are removed slowly to the lymph nodes (ICRP 2015).

According to the gastrointestinal system provided by ICRP Publication (ICRP 1979), as shown in Figure 3, the whole gastrointestinal system comprises five regions: stomach, small intestine, upper large intestine, lower large intestine, and blood.

The radon concentrations of tap water in residential buildings at different temperatures are shown in Table 1. The radon concentration of residential tap water ranges from 280 to 750 mBq L⁻¹, with an average value of 548.16 mBq L⁻¹, among them, the radon concentration of tap water sample L5 is the highest, and that of L6 is the lowest, which is 73820.2 and 2887.6 mBq L⁻¹, respectively. Radon concentrations in all tap water samples were consistent with the USEPA's maximum contaminant level of 11.11 Bq L⁻¹ and the drinking water radon parameter (100 Bq L⁻¹) set by the EURATOM Drinking-Water Directive (USEPA 1999; Council Directive 2013/51/Euratom 2013). Radon concentrations in tap water in some countries and regions are shown in Table 2, and it is obvious that the measurement results of this study are within the range of those measured in these countries and regions (Sarrou & Pashalidis 2003; Marques et al. 2004; Rusconi et al. 2004; Pagava et al. 2008; Nita et al. 2013; Ahmad et al. 2015; Erdogan et al. 2015; Fakhri et al. 2015; Le et al. 2015).

In this study, tap water samples from different residential buildings will be gradually heated to 25, 40, 55, 70, 85, and 100 °C by water bath heating. Radon concentrations at different temperatures are shown in Table 3. The radon concentration in the six groups of tap water samples showed an obvious downward trend on the whole with the increase of the water bath temperature. The average radon concentration at 25 °C was 543.33 mBq L⁻¹, and it decreased to 116.11 mBq L⁻¹ at 100 °C. The variation of the radon retention rate (radon concentration at a certain water bath temperature/initial radon concentration × 100%) in tap water at different water bath temperatures is shown in Figure 4, radon retention decreases with the gradual increase of temperature too, when the temperature reaches 100 °C, the average radon retention rate in the sample group is only 21.99%. The results indicate that heating tap water can effectively reduce the concentration of radon in water and reduce the harm of radon intake to public health.

Despite there were two different trends, the concentration of radon in the samples decreased after water bath heating. Due to the low initial radon concentration (the average value is 368.89 mBq L⁻¹), the radon retention rates of samples L1, L2, and L6 decrease continuously from 40 to 70 °C and then change gently. However, the radon retention rate of samples L3, L4, and L5 (with an average radon concentration of 717.78 mBq L⁻¹) with a high initial radon concentration decreased rapidly between 25 and 40 °C, and then the change was flat. The radon concentration and retention rate of the six sample groups increased slightly from 70 to 85 °C. Statistical analysis showed that there was no significant difference in the radon concentration value and the radon retention rate between 70 and 85 °C (p < 0.05).

The radon retention rates of all samples at different temperatures were fitted by the method of locally weighted regression (LOESS), as shown in Figure 5. When the water bath heating temperature changes from 25 to 70 °C, with the increase in temperature, radon retention decreases continuously, dropping to only about 30%. With the increase of water bath temperature, the declining trend of radon retention is gentle, and the fitting curve is flat between 70 and 85 °C, followed by a slow decline between 85 and 100 °C, which possibly was because blisters appeared in the heated samples as the water bath temperature increased, accelerating radon degassing in the water.

The annual committed equivalent dose and effective dose of radon and its progenies in tap water to respiratory and alimentary tract organs or tissues were estimated.

According to the actual situation of resident residence, bathroom space is about 14 m³. Meanwhile, according to the actual water consumption habits of residents, the temperature of shower water is assumed to be 40 °C, 30 min of shower three times a week, the water consumption is about 0.36 m³·h⁻¹, and the decay progenies of radon released into the air during the shower are attached to the particles with an activity median aerodynamic diameter (AMAD) of 1 μm. The annual drinking water quantity of adult residents is 500 L, and the tap water will be boiled and cooled for drinking, ignoring the part of the indoor air radon dissolves into the tap water during cooling, the study assumes that all the radon concentrations taken in are the values at 100 °C and appear in the stomach (UNSCEAR 2000; ICRP 2002a).

The committed equivalent dose of inhalation of α decay short-lived progenies (²¹⁸Po and ²¹⁴Po) and β decay short-lived progenies (²¹⁴Pb and ²¹⁴Bi) to the respiratory tract and alimentary tract target tissues during a shower is shown in Tables 4–6. The committed equivalent dose contribution of α-particles to ET₁, ET₂, BB, bb, and AI in the respiratory tissue regions was higher than that of β-particles. The committed equivalent doses of β-particles to the target tissues of the respiratory tract are close to each other in order of magnitude, and the committed equivalent dose to LN_ET and LN_TH regions is higher than that of the α-particles. In the alimentary tract, the committed equivalent dose of ²¹⁴Pb to the target tissues of the esophagus was higher than that of ²¹⁴Bi, while that to other organs of the alimentary tract was lower than ²¹⁴Bi.

Due to different respiratory rates (0.54 m³·h⁻¹ for adult males and 0.39 m³·h⁻¹ for adult females) and SAF, the committed equivalent dose contribution of short-lived progenies to all tissue regions of the adult male respiratory tract (except region LN_TH) was higher than that of the adult female. In the alimentary tract, except for the rectosigmoid, the committed equivalent dose in the target region of other organs was higher in adult males than in adult females.

The average annual cumulative effective dose of the respiratory tract and digestive tract in males was 0.125 and 6.97 × 10⁻⁴ μSv a⁻¹, respectively, and that of females was 0.104 and 5.43 × 10⁻⁴ μSv a⁻¹. The average total annual committed effective dose was 0.126 μSv a⁻¹ for adult male and 0.105 μSv a⁻¹ for an adult female (as shown in Table 7), the annual committed effective contribution of sample group L2 to residents was the highest, and that of adult male and an adult female was 0.152 and 0.127 μSv a⁻¹, respectively, and the annual committed effective dose of sample group L6 was the lowest, 0.094 and 0.078 μSv a⁻¹, respectively, which were all lower than the upper reference value of about 10 mSv a⁻¹ set by ICRP (ICRP 2014).

Table 8 shows the annual committed effective dose and annual committed equivalent dose caused by ²²²Rn in tap water entering the residents' gastrointestinal tract. The annual committed effective dose of sample group L3 is the highest, and that of sample group L6 is the lowest, which is 0.2722 and 0.0514 μSv a⁻¹, respectively. Among all the regions of the gastrointestinal tract, the lower large intestine region had the highest average annual committed equivalent dose (1.4606 μSv a⁻¹), followed by the upper large intestine region (1.1502 μSv a⁻¹), and the small intestine region was the lowest (0.1424 μSv a⁻¹).

In this study, the radon concentration in the tap water of six residential areas was measured, and the average radon concentration was 548.16 mBq L⁻¹. All samples were under the radon concentration limits recommended by USEPA and EURATOM (11.11 Bq L⁻¹ and 100 Bq L⁻¹). Compared with the radon concentration in the tap water of other countries or regions, the radon concentration in this study is within the range of other measurement results. At the same time, the radon concentration and the retention rate under different temperatures were obtained by heating tap water in the water bath, and the radon concentration generally showed a downward trend with the increase in temperature. Finally, the dose calculation model provided by ICRP was used to estimate the dose of radon exposure in some water-use scenarios. In the process of the shower, the annual committed effective dose of radon progenies to the male respiratory tract and the alimentary tract is higher than that of female, and in the respiratory tract, the committed equivalent dose caused by α activities to the respiratory tract target tissue is higher than that of β activities, while the average annual committed effective dose of radon from drinking water ingestion to the resident gastrointestinal tract is 0.1789 μSv a⁻¹. Urban water supply plants, therefore, can reduce the concentration of radon in the water source by centralized aeration, while residents can reduce the concentration of radon in the water by heating and degassing to avoid further increase of public exposure to radon.

The work is supported by the National Natural Science Foundation of China (Project No. 32060292). The authors are thankful to the Xinjiang Hongfu Nuclear Safety Technology Co., Ltd of China for providing facilities and support.

All relevant data are included in the paper or its Supplementary Information.