Levels and Distributions of 210Pb and 210Po in Selected Seafood Samples in China and Assessment of Related Dose to Population

In this study, the activity concentrations levels of 210Pb and 210Po in the edible portions of eight seafood samples collected from the Fujian coast of China were determined. The activity concentrations ranged from 0.74 ± 0.08 to 12.6 ± 1.0 Bq/kg for 210Po and from the minimum detectable limit (MDL, 0.80 Bq/kg) to 11. 7 ± 1.1 Bq/kg for 210Pb. The 210Po activity concentration in all the fish organs ranged from 0.68 to 204 Bq/kg (w.w.), and the 210Po activity was mainly concentrated in the stomach, spleen, heart, liver, gonad, and intestine samples. The 210Pb activity concentration in all the fish organs ranged from the MDL to 15.2 Bq/kg (w.w.), and the 210Pb activity was concentrated in the head, fish scale, and gill samples. The annual effective ingestion doses ranged from 82.8 to 255 μSv/a for all age groups, and the lifetime risk of cancers were estimated. Both the effective ingestion doses and cancer risk to humans were within the acceptable ranges.


Introduction
Natural radionuclides 210 Pb (T 1/2 = 22.3 a) and 210 Po (T 1/2 = 138.4 d) are members of the 238 U decay chain [1]. 210 Pb and 210 Po enter the marine environment from atmospheric deposition at the ocean surface, from the in situ radioactive decay of 226 Ra dissolved in seawater, from the decay of 222 Rn gas exhaled by the seafloor, and from river and anthropogenic discharges. Polonium ions in the marine environment are rapidly adsorbed onto suspended particles and are accumulated by marine organisms while lead ions are adsorbed onto inorganic particles; thus, 210 Po can be accumulated in marine biotas more effectively than 210 Pb [2]. Early research on 210 Po and 210 Pb levels and their distribution in marine organisms indicated that ingestion of seafood could be the main exposure pathway for humans to receive radiation [3]. According to the UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) report, the annual effective doses caused by the ingestion of 210 Pb and 210 Po were approximately 80% of that caused by the ingestion of uranium-and thorium-series radionuclides [1]. Consequently, investigating the radiological impact of 210 Po and 210 Pb in marine products is necessary for protecting human health.
The 210 Pb and 210 Po activity concentrations in marine biota are spread over several orders of magnitude in the literature [4][5][6][7]. Previous studies focused on the activity concentrations of edible portions of seafood and the related dose assessment to the human [4,[8][9][10][11][12][13][14][15][16]. The 210 Pb and 210 Po activity concentrations and 210 Po/ 210 Pb ratios in separate organs of different marine biota are essential for establishing reference individuals/reference marine biota for use in radiation protection [17], are important for understanding of the 210 Po-enriched biochemistry and are critical for establishing the relevant standards for the limits/reference level of radionuclides in food samples.
In China, investigation of the 210 Po and 210 Pb activity in marine biota started relatively late, and few studies have been performed. In recent years, studies have focused on the edible portions of common marine biota [18][19][20]. However, the distributions of the 210 Pb and 210 Po activity concentrations in selected marine organisms have rarely been reported, and the current understanding of the 210 Po-enriched biochemistry is poor. Thus, the main objectives of the present study were to determine the activity concentrations of 210 Po and 210 Pb in edible portions and all other organs of various marine biota species in China and to evaluate the annual effective ingestion dose and lifetime risk of cancer to the public in terms of health and safety.

Sample Collection and Preparation
Eight seafood samples, including fish, crustacean, and algae samples, were collected from the Fujian coast, China, in August 2020. The research area is shown in Figure 1. The sampling site, corresponding samples, and feeding habitat are presented in Table 1. All the seafood samples were classified, marked, and transfer to the laboratory on the same day. These samples were cleaned with ultrapure water to eliminate any possible residues and impurities. The edible portions (muscles) of the fish samples were extracted from every biota, and the non-edible portions of the fish were divided into 12 organs: the head, fish fins, bones, scales, gills, kidney, stomach, spleen, heart, liver, gonad, and intestine. The non-edible portions of the crustaceans were divided into the head and carapace. All the samples were labeled, weighed, and dried at 80 • C to a constant weight (to avoid 210 Po losses) and finally crushed and homogenized for analysis.
Po-enriched biochemistry and are critical for establishing the relevant standard limits/reference level of radionuclides in food samples.
In China, investigation of the 210 Po and 210 Pb activity in marine biota star tively late, and few studies have been performed. In recent years, studies have foc the edible portions of common marine biota [18][19][20]. However, the distribution 210 Pb and 210 Po activity concentrations in selected marine organisms have rarely ported, and the current understanding of the 210 Po-enriched biochemistry is poo the main objectives of the present study were to determine the activity concentr 210 Po and 210 Pb in edible portions and all other organs of various marine biota sp China and to evaluate the annual effective ingestion dose and lifetime risk of c the public in terms of health and safety.

Sample Collection and Preparation
Eight seafood samples, including fish, crustacean, and algae samples, were c from the Fujian coast, China, in August 2020. The research area is shown in Figu sampling site, corresponding samples, and feeding habitat are presented in Tab the seafood samples were classified, marked, and transfer to the laboratory on t day. These samples were cleaned with ultrapure water to eliminate any possible and impurities. The edible portions (muscles) of the fish samples were extract every biota, and the non-edible portions of the fish were divided into 12 organs: t fish fins, bones, scales, gills, kidney, stomach, spleen, heart, liver, gonad, and i The non-edible portions of the crustaceans were divided into the head and carap the samples were labeled, weighed, and dried at 80 °C to a constant weight (to av losses) and finally crushed and homogenized for analysis.

Apparatus and Reagents
Liquid scintillation counting (Tri-Carb 3170TR/SL, PerkinElmer, Walsham, MA, USA) was used to measure the 210 Pb and 210 Po activity. The elemental concentrations of Pb were measured via inductively coupled plasma mass spectrometry (Element II, Thermo Fisher Scientific, Bremen, Germany). A microwave digestion system (Preeken EXCEl, Shanghai, China) was used for digestion of the seafood samples.
All the chemicals used were of analytical grade (AR, Beijing, China). Sr·spec resin (100-150 µm) was purchased from Triskem International (Rennes, France). A 210 Pb reference solution (equilibrium with 210 Bi and 210 Po, 363.8 Bq/g) purchased from National Physical Laboratory (NPL, Tdington, London) was used for 210 Pb and 210 Po efficiency correction. A Pb carrier was prepared via the dissolution of Pb(NO 3 ) 2 (AR, Beijing, China) for Pb chemical recovery correction. Ultima Gold™ AB was purchased from PerkinElmer (USA). Ultrapure water (18.2 MΩ·cm −1 ) obtained from a Milli-Q system (Millipore, Billerica, MA, USA) was used.

Sample Digestion, Separation, and Measurement
The radiochemical procedure used for the 210 Pb and 210 Po activity concentrations in seafood samples is given elsewhere [21]. In brief, 0.5 g dried seafood samples were digested by the microwave digestion system. The 210 Pb and 210 Po in the samples were separated from the digestive liquid using a previously prepared Sr-Spec column (2 mL). 20 mL of 2 M HCl solution, 5 mL of 1 M HNO 3 + 25 mL 0.1 M HNO 3 and 20 mL 0.1 M (NH 4 ) 2 C 2 O 4 successively passed through the column for 210 Bi (discard), 210 Po and Pb fraction striping, respectively. The 210 Pb eluents and 210 Po eluents were heated gently to near dryness, and several small amounts of 0.1 M HNO 3 were also added to homogeneity. 2 mL 0.1 M HNO 3 and 18 mL of Gold AB liquid scintillation cocktail were added, shaken well, and placed in the dark for 2 h, the 210 Pb and 210 Po activity concentrations were measured via liquid scintillation counting (LSC) in the α/β pulse discrimination analysis (PSA) mode (PSA value = 145) for 1000 min, the Region of Interest (ROI) for 210 Pb and 210 Po were 1-28 channels and 0-1000 channels, respectively. Then, 2 mg stable Pb carrier was added in each samples before seafood samples digestion for Pb recovery correction, Pb recovery was calculated by initial concentration and final concentration after isolated by Sr-Spec column. The concentration of Pb was determined by inductively coupled plasma mass spectrometry (ICP-MS). The Po yield of this procedure was expressed as overall efficiency, which was determined by measuring the net counts of 210 Po before and after separated on Sr·spec column. Batch experiments indicate that overall efficiency of Po were fluctuated in a small range (66.7 ± 2.5%), 210 Po overall efficiency was recommended for calculation the activity concentrations of 210 Po in further experiments.

Method Validation
To validate the method, a reference material (IAEA-447, Moss Soil reference material, International Atomic Energy Agency, Vienna, Austria) was used. The results obtained using our method agreed well with certified values. Additionally, to validate the results, the 210 Po activity concentration of every sample was determined via α spectrometry after source preparation through spontaneous deposition onto a copper plate, which is the most commonly used method for monitoring the activity of 210 Po [22].

Evaluation of Effective Ingestion Dose and Lifetime Risk of Cancer to Humans
The effective ingestion dose to humans from marine seafood ingestion was evaluated according to the seafood consumption per year, dose conversion factors for humans and activity concentrations of seafood. The formula for radionuclide estimation is [23,24]: where C D represents the annual effective ingestion dose (Sv/a), R C represents the activity concentration of the specific radionuclides in the seafood samples (Bq/kg), I R represents the annual ingestion rate (kg/a), and D F is the dose conversion factor (Sv/Bq). The parameter used for annual intake rate (I R ) in this study was adopted from a previously reported method [25]. I R for adults (>18 years old), juveniles (14-17 years old) are 14.60 kg/a, and that for children (7-13 years old) is 10.95 kg/a. The parameters in Table 2 for the dose conversion factor (D F ) were obtained from International Atomic Energy Agency (IAEA) [26]. The lifetime risk of cancer was also calculated via the formula [23,24]: where R K represents the lifetime risk of cancer, I R represents the annual seafood ingestion rate (kg/a), f 1 represents the estimated gastrointestinal absorption fraction of a specific radionuclide, T represents the exposure duration (50 a. for adults, 60 a. for juveniles, and 70 a. for children), R coe is the risk coefficient from ICRP [27], and R C represents the activity concentration of the specified radionuclide in the seafood samples (Bq/kg).

210 Po and 210 Pb Concentrations in Edible Tissues of Different Biota Species
The activity concentrations of 210 Po and 210 Pb in the edible portions of seafood samples are presented in Table 3. All the data have units of Bq/kg wet weight (w.w.). The average 210 Po activity concentration was 5.31 ± 0.52 Bq/kg for all the edible portions of the seafood samples and ranged from 0.74 ± 0.08 Bq/kg (Red Sea bream muscle samples) to 12.6 ± 1.0 Bq/kg (eel samples (Anguillidae)), while the 210 Pb activity concentrations ranged from the minimum detectable limit (MDL), i.e., 0.8 Bq/kg wet weight for 210 Pb) to 11.7 Bq/kg (eel samples), with a mean value of 3.36 ± 0.41 Bq/kg. The 210 Po and 210 Pb concentrations in seafood samples found in our study were compared to those determined in previous studies from China and other countries. Our results are consistent with the previously reported ranges of 0.13-3.26 Bq/kg (w.w.) and 0.2-25.8 Bq/kg (w.w.) for the 210 Pb and 210 Po activity concentrations, respectively, in seafood samples from the coast of Guangdong, China [20]. They are also similar to a previously reported finding that the 210 Po level of various seafood edible portions ranged between 1.17 × 10 −1 and 6.58 × 10 Bq/kg (w.w.) [18]. Additionally, the 210 Po activity concentration in the seafood samples agreed well with the results of a seafood radionuclide survey conducted in 1977-1978 [28].  [11,29,30], the concentrations of 210 Po in our survey were lower, and there were no significant differences in the concentration range of 210 Pb. The 210 Po and 210 Pb activity concentrations in our survey were similar to those reported for other countries [4,8,[12][13][14]16,24,[31][32][33][34][35]. Significant differences in the 210 Po and 210 Pb activity concentrations among different countries were observed, possibly owing to the marine species evaluated and the variations in the geochemistry of the regions [8]. The average activity concentration of 210 Po in fish samples was slighter lower than the representative concentrations reported by UNSCEAR (2.4 Bq/kg) [1]. The average activity concentrations of 210 Po in the crustacean samples were slightly higher than the representative concentrations reported by UNSCEAR (6.0 and 15 Bq/kg, respectively). The 210 Po concentration decreased in the following order: crustaceans > fish > algae. The higher 210 Po activity concentrations in the crustaceans are explained as follows: (1) the edible portions of the crustaceans contained the digestive system, and the 210 Po activity concentrations were generally higher in the internal organs than in the muscles [17]; (2) the crustaceans were captured in nature and were not subjected to artificial feeding. The order of the 210 Pb activity was identical to that for 210 Po. The 210 Po/ 210 Pb ratio was >1 for almost all the samples, with the exception of the Carassius auratus auratus and Largehead hairtail (Trichiurus lepturus) samples. This may be because the 210 Po was more easily concentrated in the internal organs than the 210 Pb.

Distributions of 210 Po and 210 Pb Activity Concentrations in Selected Organs of Different Species
The 210 Po and 210 Pb activity concentrations and 210 Po/ 210 Pb ratios in selected organs of four types of fish are shown in Figure 2.
As shown in Figure 2a, The 210 Po activity concentrations in all the fish organs ranged from 0.68 to 204 Bq/kg (w.w.). The lowest and highest values were obtained for the fish scale sample of the Red sea bream and the intestine sample of the yellow croaker, and relatively high values were obtained for the stomach, spleen, heart, liver, gonad, and intestine samples. These results are consistent with previous studies [2,3]. As shown in Figure 2b, the 210 Pb activity concentrations in all the fish organs ranged from the MDL to 15.18 Bq/kg (w.w.), and the highest value was obtained for the head sample of the Common sea perch. Relatively high values were obtained for the head, fish scale, and gill samples. Regarding the 210 Po/ 210 Pb ratio, the values for the stomach, liver, gonad, and intestine samples were significantly higher than one, and those for the bone, gill, head, fish scale, and fish fin samples were generally lower than one. This may be because 210 Pb and 210 Po accumulated in the organisms through the food chain (less 210 Po absorption in the form of inorganic ions but more organic 210 Po). 210 Pb was mainly deposited on bones, and 210 Po was mainly deposited on internal organs such as the liver, gastrointestinal tract, and gonad [17]. Similarly, the 210 Po activity concentration of muscle samples in crustaceans (shrimp and Oratosquilla oratoria) was significantly lower than that of head samples, which may contributed to the viscera and digestive system of these two species are inside the head samples, and the viscera and digestive system were strongly bonded to 210 Po, while the 210 Pb activity concentrations exhibited no significant differences.

Distributions of 210 Po and 210 Pb Activity Concentrations in Selected Organs of Different Species
The 210 Po and 210 Pb activity concentrations and 210 Po/ 210 Pb ratios in selected organs of four types of fish are shown in Figure 2. As shown in Figure 2a, The 210 Po activity concentrations in all the fish organs ranged from 0.68 to 204 Bq/kg (w.w.). The lowest and highest values were obtained for the fish scale sample of the Red sea bream and the intestine sample of the yellow croaker, and relatively high values were obtained for the stomach, spleen, heart, liver, gonad, and intestine samples. These results are consistent with previous studies [2,3]. As shown in Figure 2b, the 210 Pb activity concentrations in all the fish organs ranged from the MDL to 15.18 Bq/kg (w.w.), and the highest value was obtained for the head sample of the Common sea perch. Relatively high values were obtained for the head, fish scale, and gill samples. Regarding the 210 Po/ 210 Pb ratio, the values for the stomach, liver, gonad, and intestine samples were significantly higher than one, and those for the bone, gill, head, fish scale, and fish fin samples were generally lower than one. This may be because 210 Pb and 210 Po accumulated in the organisms through the food chain (less 210 Po absorption in the form of inorganic ions but more organic 210 Po). 210 Pb was mainly deposited on bones, and 210 Po was mainly deposited on internal organs such as the liver, gastrointestinal tract, and gonad [17]. Similarly, the 210 Po activity concentration of muscle samples in crustaceans

Effective Ingestion Dose and Risk to Humans via Seafood Consumption
The effective ingestion doses due to the ingestion of 210 Pb and 210 Po through seafood consumption for different ages were estimated, as shown in Figure 3. Because seafood consumption varies significantly among individuals, the weighted average activity concentration of 210 Pb and 210 Po in seafood samples were adopted for effective ingestion dose evaluation based on the representative values (fish:crustacean:mollusk = 13:1:1) [1]. The total effective ingestion doses ranged from 82.8 to 255 µSv/a for all the age groups. The total effective ingestion dose decreased in the following order: children > juveniles > adults. Among the radionuclides studied, 210 Po was the highest contributor and accounted for >85% of the total dose. The total effective ingestion dose was also found below the average natural ingestion radiation dose received by humans around the world (300 µSv/a) [1], and the total effective ingestion dose for adults was 1/30 of the public annual effective dose (2.4 mSv/a) caused by natural radiation sources according to the UNSCEAR report [1]. Furthermore the effective dose caused by 210 Pb and 210 Po was below the legal dose limit of 1 mSv per year for members of the public recommended by Centre for Environment Fisheries and Aquaculture Science (CEFAS) [36]. Therefore, seafood from the Fujian coast of China is considered to be safe for human consumption. evaluation based on the representative values (fish:crustacean:mollusk = 13:1:1) [1]. The total effective ingestion doses ranged from 82.8 to 255 μSv/a for all the age groups. The total effective ingestion dose decreased in the following order: children > juveniles > adults. Among the radionuclides studied, 210 Po was the highest contributor and accounted for >85% of the total dose. The total effective ingestion dose was also found below the average natural ingestion radiation dose received by humans around the world (300 μSv/a) [1], and the total effective ingestion dose for adults was 1/30 of the public annual effective dose (2.4 mSv/a) caused by natural radiation sources according to the UNSCEAR report [1]. Furthermore the effective dose caused by 210 Pb and 210 Po was below the legal dose limit of 1 mSv per year for members of the public recommended by Centre for Environment Fisheries and Aquaculture Science (CEFAS) [36]. Therefore, seafood from the Fujian coast of China is considered to be safe for human consumption. The lifetime risk of cancer levels associated with the ingestion of 210 Pb and 210 Po in seafood were estimated, as shown in Table 4. The risks varied in the range of 6.04 × 10 −6 (adults, for 210 Pb) to 1.24 × 10 −4 (juveniles, for 210 Po). All the total lifetime risk of cancer from ingestion of 210 Pb and 210 Po observed in this study was below the world mean value of 5.3 × 10 −3 [27], and also much lower the lifetime risk of all cancer (27.77%) in China estimated by [37]. From this point of view, the lifetime risk of cancer due to 210 Pb and 210 Po was acceptable. The lifetime risk of cancer levels associated with the ingestion of 210 Pb and 210 Po in seafood were estimated, as shown in Table 4. The risks varied in the range of 6.04 × 10 −6 (adults, for 210 Pb) to 1.24 × 10 −4 (juveniles, for 210 Po). All the total lifetime risk of cancer from ingestion of 210 Pb and 210 Po observed in this study was below the world mean value of 5.3 × 10 −3 [27], and also much lower the lifetime risk of all cancer (27.77%) in China estimated by [37]. From this point of view, the lifetime risk of cancer due to 210 Pb and 210 Po was acceptable.

Conclusions
The activity concentrations of 210 Po and 210 Pb were evaluated in seafood samples collected near the Fujian coast of China. The 210 Po and 210 Pb activity concentrations in edible portions of the marine biota were similar to those in the majority of the world's countries, with the exceptions of Turkey, Spain, and India. 210 Po was mainly concentrated in the liver, gonad, and intestine samples, and 210 Pb was mainly concentrated in the bone, fish scale, and head samples. The 210 Po and 210 Pb activity concentrations and 210 Po/ 210 Pb ratios observed in this study are valuable references for evaluating the radiation risk of marine biota. In the next research, the relationship between higher 210 Po activity concentrations and biomarkers (such as H 2 O 2 , Superoxide dismutase and Malondialdehyde) would be also discussed in order to explore the 210 Po-enriched mechanism in internal organs further. The annual effective ingestion dose and lifetime risk of cancer to humans due to 210 Po and 210 Pb for seafood consumption from the Fujian coast were consistent with previous studies performed around the world and were lower than the global mean values. Therefore, the