210Po in the Environment: Reassessment of Dose to Humans

Abstract

Significant efforts have been made by the International Commission on Radiological Protection (ICRP) to establish a reliable basis of equivalent and effective doses due to radionuclides. The ICRP over years has been updating the dose coefficients to include recent developments and make it more realistic. This perspective highlights some issues that warrant updating the methodology used for estimating ²¹⁰Po dose to humans. The need to underpin these dose coefficients with ever-increasing literature has encouraged us to share the observation on the significant loss of ²¹⁰Po due to seafood cooking, considering the loss due to cooking warrants changing the factor for the dose from seafood ingestion. Most dose assessment approaches use whole-body concentration, while most ²¹⁰Po is present in the liver and digestive system that often are not part of the edible portion. The other factor is the extremely high ²¹⁰Po concentration in aerosols as a result of coal and oil-fired power plants, forest fires, and volcanic activities, especially in the inhalable fraction. The ²¹⁰Po/²¹⁰Pb concentration ratio in the Gulf was observed to be between 1.6 and 1.9 in contrast to the 0.1 ratio observed in non-impacted areas. This reassessment of the inhalation dose is also relevant globally due to increasing incidences of forest fires where a much higher than 0.1 ²¹⁰Po/²¹⁰Pb ratio is expected and will result in a significant inhalation dose.

1. Introduction

There has been considerable interest in polonium ever since its discovery in 1898. There are forty three isotopes of polonium and all of which are radioactive [1]. Seven of these isotopes are naturally occurring, of which ²¹⁰Po is the longest lived with a 138.4-day half life [2]. ²¹⁰Po is one of the most toxic naturally occurring radioactive material (NORM) due to its high specific activity of 1.66 × 10¹⁴ Bq g⁻¹ [1]. The toxicity of ²¹⁰Po can be appreciated from the fact that the lethal dose for humans is ~10 µg [3]. The main source of ²¹⁰Po and its progeny in the environment is ²³⁸U. The average concentration of uranium in earth is about 2.8 ppm of which 99.284% is ²³⁸U. The exhalation of ²²²Rn from the soil due to natural decay of ²³⁸U is a major source of ²¹⁰Po in the atmosphere [4,5]. Other conspicuous natural sources of ²¹⁰Po in the environment are volcanic eruptions, forest fires [6,7], and atmospheric dust [5], while fossil fuel production and burning [5,8], effluent and tailings from uranium mining, phosphate fertilizer, and coal fired power plants are the major artificial sources. There was an incident of a 42 TBq ²¹⁰Po release from a nuclear power plant fire in Sellafield, United Kingdom in 1957 [9]. ²¹⁰Po is also artificially produced by irradiating ²⁰⁹Bi with thermal neutrons to obtain ²¹⁰Bi, which decays to ²¹⁰Po.

The overriding interest in ²¹⁰Po assessment emanates from the fact that over 90% of the natural radiation dose received by living organisms comes from ²¹⁰Po ingestion and inhalation [10,11]. The initial assessment on ²¹⁰Po dose indicated that 78.1% of total ²¹⁰Po intake is from the ingestion of food, 17.1% from cigarette smoke, 4.2% from water and 0.6% from air inhalation [12]. These estimates have been updated and revised in a recently developed biokinetic model developed following the mysterious death of Mr. Alexander Litvinenko, and it was concluded that the daily committed effective dose range from ingestion ranges between 2.1 × 10⁻³–1.7 × 10⁻² mSv and 5.5 × 10⁻²–4.5 × 10⁻¹ mSv by inhalation [13]. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) estimates suggest that the average natural radiation dose received by humans on earth is 2.4 mSv y⁻¹ [14] of which 52% is from internal radiation, 20% from external terrestrial radiation, 16% due to cosmic radiation, and 12% from ingestion. Details of likely ²¹⁰Po dose from seafood ingestion and aerosol inhalation are discussed in this perspective, and that should provide insight on whether the dose estimates and significant pathways need reassessment.

2. ²¹⁰Po Levels in Environment

There is a consensus that ²³⁸U in earth’s crust is the source of ²¹⁰Pb and ²¹⁰Po in environment. There is significant variation in uranium concentration in different geologic formations, with 0.1 mg kg⁻¹ in some basalts to 6.1 mg kg⁻¹ in some granites, and a maximum of 300 mg kg⁻¹ in phosphates and up to 1250 mg kg⁻¹ in some shale formations. In an ideal condition ²¹⁰Pb and ²¹⁰Po are more or less in secular equilibrium with ²³⁸U. However, due to various geological and geomorphological processes, this equilibrium is disturbed mainly due to decay of uranium and release of ²²²Rn from soils into the atmosphere. The ²¹⁰Pb and ²¹⁰Po are very close to secular equilibrium in top soils, with considerable spatial variation globally. Several studies have reported ²¹⁰Po concentrations of 2–22000 Bq kg⁻¹ (DW) [15,16,17,18,19,20,21,22,23,24] in surface soil with significant spatial distribution. A significant quantity of ²¹⁰Po is known to be added by fracking waste from the oil industry [25].

In addition to ²²²Rn exhalation from soil into the atmosphere, there are other significant pathways, e.g., volcanic eruptions, industrial activities, power plants (coal and oil fired), forest fires, and fossil fuel burning [4,26,27,28,29,30,31,32]. Investigations by such researchers have found the oil industry and power and desalination plants reliant on fossil fuel to be a significant source of ²¹⁰Po for the atmosphere [5] in those regions. Due to their particle reactive nature, ²¹⁰Po and ²¹⁰Pb quickly attach to the particles in atmosphere and are scavenged and deposited in terrestrial and marine areas due to wet and dry deposition. The ²¹⁰Pb and ²¹⁰Po deposited over these areas becomes incorporated into various plants due to uptake by roots or foliar uptake by leaves [9,33]. Carvalho et al. [9] provided a detailed summary of the activity concentration of ²¹⁰Po in terrestrial plants and they vary both spatially and with plant type, with the lowest values in vegetables, fruits, and trees in Portugal, and the highest levels in lichens in the United Kingdom.

There are umpteen studies over last 5–6 decades that have looked at ²¹⁰Pb and ²¹⁰Po levels in organisms (animals). ²¹⁰Pb is commonly retained in bones while ²¹⁰Po is located in soft tissues, with the highest concentrations in liver > kidney > meat [34,35,36,37]. Both ²¹⁰Pb and ²¹⁰Po have been extensively studied in marine biota [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52]. Elevated levels of ²¹⁰Po in most marine biota are considered a potent route of enhanced radiation dose to humans consuming seafood [26,46,53,54,55].

Being particle reactive, significant amounts of ²¹⁰Pb and ²¹⁰Po are associated with suspended particulate matter in the aquatic environment. In the marine environment, higher concentrations of ²¹⁰Po and ²¹⁰Pb are observed at depth due both to the release of ²²⁶Ra from the oceanic sea floor and the downward flux of phytoplankton and zooplankton debris, such as zooplankton molts, carcasses and fecal pellets.

3. ²¹⁰Po Dose to Humans

²¹⁰Po is a highly radiotoxic naturally occurring radioisotope with a specific activity of 166 TBq g⁻¹. The International Commission on Radiological Protection (ICRP) has stipulated the fractional intestinal uptake as 50%. Seafood ingestion is considered a significant route of ²¹⁰Po dose to humans [14]. Reports show that intake via inhalation is much smaller than ingestion [56], even at uranium milling facilities [57]. The International Atomic Energy Agency (IAEA) under its coordinated research project “Sources of Radioactivity in the Marine Environment and Their Relative Contributions to Overall Dose Assessment from Radioactivity (MARDOS)” suggested two methods for dose estimation from seafood ingestion [58]. The dose estimates using Method 1 use the activity concentration of ²¹⁰Po in water (for 1990) for different fishing areas and applies recommended concentration factors which are equivalent to 1.5 × 10⁻⁴ C_wF_c for fish and 3.2 × 10⁻³ C_wF_c for shellfish. For Method 2, the doses were estimated using IAEA estimated concentrations in fish and shellfish (for 1990), equivalent to 7.6 × 10⁻⁸ C_bF_c for fish and 1.1 × 10⁻⁷ C_bF_c for shellfish, where C_w is concentration in water, C_b is concentration in edible part of the biota, and F_c is the catch from the FAO fishing area in kg yr⁻¹. It is reported that ingestion is the main route of Po reaching the gastrointestinal tract, being absorbed in plasma and distributed to soft tissues, with accumulation in the liver and kidneys [56,59]. Table 1 presents annual ²¹⁰Po ingestion in different countries. The graphical depiction is presented in Figure 1.

4. Need for an Improved Dose Estimate

For a realistic dose assessment, it was highlighted way back in 1995 to consider the effect of cooking on levels of ²¹⁰Po and ²¹⁰Pb in seafood [72]. It was only in 2018–2019 that an assessment of ²¹⁰Po loss in cooked seafood was published by this group which showed a significant loss of ²¹⁰Po in cooked seafood [68]. The ²¹⁰Po loss in fish varies between 14% and 58%, with significant variation among the fish studied, with no difference in most fish between the way they were cooked. The wet weight (wwt) concentration ranged from 0.73 to 50.9 mBq g⁻¹ for uncooked edible tissue; 0.43 to 46.3 mBq g⁻¹ for grilled; 0.38 to 44.3 mBq g⁻¹ for boiled fish muscle. The ²¹⁰Po concentrations in fish stock varied between 0.05 and 3.84 mBq g⁻¹. The ²¹⁰Po concentration in whole shrimp varied between 68.6 and 484 mBq g⁻¹ wwt (uncooked), 78.1 and 310 mBq g⁻¹ wwt (grilled), 35.6 and 157 mBq g⁻¹ wwt (boiled), and the stock range was 14.4 to 27.6 mBq g⁻¹. The study also highlighted that the bulk of ²¹⁰Po concentration is in the heptopancreas and digestive systems. Once the head was removed and shrimp were deveined, the highest ²¹⁰Po concentrations of 25.1–79.7 mBq g⁻¹ wwt were in uncooked shrimps, followed by grilled samples in which the concentrations were 19.4 and 59.1 mBq g⁻¹ wwt, and boiled shrimp concentrations at 18.1–42.6 mBq g⁻¹ wwt, and the lowest concentrations were in the stock where the ²¹⁰Po was only 3.70 and 4.52 mBq g⁻¹.

The reduction in ²¹⁰Po concentration were more significant in shrimps where a 75% decrease in ²¹⁰Po concentration was observed in deveined shrimps and 84% in cooked deveined shrimps than in uncooked whole shrimps. These results strongly confirm a significant loss of ²¹⁰Po due to cooking. This finding elicits a discussion on reassessing how doses are estimated for humans ingesting seafood. The safe consumption advisories are based on the concentration in fresh seafood. With such losses due to cooking now known, they need to be factored in the dose models for determining realistic dose assessments and safe consumption limits for humans.

Another aspect that needs addressing is the inhalation dose, considering that the ICRP has made efforts to assess radiation dose, several publications have been produced over the years, the latest of them being ICRP Publication 119 [74,75]. The inhalation dose is based on the assumption of absorption scenarios classified as slow, moderate, and fast. The highest dose coefficient is for slow and is seven times higher than the fast scenario. The ICRP Publication 119 has provided inhalation dose coefficients for 1 µm and 5 µm particle sizes under moderate and fast absorption scenarios. The inhalation dose to workers for a 1 µm fast scenario was 6.0 × 10⁻⁷ Sv Bq⁻¹ and for moderate is 3.0 × 10⁻⁶ Sv Bq⁻¹; whereas for a 5 µm size fast scenario it was 7.1 × 10⁻⁷ Sv Bq⁻¹ and for moderate is 2.2 × 10⁻⁶ Sv Bq⁻¹. Compared to the inhalation dose, the ingestion dose coefficient was suggested to be 2.4 × 10⁻⁷ Sv Bq⁻¹.

In some recent investigations, concentrations of ²¹⁰Po were determined for size-fractionated aerosols in Kuwait. The results showed 91% of the aerosol load was in the inhalable size fraction with the ²¹⁰Po/²¹⁰Pb ratios between 1.2–1.9, much above typical activity Po/Pb ratios of <0.1 in areas free of anthropogenic inputs. The highest concentrations were found downwind of an industrial area that houses petroleum industries, a cement factory, and a power and desalination plant [5]. Due to sparse precipitation, often, the ²¹⁰Pb and ²¹⁰Po produced from ²²²Rn in atmosphere remains in aerosol. The dry deposition of this ²¹⁰Po will be a more significant pathway in this case.

The volcanic gasses and the recurrent forest fires reported globally has destroyed about 1,207,649 Km² forest between 2001 and 2019 [76]. The country-wise area is provided in Supplementary file S1; However significantly, forest fires are reported from Australia, Argentina, Brazil, Bolivia, Chile, China, Colombia, Congo, Indonesia, Madagascar, Mongolia, Mexico, Malaysia, Peru, Paraguay, Portugal, Russia, Spain, South Africa, and United States of America, which can be another significant source of atmospheric polonium. The ²¹⁰Po/²¹⁰Pb and ²¹⁰Bi/²¹⁰Pb ratios can be used for assessing the suspended particulate matter deposition rates and factored in for dose calculation [29,77]. The oil operations and coal-fired power plants are to be considered. In Kuwait, radon concentrations in air samples collected downwind from industrial areas associated with oil operations are in the 2523–131,618 Bq/m³ range [78]. Such levels could result in higher atmospheric ²¹⁰Pb and ²¹⁰Po concentrations and likely lead to higher inhalation exposure.

Given the higher concentration of ²¹⁰Po in aerosol in oil producing countries, areas affected by forest fires and volcanic eruptions, and coal-fired power plants, the inhalation doses in these regions are likely to be much higher than the ingestion dose. Keeping in mind that a significant portion of polonium is lost due to cooking, we feel there is a definite need to reassess the ingestion and inhalation doses that humans and non-human terrestrial biota are receiving.

5. Discussion

The need for reassessing the dose from ²¹⁰Po emanates from the fact that none of the currently applied approaches, i.e., ERICA (Environmental Risk from Ionising Contaminants: Assessment and Management), MARDOS-IAEA, and ICRP, take into account the ²¹⁰Po losses due to cooking of seafood. Most of the ²¹⁰Po concentrations used are whole-body concentrations; whereas studies have shown that the highest ²¹⁰Po concentrations in marine biota are in the liver and digestive systems that are often removed before cooking, which will result in significantly lower ²¹⁰Po in edible tissue. The methods proposed by MARDOS include the activity concentration of ²¹⁰Po in water (for 1990) for different fishing areas and applying concentration factors equivalent to 1.5 × 10⁻⁴ C_wF_c for fish and 3.2 × 10⁻³ C_wF_c for shellfish, or the doses estimated using IAEA estimated concentrations in fish and shellfish, both of which are very coarse approximations.

The other very critical issue is the weight to be given to the ²¹⁰Po inhalation dose. The growing amount of literature on high levels of ²¹⁰Po in aerosol in the proximity of coal and oil-fired power plants, the oil industry, volcanic eruptions, and forest fires certainly warrants reconsideration of inhalation pathways in dose estimations. The increasing incidences of forest fires can result in significantly higher ²¹⁰Po concentration in the atmosphere and rationalize the need to look at inhalation pathways more seriously. This perspective intends to open a discussion among groups interested in radioecology and risk assessment to refine and modify the current risk assessment tools.

6. Conclusions

Based on the information summarized in this perspective, we think there is an obvious need to reconsider how ²¹⁰Po dose to humans is assessed. The ICRP is continuously updating the guidelines, with more and more regional data generated, much of which is summarized in the IAEA’s MARIS database. The fact that usually the highest concentrations of ²¹⁰Po are in organs such as the liver, hepatopancreas, digestive system, and fecal matter, which are most often not part of the human diet, it will be prudent to consider a concentration in edible tissue only. Moreover, the significant loss of ²¹⁰Po which takes place due to cooking is not presently accounted for in dose estimations. Unequivocal evidence of ²¹⁰Po emanating from industrial sources and forest fires has not attracted due attention and can be a more significant pathway for ²¹⁰Po dose to humans via inhalation. With recent developments, some countries in Europe are considering the utilization of coal to meet the energy demands which would further exacerbate ²¹⁰Po emissions to the atmosphere. The recurrence of forest fires across the globe is also likely to add an unprecedented amount of ²¹⁰Po to the atmosphere and likely result in a large dose contribution to humans. With the evidence provided in this communication, we wish to initiate a discussion within the current scenario on whether inhalation is a dominant pathway for the population living in regions impacted by forest fires and downwind of coal-fired power plants and oil operations, instead of only considering seafood ingestion which itself needs correction and refinement.

Author Contributions

Conceptulization, S.U.; Writing the original draft, S.U., S.W.F. and M.B.; Reviewing and editing, S.U. and S.W.F. All authors have read and agreed to the published version of the manuscript.

Informed Consent Statement

Not Applicable.

Data Availability Statement

All the data reported and used is duly referenced and will be made available on request.

Acknowledgments

The authors thank Mohammad Faizuddin of GGIS UAE for providing a graphical representation of the polonium ingestion data.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figure and Table

Figure 1. 210Po ingestion rates globally (Data was obtained from Table 1).

Table 1

Annual ²¹⁰Po ingestion rates in different countries.

210Po Ingestion Rate (Bq y−1)	Location	References
1351	Alaska—Arctic dwellers	[60]
18	Buenos Aires, Argentina	[61]
1351	Canada—Arctic dwellers	[62]
68–130	China	[14]
932	Finland—Arctic dwellers	[63]
260	France	[64]
40–62	Germany	[14,65]
145	Greece	[64]
20	India	[14]
126	Italy	[66]
175–252	Japan	[67]
37–347	Kuwait	[68]
38	Leningrad, erstwhile USSR	[69]
796–3033	Marshall Islands	[70]
32–56	Poland	[71]
299	Portugal	[72]
51	Romania	[14]
54	Rostov-on-Don, erstwhile USSR	[56]
310	Spain	[64]
28–44	United Kingdom	[14]
22	United States of America	[73]
540	USSR—Arctic dweller

Supplementary Materials

The Following upporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su15021674/s1, Table S1: Forest Area lost due to fires (2001 – 2019) in Km².

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