Climate change - mediated atmospheric ²¹⁰Po and ²¹⁰Pb distribution: How significant can it be for the inhalation dose to humans?

INTRODUCTION

The ubiquitous presence of ²³⁸U and ²³²Th in the environment and vegetation has been reported. Polonium-210 and lead-210 are naturally occurring radioactive isotopes and are part of the uranium radioactive decay series. Polonium-210 has a 138-day half-life, while radioactive lead-210 has a much longer half-life of 22.2 years. Both ²¹⁰Po and ²¹⁰Pb are produced as part of the decay chain of radon gas (²²²Rn), which itself has a half-life of 3.8 days^[1]. Radon, a noble gas, has a neutral electric charge, but its radioactive daughters, ²¹⁰Po and ²¹⁰Pb, are positively charged ions. These ions are highly reactive and quickly attach to airborne particles. Polonium-210 ions in the atmosphere rapidly become attached to aerosol particles within a relatively short time frame, ranging from 40 to 180 seconds after their formation from the radioactive decay of precursor radon^[2]. This attachment to particles is a key factor in their atmospheric dispersion. Dry and wet atmospheric depositions continuously remove radon daughters from the atmosphere. This removal process prevents the establishment of a secular radioactive equilibrium between radon and its progeny, including ²¹⁰Po and ²¹⁰Pb. As a result, there is a significant radioactive disequilibrium between ²¹⁰Po and ²¹⁰Pb in the environment.

The ²¹⁰Po/²¹⁰Pb ratio in the aerosols is generally much lower than 0.1 due to the ongoing removal of radon daughters from the atmosphere^[3,4]. Consequently, the concentration of ²¹⁰Po is typically much lower than that of ²¹⁰Pb in aerosol samples. This aspect highlights the dynamic behavior of these radionuclides in the atmosphere and their connection to the decay of radon gas, leading to a significant radioactive disequilibrium between ²¹⁰Po and ²¹⁰Pb in natural settings^[5]. This disequilibrium is vital to consider when studying environmental radioactivity, including the mean residence time of aerosols (²¹⁰Bi/²¹⁰Pb ratios for recent air mass up to a few days^[5,6], and ²¹⁰Po/²¹⁰Pb for older air mass of a month or more^[7,8]) and their potential health effects^[9,10].

While the radioactive decay of atmospheric radon is one source of ²¹⁰Po and ²¹⁰Pb, later research has identified additional sources, including volcanic emissions, industrial facilities, forest fires, coal burning, and nuclear weapons tests^[11-17]. These sources can release significant quantities of ²¹⁰Po and sometimes ²¹⁰Pb into the atmosphere. Fossil fuel and coal-based Power and Desalination plants and Oil refineries are known to contribute to significantly higher concentrations of ²¹⁰Po and ²¹⁰Pb^[18,19]. Importantly, these additional sources can significantly increase the ²¹⁰Po/²¹⁰Pb ratios in the atmosphere^[14,17]. Both ²¹⁰Po and ²¹⁰Pb can be inhaled when they are adhered to ultra-fine particles in the air. This type of exposure to these radionuclides can result in internal radiation doses to human beings [Figure 1], particularly from ²¹⁰Po, which is generally higher than other naturally occurring radionuclides, except for radon^[20]. The activity-to-dose conversion factor is 2.2 µSv/Bq for inhaled ²¹⁰Po^[21], which is higher than other naturally occurring radionuclides, including ²¹⁰Pb.

Figure 1. Graphical representation of ²¹⁰Po and ²¹⁰Pb sources and a likely inhalation dose.

The redistribution of ¹³⁷Cs was first noticed in Europe after the extensive forest fires in Ukraine, Belarus, and Russia during 2010^[22]. However, not much attention has been given to natural radionuclides associated with vegetation that are released into the atmosphere during forest fires^[14,23]. Given the potential health risks associated with exposure to ²¹⁰Po and ²¹⁰Pb, monitoring their presence and redistribution in the environment consequent to forest fires is crucial. With an increase in the frequency of forest fires globally, mainly due to climate change, about 9.3 × 10⁶ hectares of tree cover were lost in 2021, and Russia alone lost 5.4 × 10⁶ hectares of tree cover to fire in 2021^[24]. The more recent statistics are even more striking; the Forest Fire Centre, Canada estimated an area of 9.5 × 10⁶ hectares was burned in the seven months of 2023, between January and July, keeping in view the recurrence and increasing spatial scale of the forest fires, it is quite likely that the activity concentration of ²¹⁰Po, ²¹⁰Pb and other radionuclides could be considerably enhanced in the surface air over areas beyond the charred forests.

RESULTS AND DISCUSSION

Certain investigations have documented ²¹⁰Po/²¹⁰Pb ratios surpassing one. These escalated ratios are frequently linked to the release of ²¹⁰Po from human-made sources involving high temperatures, such as metal smelters, ceramic kilns, incinerators, and forest fires^{[2,3,10,17,25-27]}. Studies analyzing radionuclides in aerosols have revealed that a significant amount of polonium activity is associated with fine and ultrafine aerosol particles. In Japan, over 70% of ²¹⁰Po activity in aerosols was detected in particles smaller than 0.7 µm^[28]. In Poland, 82% of ²¹⁰Po found in aerosols was measured within the particle size range of 0.1 - 0.3 µm, primarily attributed to emissions from industrial sources^[27]. Similarly, in Portugal, research on smoke from vegetation and forest fires indicated that most of the ²¹⁰Po in aerosols were linked to particles smaller than 1 µm. Kuwait has also reported relatively high levels of ²¹⁰Po in aerosols countrywide. The major fraction of this activity was identified in the fine fraction of aerosol particulates, specifically in the size range of 0.39 - 2.5 µm^[18,19,29], consistent with observations in other studies^{[2,3,10,14-16,19,27,30-36]}.

Several studies have highlighted the concentration of ²¹⁰Po and ²¹⁰Pb in vegetation [Table 1] and aerosols [Table 2]. The concentration in vegetation samples exhibited ²¹⁰Pb concentrations ranging from 0.98 to 20.27 Bq kg^-1 and ²¹⁰Po concentrations ranging from 0.97 to 49.4 Bq kg^-1, where the aerosol-containing combusted particles showed a significant increase in ²¹⁰Po and ²¹⁰Pb concentrations. A significant variation in ²¹⁰Po levels was observed in non-fire impacted aerosol in which the ²¹⁰Po on the filter sample was 111 Bq kg^-1[17], while in fly ash collected on filters of an aerosol sampler, concentrations varied between 3,604 - 7,255 Bq kg^-1 and was 1,115 Bq kg^-1 in ground ash from forest fires. The enhanced concentration of ²¹⁰Po in fly ash was a result of forest fire-contributed aerosol.

Since ²¹⁰Po and ²¹⁰Pb are non-essential for any growth and metabolic functions in plants, a detailed study has provided data to underpin the hypothesis that both ²¹⁰Po and ²¹⁰Pb are taken up by plants from soil^[37]. In spite of the restrictive uptake of potentially toxic elements by plants in metalliferous soils, ²¹⁰Po and ²¹⁰Pb accumulation has been observed in both metal-tolerant and non-tolerant plants^[38]. ²¹⁰Pb and ²¹⁰Po enter the vegetation through root uptake and aerial deposition on foliage^[39].

The evidence of the accumulation of these radionuclides in vegetation is explicit. The edibles (fruits and vegetables) are likely to contribute to the internal radiation dose to consumers. The data are compelling that ²¹⁰Po and ²¹⁰Pb are significantly incorporated in these fruits and vegetables, which follow a non-linear uptake pattern mathematically. Plant concentration for ²¹⁰Pb is expressed as

while for ²¹⁰Po, the concentration is expressed as:

The data depict that bioaccumulation takes place at even lower substrate concentrations and ²¹⁰Po accumulation exceeds ²¹⁰Pb by about a factor of 2^[37]. The much higher ²¹⁰Po and ²¹⁰Pb concentrations in aerosols emanating from forest fires, power and desalination plants, and oil installations highlight the considerable dose humans can get from inhalation. The issue is exacerbated given that frequent forest fires are linked to climate change. An extensive area is lost to forest fires year after year globally, most recently in Australia, Canada, Spain, Belarus, Russia, Portugal, and Turkiye. A very detailed spatial distribution of forest fires^[24] provides an overview of approximately 10 million hectares of forest lost yearly.

It will be quite imperative to conduct aerosol sampling and define the concentration of these radionuclides in size-fractionated aerosols. We have found a six-stage cascade impactor mounted on a high-volume air sampler to be very effective in determining radioactivity in respirable and inhalable fractions^{[19,29,40,41]}. The likelihood of volatilization of ²¹⁰Po in forest fires is relatively high as temperatures above 1,000 ^oC have been reported from forest fires^[17]. The considerable reduction of volume due to fire and the positive charge of gaseous ²¹⁰Po can result in the recapture of ²¹⁰Po and ²¹⁰Pb on ash particles, resulting in higher concentrations of these radionuclides in ash and aerosols.

The highest concentration of ²¹⁰Po in the vicinity of forest fires and downwind industrial sites is associated with PM_{0.39 - 2.5} and PM_2.5-10 sizes. Considering an average breathing rate of 6 L min^-1, an adult breathes about 8.64 m³ daily. The ²¹⁰Po concentration in forest fires in Portugal was reported as 70 mBq m^-3, resulting in an inhalation rate of 0.605 Bq d^-1. Using the dose conversion factor of 3.3 × 10^-6 Sv Bq^-1, a 2 µSv d^-1 dose is imparted due to inhalation; although it is not a permanent event, it can extend for weeks and months. It is interesting to put it in perspective: an individual who smokes a packet of cigarettes daily gets about 0.099 µSv d^-1, which is significantly lower than the population exposure during forest fires.

CONCLUSION

Forest fires play a significant role in the redistribution of ²¹⁰Po and ²¹⁰Pb. These fires not only release these isotopes into the atmosphere but also facilitate their transport over considerable distances. The combustion of organic matter during forest fires liberates substantial quantities of ²¹⁰Po and ²¹⁰Pb into the air, where they can attach to aerosol particles. Studies have shown that these isotopes often exhibit elevated concentration ratios in the aftermath of forest fires, surpassing unity in some instances.

The resultant aerosols, laden with ²¹⁰Po and ²¹⁰Pb, disperse throughout the atmosphere. Fine and ultrafine aerosol particles become carriers for a significant portion of these isotopes. Research has indicated that a substantial percentage, sometimes over 70%, of ²¹⁰Po activity in aerosols is associated with particles smaller than 2.5 µm. The ultrafine particles are reported to have the highest levels of these isotopes post-forest fires. This pattern aligns with observations not only in forests but also in areas affected by other combustion sources, such as industrial emissions across different geographical locations. The highest concentrations are observed in inhalable and respirable fractions of the aerosols that are more likely to enhance the radiation dose humans receive due to inhalation, i.e., the ~2 µSv d^-1 dose from forest fires and ~ 0.002 - 0.042 µSv d^-1 dose from industrial emissions. From the dose perspective, the climate change-mediated fires and increase in fossil fuel and coal-based power generation are likely to result in the redistribution of ²¹⁰Po and ²¹⁰Pb and impart a dose that is several orders of magnitude higher than the normal background doses and likely to significantly contribute to the 1 mSv permissible annual dose.

Understanding the dynamics of ²¹⁰Po and ²¹⁰Pb redistribution in the aftermath of forest fires is crucial for comprehending their environmental impact and potential implications for human health. Tracking the dispersion of these radionuclides and their attachment to aerosols can shed light on the broader implications of these natural events on atmospheric radioactivity and the subsequent exposure risks to ecosystems and populations downwind from these fire-affected regions.