Following the atmospheric nuclear tests in the 1950s and early 1960s, several radioecological research projects focusing on the (sub)arctic food-chain lichen-reindeer/caribou-man were initiated in Scandinavia and North America [1,2,3]. Lichen collects deposited radionuclides efficiently and is the main fodder for reindeer during the winter season. The enrichment of radionuclides in this food chain can lead to exceptionally high body burdens among the indigenous Sami and Inuit populations consuming large quantities of the meat and edible organs of reindeer and caribou. This concerns both artificial and natural radionuclides, even though the first research projects dealt with ¹³⁷Cs and other fission product nuclides. However, it was observed that the natural radionuclides ²¹⁰Pb and ²¹⁰Po are enriched in this food chain as well.

²¹⁰Pb is formed in the atmosphere from the radioactive noble gas ²²²Rn, emanating from the Earth’s crust. In total, 99% of the airborne ²²²Rn originates from land and only 1% from the sea [4], and consequently high ²¹⁰Pb concentrations are found in continental air masses. Owing to the long half-life of ²¹⁰Pb (t½ = 22.3 a), its removal from the atmosphere is governed by the wet and dry deposition processes affecting the aerosol particles carrying it rather than radioactive decay. ²¹⁰Pb decays to ²¹⁰Po (t½ = 138.4 d) via the short-lived beta emitter ²¹⁰Bi (t½ = 5.013 d). Being an alpha emitter ²¹⁰Po is highly radiotoxic.

From the radiation protection point of view, less important natural radionuclides are ²³⁸U, ²²⁶Ra, ⁴⁰K, and Th isotopes. However, ²³⁸U is a long-lived (t½ = 4.5 × 10⁹ a) alpha emitter whose chemical toxicity exceeds its radiological toxicity. Although isotope ²³⁸U is not considered to cause significant radiation exposure, it is still the mother nuclide of a whole uranium decay series, producing isotopes of astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, and thorium while decaying. ⁴⁰K also has a very long half-life (t½ = 1.28 × 10⁹ a). A human body has a ⁴⁰K whole body content of a few kBqs. The alpha emitter ²²⁶Ra (t½ = 1600 a) is an important radionuclide considering ingested radiation doses. However, the population in Finland receives exposure to ²²⁶Ra mainly via uptake from drinking water, and there is hardly any published data about ²²⁶Ra in terrestrial environment and biota in Finland, except ²²⁶Ra concentration in cereals [5], animal-based food products [6], and timber [7]. Therefore, ²²⁶Ra is left outside the scope of this article. Thorium has several isotopes with widely different half-lives. The longest half-life belongs to isotope ²³²Th (t½ = 1.405 × 10¹⁰ a). Owing to the long half-life, the specific activity of ²³²Th is low and thus its potential threat against human health might be due to its chemical characteristics (heavy metal) rather than radiological hazards.

According to Paatero et al. [9], the ²¹⁰Pb deposition in northern Finland is about 20–40 Bq/m²/a (Figure 1). The deposition level is lower than in southern Finland, where the maximum deposition is about 80 Bq/m²/a. In the atmosphere the ²¹⁰Po:²¹⁰Pb, activity ratio is usually about 5–10%, so the amount of ²¹⁰Po deposition in Finnish Lapland is about 2 Bq/m²/a [10]. However, the ²¹⁰Po:²¹⁰Pb activity ratio continues to increase after the deposition.

²¹⁰Po is enriched from soil to plants via root uptake [11,12] and in less extent via foliar uptake. Lichens behave differently than plants as ²¹⁰Po transfers and enriches from air and surface soil to lichens mainly by surface (aerial) uptake [13]. ²¹⁰Pb, a grandmother radionuclide of ²¹⁰Po, has a different chemical and radiochemical behaviour than ²¹⁰Po, although some similarities occur. Just like ²¹⁰Po, ²¹⁰Pb has also been found to accumulate efficiently to lichen, similarly with stable lead [14]. However, as will be discussed next and can be seen from the data in Table 1 and Table 2, there are clear differences between ²¹⁰Po and ²¹⁰Pb in their bioavailability and enrichment in the food chains.

Lichen is the main fodder of reindeer during the winter. In the 1960s, the ²¹⁰Pb activity concentration in lichen (Cladonia alpestris) varied between 210 and 380 Bq/kg dry weight (d.w.) in Finnish Lapland and between 190 and 300 Bq/kg d.w. in southern Finland [15]. The ²¹⁰Po:²¹⁰Pb activity ratio in lichen varied from 0.78 to 0.98. The activity concentrations of ²¹⁰Pb and ²¹⁰Po were about a factor of 50 higher in lichen compared to those in other plants that reindeer eat during the summer season, such as birch (Betula verrucosa/Betula pendula) leaves. Persson [19] found similar ²¹⁰Pb activity concentration levels in lichen in Sweden. According to his studies, the effective half-life (physical + biological) of ²¹⁰Pb in lichen is 7 ± 2 years. As ²¹⁰Pb and ²¹⁰Po are natural radionuclides, their amount in the environment is very stable in the long-time scale. Due to this, the ²¹⁰Pb content in lichen has not changed during the several decades of monitoring. The activity concentration of ²¹⁰Pb in ground lichen (Cladonia sp.) was 250 Bq/kg d.w. in western Lapland in 1980 and 170 Bq/kg d.w. in eastern Lapland in 2004 [16].

Reindeer consume mushrooms in late summer and autumn. The ²¹⁰Pb content in brown-yellow boletus (Suillus luteus) is rather low, only a few Bq/kg d.w. However, they contain 100–140 Bq/kg d.w. of ²¹⁰Po, which significantly increases the intake of this nuclide into reindeer [16]. The content of ²¹⁰Pb and ²¹⁰Po varies two orders of magnitude from one species to another in mushrooms sampled in northern Finland (Figure 2a,b) [17]. The activity concentration of ²¹⁰Pb ranged from 1.55 (Leccinum vulpinum) to 16.2 (Cortinarius armillatus) Bq/kg d.w. For ²¹⁰Po, the corresponding range was from 8.98 (Hygrophorus camarophyllus) to 2192 (Leccinum vulpinum) Bq/kg d.w., respectively. Additionally, the type of forest and forest soil affects the content of ²¹⁰Pb and ²¹⁰Po in mushrooms. For example, the concentration level of radionuclides in the organic-rich litter layer affects the radionuclide content of mushrooms, but a clear connection between soil type and concentrations of ²¹⁰Pb and ²¹⁰Po in mushrooms has not been established yet. Furthermore, mushroom species are suggested to be the most important factor in radionuclide accumulation [17].

The activity concentration of ²¹⁰Pb and ²¹⁰Po in different parts of blueberry and lingonberry in northern Finland is depicted in Figure 3 [17]. The activity concentration of ²¹⁰Po is higher than that of ²¹⁰Pb in stems, berries, and leaves of both berry species, indicating a higher bioavailability of polonium compared to lead. The berries have much smaller activity content of both radionuclides compared to other parts of the plants. Plants are known to have a selective ability to metal uptake, capacity to store toxic metals in inert parts (e.g., tree stems) and seedlings, and prevention mechanism for blocking accumulation of toxic metals in their reproductive organs, e.g., seeds and berries [20,21,22,23,24].

The seasonal variation of reindeer’s feeding habits are also reflected in the content of ²¹⁰Pb and ²¹⁰Po in various reindeer tissues (Figure 4). The activity concentrations are in their minimum in summer when reindeers have consumed vascular plants, birch leaves, etc. (Figure 5 and Figure 6). The activity concentrations are higher in autumn and winter after the consumption of first mushroom and, later, lichen [15].

According to results by Solatie et al. [16], ²¹⁰Pb mainly enriches reindeer bones after entering to reindeer body. There was a 10-fold concentration of ²¹⁰Pb in the reindeer bones compared to muscles. In the same study, the concentration of ²¹⁰Po was two times higher in bones than in muscles, at most. Therefore, relative enrichment of ²¹⁰Po to bones was not as high as of ²¹⁰Pb in investigated samples. Alongside bones, both ²¹⁰Pb and especially ²¹⁰Po enrich the liver and kidney in reindeer. The same preference for enrichment of ²¹⁰Pb was also observed with aribous in Canada [13]. In past centuries when reindeer was the foundation of diet among reindeer herders, consumption of inner organs and bone marrow of reindeer was also considerable; therefore, it has had a significant effect on intake of both ²¹⁰Pb and ²¹⁰Po, increasing the internal radiation dose in the Lapland region.

Enrichment of ²¹⁰Pb and ²¹⁰Po from diet to humans is roughly similar to reindeer, since both radionuclides accumulate to human bone, liver, and kidney. Next, some differences between the two regions (Lapland and southern Finland) with different diets in the past, in the concentrations of ²¹⁰Pb and ²¹⁰Po in human organs, are discussed. A related review about possible increased cancer risk among Sami people due to anthropogenic radionuclides from nuclear weapons tests, diet, and life habits has been recently published by Soininen and Mussalo-Rauhamaa [25].

Kauranen and Miettinen [15] estimated that the intake of ²¹⁰Po via ingestion within the reindeer herding population, 2.6 Bq/d, was 20 times higher than in the case of the general population, having an average western diet of 0.12 Bq/d. The corresponding intake of ²¹⁰Pb was three times higher, 0.32 Bq/d, within the reindeer herding population, than in the average western diet of 0.12 Bq/d. The high intake of ²¹⁰Po was mainly due to ²¹⁰Po in reindeer meat and liver. In recent decades, the difference in diet habits between reindeer herders vs. the average population has probably decreased but not entirely disappeared. The high ²¹⁰Po content in diets is also reflected in the body content of ²¹⁰Po within the reindeer herding population. The ²¹⁰Po activity concentration in the blood of reindeer herders was almost 20 times higher than within the population of Helsinki. The difference in the case of ²¹⁰Pb was almost threefold. Correspondingly, the activity concentration of ²¹⁰Po in human placenta in Inari and Utsjoki was over 12 times higher than within the population of Helsinki. In the case of teeth, the difference was only about a factor of two; the same as with ²¹⁰Pb. It is clear that ²¹⁰Po is not accumulated into teeth but is formed in situ from the decay of ²¹⁰Pb.

In a later study by Mussalo-Rauhamaa et al. [18], a set of tissue samples of five autopsy cases from Inari and Utsjoki region, deceased between 1977 and 1979, were analyzed. It was found that the ²¹⁰Pb activity concentration in human liver ranged from 0.13 to 1.0 Bq/kg wet weight (w.w.) with an average value of 0.48 Bq/kg w.w. In autopsy cases from southern Finland, an average ²¹⁰Pb activity concentration of 0.27 Bq/kg w.w. was obtained. ²¹⁰Po activity concentration in human liver in Lapland ranged from 0.40 to 8.0 Bq/kg w.w., with an average of 3.2 Bq/kg w.w. In autopsy cases from southern Finland, an average ²¹⁰Po activity concentration of 0.57 Bq/kg w.w. was obtained. Thus, the activity concentration was two times higher in the residents of Lapland compared to those in southern Finland in the case of ²¹⁰Pb. In the case of ²¹⁰Po, the average activity concentration was over five times higher in the residents of Lapland compared to those in southern Finland. In a recent study, Muikku et al. [26] reported that the ²¹⁰Po content in urine was three times higher in reindeer herders than the control group representing the average population. However, in the case of ²¹⁰Pb, there was no difference.

The work of Holm and Persson [27] indicates clearly how ²¹⁰Po dominates the total alpha activity of lichen (Table 3). The activity concentration of ²¹⁰Po in 1972 was 50 times higher than the infamous plutonium isotopes from the atmospheric nuclear tests. In addition, the deposition of ²¹⁰Pb has recurred every year after the ice age and will recur in the future too, contrary to plutonium deposition, which was in a longer timescale and was practically a single event. Furthermore, the half-lives of artificial radionuclides ²³⁸Pu (t½ = 87.7 a), ²⁴⁴Cm (t½= 18.1 a), and ²⁴²Cm (t½ = 163 d) are so short that they have decayed more or less significantly since their introduction to the environment.

After 1972, there have been several other nuclear events releasing artificial radionuclides to the environment, most importantly the nuclear reactor accidents of Chernobyl in 1986 and Fukushima in 2011. This kind of accidental release of radionuclides creates local and regional elevated radiation exposure. The radionuclide(s) in question, as well as the released activity level, environmental conditions, and countermeasures after the accident, etc., determine how high fractions of the received radiation dose are from natural radionuclides and from accidentally released (often artificial) radionuclides. The early situation after a nuclear accident, where artificial radionuclides often dominate the natural radionuclides as an exposure source, may change during time, when shorter-lived artificial radionuclides have been decaying and both spontaneous environmental processes, as well as remediation actions, have occurred.