Introduction

Honey is produced by honeybees from nectar of different plants, as well as from honeydew. As a foodstuff used for healing purposes, honey must be free of any of objectionable contents, and it should contain only small amounts of pollutants [1]. Environmental samples are complex most often in terms of composition and structure, very often heterogeneous and sometimes variable over time, resulting in lack of repeatability at sampling due to rapid changes in material being analyzed. Bioindicators are organisms used as an indicator of the state of the environment. These are species with a low tolerance index, or in a specific way responsive to the substance. Honey is a popular bioindicator. Bees collecting nectar, pollen and honeydew are often exposed to harmful substances planted on plants used in agriculture and carry these contaminants on the body surface to the hive, and as a consequence the impurities get into the honey. The results showed that trace element concentrations in the honeys from different regions generally reflect the degree of trace element contamination of the environment [1].

Bee honey can be contaminated by various treatments related to apiculture and apiculture itself as well as by xenobiotics in the environment. Taking into account the fact that bees use the benefits of a radius of up to 3 km from the hive, the origin of the nectar can be determined with great accuracy. With that in mind, bee products can be used as indicators of environmental pollution in a given area.

Research and evaluation of radiological conditions existing in the environment and the ability to track and predict the changes that may occur in it as a result of human activity are significant elements of population exposure control. This exposure is determined by the size of the effective dose which people receive from all sources and types of ionizing radiation during the year. The largest share (73.8%) in the value of the effective dose received by a statistical Pole is natural radiation [2]. Among the sources which have an impact on environmental pollution by natural radionuclides are: activities of mining and power industry and the presence of various types of landfills, fertilization; exhalation of radon 222Rn from the surface of the earth and building materials [2,3,4,5,6,7,8].

The presence of radionuclides can pose a health risk for humans but their presence in honey have not been studied much. The main aim of this study was determination of 210Po concentration as well as calculation of annual effective dose for 210Po from honey consumption in honey samples collected by courtesy of beekeepers from northern Poland. Moreover, the aim of this work was the diversification of 210Po sources in the analyzed honey samples, confirmation of the potential contaminants of the increased levels of natural radionuclides and morphological structure of plants, as well as indication of the usefulness of honeys as bioindicators of environmental pollution.

Methods and materials

140 different honey samples obtained from private apiaries of northern Poland were examined. 31, 30, 21, 27 and 31 honey samples respectively were obtained by courtesy of beekeepers or purchased in the area of West Pomeranian Voivodeship, Pomeranian Voivodeship, Podlaskie Voivodeship, Warmian-Masurian Voivodeship and Kuyavian-Pomeranian Voivodeship between May and October 2017.

About 10 g samples of honey were weighed, added with known amount of 209Po tracer and dissolved in 100 ml of 1 M HCl. After 1 h of stirring on magnetic stirrer, samples were filtered and diluted by addition of 400 ml H2O. The obtained solution was added with 3 ml MnCl2 and 5 ml KMnO4 and pH was set to 9 by addition of ammonia. After 1 h of stirring on magnetic stirrer, samples were left overnight to allow complete precipitation of MnO2. The precipitate was collected by centrifugation (4000 RPM, 5 min), dissolved in 15 ml of 1 M HCl with a drop of H2O2 then evaporated to near dryness. The remains of organic matrix were decomposed by subsequent evaporation to wet salts with 20 ml of concentrated HNO3 and then 20 ml of concentrated HCl. Finally the residue was dissolved in 15 ml of 0.5 M HCl and after addition of 0.05 g of ascorbic acid, solution was transferred to autodeposition cell. Polonium was deposited on silver in time of 1.5 h with continuous stirring. Obtained polonium was counted by alpha spectrometry between 2 and 6 days.

The activities of 210Po were measured using an alpha spectrometer (Alpha Analyst S470) equipped with a surface barrier PIPS detector with an active surface of 300–450 mm2 placed in a vacuum chamber connected to a 1024 multichannel analyzer (Canberra–Packard, USA). Detector yield ranged from 0.30 to 0.40. In most of the used detectors with the surface of 300–450 mm2 the resolution was 17–18 keV. Minimum detectable activity (MDA) for 210Po was 0.003 mBq g−1. The accuracy and precision of the radiochemical method were estimated to be less than 7% by participation in international intercomparative exercises and analysis of IAEA materials (IAEA-327, 384, 385, 414, IAEA-TEL-2011-03 and MODAS-2015). The precision between subsamples was estimated to be less than 3% for all analyzed radioisotopes. 210Po activities were corrected for decay between the time of deposition on silver discs and the time of measurement on alpha spectrometer. On the basis of the 210Po concentration and the average honey consumption in Poland (0.61 kg/person), the so-called effective annual dose from honey consumption was calculated according to Meli and co-workers [9]:

$$D_{{\text{Po-210}}} = Q \cdot C_{{\text{Po-210}}} \cdot I$$

where \(D_{\text{Po-210}}\)—annual effective dose (µSv rok−1), Q—conversion factor which is 1.2, \(C_{\text{Po-210}}\)—concentration of 210Po (Bq kg−1), I—average honey consumption.

Results and discussion

The results of 210Po concentration in honey samples collected from West Pomeranian Voivodeship, Pomeranian Voivodeship, Podlaskie Voivodeship, Warmian-Masurian Voivodeship and Kuyavian-Pomeranian Voivodeship differed in the range between 0.009 ± 0.001 and 0.328 ± 0.013 Bq kg−1 (with the average value 0.074 ± 0.055 Bq kg−1) (Table 1 and Fig. 1). The values of effective annual dose from honey consumption for analyzed voivodeships were from 0.006 ± 0.001 to 0.240 ± 0.009 μSv year−1 (with the mean value 0.054 ± 0.041 μSv year−1).

Table 1 210Po concentration and annual effective dose from the honey consumption in honey samples taken from Kuyavian-Pomeranian Voivodeship (KP), Podlaskie Voivodeship (PD), Pomorskie Voivodeship (PM), Warmian-Masurian Voivodeship (WM) and West Pomeranian Voivodeship (WP)
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Fig. 1
figure 1

210Po concentration in analyzed honey samples taken from Kuyavian-Pomeranian Voivodeship (KP), Podlaskie Voivodeship (PD), Pomorskie Voivodeship (PM), Warmian-Masurian Voivodeship (WM) and West Pomeranian Voivodeship (WP)

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The results of 210Po concentration and effective annual dose from honey consumption in West Pomeranian Voivodeship were presented in Table 1 and ranged between 0.011 ± 0.002 and 0.157 ± 0.012 Bq kg−1 (with the average value 0.068 ± 0.045 Bq kg−1), while values of effective annual dose from honey consumption varied from 0.008 ± 0.002 to 0.115 ± 0.009 μSv year−1 (with the average value 0.047 ± 0.033 μSv year−1) (Fig. 1). The higher values of 210Po concentrations and effective annual dose from honey consumption (between 0.099 ± 0.006 and 0.157 ± 0.012 Bq kg−1 and between 0.073 ± 0.004 and 0.115 ± 0.009 μSv year−1 respectively) were found near Szczecin city, where chemical plant “Fosfan S.A.” is situated. “Fosfan S.A.” manufactures, sells, distributes, and exports compound agricultural fertilizers and gardening fertilizers. A lot of authors show that chemical industry connected with production of fertilizers for agriculture provides higher activities of natural radionuclides [10,11,12,13,14,15,16,17,18]. The higher 210Po concentration and effective dose were observed also for samples collected near phosphogypsum heap in Police (from 0.123 ± 0.004 to 0.132 ± 0.017 Bq kg−1 and from 0.090 ± 0.003 to 0.097 ± 0.012 μSv year−1). Other authors suggests that higher content of polonium and uranium in various components of environment can be considered as an effect of their transfer from heap to plants and soils caused by leaching and lixiviation [15, 19, 20].

Generally, the average 10Po concentrations and effective annual dose from honey consumption were higher for industrial areas of this province (0.128 ± 0.021 Bq kg−1 and 0.094 ± 0.015 μSv year−1 respectively) than for non-industrial areas (0.053 ± 0.037 Bq kg−1 and 0.036 ± 0.025 μSv year−1 respectively) (Table 2) (Fig. 2).

Table 2 210Po concentration and annual effective dose from the honey consumption in honey samples taken from industrial and non-industrial areas of Kuyavian-Pomeranian Voivodeship (KP), Pomorskie Voivodeship (PM) and West Pomeranian Voivodeship (WP)
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Fig. 2
figure 2

210Po concentration in honey samples taken from industrial and non-industrial areas of Kuyavian-Pomeranian Voivodeship (KP), Pomorskie Voivodeship (PM) and West Pomeranian Voivodeship (WP)

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The results of 210Po concentration and effective annual dose from honey consumption in Pomeranian Voivodeship were presented in Table 1. The 210Po concentration in this province was between 0.016 ± 0.002 and 0.238 ± 0.006 Bq kg−1 (with the average value 0.090 ± 0.072 Bq kg−1) (Fig. 1), while values of effective annual dose from honey consumption ranged from 0.012 ± 0.002 to 0.174 ± 0.004 μSv year−1 (with the average value 0.066 ± 0.053 μSv year−1). The average 10Po concentrations and effective annual dose from honey consumption were higher for industrial areas of this province (0.175 ± 0.041 Bq kg−1 and 0.129 ± 0.030 μSv year−1 respectively) than for non-industrial areas (0.040 ± 0.021 Bq kg−1 and 0.030 ± 0.015 μSv year−1 respectively) (Table 2, Fig. 2). The higher values of 210Po concentrations and effective dose were measured in this region around phosphogypsum waste heap in Wiślinka and varied from 0.123 ± 0.013 to 0.207 ± 0.007 Bq kg−1 and from 0.090 ± 1.010 to 0.152 ± 0.005 μSv year−1 respectively. Long-term studies of the impact of phosphogypsum heap in Wiślinka have proved that this area can be a threat to the surrounding environment. Detailed studies of Wiślinka phophogypsum heap were described by Boryło and co-workers [6,7,8, 21] and Olszewski and co-workers [22, 23]. In this province higher 210Po concentration and effective dose were also measured in Gdańsk city, where the petrochemical industry is developing (from 0.128 ± 0.005 to 0.142 ± 0.012 Bq kg−1 and from 0.094 ± 0.004 to 0.104 ± 0.009 μSv year−1 respectively.

The results of 210Po concentration and effective annual dose from honey consumption in Warmian-Masurian Voivodeship were presented in Table 1. The 210Po concentration in this province was between 0.012 ± 0.001 and 0.104 ± 0.008 Bq kg−1 (with the average value 0.060 ± 0.031 Bq kg−1) (Fig. 1), while values of effective annual dose from honey consumption ranged from 0.009 ± 0.001 to 0.082 ± 0.012 μSv year−1 (with the average value 0.044 ± 0.023 μSv year−1). The similar values of 210Po concentration and effective annual dose from honey consumption were obtained in Podlaskie Voivodeship and presented in Table 1. The 210Po concentration in this province was between 0.013 ± 0.002 and 0.109 ± 0.007 Bq kg−1 (with the average value 0.053 ± 0.031 Bq kg−1) (Fig. 1), while values of effective annual dose from honey consumption ranged from 0.010 ± 0.002 to 0.080 ± 0.005 μSv year−1 (with the average value 0.039 ± 0.023 μSv year−1). Due to the values of the natural environment the region of Warmian-Masurian voivodeship is often called the “green lungs” of Poland. It results among others, from the low level of industrialization in the voivodeship, which is primarily limited to agricultural and forestry production. Other branches of industry are located mainly in Olsztyn and Elbląg—the two largest cities in the region. Also the assessment of the environmental condition of the Podlaskie voivodeship, which has unique natural values, indicates that it is a region with the highest environmental quality standards in Poland. In both voivodeships, no area was found for which higher concentrations of 210Po were noted. It seems, however, that slightly higher 210Po concentrations were measured among plants covered with thick, tangled hairs that cover the leaves, stem and fruit (e.g. raspberry, acacia, yellow sweet clover). On the other hand, the above mentioned plants belong to collective clade [24]. This clade is called the rosids and is a member of a large clade (monophyletic group) of flowering plants [25, 26]. They are classified according to their cup and crown or single or disappearing flower. The average values of 210Po concentration and effective dose for these plants were 0.058 ± 0.020 Bq kg−1 and 0.045 ± 0.019 μSv year−1 for Warmian-Masurian Voivodeship and 0.040 ± 0.005 Bq kg−1 and 0.029 ± 0.004 μSv year−1 for Podlaskie Voivodeship (Table 3, Fig. 3). The smaller values of analyzed 210Po were observed for plants not covered by tomentose and they do not belong to rosids (0.031 ± 0.022 Bq kg−1 and 0.023 ± 0.016 μSv year−1 for Warmian-Masurian Voivodeship and 0.026 ± 0.008 Bq kg−1 and 0.019 ± 0.006 μSv year−1 for Podlaskie Voivodeship) (Table 3, Fig. 3). This effect was also observed for Kuyavian-Pomeranian Voivodeship and West Pomeranian Voivodeship (Table 3, Fig. 3). Also research conducted in northern Poland confirm that plants that are tomentose or hairs covered have ability to accumulate higher amounts of polonium and uranium [19].

Table 3 210Po concentration and annual effective dose from the honey consumption in honey samples taken from plants (other and rosids) of Kuyavian-Pomeranian Voivodeship (KP), Podlaskie Voivodeship (PD), Warmian-Masurian Voivodeship (WM) and West Pomeranian Voivodeship (WP)
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Fig. 3
figure 3

210Po concentration in honey samples taken from plants (other and rosids) of Kuyavian-Pomeranian Voivodeship (KP), Podlaskie Voivodeship (PD), Warmian-Masurian Voivodeship (WM) and West Pomeranian Voivodeship (WP)

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However, it should be remembered that honey may not be the most sensitive tool for evaluating environmental contamination with heavy metals due to the low concentration present, and the great variability caused by several factors, e.g. botanical origin, floral density, season of the year and rainfall [1]. Also in the territories of these voivodeships, higher concentrations of 210Po were measured for honeydew samples. This is also noticeable for all other provinces. Honeydew honey arises from the juice of some trees. This is a viscous, sweet discharge mainly in summer in the form of droplets on trees, secreted by aphids and the scale insects sucking juices from young shoots and leaves. Honeydew can be collected in two ways. The first one is direct collection from plants, while the second one—collecting from aphids biting into plants and excreting also honeydew. This may suggest that aphids and the scale insects may have some effect on higher concentrations of 210Po.

The results of 210Po concentration and effective annual dose from honey consumption in Kuyavian-Pomeranian Voivodeship were presented in Table 1. The 210Po concentration in this province was between 0.009 ± 0.001 and 0.208 ± 0.011 Bq kg−1 (with the average value 0.092 ± 0.067 Bq kg−1), while values of effective annual dose from honey consumption ranged from 0.007 ± 0.001 to 0.152 ± 0.008 μSv year−1 (with the average value 0.067 ± 0.049 μSv year−1). The average 10Po concentrations and effective annual dose from honey consumption were higher for industrial areas of this province (0.156 ± 0.031 Bq kg−1 and 0.114 ± 0.023 μSv year−1 respectively) than for non-industrial areas (0.052 ± 0.050 Bq kg−1 and 0.038 ± 0.036 μSv year−1 respectively) (Table 2, Fig. 2). The higher values of 210Po concentrations and effective dose (between 0.101 ± 0.010 and 0.199 ± 0.012 Bq kg−1 and between 0.074 ± 0.007 and 0.146 ± 0.009 μSv year−1 were measured in this region around salt mine “Solina” (where the salt is obtained as a result of leaching) within which there are: borehole mine of salt “Mogilno” in Przyjma and borehole mine of salt “Góra” in Radojewice. Additionally, the stage of reconnaissance works was completed in Lubień Kujawski and the boreholes confirmed the presence of salt. In 2018, the construction of a salt mine will commence, and it is planned to be completed in 2020. The obtained results of 210Po concentration in this mining area are smaller than the results of 210Po concentration around the mining area in Lower Silesian Voivodeship and Lesser Poland Voivodeship [27, 28]. Moreover, in this region there are active plants, where salt is obtained as a result of evaporation of underground brine: “Ciech Soda Polska S. A.” plant in Inowrocław-Mątwy and Janikowo and the saltern in Ciechocinek. It should be remembered, however, that the “Ciech Soda Polska” plant places strong emphasis on environment-friendly solutions.

Based on the results obtained for Pomeranian, West Pomeranian and Kuyavian-Pomeranian voivodeships, it can be concluded that apiaries located near polluted areas (taking into account heavy traffic, industrial contaminants) can be helpful in monitoring the radionuclides levels from various sources. In West Pomeranian, Pomeranian and Kuyavian-Pomeranian Voivodeships the reason of higher 210Po concentration might be well-developed industry and location of apiaries near the sources of pollution. In contrast, honeys from Warmian Masurian and Podlaskie voivodeships showed lower contents of 210Po than did the other honeys, due to the fact that these regions are not industrially polluted.

Heoneybees and their products can be considered as bioindicators because honeybees are exposed to contaminants present in environment around apiary [1, 29]. The honeybee’s collects nectar and pollen from locations that are situated up to about 3 km from apiary. Because of this large surface area that honeybees and their products have been proposed as suitable bioindicators of chemical pollution [1, 29,30,31,32]. Taking into account kind of honey, our obtained results of 210Po concentration in honey samples for all analyzed provinces show, that the highest concentrations were measured in honeydew samples (between 0.058 ± 0.003 and 0.238 ± 0.006 Bq kg−1 (with the average value 0.125 ± 0.047 Bq kg−1), while smaller values were found in nectar honey samples (from 0.011 ± 0.001 to 0.198 ± 0.006 Bq kg−1 (with the average value 0.043 ± 0.033 Bq kg−1) (Table 1, Fig. 4). The obtained results show, that honey samples, due to their ability to accumulate the natural isotope 210Po, meet all the requirements that are put to good bioindicators, and especially honeydew honey, which accumulates about 3 times more of this radionuclide than other nectar honeys. Values of the estimated annual effective dose from honey consumption ranged from 0.042 ± 0.002 to 0.174 ± 0.004 μSv year−1 and from 0.008 ± 0.001 to 0.145 ± 0.004 μSv year−1 for honeydew samples and nectar samples respectively (Table 2). The average annual effective dose from honey consumption was 0.091 ± 0.035 μSv year−1 for honeydew samples and 0.031 ± 0.024 μSv year−1 for nectar honeys samples (Table 1). The presented work shows significant variations in the values of 210Po in different Polish voivodeships so we can conclude that honey can be a reliable biological marker for the assessment of radionuclides pollution.

Fig. 4
figure 4

210Po concentration in nectar and honeydew honey samples in analyzed voivodeships

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On the other hand taking into account kind of honey and type of plants from which bees have been collecting pollen or nectar our obtained results of 210Po concentration in honey samples for all analyzed provinces show, that the highest concentrations were measured in honeydew samples, higher in honey samples from plants tomentose covered and/or belonging to rosids clade and the smallest in other honey samples (Fig. 5).

Fig. 5
figure 5

210Po concentration in analyzed honey samples taking into account honey substrate

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Based on ANOVA analysis significant statistical differences were found between the concentration of polonium in honey samples in industrial and non-industrial areas of West Pomeranian Voivodeship, Pomeranian Voivodeship and Kuyavian-Pomeranian Voivodeship (p ≪ 0.05). The statistically significant differences were observed also for honeydew and nectar samples of honey for all analyzed voivodeships (p ≪ 0.05). Significant statistical differences were found for Podlaskie Warmian-Masurian, West Pomeranian and Kuyavian-Pomeranian Voivodeships between kind of honey and morphological structure of plants from which bees collected nectar (tomentose and its lack) (P ≪ 0.05).

The obtained in this study values of 210Po concentrations in honey samples are much smaller than the values for other voivodeships of southern Poland (e.g. the average 210Po concentration was 0.621 ± 0.832 Bq kg−1 for Lesser Poland Voivodeship and 0.777 ± 1.711 Bq kg−1 for Lower Silesian Voivodeship) [27, 28]. Average effective dose from the consumption of honeys from analyzed voivodeships is smaller than the average effective dose from natural sources, which is 2.4 mSv and from the average annual effective dose associated with the absorption of radioactive radionuclides (90 μSv) [33, 34]. This allows us to believe that eating honey from the analyzed voivodeships is safe and does not constitute a radiological threat.

Conclusions

The results of polonium (210Po) determination in different environmental honey samples collected in northern Poland are presented and discussed in this paper. The polonium content in honey samples was measured by alfa-spectrometer technique. The 210Po concentrations in honey samples from West Pomeranian Voivodeship, Pomeranian Voivodeship and Kuyavian-Pomeranian Voivodeship recorded in this study are higher than the contents of 210Po from Warmian-Masurian Voivodeship and Podlaskie Voivodeship. The results of the presented survey suggest that honey can be a good indicator of polonium contamination of the environment. Higher 210Po concentrations were found in three voivodeships, which is connected with the development of many branches of industry (e.g. chemical industry, petrochemical and sodium industry, as well as fertilizer industry). It also seems that a type of plant from which bees have been collecting pollen or nectar may have some effect on the bioaccumulation of this radionuclide. The present results also proved that honeydew honey contains more polonium than nectar honey. The results showing concentration of 210Po in honey samples from analyzed voivodeships obtained in this study can lead to the conclusion that consumption of honey is safe and does not adversely affect human health.