Environmental Biomonitoring of Heavy and Toxic Metals Using Honeybees and Their Products—An Overview of Previous Research

Abstract

Humans and bees share millennia of history that have resulted in ever-increasing connection and interdependence. Thus, today, it is impossible to ignore the influence of humans on bees, particularly regarding the decrease in their numbers due to environmental contamination. Although they do not cause immediate mortality, heavy and toxic metals, along with dangers such as bee diseases, pesticides, habitat destruction, and climate change, threaten the number of bees and should not be ignored. Honeybees, their colonies, and their products are recognized as accumulators of metals and biological indicators of the presence of these metals in all environmental components. This study is an overview of prominent research from the past three decades on heavy and toxic metal levels in honeybees (Apis mellifera L.), honey, wax, and pollen. This research compares metals such as Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, and Zn in natural environments and in an environment where anthropogenic pressure manifests. The presented studies represent a range of research using analytical methods to determine the presence of heavy and toxic metals in different segments of bees and their products, linking these findings with the state of the environment. It has been repeatedly established that if heavy and toxic metals are present in higher concentrations in components of the environment that are under anthropogenic pressure, then their concentrations in bees, honey, and wax will also be higher. By summarizing this research in one place, this study can provide guidelines for future scientific work on this subject, promoting sustainable development through safe beekeeping and healthy bees.

1. Introduction

Bees have been associated with humans since ancient times. The oldest depiction of bees and humans dates back 12,000 years and can be found in the Cueva de la Arana cave in Spain, showing a man climbing a rock to a bee colony from which he takes honey with a comb [1]. Each organism and its home represents a mirror pair, and it is impossible to view them separately. This mirror image allows us to use certain living beings, in relation to their biotope, as biological indicators or bioindicators of environmental conditions and events [2]. This is precisely what applies to bees, especially the honeybee, Apis mellifera L. 1758.

Bees are insects from the order Hymenoptera, of which there are about 20,000 species [3,4]. They are leading pollinators for plants that owe their reproduction to insects [5]. Thus, bees are guarantors of maintaining biodiversity and sustainable food production for the human species. This is especially important in agricultural production, where they are the primary pollinators of crops that are not pollinated by the wind. There, bees pollinate about 70% of plantations and crops [6]. The direct benefit of honeybees can also be seen in bee products such as honey, wax, pollen, propolis, royal jelly, and bee venom.

Sustainable development and sufficient food production for humans and animals cannot be achieved without bees [7]. Because of their extremely important role as plant pollinators and producers of irreplaceable natural products, bees are the link between what reaches plants and bee products through water, soil, and air. Therefore, bees are irreplaceable biological indicators of environmental conditions and contamination [2,8,9,10,11,12,13,14,15,16], as well as the health of the environment and its individual components, which are inextricably linked to human health [17]. However, since 1985, a 25% loss of honeybee colonies has been recorded in Central Europe, and in the USA, this loss has been as high as 40% since 2006, which, according to a report from 2013, resulted in the lowest number of these pollinators in North America in the last half-century [18,19]. Bees around the world are threatened by diseases, pesticides, habitat destruction, and climate change but are also burdened by toxic heavy metals. This results in a major problem: the cumulative effect of these factors is evident through disorders and the increased mortality rate in honeybee colonies [2,15].

Therefore, the importance of monitoring and protecting water, soil, and air from various contaminants, such as heavy and toxic metals, should be highlighted. If these are present in our common environment, they will reach plants, bees, and their products, posing a threat to humans as the final consumers. Consequently, honeybee health is also at risk. To prevent potential losses, understanding this inter-relationship is crucial and can be used for biomonitoring. Honeybees and their broods, honey, pollen, and propolis are useful for analyzing the heavy and toxic metal contents. This study presents decades of selected scientific research on this subject, serving as a basis to guide future explorations and to accentuate the importance of such endeavors for sustaining biodiversity and food production.

2. Heavy and Toxic Metals in the Environment

Heavy metals are found in many components of the environment, such as the hydrosphere, lithosphere, and biosphere, and have several classifications. They are defined as metals with a large mass number and a density at least five times greater than water [20,21]. In addition to their weight, this classification is associated with their toxicity, which is why metalloids like arsenic and selenium are included. Another definition states that heavy metals are those with a density greater than 5 g/cm³ and an atomic number above 20 [22,23]. For our purposes, the biological classification is also important, which divides heavy metals into those that plants need for growth and development, such as Cu, Fe, Mn, Mo, Ni, and Zn; those essential for certain organisms, such as Co, Cr, Fe, Mn, Mo, V, and Zn; and phytotoxic metals such as Cd, Hg, and Pb. Notably, the line between being a plant nutrient and becoming phytotoxic can be thin even in the same element [24,25].

Heavy metals can be found in water, soil, sediments, air, and plants. They interact with natural chemical and physical compounds, changing their form. They can bind or sorb onto natural substances, which can increase or reduce their mobility. In increased concentrations in the soil, they represent a danger to the environment, representing one of the most difficult problems without a solution. This is because heavy metals are not subject to biodegradation; instead, they accumulate in the environment. Furthermore, the soil composition is complex and inhomogeneous. Through the soil, heavy metals can reach the water through elutriation; while their transport depends on the physical and chemical properties of the metal, it relies even more so on the physical and chemical properties of the soil [26].

Heavy metals are widely used in the chemical and metal industries, mining, refineries, agriculture, pharmacy and medicine, technology, thermal power plants, fuel combustion, households, wastewater, and landfills. Thus, they are very widespread and pose a potential threat to human and environmental health. In addition, natural sources of heavy metals also exist. For example, rocks and sediments contain them, and as such, heavy metals can be washed away, worn out, eroded, or come into the environment through volcanic eruptions and evaporation from the soil and water. However, human activity primarily contributes to environmental contamination with heavy metals, and in this regard, Cd, Cr, Cu, Fe, Hg, Mn, and Pb stand out as the most significant contaminants [24]. Some of these are essential nutrients or micronutrients, while others are trace elements because of the concentration in which they appear. The essential heavy metals have important biochemical and physiological roles as parts of enzymes or in oxidation and the reduction reactions of plant and animal organisms. However, they can also have harmful effects on the cell, i.e., cell organelles such as the membrane, mitochondria, endoplasmic reticulum, lysosomes, and cell nucleus, as well as on enzymes, nuclear proteins, and DNA, causing disorders in their functions, metabolism, detoxification processes, and damage repair. The most common way for heavy metals to enter the body is through the largest organ, the skin, through inhalation and ingestion. Thus, short- and long-term effects on health are possible, including abnormalities in development, immunological and hematological disorders, cardiovascular diseases, diabetes, and various malignant diseases. Simultaneous exposure to several heavy metals can have an additive, antagonistic, or synergistic effect. However, all of this depends on the metal and its chemical form, dose, and time of exposure [20,21,24].

Heavy metals can cause damage and contamination such that their concentrations cause visible and measurable disturbances to the functions of soil, which primarily refers to its role of fertility in the normal growth and development of natural and cultivated vegetation. This also includes its filtration function for ground and surface water. The concentrations of heavy metals in a plant species depend on several factors, such as the type of plant, its dominant tissue, the metal concentrations and availability in the soil, the weather conditions and season, and its distance from sources emitting certain elements [25].

Heavy metals are continuously released into the environment, either from natural (geological activity) or anthropogenic sources (human activity). They do not break down but instead enter physical and biological cycles, where they continue to circulate. Honeybees come into contact with heavy metals with their hairy bodies in the air or bring them to their colonies through pollen, water, nectar, and honeydew. When using bees as biological indicators of heavy metals, it is necessary to consider the following factors: the botanical origin of honey (open flowers are more exposed to pollutants), the season (in spring, nectar secretion is more abundant than in summer and autumn), and weather conditions (wind and rain can transfer pollution from the atmosphere to other environmental components) [27].

Environmental contamination with heavy metals has contributed to the weakening of bee colonies and the disappearance of these insects in recent years. The greater this contamination, the more it accumulates in worker bees, although this depends on the apiary’s distance from the source of contamination [12].

3. Scientific Research on Honeybees in the Past Three Decades

Honeybees were recognized as biological indicators of environmental contamination in 1935. In 1970, they were first used to monitor heavy metal contamination, and in 1980, bees and bee products were first used to assess pesticide contamination [28]. In 1962, Svoboda published the first scientific paper on the possibility of using bees in environmental monitoring. He and his colleagues used bees to monitor increased concentrations of the radionuclide ⁹⁰Sr, which may have occurred as a result of conducting nuclear tests in the atmosphere [29]. In 1975, Kresak reported the poisoning of bees in Slovakia in 1938, stating that arsenic, beryllium, copper, lead, zinc, magnesium fluoride, silicon dioxide, hydrochloric acid, and sulfuric acid could be found in honeybees [30]. At the beginning of this millennium, there was an increase in research on bees and their products as biomarkers of environmental contamination [2]. They have even been used to detect mines [31]. They serve as important biological samplers for contaminants in their own environments and human environments, including heavy and toxic metals, as well as other substances (Figure 1) because in every segment of their activities, they are in contact with individual components of the environment (water, air, and soil), as well as plants [15,32]. Thus, trace and heavy metals, as well as metalloids, pose a threat, together with other stressors released from natural and, to a greater degree, anthropogenic sources, and they should not be ignored [33,34].

Leita et al. studied 12 bee colonies next to an extra-urban traffic intersection with traffic of 10,000 vehicles per day. They showed that bee products can be useful in assessing the presence of environmental contamination, while determining heavy metals in dead bees can confirm the dynamics of contaminant accumulation. Cd, Pb, and Zn concentrations were monitored; Cd and Zn were found on the bees bodies, and Pb was apparently retained in the body of bees. Honey and royal jelly contained higher concentrations of heavy metals. A linear relationship between Cd concentrations in honey and the flowers of the clover Trifolium pratense L. was determined. The study mentions six groups of heavy metal contamination sources, including natural sources; agricultural additives; waste from mines and mills; industrial emissions, emissions from communal services; and emissions from mobile sources, primarily cars. Petroleum fuels contain Pb as an additive, while Cd, Pb, and Zn can be found in mechanical lubricants [8].

Bees indicate the presence of contamination in the environment in two ways: through increased mortality and the residues of certain toxic substances in honey, pollen, and larvae, such as heavy metals, herbicides, fungicides, and radionuclides. Given that they possess several important morphological, ecological, and behavioral characteristics as biological indicators, bees help create so-called environmental health maps related to, for example, pesticides, using mortality rates, apicide numbers, the type and risk level of the identified compounds, etc. They are highly effective and precise in detecting the spatial range and quality of pollution comprising different compounds [2]. Bees are ubiquitous, are easy to raise, have moderate food requirements, are covered with hair, are sensitive to most plant protection products, have a very high reproduction rate, have a relatively short life span, and are very mobile. Indeed, bees can cover a wide area: by flying, they can sample almost every segment of the environment—soil, water, plants, and air. If there is a contamination source due to various hydrological processes (evaporation or rain), contamination circulates in water between individual components of the environment (inanimate nature) and reaches living components (plants, animals, and humans), including bees, which, in different ways (through inhalation or ingestion), are also exposed to this effect (Figure 2). Research shows that Pb concentrations are higher in the body of bees in urban and industrial environments, while higher Ni and Cr concentrations can be found in bees in the natural environment, as can Pb if transported and deposited through the air from a distant source [27].

Bees are excellent biological indicators of environmental contamination via plant protection products, whether they are misused, used in the wrong way (either qualitatively or quantitatively), or prohibited pesticides are used, This cannot be determined otherwise without the help of bees. In this way, the use of detrimental and prohibited compounds can be proven. Treated crops can also be determined; in addition, the area and period representing the main risk for bees can be characterized. From 1982 to 1993, a large and long-term study was carried out in northern Italy, in the province of Forlì, where intensive fruit production was present. The research demonstrated a reduction in pesticide contamination in the agro- and ecosystems due to the growers’ increased awareness of the use of pesticides as a result of education and training. They began to use pesticides more responsibly and chose those that do not harm the environment, especially beneficial insects. The average number of bee poisoning incidents dropped from 8 to 9 per year in the 1980s to 3 incidents per year in the 1990s [35].

Bogdanov lists two sources of pollutants in bee products, namely, beekeeping practices and the environment (Figure 3). Heavy metals (such as Cd, Hg, and Pb, mostly from industry and traffic), pesticides (bactericides, fungicides, herbicides, and insecticides), radioactive elements (primarily ⁴⁰K and ¹³⁷Cs), pathogenic bacteria, and genetically modified organisms can enter bee products from the environment. Beekeeping can be a source of contamination with acaricides, organic acids, substances from essential oils, antibiotics, para-dichlorobenzene, and chemical repellents. Beekeeping practices pose a greater danger than bee products that are contaminated by the environment. Thus, the greatest risks are antibiotics for honey, acaricides for wax, Pb and acaricides for propolis, pesticides for pollen, and antibiotics for royal jelly. Lead spreads through the air in areas of intense motor traffic and thus ends up in nectar and honeydew. It is not transmitted by plants. Cadmium originating from the metal industry and incinerators ends up in the soil and is transported by plants to nectar and honeydew. However, because of nectar filtration, bees are still better indicators of Pb and Cd than their products [36].

Honey, pollen, and bees were sampled at 18 apiaries in western France during four different periods in two beekeeping seasons in 2008 and 2009, and Pb was found through these means. The highest Pb concentration was in foraging bees and the lowest was in pollen and honey. More Pb was found in urban and hedgerow samples, and less Pb was found in cultivated area and island samples. A higher Pb content in the samples was confirmed during the dry period. This indicates that foragers are most exposed to Pb pollution from the air, through food (nectar and pollen), and in water [37]. In the analyzed period and area, samples from foraging bees, pollen, and honey were analyzed for the presence and concentration of polycyclic aromatic hydrocarbons, which depend on landscape characteristics [38].

In four districts of the Kurdistan province in western Iran, a survey of Hg, Ba, Ca, Fe, Mn, Li, As, Na, and K in the bodies of bees was conducted. Contaminants from the soil can accumulate in plants; bees collect these contaminants with nectar and pollen, and certain amounts can accumulate in a bee’s body. The highest concentrations of metals were found in bees; those in honey were lower, possibly indicating that bees purify the nectar or honey in a particular way. Through this research, three groups of elements were differentiated: very abundant (K and Na), elements in medium concentrations (Ca, Hg, Ba, and Fe), and trace elements (Mn, Li, and As). Bees can warn of changes in environmental conditions at an early stage and aid in assessing anthropogenic impacts over a longer period, for example, in cases of heavy metal contamination, where heavy metals do not cause mortality but instead bioaccumulate [39].

Bees react quickly to external conditions and can, therefore, be used to monitor the quality of environmental components. These can be monitored through bee colony development, behavior, and mortality rates or through comparative analyses of certain indicators, such as heavy metals and pesticides [40]. The biomonitoring of heavy metals in the soil, air, and plants is possible with the help of bees, a fact that has been confirmed by research near the town of Stara Zagora at the military–technical test site Zmeyevo, Bulgaria. Samples of bees and secretions taken from their hindguts (56) were collected during the beekeeping season of 2010 and 2011 and analyzed for Cu, Zn, Pb, Cd, Co, Ni, Mn, and Fe through atomic absorption spectrometry. A higher concentration of these metals was found in the fecal excretions of bees than in their bodies, indicating that the hindgut is a biological barrier. Ni, Cd, Co, and Pb were the most abundant in bee feces. Cu, Fe, and Zn were deposited in other body parts and were not efficiently excreted in the feces. For example, Fe can be deposited in trophocytes under the hypodermis of the abdomen, and Cu and Zn can accumulate in hemolymph and pectoral muscles [41]. Recent research in Poland in the cities of Lublin and Poznań also indicates that honeybees filter heavy metals out of nectar while transforming it into honey; these metals are transported to other parts of a bee’s body or excreted through feces [42].

In another study, atomic absorption spectrometry identified 12 elements in 59 samples of multifloral honey from fruit trees, meadow honey, chestnut, acacia, linden, and sage. Ca, Fe, K, Mg, Na, and Zn were found using the flame technique; As, Cd, Cu, Pd, and Se through the graphite technique; and Hg using a mercury analyzer. The highest concentrations of K, Ca, Mg, and Hg were found in chestnut honey, which contained the least As and Cd. The highest concentrations of Cu, Zn, Cd, and Pb were found in linden honey. The least Fe, K, Mg, and Hg were in acacia honey. Sage honey had the least Ca and Na. The Se concentration was below the detection limit in all cases. Multifloral fruit tree honey contained the most Fe and As and the least Pb, while meadow honey contained the most Na and the least Cu [11].

Bees and their brood are often exposed to environmental contamination. There is a proven connection between the concentrations of metals in soil and plants and their content in bees and their products, which affects their health and mortality. The aforementioned bioaccumulation and harm refer to the most frequently studied toxic elements, Cd, Hg, and Pb, which bees are exposed to in industrial development areas and have become increasingly prevalent in road traffic areas [12].

Bargańska and colleagues confirmed that bees and their products are good indicators of environmental contamination with toxic substances such as heavy metals, pesticides, and radioactive elements. Bees are excellent bioindicators because of their morphology, which allows them to monitor contamination in areas of several square kilometers around their hives. Thus, bees can be used to monitor the level of environmental contamination with heavy metals, pesticides, and radioactive elements by covering large areas, visiting many useful plants, and capturing and accumulating contaminants. However, when analyzing complex matrices such as the bee body, honey, wax, pollen, bee bread, and propolis, it is necessary to bear in mind possible interferences and their influence on the results, as well as the fact that contaminants can enter bees and their products in addition to the environment through beekeeping [28].

The most interesting heavy metal in the environment is Hg, which comes from natural and anthropogenic sources in an inorganic form as elemental mercury or mercury cation or in an organic form as methylmercury. Research in eastern Slovakia aimed to determine the total mercury content in the bodies of bees and their products, honey, and pollen. Sampling was performed at the apiaries of the University of Veterinary Medicine and Pharmacy in Košice and the apiary in Rozhanovce. Samples for mercury identification were mineralized using aqua regia, decomposed via microwave digestion, and analyzed on an AAS (atomic absorption spectrometry) analyzer. The Hg concentration found in honey and pollen samples was low and did not threaten the food chain or human health. A statistically significant relationship was established between both location and bees and both location and pollen [43].

Concentrations of 24 elements were identified for seven types of honey using ICP-MS (inductively coupled plasma mass spectrometry). The results support the evidence of a botanical origin related to significant differences in the elemental composition of honey. Honey is an excellent bioindicator of environmental pollution from heavy metals, as noted by Bilandžić et al. in 2017 [44].

Owing to heavy metal bioaccumulation, bees have become important in ecotoxicological assessments of soil, water, and air contamination. Thus, they were used to provide insights into the state of contamination in the city of Trieste in northeastern Italy, which is a large port and industrial center. The condition was monitored at two locations, in the urban part of the area in the city garden and in the industrial suburb. As, Bi, Cd, Co, Cr, Cu, Ni, Pb, Sr, V, and Zn were analyzed to determine their qualitative and quantitative nature in bee tissue samples, considering the distance from the industrial center. Sources of heavy metals included heating systems, incinerators, traffic, industrial emissions, waste disposal, combustion of fuel and additives, and coal, iron, and steel production. These can enter the body through inhalation and ingestion and cause respiratory infections, lung cancer, heart disease, irritation, and oxidative stress. The study showed elevated Cr and Cu concentrations in the bodies of bees in the urban environment, which can be attributed to contamination from traffic and heating systems. Conversely, elevated Cd concentrations were found in samples from the industrial area [45].

Ciobanu and Rădulescu (2018) researched the heavy metals Cd, Cr, Fe, Mn, Ni, Pb, and Zn in polluted and nonpolluted areas of Timiş County, Romania. The samples were taken from linden tree and mixed flower honey. The authors indicated the importance of honey quality regarding environmental pollution from soil, plant, air, beekeeping practices and production, and the environment. Furthermore, the botanical origin of honey, together with season and weather, should not be ignored. The study showed concentrations higher than the maximum permissible by EU standards. Cd and Zn were found in high concentrations in both kinds of honey. Honey from polluted areas had higher heavy metal concentrations, except Fe in mixed flower honey, regarding the micropollutants present and the location of hives [46].

In Serbia, research was conducted on honey (23), pollen (13), and nectar (6) samples collected in 2015 and 2016, in which toxic or heavy metals, pesticides, and PAHs were analyzed. The samples were taken from the center of Zemun, where the experimental stationary apiary of the Faculty of Agriculture of the University of Belgrade is located. Pb, Cd, As, Cu, Zn, Fe, Mn, Ni, Cr, and Hg were identified. The results showed that their concentrations in the samples were generally within the limits of European and Serbian regulations, although the elevated concentrations of Cr, Hg, and PAHs in the pollen samples indicated air contamination. This shows that, in this case, the city environment does not pose the threat of honey contamination if good beekeeping practices are followed. The study states that the loss of bees in Europe since 1985 is 25%, and in the United Kingdom alone since 2010, it is 45%. The rate of bee loss in the USA since 2006 is 40% [18].

Heavy metals accumulate in beeswax over many years and multiple processes without the possibility of decomposition. This can affect the physiology of the bee colony. Thus, in one study, As, Cd, and Hg were analyzed during the production of honeycomb bases by casting these bases on cooled rollers and printing a Maraldi pyramid on them after a double phase of extended sedimentation and cooling. The results showed that the difference in Hg concentration is statistically significant in different layers of wax and the remaining dark waste material. The concentrations of the analyzed elements were lower than the maximum allowed by European regulations [47].

Cr, Cu, Ni, Pb, Ca, Fe, Mn, Rb, Sr, and Zn concentrations were determined in 18 honeycomb samples from three apiaries in continental Croatia. The age of a honeycomb and its geographical origin (along with exposure to certain environmental contaminants) can impact honeycomb quantity. This is evident from the toxic elements Cr, Cu, Ni, and Pb and the essential elements Fe, Mn, Sr, and Zn but not Rb. Honeycomb-shaped beeswax is an excellent bioindicator and suitable for monitoring the toxic metal levels in the environment to determine the degree of contamination of an area. In this study, Cd, Cu, and Pb harmed the entire bee colony by slowing brood development with reduced weights in the pre-pupal and pupal stages, depending on the dose; reducing the survival rate; and reducing the energy level in adult bees. Al can negatively impact bee foraging behavior by reducing the number of bees returning from collecting and limiting their ability to find carbohydrate-rich pastures [48,49]. As, Cd, Pb, and Hg were analyzed with a graphite furnace using an AAS on wax from different layers. The wax was extracted as it was processed into honeycomb bases using the extended cooling and sedimentation phase techniques. The lowest concentrations were found in the two upper layers, from which the comb bases are made, and the highest concentrations of the tested elements were in the lowest layer, representing waste material. This indicates that this method can remove a certain amount of heavy and toxic metals from wax [13].

Heavy metal bioaccumulation in honeybees (Apis mellifera ligustica) was investigated in 35 localities of the Umbria area in central Italy. Based on this, the honeybee contamination index (HCI) was developed, which showed a low level of metallic contamination in 16 locations, a medium level in 18 locations, and a high pollution level in 1 location. In addition, the metal concentrations in air particles were determined at three regional stations. Enriching metals such as Cd, Cu, Mn, and Zn in bees apparently depends on local conditions, which are probably related to pesticide and fertilizer use and the resuspension of contaminated soils and agricultural residues [50]. Research conducted in the urban area of Milan, Italy, by Costa et al. (2018) confirmed that air pollution with Pb, Ni, and Cd is connected to the same heavy metal concentrations in bees. The results showed a quantitative connection between the concentration of these elements in the air in the form of atmospheric particulate matter and in dead bees collected from a hive at the Triennale Museum in Milan [51]. A similar conclusion was drawn by the authors of research on variability in heavy and toxic metals through time and space aimed at assessing the health status of the Molise Region, Italy, suggesting that honeybees are good bioindicators of heavy metal pollution in the air [52].

Honey’s composition is influenced by natural and anthropogenic factors with a botanical and geographical origin, and it contains heavy metals which, although present in very small quantities, have an important impact on quality. In total, 54 different elements have been found in honey as macroelements (Na, K, Ca, Mg, P, S, and Cl), microelements or trace elements (Al, Cu, Pb, Zn, Mn, Cd, Tl, Co, Ni, Rb, Ba, Be, Bi, U, V, Fe, Pt, Pd, Te, Hf, Mo, Sn, Sb, La, I, Sm, Tb, Dy, Sd, Th, Pr, Nd, Tm, Yb, Lu, Gd, Ho, Er, Ce, Cr, As, B, Br, Hg, Se, and Sr), or heavy metals (densities five times higher than that of water and of inorganic origin) [53,54].

In Bosnia and Herzegovina (BiH), an analysis of 33 samples of different honey types was carried out to determine their physicochemical and microbiological properties. The samples came from registered beekeepers from an area that covers more than 60% of the surface of BiH, i.e., its continental, sub-Mediterranean, and Mediterranean parts. This research did not confirm the correlation between physicochemical indicators and microbiological properties in samples that did not meet the quality criteria [55].

Recent research on Cd, Cr, Cu, Ni, and Pb concentrations in honey from Lithuania is also illustrative. Honey samples were taken from 12 potentially polluted locations near industrial plants, landfills, railways, and highways. The honey was burned to ash, which was then decomposed using aqua regia in a microwave digestion system used for samples. The individual metal concentrations were determined via atomic absorption spectrometry using a flame technique with acetylene and the graphite furnace technique. Low metal concentration values were found in the tested samples, and it was concluded that they did not threaten human health. While it is difficult to compare these results with those from other countries, as they differ owing to soil composition, botanical origin, weather conditions during flowering, and the degree of anthropogenic contamination, this research supports honey as a good biological indicator of contaminant levels in the environment, such as heavy metals. There was also a strong negative correlation between the heavy metal concentrations and the distance from the pollution source [16].

In Turkey, at different locations in Konya province, research was conducted on heavy metals Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn. Honeybee, honey, and pollen samples were gathered from eight different locations, four in urban areas near the city center and four in rural parts of this province, where 40 honeybee colonies were placed, with 5 per site. The two types of multifloral locations were compared. The results showed no significant variations in Cd and Pb in honey samples or Cd in pollen samples; however, for other heavy metals (Cr, Cu, Fe, Mn, Ni, and Zn), a statistically significant difference was established. Urban areas generally had higher metal concentrations than rural areas. Cd and Pb concentrations at all the locations conformed with the International Food Standard [56].

Environmental heavy and toxic metal contamination has also been confirmed by a research project in Varaždin county in NE Croatia. At five locations, different anthropogenic impacts were assumed regarding urban areas and traffic near agricultural plots and forest hill sites. Water, soil, honeybees, and honey were sampled and analyzed for Cd, Pb, and Hg content using atomic absorption spectroscopy and inductively coupled plasma mass spectrometry. The study found high Pb concentrations in the honey and high Hg concentrations in the water. In the case of honey samples containing more Pb than allowed by the EU Directive [57], this can be explained by environmental contamination spread through the air. A high Hg content in water samples near apiaries in the city can be attributed to painted plastic contamination [58].

Given that the number of honeybees in Europe and North America is declining, to gain insight into this situation, a survey in East Africa and Kenya was conducted 10 years ago. There, the honeybee is important as a pollinator and a source of income for small farmers and rural households. Therefore, in 2010, the number and size of honeybee colonies, the state of invasion by parasites and viral infections, the presence of pesticides, and the degree of hygiene were investigated at 24 locations. The presence of Varroa was proven throughout the country, except in the north, but it does not seem to affect colony sizes. Nosema apis was found in four locations. Three of the seven most common viruses in Europe and North America were detected (like Varroa, they were not found in the north of the country), and a small number of pesticides were found in low concentrations. Thus, chemical control for Varroa and Nosema was not necessary, as apparently, along with viruses, they have been recently introduced and have not affected the health of bee colonies in Kenya, which show some resistance or tolerance against them. The study emphasized the great importance of pollinators in three-quarters of the world’s food production and an increasing dependence on pollinators in developed and developing countries [59].

For easier navigation and a clearer overview of the scientific research presented in this section,

Table 1. Summarized research data according to the cited studies.

Year of Publication	Research Area	Material Sampled	Analysis Parameters	Country	Author(s)
1962	Atmospheric nuclear pollution	Honeybees	Radionuclide 90Sr	France	Svoboda
1975	Industrial areas	Honeybees	As, Be, Cu, Pb, Zn, magnesium fluoride, silicon dioxide, hydrochloric acid, and sulfuric acid	Slovakia	Kresak
1996	Urban traffic pollution	Honeybees	Cd, Pb, Zn	Italy	Leita et al.
2003	Environment	Honeybees, larvae, honey, and pollen	Heavy metals, pesticides, and radionuclides	Italy	Celli and Maccagnani
2003	Urban and industrial pollution	Honeybees	Cr, Ni, Pb	Italy	Porrini et al.
2003	Agro- and ecosystems	Honeybees	Pesticide residues	Italy	Porrini et al.
2006	Environment and beekeeping	Honeybees and their products	Heavy metals, pesticides, and antibiotics	Switzerland	Bogdanov
2012	Urban and cultivated areas	Honeybees, pollen, and honey	Pb and polycyclic aromatic hydrocarbons	France	Lambert et al.
2012	Environment	Honeybees	As, Ba, Ca, Fe, Hg, K, Li, Mn, and Na	Iran	Sadeghi et al.
2012	Military test site	Honeybees and their feces	Cd, Co, Cu, Fe, Mn Ni, Pb, and Zn	Bulgaria	Zhelyaskova
2014	Environment and orchards	Honey	As, Cd, Ca, Cu, Fe, Hg, K, Mg, Na, Pd, Se, and Zn	Croatia	Bilandžić et al.
2014	Industrial and traffic areas	Honeybees and their brood	Cd, Hg, and Pb	Poland	Madras-Majewska et al.
2014	Environment	Honeybees	Pathogens, parasites, and pesticides	East Africa	Muli et al.
2015	Environment	Honey	Ag, Al, As, Ba, Cd, Cr, Ni, Pb, Sb, Th, U, and V	Croatia	Bilandžić et al.
2015	Environment	Honey	Na, K, Ca, Mg, P, S, Cl, Al, Cu, Pb, Zn, Mn, Cd, Tl, Co, Ni, Rb, Ba, Be, Bi, U, V, Fe, Pt, Pd, Te, Hf, Mo, Sn, Sb, La, I, Sm, Tb, Dy, Sd, Th, Pr, Nd, Tm, Yb, Lu, Gd, Ho, Er, Ce, Cr, As, B, Br, Cd, Hg, Se, and Sr	Bangladesh	Solayman et al.
2016	Environment	Honeybees, honey, beeswax, pollen, bee bread, and propolis	Heavy metals, pesticides, and radionuclides	Poland	Bargańska et al.
2016	Beeswax production	Beeswax	As, Cd, Hg, and Pb	Croatia	Tlak Gajger et al.
2016	University apiaries	Honeybees, honey, and pollen	Hg	Slovakia	Toth et al.
2017	Environment	Honey	Essential and toxic metals	Croatia	Bilandžić et al.
2017	Industrial/urban areas and ports	Honeybees	As, Bi, Cd, Co, Cr, Cu, Ni, Pb, Sr, V, and Zn	Italy	Giglio et al.
2018	Environment	Honey	Cd, Cr, Fe, Mn, Ni, Pb, and Zn	Romania	Ciobanu and Rădulescu
2018	Urban areas and university apiaries	Honey, pollen, nectar	As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Zn, and polycyclic aromatic hydrocarbons	Serbia	Jovetić et al.
2018	Urban environment	Honeybees	Cd, Ni, and Pb	Italy	Costa et al.
2019	Beeswax production	Beeswax	As, Cd, and Hg	Croatia	Kosanović et al.
2019	Honeybee colonies/hives	Beeswax	Ca, Cr, Cu, Fe, Mn, Ni, Pb, Rb, Sr, and Zn	Croatia	Tlak Gajger et al.
2020	Terrestrial environment	Honeybees	Cd, Cu, Mn, and Zn	Italy	Goretti et al.
2021	Urban environment	Soil, plants, pollen, nectar, honey, and honeybees	Cr, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, Pb, Cd, Sn, and Hg	Poland	Grzegorz et al.
2022	Environment	Honey	Physicochemical, and microbiological properties	Bosnia and Herzegovina	Landeka et al.
2022	Industrial areas, landfills, railways, and highways	Honey	Cd, Cr, Cu, Ni, and Pb	Lithuania	Šerevičiene et al.
2022	Urban, city, and rural areas	Honeybees, honey, and pollen	Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn	Turkey	Bayir and Aygun
2022	Urban and rural areas	Honeybees	Be, Cd. Co, Cr, Cu, Ni, Pb, and V	Italy	Di Fiore et al.
2024	Beeswax production	Beeswax	Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Se, V, and Zn	Croatia	Kosanović
2024	Urban areas, traffic, agriculture, and forest hills	Water, soil, honeybees, and honey	Cd, Hg, and Pb	Croatia	Zavrtnik and Loborec

lists the cited works.

4. Conclusions

Honeybees provide humans with multiple benefits, some of which are impossible to assign an exact material value to. These include preserving biological diversity through pollination, sustainably producing quality food for humans and animals, producing irreplaceable honeybee products, positively impacting physical health through the consumption of bee products, and many others. The review of three decades of scientific research presented in this study confirmed that honeybees (Apis mellifera L.) are good indicators when biomonitoring heavy metals and other compounds present in various components of our environment. The summarized research on heavy and toxic metals (such as Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, and Zn) in honeybees, broods, honey, and pollen provides guidelines for future scientific work on this subject. Although heavy and toxic metals do not cause immediate bee deaths, they do accumulate in bee bodies and products, which can cause physiological disorders. Thus, they should not be ignored. Today, bees face different threats, of which toxic metals are just one factor. In synergy with other issues, this can have a negative cumulative effect. Hopefully, this research overview will serve as a common platform by contributing to future studies in sustainable development and healthy beekeeping practices concerning environmental contamination. The survival of the human species would be greatly threatened if bees became extremely endangered or extinct; therefore, they must be protected from contaminants like heavy and toxic metals. There can be no healthy planet, sufficient food production, or sustainable development without healthy honeybees.