**3. Heavy metal content of animal tissues and raw materials of animal origin**

Among animal products, cow's milk generally contains a number of trace elements in its composition, and its value as an environmental bioindicator is quite low. The mammary gland of cows forms a natural biological barrier that limits the passage of toxic elements from the mother's body to the food. However, milk may contain higher amounts of heavy metals, as a result of their breeding in industrial areas or secondary contamination in technological processes. The conducted studies on the content of heavy metals in animal tissues and raw materials of animal origin mostly confirm bioaccumulation of these elements in living organisms and products of animal origin.

For example, in Poland, on farms located in the direct vicinity of the Turów power plant, high concentrations of Pb, reaching even 1.865 mg kg−1,were detected in milk. Also, in milk collected directly from cows on smallholder farms in Lower Silesia, Pb content in 22.6% of samples exceeded the permissible level, Cd in 29.4%, and Hg in 50% of samples [30].

In pigs, the main source of possible contamination of pig tissues is feed, which due to the use of various additives may increase the concentration of heavy metals. The sources may be also dust and gas emissions from the industry caused by intensive air exchange in the livestock building. The average Pb content in the muscle tissue of fattening pigs is 0.05–0.58 mg kg−1 d.m. The concentration of Cd in muscles ranges from 0.02 to 0.04 mg kg−1 [30].

Sheep belong to good bioindicators of the environment because they are kept in extensive and pasture systems. Research shows that in industrial areas, cadmium accumulated in kidneys and udder. On the other hand, lead was found in the ribs, liver, and long bones. Zinc was most abundant in ribs and long bones, and Cu in kidneys, bones, and udder [30].

Conventional farms in central Greece were analyzed for the heavy metal content of copper (Cu), zinc (Zn), cadmium (Cd), lead (Pb), nickel (Ni), and chromium (Cr) in muscle tissues, livers, kidneys, feces and staple mixtures for livestock, such as cows and sheep. The study showed that the transfer of heavy metals from feed to animal products varied below acceptable risk levels. In experiment 2, feed for animals kept under different feeding systems and seasons on different farms (sheep, dairy cows, and pigs) was studied. The analyses conducted showed relatively high concentrations of Cu in pig feces (155 ± 9.13) and Zn in sheep feces (144.56 ± 5.78) [31].

Excessive accumulation of heavy metals can occur in poultry, particularly from backyard rearing. Poultry is exposed to landfills (farm waste), animal feces, and contaminated roadside ditches. It may eat contaminated soil, plants, and geohelminths. The study by Kołacz et al. [30] shows that in hens kept in the Lower Silesia region exceedances of Pb in chicken muscles occurred in 20% of samples, and chicken eggs in I4.3% of samples. In the case of Cd, exceedances concerned I4.8% and 7.9%, whereas Hg exceedances concerned 45.5% and 64.5%, respectively. Relatively high accumulation of mercury occurred in hen eggs, with averages often exceeding the nationally permissible limit of 0.02 mg Hg kg−1. Kołacz et al. [32] performed an assessment of the degree of bioaccumulation of heavy metals (Cd, Cu, Hg, Pb, and Zn) in the muscles and livers of free-range hens in the copper Belt region. The results showed that the organ accumulating the higher amounts of heavy metals was chicken liver, in which the permissible content of Pb and Cd was exceeded. In the muscles, however, the Pb content was exceeded.

Duck and goose eggs are a good indicator of environmental contamination with heavy metals in the rural environment. Studies performed in industrialized areas exposed to metal-bearing dust emissions and other sources have shown many times higher concentrations of As, Cd, and Hg, slightly less Pb and Cu compared to sites not exposed to industrial emissions [33].

Bees are a very good bioindicator of the heavy metal content of micronutrients and toxic metals in the environment. Roman [34] conducted a study in two regions industrial and agroforestry, with the aim of indicating whether worker bees and drones accumulate toxic metals. Toxic elements (Ni, Cr, Pb, Cd, and Se) were found in all samples, with higher concentrations of Ni, Cr, Cd, and Pb detected in the organisms of worker bees, while higher concentrations of Se were found in the organisms of drones. The organism of worker bees is a filter that retains from 20.45 (Pb) to 36.36%

*Influence of Heavy Metals on Quality of Raw Materials, Animal Products, and Human… DOI: http://dx.doi.org/10.5772/intechopen.102497*

(Cd) of toxic metals from the honey raw material in the process of processing it into honey. In a study conducted in Wroclaw, Poland, concerning the content of heavy metals in multiflower honey and propolis, it was found that the highest concentration of elements was in propolis: Zn—48.1, Cu—6.95, Pb—5.74, As—0.66, and Cd 0.19 mg kg−1. Statistically, a significantly lower concentration of zinc, copper, arsenic, and cadmium was in honey, except for lead, whose content was higher than permissible values in 85% of samples [35].

Horses due to their use in different environmental conditions belong to a group of animals potentially exposed to bioaccumulation of heavy metals, especially in industrialized and urbanized areas. Few studies show that horse muscles do not contain excess As, Cd, Hg, and Pb, while kidneys almost always accumulate large amounts of Cd, and liver Zn and Cd [30].

Szkoda et al. [36] also conducted studies on the content of lead, cadmium, and mercury in tissues of game animals from selected main industrial areas in Poland roe deer (*Capreolus capreolus*), red deer (*Cervus elaphus*), and wild boar (*Sus scrofa*). Studies are a useful source of information about the quality of ecosystems in which they live. The highest acceptable level of lead in meat was exceeded in 21% of analyzed samples. High cadmium concentration was determined in kidneys (above the maximum level of 1 mg kg−1 in 88% of samples). The highest concentration of toxic elements was found in roe deer and wild boar from Upper Silesia, which indicates high environmental contamination in comparison with other areas.

Heavy metal levels of wildlife biotopes from two different industrially exploited areas in Slovakia were studied in the Zemplín region of Slovakia. Various tissues (lungs, liver, kidneys, spleen, heart, and muscles) were sampled from animals, such as red deer, roe deer, mouflon, chamois, wild boar, European hare, among others. The content of elements exceeded the legal limits allowed for human consumption mercury in 29%, cadmium—28%, and lead—23%. Chromium concentration did not exceed the limit in any sample. Of the wildlife living there, wild boar was the most heavily burdened species [37].
