# Particulate Matter Contamination of Bee Pollen in an Industrial Area of the Po Valley (Italy)
## 4 Discussion  
Airborne PM of natural and anthropogenic origin was observed on pollen grains collected by worker bees from the monitoring station. Naturally derived minerals included calcite, clay minerals, quartz, and feldspars, which are abundant in the neighboring soils [16]. Natural erosion and agricultural activities, such as sowing and harvesting that may have occurred in the adjacent crop fields, may have contributed to the environmental spread of soil minerals. Regarding calcite, this mineral is also widespread in incinerator bottom ash and represents one of the most common products of ash weathering [25]. Even if our data suggest that handling ash may represent a critical step for dust leakage, further work is needed to assess specific contributions of the waste-treatment process. The incinerator plant may also be the source of the spherical dusts of SiO\({}_{2}\) contaminating bee pollen [16]. The spherical morphology is, indeed, a well-known marker of high-temperature combustion, such as that occurring at the incinerator plant [25; 26]. Furthermore, the unexpected presence of gold particles on bee pollen could also be attributed to the incineration plant, as bottom ash may contain economically significant levels of this mineral [27; 28]. According to the literature, the presence of precious metals  
Figure 6: Fine and ultrafine gold PM found on the pollen surfaces. (**a**) Secondary electron micrograph of corn pollen contaminated by gold particles. (**b**) Corresponding backscattered electron (BSE) micrograph of corn pollen, highlighting bright spots that correspond to gold particles. (**c**) BSE micrograph showing PM of gold scattered on several pollen grains. (**d**) Higher magnification of gold PM on pollen grains showing the presence of fine and ultrafine PM. is mainly derived from waste of electrical and electronic equipment and many efforts are currently being made to recover precious metals in bottom ash [27; 28]. Airborne PM of quartz contaminating bee pollen may also have originated from the incineration plant, as SiO\({}_{2}\) fragments are often abundant in bottom ash [25; 29]. Other mineral phases frequently found in bee pollen included Fe oxides/hydroxides, metallic Fe, and Fe alloys (e.g., Fe, Cr, \(\pm\)Mn, \(\pm\)Sn). In natural soils, Fe oxides/hydroxides are common in mafic and ultramafic rocks, in lateritic horizons, and following alteration of sulfide ores, but they are rare in the alluvial deposits that characterize the area of investigation [16]. Thus, the natural origin of such dusts should be excluded. In addition, an anthropogenic origin should be considered for metallic Fe and Fe alloys. These compounds are present in several manufactured goods, especially in the automotive industry [30; 31]. According to previous studies, Fe compounds and their alloys may therefore originate from the wearing of mechanical parts or the braking systems of vehicles [5; 16; 22; 31]. Antimony oxides contaminating pollen grains may also be derived from the braking system. This compound is known to originate from the oxidation of antimony sulfide used to improve friction stability at elevated temperatures [32]. One possible origin of metallic Zn is vehicular traffic, as pure Zn is widely used in the automotive industry to protect steel from corrosion (International Zinc Association, www.zinc.org, 19 January 2021). Another mineral phase frequently found in pollen grains was barite (BaSO\({}_{4}\)). Barite occurs naturally in hydrothermal deposits; therefore, this mineral should not be present in the area of investigation. However, barite (BaSO\({}_{4}\)) is widely used in vehicle tires and brakes as it improves the thermal and wear resistance and reduces noise [33; 34; 35].