Chemical Composition of PM₁₀ in a Classroom near the Copper Smelter in Bor, Serbia

Abstract

An analysis was conducted on the influence of advancements in smelting technology at the copper smelter in Bor, Serbia, and seasonal changes on the level and chemical composition of PM₁₀ inside and outside a classroom at the Technical Faculty in Bor in 2015 and 2019. The results of chemical analyses revealed that the average level of arsenic in PM₁₀ within the classroom reached 11.9 ng/m³, nearly double the annual target value. In comparison, the average level of arsenic (As) in PM₁₀ in ambient air stood at 15 ng/m³. A notable enrichment factor was observed for elements present in copper concentrates (Se > Ag > Bi > Pb > Cd > As > Sb > S > Cu > Sn > Zn) in both the classroom and outdoor air, underscoring their predominantly anthropogenic origin. Higher concentrations of As in PM₁₀ were recorded during the non-heating season and the operation of the copper smelter with outdated smelting technology (2015). These findings hold significant implications for health protection for all citizens in the investigated area surrounding the Technical Faculty in Bor. The study highlights the need for additional measures to reduce As content in PM₁₀ in ambient air and prevent the infiltration of suspended particles from outdoor air into classrooms.

1. Introduction

The city of Bor is situated in the southeastern region of the Republic of Serbia, close to the borders with Bulgaria and Romania. It occupies an area of 856 km² where 41,280 inhabitants live (according to the census of 2022), a density of 48 inhabitants per km². At the beginning of the last century, in 1903, a copper ore deposit was discovered in Bor, meaning that the city of Bor has been known for the excavation and processing of copper ore and noble metals for over a century. Initially, it was underground exploitation, although later excavations were carried out at surface mines near Bor. After World War II, the mine was nationalized, and a new enterprise was named the Mining and Smelting Basin Bor—RTB Bor. During the 1960s and 1970s, RTB Bor production exceeded 150,000 tons of cathode copper annually. During that period, metallurgical and industrial capacities were being built, new mines were being opened, and apartments, railways, roads, and other infrastructure objects were being constructed. The industrial endeavors within the confines of Bor have not been without consequences for the local environment, leaving discernible negative imprints on the air, water, and soil quality in the vicinity. Moreover, a heightened awareness about the potential health repercussions stemming from the widespread pollution has taken root. The airborne particulate matter (PM) and sulfur dioxide (SO₂) contamination in Bor are predominantly sourced from the release of fugitive dust from open pits, ore waste heaps, flotation waste heaps, and point sources within the copper smelter. These cumulative emissions exacerbate environmental contamination, with a heavy emphasis on PM and SO₂ [1,2,3,4,5,6,7].

The copper ore subjected to smelting in the copper smelter in Bor originated from nearby open pits and is primarily composed of chalcopyrite-pyrite minerals, enriched with As compounds such as FeAsS, Cu₃AsS₄, CuFeS₂, Cu₅FeS₄, and Cu₂S. The subsequent oxidation, roasting, and smelting processes precipitate a surge in heavy metal oxide concentrations and the release of SO₂ gas, thereby contributing to the environmental burden [1].

The outdated technology of copper production persisted in the smelter until 2015, inevitably intensifying the pollution rates with elevated concentrations of SO₂ and PM [7]. In response, the Serbian Government provided financial support for building a modern copper smelting facility, which commenced full-scale operations in 2016. This transformation aimed to reduce SO₂, PM, and other pollutant emissions associated with copper smelting waste gases.

The novel smelting approach harnessed flash smelting technology [8], reshaping the intensity and composition of waste gas emissions from the smelting process. Despite these proactive efforts, air quality assessments between 2016 and 2020 showed an increased concentration of As within PM₁₀ [5]. The air quality in Bor was significantly exacerbated during 2019–2020, attributed to shifts in ownership of the metallurgical complex.

Notably in 2018, ZiJin, a globally prominent mining conglomerate specializing in the production of copper and precious metals [9], emerged as a strategic partner for RTB Bor, acquiring a controlling stake. This alliance gave rise to the establishment of the Serbia ZiJin Bor Copper doo company, in which ZiJin held a commanding 63% share, while the Republic of Serbia retained a 37% stake.

The aerosols embedded within metals and metalloids, discharged from copper smelting plumes, manifest as a formidable threat to human well-being [10,11,12]. Extensive research spanning decades has culminated in heightened awareness regarding the toxicity of trace elements, prompting regulatory initiatives in both the Republic of Serbia and European Union nations. The scrutiny is centered on elements such as As, cadmium (Cd), lead (Pb), and nickel (Ni), whose PM₁₀ concentration levels in ambient air are subject to regulatory oversight.

Numerous studies have underscored a connection between particulate air pollution and mortality across all age groups [13,14,15,16]. In response, items of national legislation have been enacted to safeguard human health, mandating an annual limit of 40 µg/m³ for PM₁₀, a ceiling of 500 ng/m³ for Pb in PM₁₀, and annual target values of 5 ng/m³ for Cd, 20 ng/m³ for Ni, and 6 ng/m³ for As content in PM₁₀ [17].

The copper smelter waste gases are enriched with Cu and Fe oxides and other elements, such as As, Sb, Pb, and Zn [18,19]. Typical concentrations of As range from 1–10 ng/m³ in rural areas, and up to 30 ng/m³ in uncontaminated urban areas [20]. Substances such as inorganic As, Cd and Cd compounds, and Ni compounds are classified as carcinogens to humans (group 1) [21].

Average Pb concentrations in the ambient air are usually below 0.15 µg/m³ at rural sites. In most European cities, Pb levels in ambient air typically range between 0.15 and 0.5 µg/m³. In rural areas, typical Cd levels in the airborne aerosol are 0.1–0.4 ng/m³ and up to 20 ng/m³ at industrial sites [22]. In rural areas, typical Ni levels in the airborne aerosol are 0.4–2 ng/m³ and up to 50 ng/m³ near industrial sites. Ni compounds are considered human carcinogens if exposed through inhalation [22].

Good air quality in educational buildings is necessary since children and students spend a large part of their time in classrooms [23]. The increase in indoor PM₁₀ levels can be attributed to internal and external sources. Internal sources include the generation of particles (related to combustion processes, use of sprays, and other household items) and their resuspension during intense movement and other activities in the observed enclosed space [23].

The aim of this work is an analysis of the influence of changes in smelting technology in the copper smelter in Bor, and seasonal changes on the level and chemical composition of PM₁₀ particles in a classroom at the Technical Faculty in Bor, Serbia.

Due to the lack of sampling equipment and financial resources for sample analysis, we sampled and analyzed the PM₁₀ fraction for only two years: 2015 (old smelting technology in the copper smelter) and 2019 (new smelting technology in the copper smelter). The period from 2020 until 2024 was not analyzed because of online classes during the COVID-19 pandemic, and the copper smelter reconstruction/testing period from April 2022 to April 2024.

The results of chemical analyses revealed that the average content of As in PM₁₀ within the classroom reached 11.9 ng/m³, nearly double the annual target value. In comparison, the average content of As in PM₁₀ in ambient air stood at 15 ng/m³.

This study highlights the urgent need for additional measures to reduce As content in PM₁₀ in ambient air and prevent the infiltration of suspended particles from outdoor air into the classroom.

2. Materials and Methods

2.1. Sampling Location

The Technical Faculty in Bor (TF) is part of the University of Belgrade, the oldest, biggest, and the most prestigious state university in the Republic of Serbia. The number of students who have graduated since the establishment of the faculty is 2850, 20 students have completed specialist studies, 180 have completed master’s studies, and 119 students have defended doctoral theses [24]. Positioned approximately 1 km northwest of the copper smelter (as shown in Figure 1), the TF comprises several edifices.

The PM₁₀ mass concentration measurements encompassed the measurement campaigns in 2015, when the copper smelter used old smelting technology, and in 2019, when the copper smelter operated with new smelting technology. The measurements span both the heating (15 October to 15 April) and non-heating seasons (15 April to 15 October). The selected classroom typically accommodates up to 20 students during lectures between 8 AM and 6 PM. The classroom’s 50 m³ space boasts a vinyl-covered floor, lacks an air conditioning system, and features windows totaling approximately 4 m². During the non-heating season, windows in the classroom remained mostly open or partially ajar during working hours.

2.2. Sampling Equipment and Methods for Chemical Analysis

The sampling campaigns were carried out in 2015 and 2019. The PM samples were collected by using SVEN/LECKEL LVS3/6RV reference gravimetric samplers [25] with an airflow rate of 55 m³/24 h and PM₁₀ sampling heads. A reference flow meter verified the flow rates of all sampling instruments before and after each measurement campaign. Each measurement campaign lasts a minimum of 14 days and a maximum of 30 days. A total of 82 indoor and 83 outdoor PM₁₀ samples were collected during sampling campaigns.

One PM₁₀ sampler was placed in the middle of the classroom, while another occupied the first-floor balcony of the same edifice. Quartz filters of the Whatman QM-A 47 mm variant facilitated PM₁₀ sample collection. Samples were collected daily (from 9 AM to 9 AM on the next day). The weight of the filters was measured according to the procedure prescribed by standard SRPS EN12341:2015 [26]. The EN 14902:2005 [27] methodology was adhered to in preparing filters for chemical analysis. Inductively Coupled Plasma Mass Spectrometry—ICP MS (Agilent model 7700)—was applied for chemical element level determination in PM₁₀ samples. In order to control the quality and verify the dissolution and analysis procedure, the standard reference material, NIST 1648a [28], was analyzed in the same way as the PM samples, with recovery rates spanning 75 to 125% for all presented elements.

3. Results and Discussion

3.1. PM₁₀ Levels, Chemical Composition, and I/O Ratios

Table 1. Daily average PM₁₀ levels, the chemical composition of PM₁₀, and I/O ratios for the entire observation period inside and outside the selected classroom at the TF (NS—number of samples analyzed, stdev—standard deviation).

NS	Average	Stdev	Min	Max	NS	Average	Stdev	Min	Max	I/O
82	µg/m3				82	µg/m3
PM10 in	32.6	9.9	10.0	49.2	PM10 out	37.0	16.4	10.2	84.1	0.9
	ng/m3					ng/m3
As	11.9	11.7	0.6	55.0	As	15.0	17.8	0.4	70.9	0.8
Cd	1.5	1.8	0.0	9.5	Cd	1.6	2.5	0.1	11.8	0.9
Mn	12.8	13.5	0.3	56.2	Mn	11.3	10.1	0.8	46.8	1.1
Pb	138.1	216.4	4.1	937.2	Pb	97.8	163.3	3.5	1062.8	1.4
Cu	115.5	101.5	7.2	459.7	Cu	196.3	171.2	8.5	653.7	0.6
Zn	69.4	52.6	1.0	270.6	Zn	70.0	75.5	1.2	531.6	1.0
Ni	8.1	5.1	0.1	24.2	Ni	7.3	5.1	0.5	24.3	1.1
Cr	11.8	11.8	0.2	72.5	Cr	10.1	12.6	0.2	95.5	1.2
Ti	24.9	18.3	4.2	91.0	Ti	20.2	14.7	3.7	68.6	1.2
Sr	7.1	6.3	0.8	33.8	Sr	7.6	7.6	0.3	33.8	0.9
S	1524.4	780.2	375.9	2933.5	S	1588.4	948.9	148.6	3645.5	1.0
Fe	566.3	442.9	18.9	1516.5	Fe	791.7	790.0	22.7	2951.8	0.7
Ca	1358.4	1015.3	122.0	4396.9	Ca	1312.3	1202.4	96.7	6292.8	1.0
Mg	287.4	200.3	47.4	976.3	Mg	415.2	299.4	5.4	1439.0	0.7
Na	518.9	733.5	45.5	4126.8	Na	430.1	611.1	10.8	2815.7	1.2
K	600.4	413.0	23.6	1828.3	K	533.0	424.6	21.7	1504.4	1.1
P	247.0	125.4	1.2	547.3	P	274.8	92.9	148.0	513.5	0.9
Co	0.8	0.9	0.1	3.3	Co	0.6	0.7	0.1	3.5	1.4
Zr	3.0	2.8	0.4	11.1	Zr	2.8	2.2	0.7	10.2	1.1
V	1.7	2.1	0.1	10.1	V	2.0	2.1	0.1	12.0	0.9
Se	6.7	8.8	0.2	53.4	Se	10.4	12.2	0.4	48.3	0.6
Rb	1.1	0.7	0.2	3.7	Rb	1.0	0.6	0.2	2.8	1.2
Ag	3.1	2.9	0.2	12.3	Ag	3.8	3.3	0.2	12.8	0.8
Sn	3.1	2.4	0.2	8.6	Sn	2.5	2.4	0.2	11.6	1.3
Sb	1.2	1.0	0.2	5.7	Sb	1.6	4.1	0.1	30.3	0.8
Bi	1.7	2.9	0.2	13.6	Bi	1.9	2.7	0.2	13.0	0.9
Ba	27.0	14.5	1.2	59.1	Ba	25.2	15.4	0.1	65.9	1.1

shows the average concentrations of PM₁₀, the chemical composition, and indoor/outdoor (I/O) ratios for the entire observation period. Data outlined in Table 1 highlight that average PM₁₀ concentrations within the classroom and ambient air remained compliant with prescribed annual limits (40 µg/m³). Good matching is observed with prior periods’ PM₁₀ levels in ambient air [1].

Serbian Environmental Protection Agency (SEPA) annual reports presented similar values for Bor’s PM₁₀ levels in ambient air [29]. For example, in 2019, the TP site (less than 1 km away from the TF site) averaged annual PM₁₀ levels of 36 µg/m³.

Chemical element levels within PM₁₀ are closely aligned with those near Spain’s Huelva copper smelter [30,31,32] (I/O ratios of PM₁₀ element concentrations in Table 1 exhibit minor deviations from 1.0, indicating negligible indoor sources and attributing their presence primarily to external infiltration).

Within the scope of elements with regulated content thresholds in PM₁₀, As concentrations exceeded the annual target (6 ng/m³) both indoors (11.9 ng/m³) and outdoors (15.0 ng/m³), as depicted in Figure 2. This trend persists, reflecting heightened levels indoors and outdoors [5].

As expected, As, Cd, Cu, Zn, S, Sb, and Pb levels in PM₁₀ were elevated because of their presence in the copper concentrates processed in the copper smelter in Bor. High concentrations of Se, Ag, and Bi in PM₁₀ were also determined in both indoor and outdoor environments.

3.2. Enrichment Factor

To determine the anthropogenic and crustal origin of elements in aerosols, the enrichment factor is widely used [4]. As a common reference element for crustal particles in enrichment factor (EF) calculations, we used Mn because its anthropogenic sources in the studied area are negligible. The EF is defined as:

$$\mathrm{EF}={\displaystyle \frac{{\left(\frac{\mathrm{X}}{\mathrm{Mn}}\right)}_{\mathrm{air}}}{{\left(\frac{\mathrm{X}}{\mathrm{Mn}}\right)}_{\mathrm{crust}}}}$$

(1)

where (X/Mn)_air and (X/Mn)_crust refer to the ratio of the concentration of element X to that of Mn in the air and in the average crustal material, respectively.

Based on the EF values, the elements can be grouped as follows: very enriched (EF > 100), medium-enriched (10 < EF < 100), and low-enriched (EF < 10) [33]. The EF calculated from the filter samples collected around four of the biggest smelters in Chile showed strong enrichment of Pb, As, Cu, and Ni [34]. The EF calculated for selected elements in Bor is shown in

Table 2. Average values of EF and EF I/O ratios in non-heating (NHS) and heating seasons (HS) at the TF site during the entire observation period.

Chemical Element	NHS EF In	NHS EF Out	NHS EF I/O	Chemical Element	HS EF In	HS EF Out	HS EF I/O
Se	36,380	18,663	1.9	Se	18,038	21,712	0.8
Ag	33,424	13,042	2.6	Ag	4931	10,344	0.5
Bi	12,085	6971	1.7	Bi	3467	13,472	0.3
Pb	5128	1649	3.1	Pb	1500	1414	1.1
Cd	2727	2870	1.0	Cd	1122	1112	1.0
As	1453	1648	0.9	As	728	953	0.8
Sb	1338	1001	1.3	Sb	965	1259	0.8
S	786	389	2.0	S	319	347	0.9
Cu	321	327	1.0	Cu	441	595	0.7
Sn	436	180	2.4	Sn	233	275	0.8
Zn	425	196	2.2	Zn	136	166	0.8
P	25	30	0.8	P	41	48	0.8
Ni	28	7	4.0	Ni	13	22	0.6
Ba	24	6	3.9	Ba	11	20	0.6
Cr	13	8	1.7	Cr	15	13	1.1
Ca	6	3	1.8	Ca	2	4	0.6
Zr	5	5	0.9	Zr	3	3	0.9
K	3	2	1.5	K	6	6	0.9
Co	4	3	1.3	Co	4	3	1.2
Sr	3	2	1.5	Sr	3	4	0.9
Na	3	2	1.4	Na	3	3	1.0
Rb	2	3	0.9	Rb	3	3	1.0
Mg	2	2	1.0	Mg	1	2	0.6
V	1	2	0.7	V	1	1	0.7
Mn	1	1	1.0	Mn	1	1	1.0
Fe	1	1	0.8	Fe	1	1	0.9

.

The data in Table 2 clearly show that most of the elements that are part of the copper concentrate processed in the copper smelter (Se > Ag > Bi > Pb > Cd > As > Sb > S > Cu > Sn > Zn) show very high EF values, which indicates the predominantly anthropogenic origin of those elements.

In contrast, the elements originating from the earth’s crust (Ca, K, Na, Mg, Mn, and Fe) have EF values less than 10. This indicates the background level for those elements around the TF site.

In addition, based on the data from Table 2, seasonal changes in EF can be observed, and this can be best demonstrated when comparing the I/O ratios of EF in the non-heating and heating seasons. It is noticeable that for most of the elements, the EFs and the EF I/O ratios are higher in the non-heating season compared to the values obtained in the heating season.

The changes in EF are influenced by two different phenomena. First, the changes are influenced by meteorological conditions, such as changes in wind speed and direction in the non-heating season and heating season. Namely, in the heating season, the winds that blow mostly from the north/northwest direction contribute to reducing PM pollution at the TF site. In contrast, in the non-heating period, the frequency and speed of winds from these directions are lower [1].

Additionally, the natural ventilation of the classroom is better during the non-heating season than during the heating season. During teaching hours in the non-heating season, the windows are mostly open or half-open, resulting in a higher infiltration of external PM₁₀ pollution into the classroom. In contrast, during the heating season, the windows predominantly remain closed, limiting the infiltration of external PM₁₀ pollution into the classroom.

3.3. The Impact of Changes in the Copper Smelting Technology on the Level of PM₁₀ and Content of Chemical Elements in PM₁₀ at the TF Site

During the period of using old smelting technology in the copper smelter in Bor (2015), the average PM₁₀ levels in the classroom and ambient air at the TF site were below the prescribed annual limit, as indicated in

Table 3. Average PM₁₀ concentrations, chemical composition, and I/O ratios determined at the TF site during the period of application of old smelting technology in the copper smelter in Bor (2015), (NS—number of samples analyzed, stdev—standard deviation).

NS	Average	Stdev	Min	Max	NS	Average	Stdev	Min	Max	I/O
45	µg/m3				45	µg/m3
PM10 in	36.3	8.4	20.8	49.2	PM10 out	39.5	17.4	10.2	84.1	0.9
	ng/m3					ng/m3
As	15.5	13.3	0.7	55.0	As	15.7	17.7	0.4	70.9	1.0
Cd	1.9	2.0	0.2	9.5	Cd	1.7	2.6	0.1	10.3	1.1
Mn	5.7	4.9	0.3	18.8	Mn	7.8	6.2	0.8	25.2	0.7
Pb	102.3	111.5	4.3	456.3	Pb	112.1	182.9	3.5	1062.8	0.9
Cu	116.0	98.5	7.2	459.7	Cu	203.0	130.0	11.0	425.9	0.5
Zn	77.9	59.7	5.2	270.6	Zn	77.5	89.3	1.2	531.6	1.0
Ni	6.4	4.2	0.9	16.8	Ni	7.4	4.8	0.5	23.3	0.9
Cr	9.0	14.3	0.2	72.5	Cr	13.5	16.5	0.3	95.5	0.7
Ti	25.5	24.4	4.2	91.0	Ti	21.8	17.1	4.8	68.6	1.2
Sr	6.7	7.6	0.8	33.8	Sr	8.7	9.5	0.3	33.8	0.8
S	1560.2	783.4	420.5	2745.4	S	1720.0	890.3	148.6	3279.6	0.9
Fe	463.4	400.5	22.7	1296.3	Fe	716.6	766.3	22.7	2618.5	0.6
Ca	1618.2	1171.7	122.0	4396.9	Ca	1394.9	1512.3	96.7	6292.8	1.2
Mg	280.2	182.8	51.5	660.3	Mg	513.9	351.0	78.7	1439.0	0.5
Na	752.5	1015.0	45.9	4126.8	Na	810.7	786.7	86.8	2815.7	0.9
K	572.6	437.5	23.6	1828.3	K	621.5	509.5	21.7	1504.4	0.9
P	227.6	71.7	146.4	397.8	P	247.1	73.6	148.0	440.9	0.9
Co	0.8	1.0	0.1	3.3	Co	0.5	0.4	0.1	2.0	1.6
Zr	3.0	3.4	0.4	11.1	Zr	2.7	2.2	0.7	9.5	1.1
V	2.1	3.0	0.1	10.1	V	1.9	2.5	0.1	12.0	1.1
Se	8.7	12.2	0.5	53.4	Se	12.9	15.3	0.4	48.3	0.7
Rb	0.9	0.7	0.2	2.9	Rb	0.9	0.6	0.2	2.8	1.0
Ag	4.8	3.4	0.7	12.3	Ag	4.7	3.3	0.7	12.8	1.0
Sn	3.2	2.2	0.2	6.8	Sn	2.2	2.7	0.2	11.6	1.5
Sb	0.9	0.6	0.3	2.4	Sb	1.9	5.2	0.2	30.3	0.5
Bi	1.8	3.6	0.2	13.6	Bi	1.2	1.3	0.2	4.3	1.4
Ba	33.8	14.8	1.3	59.1	Ba	33.9	13.0	2.1	65.9	1.0

.

The I/O ratios of concentrations of chemical elements in PM₁₀, displayed in Table 3, demonstrate variations from element to element. However, except for Co, Sn, and Bi, there are no significant deviations in I/O ratios from 1.0.

This implies that there are no notable indoor sources, except for particle resuspension, and the presence of chemical elements in the classroom primarily originates from infiltration from the external environment.

Figure 2 illustrates that out of the elements Pb, Cd, Ni, and As, which have prescribed annual limits or target values for their concentrations in PM₁₀, only As concentrations exceeded the annual target value both indoors (15.5 ng/m³) and in outdoor air (15.7 ng/m³).

The elevated concentrations of elements such as As, Cd, Cu, Zn, S, Sb, and Pb in PM₁₀ result from their presence in the copper concentrates processed in the copper smelter in Bor. Furthermore, the inadequate treatment of waste gases generated in the precious metals extraction plant (Zlatara), a part of the copper smelter facilities, contributes to the heightened concentrations of Se and Ag in PM₁₀ [35].

During the period of applying new smelting technology (2019) in the copper smelter in Bor, the average PM₁₀ levels in the classroom and ambient air at the TF site were below the prescribed annual limit, as presented in

Table 4. Average PM₁₀ concentrations, chemical composition, and I/O ratios determined at the TF site during the period of application of new smelting technology (2019), (NS—number of samples analyzed, stdev—standard deviation).

NS	Average	Stdev	Min	Max	NS	Average	Stdev	Min	Max	I/O
37	µg/m3				37	µg/m3
PM10 in	29.6	10.1	10.0	49.0	PM10 out	37.0	16.4	10.2	84.1	0.8
	ng/m3					ng/m3
As	9.3	9.8	0.6	36.6	As	14.8	17.7	0.4	70.9	0.6
Cd	1.2	1.7	0.1	6.3	Cd	1.6	2.5	0.1	11.8	0.7
Mn	17.7	15.3	2.1	56.2	Mn	11.3	10.1	0.8	46.8	1.6
Pb	166.8	270.9	4.1	937.2	Pb	97.8	163.3	3.5	1062.8	1.7
Cu	115.2	104.9	11.2	381.7	Cu	189.6	205.8	8.5	653.7	0.7
Zn	62.9	46.2	1.0	204.2	Zn	70.0	75.5	1.2	531.6	0.9
Ni	9.2	5.4	0.1	24.2	Ni	7.3	5.1	0.5	24.3	1.3
Cr	13.6	9.7	0.2	30.1	Cr	10.1	12.6	0.2	95.5	1.3
Ti	24.5	14.2	7.8	58.9	Ti	20.2	14.7	3.7	68.6	1.2
Sr	7.4	5.2	1.3	22.9	Sr	7.6	7.6	0.3	33.8	1.0
S	1499.3	790.3	375.9	2933.5	S	1588.4	948.9	148.6	3645.5	0.9
Fe	635.6	460.9	18.9	1516.5	Fe	791.7	790.0	22.7	2951.8	0.8
Ca	952.5	513.9	351.5	1961.1	Ca	1312.3	1202.4	96.7	6292.8	0.7
Mg	293.5	217.2	47.4	976.3	Mg	415.2	299.4	5.4	1439.0	0.7
Na	327.0	268.4	45.5	895.6	Na	430.1	611.1	10.8	2815.7	0.8
K	618.2	403.6	69.7	1719.2	K	533.0	424.6	21.7	1504.4	1.2
P	256.7	145.1	1.2	547.3	P	274.8	92.9	148.0	513.5	0.9
Co	0.9	0.9	0.1	3.3	Co	0.6	0.7	0.1	3.5	1.5
Zr	2.9	2.2	0.4	9.3	Zr	2.8	2.2	0.7	10.2	1.0
V	1.5	1.5	0.1	6.1	V	2.0	2.1	0.1	12.0	0.8
Se	5.8	6.8	0.2	26.1	Se	8.4	8.3	0.5	30.9	0.3
Rb	1.2	0.8	0.3	3.7	Rb	1.0	0.6	0.2	2.8	1.3
Ag	1.7	1.1	0.2	4.2	Ag	3.8	3.3	0.2	12.8	0.4
Sn	3.1	2.6	0.5	8.6	Sn	2.5	2.4	0.2	11.6	1.3
Sb	1.3	1.1	0.2	5.7	Sb	1.6	4.1	0.1	30.3	0.8
Bi	1.7	2.4	0.2	10.9	Bi	1.9	2.7	0.2	13.0	0.9
Ba	24.4	13.8	1.2	58.4	Ba	25.2	15.4	0.1	65.9	1.0

.

The I/O ratios of concentrations of chemical elements in PM₁₀, displayed in Table 4, demonstrate variations from element to element. However, except for Co, Mn, and Pb, there are no significant deviations in I/O ratios from 1.0. This implies that there are no notable indoor sources, except particle resuspension, and the presence of chemical elements in the classroom primarily originates from infiltration from the external environment.

Figure 2 illustrates that out of the elements Pb, Cd, Ni, and As, which have prescribed annual limits or target values for their concentrations in PM₁₀, only the average concentration of As exceeded the annual target value both indoors (9.3 ng/m³) and in outdoor air (14.8 ng/m³).

The concentrations of elements such as As, Cd, Cu, Zn, S, Sb, and Pb in PM₁₀ were elevated due to their presence in the copper concentrates processed in the copper smelter in Bor.

As previously explained, during the operation of the old copper smelter, high levels of Se and Ag in PM₁₀ mainly occurred due to inadequate treatment of waste gases generated in the precious metals extraction plant. The reconstruction of this plant was not included in the old copper smelter reconstruction, resulting in the emissions of waste gases remaining the same as in the previous period.

Based on the data presented in Table 3 and Table 4, it can be concluded that the concentrations of most chemical elements found in the copper concentrates (such as As, Cd, Zn, Cu, S, Se, Ag, and Ba) decreased during the operation of the copper smelter with new smelting technology, compared to their concentrations in PM₁₀ at the TF when the copper smelter operated with old smelting technology.

Additionally, it can be asserted that during the operation of the copper smelter with new smelting technology, there was a significant reduction in SO₂ emissions compared to the period before 2016 [5,6].

3.4. Seasonal Variations of PM₁₀ Levels and Content of Chemical Elements in PM₁₀ at the TF Site

Table 5. Average PM₁₀ concentrations, chemical composition, and I/O ratios determined at the TF site in non-heating seasons during the entire observation period (NS—number of samples analyzed, stdev—standard deviation).

NS	Average	Stdev	Min	Max	NS	Average	Stdev	Min	Max	I/O
40	µg/m3				40	µg/m3
PM10 in	36.8	12.7	14.8	49.2	PM10 out	38.5	20.9	8.3	66.0	1.0
	ng/m3					ng/m3
As	18.3	25.9	0.7	96.2	As	19.4	18.1	0.7	66.5	0.9
Cd	1.2	2.0	0.0	9.5	Cd	3.0	6.1	0.1	37.1	0.4
Mn	8.1	8.9	0.3	38.0	Mn	9.5	7.5	0.8	27.3	0.9
Pb	299.9	506.7	4.3	2513.6	Pb	138.8	225.1	3.5	1062.8	2.2
Cu	302.5	308.3	20.0	1204.6	Cu	246.7	202.3	11.0	1052.4	1.2
Zn	87.3	54.8	5.2	270.6	Zn	86.5	70.0	1.2	296.8	1.0
Ni	12.9	10.0	0.9	39.9	Ni	10.3	10.8	0.5	62.7	1.3
Cr	15.2	14.1	0.2	72.5	Cr	13.6	15.3	0.3	95.5	1.1
Ti	23.5	18.6	2.2	73.5	Ti	20.0	13.9	2.0	66.7	1.2
Sr	5.8	6.4	0.7	26.3	Sr	5.9	5.8	0.3	23.2	1.0
S	2104.8	1187.1	420.5	3745.4	S	1973.3	1313.0	148.6	4933.8	1.0
Fe	988.8	1807.9	22.7	8794.2	Fe	681.5	741.5	22.7	2951.8	1.5
Ca	1706.8	612.8	1089.8	2694.5	Ca	2267.2	1379.6	96.7	6292.8	0.8
Mg	302.9	191.8	125.6	660.3	Mg	634.5	417.8	78.7	1693.9	0.5
Na	141.8	74.1	48.2	243.9	Na	636.9	909.2	86.8	2815.7	0.2
K	626.3	348.4	232.1	1039.7	K	391.5	350.9	21.7	1147.7	1.6
P	72.0	121.9	0.0	397.8	P	146.5	122.3	0.0	440.9	0.5
Co	0.3	0.2	0.1	0.7	Co	0.4	0.4	0.1	2.0	0.8
Zr	1.3	1.3	0.2	7.4	Zr	2.4	2.2	0.2	9.5	0.6
V	0.8	0.5	0.0	1.9	V	2.1	2.4	0.1	12.0	0.4
Se	14.5	26.7	0.3	97.0	Se	10.2	19.8	0.3	83.1	1.4
Rb	1.5	1.8	0.2	7.0	Rb	1.9	5.0	0.2	28.0	0.8
Ag	5.5	3.0	1.3	14.0	Ag	5.4	3.4	0.7	14.8	1.0
Sn	2.0	2.3	0.2	8.2	Sn	2.1	2.8	0.2	12.0	1.0
Sb	0.8	0.5	0.2	2.4	Sb	2.5	6.6	0.2	30.3	0.3
Bi	1.4	1.2	0.2	3.3	Bi	1.3	1.4	0.2	4.3	1.1
Ba	32.6	9.6	1.3	59.1	Ba	25.8	15.7	2.1	65.9	1.3

and

Table 6. Average PM₁₀ concentrations, chemical composition, and I/O ratios determined at the TF site in heating seasons in the entire observation period (NS—number of samples analyzed, stdev—standard deviation).

NS	Average	Stdev	Min	Max	NS	Average	Stdev	Min	Max	I/O
42	µg/m3				42	µg/m3
PM10 in	32.9	9.9	10.0	49.2	PM10 out	37.7	17.3	12.7	84.1	0.9
	ng/m3					ng/m3
As	6.9	8.1	0.6	36.6	As	11.2	16.1	0.7	70.9	0.6
Cd	0.9	1.1	0.2	4.8	Cd	1.0	1.6	0.1	9.4	1.0
Mn	16.1	16.1	1.3	56.2	Mn	11.8	11.9	1.5	46.8	1.4
Pb	49.8	60.2	4.1	238.3	Pb	65.7	84.0	4.9	362.1	0.8
Cu	128.1	117.9	11.2	459.7	Cu	216.0	182.7	8.5	640.8	0.6
Zn	49.2	26.0	1.0	109.3	Zn	64.9	76.6	6.3	531.6	0.8
Ni	7.1	3.6	0.1	15.3	Ni	9.1	5.3	0.6	24.3	0.8
Cr	12.3	6.9	0.2	28.9	Cr	10.3	6.1	0.2	20.0	1.2
Ti	21.2	12.9	5.2	58.9	Ti	22.7	15.6	6.2	68.6	0.9
Sr	8.2	7.0	0.9	33.8	Sr	7.3	7.9	0.7	33.8	1.1
S	1420.6	743.7	375.9	2933.5	S	1238.6	797.6	365.0	3419.2	1.1
Fe	578.7	449.9	18.9	1516.5	Fe	702.0	749.1	22.7	2641.0	0.8
Ca	598.9	418.1	122.0	1794.1	Ca	1045.1	797.2	165.6	3220.9	0.6
Mg	307.5	231.9	47.4	976.3	Mg	315.0	320.5	5.4	1439.0	1.0
Na	436.9	345.4	45.5	1164.6	Na	377.5	357.8	46.5	1126.3	1.2
K	658.5	460.2	23.6	1828.3	K	553.4	375.8	84.4	1302.5	1.2
P	258.1	144.8	1.2	547.3	P	292.4	93.2	161.2	513.5	0.9
Co	1.0	0.9	0.1	3.3	Co	0.7	0.8	0.1	3.5	1.5
Zr	3.5	2.2	0.7	9.3	Zr	3.4	2.2	1.1	10.2	1.0
V	1.6	1.5	0.1	6.1	V	1.9	1.7	0.2	6.7	0.9
Se	7.0	10.0	0.2	53.4	Se	12.0	13.4	0.5	48.3	0.6
Rb	1.3	0.8	0.4	3.7	Rb	1.1	0.7	0.2	2.8	1.3
Ag	2.2	2.5	0.2	12.3	Ag	3.7	3.6	0.2	12.8	0.6
Sn	3.6	2.5	0.5	8.6	Sn	2.7	2.4	0.5	8.1	1.3
Sb	1.5	1.1	0.3	5.7	Sb	1.3	1.7	0.2	8.6	1.2
Bi	2.0	3.3	0.2	13.6	Bi	2.3	3.0	0.2	13.0	0.9
Ba	23.3	14.4	1.2	58.4	Ba	26.5	14.6	0.1	47.1	0.9

present the average concentrations of PM₁₀, their chemical composition, and I/O ratios during both non-heating and heating periods over the entire measurement period.

Table 5 shows the average ratio of concentrations of chemical elements during the non-heating period in the classroom and outdoor air (I/O), revealing that most of the chemical elements in PM₁₀ indoors predominantly originate from outdoor air. Elevated levels of certain elements (Pb, Ni, Fe, K, Se, and Ba) in the classroom, compared to outdoor air, result from the resuspension of particles caused by student activities.

Among the chemical elements with prescribed limit and target values for annual PM₁₀ content, only As exceeds the target value, by an average of 3.1 times indoors and 3.2 times outdoors, as illustrated in Figure 2.

Similarly, Table 6 displays the I/O ratios during the heating period, indicating that the majority of chemical elements in PM₁₀ indoors predominantly originate from outdoor air.

Elevated levels of specific elements (Mn, Co, Rb, and Sn) in the classroom, compared to outdoor air, are attributed to particle resuspension caused by student activities. Among the chemical elements with prescribed limit and target values for annual PM₁₀ content, only As surpasses the target value, by an average of 1.2 times indoors and 1.9 times outdoors, as shown in Figure 2.

Comparing the concentrations of elements found in the copper concentrate (As, Cd, Pb, Cu, Zn, S, Cr, Se, and Ag) in PM₁₀ within the classroom during the non-heating season to those in the heating season, higher levels were identified, as indicated by the data in Table 5 and Table 6. A parallel observation can be drawn by examining the average concentrations of these elements in PM₁₀ in outdoor air; during the observed period, their presence in PM₁₀ in outdoor air was greater in the non-heating season than in the heating season.

As discussed earlier (in Section 3.2 concerning EF), two significant factors influence the seasonal variations in chemical element concentrations in PM₁₀ at the TF site. The alterations in PM₁₀ levels stem from shifts in meteorological conditions. Specifically, during the heating season, there is a higher frequency of winds from the west and northwest compared to the non-heating season [1]. This fosters aeration and diminishes the likelihood of particle resuspension from the soil during the heating season. Additionally, the natural ventilation pattern in the classroom undergoes a seasonal shift, transitioning from open windows during classes in the non-heating season to closed or nearly closed windows during the heating season.

3.5. Correlations between the Chemical Elements Determined in PM₁₀ inside and outside the Classroom at the TF Site

The investigation into the relationships between average concentrations of chemical elements determined in PM₁₀ at the TF involved correlation analysis over the entire observation period. This analysis was conducted separately for the non-heating and heating periods, and the resulting Pearson’s correlation coefficients (r) are presented in

Table 7. Pearson’s correlation coefficients between levels of chemical elements determined in PM₁₀ at the TF site in non-heating seasons (NHS) over the entire observation period.

NHS Outdoor

As

Cd

Mn

Pb

Cu

Zn

Ni

Cr

Ti

Sr

S

Fe

As

1

Cd

0.709 **

1

Mn

1

Pb

0.491 **

0.825 **

1

Cu

0.454 *

0.614 **

0.666 **

1

Zn

0.413 *

0.592 **

0.609 **

0.741 **

0.647 **

1

Ni

0.401 *

0.496 *

1

Cr

0.430 *

0.465 *

1

Ti

0.670 **

0.555 *

0.570 *

0.821 **

1

Sr

0.844 **

0.536 **

0.539 *

1

S

0.547 *

0.729 **

0.723 **

0.671 **

1

Fe

0.589 **

0.750 **

0.760 **

0.655 **

0.777 **

0.761 **

0.553 **

0.851 **

1

NHS indoor

As

Cd

Mn

Pb

Cu

Zn

Ni

Cr

Ti

Sr

S

Fe

As

1

Cd

0.453 **

1

Mn

0.522 **

1

Pb

1

Cu

0.447 **

0.408 *

1

Zn

0.414 *

0.589 **

0.437 **

1

Ni

0.601 **

1

Cr

0.408 *

0.738 **

1

Ti

0.677 **

1

Sr

0.479 **

0.721 **

0.541 **

1

S

0.563 **

0.586 **

0.712 **

1

Fe

0.648 **

0.872 **

0.492 *

0.838 **

1

** Correlation is significant at the 0.01 level (2-tailed). * Correlation is significant at the 0.05 level (2-tailed).

for the non-heating period and

Table 8. Pearson’s correlation coefficients between levels of chemical elements determined in PM₁₀ at the TF site in heating seasons (HS) over the entire observation period.

HS Outdoor

As

Cd

Mn

Pb

Cu

Zn

Ni

Cr

Ti

Sr

S

Fe

As

1

Cd

0.879 **

1

Mn

1

Pb

0.695 **

0.644 **

1

Cu

0.407 **

1

Zn

0.702 **

0.876 **

0.595 **

1

Ni

1

Cr

1

Ti

0.480 **

0.495 **

0.515 **

1

Sr

0.631 **

0.647 **

0.430 **

0.601 **

0.652 **

1

S

0.812 **

1

Fe

0.415 **

0.783 **

0.547 **

0.462 **

1

HS indoor

As

Cd

Mn

Pb

Cu

Zn

Ni

Cr

Ti

Sr

S

Fe

As

1

Cd

1

Mn

1

Pb

0.537 **

1

Cu

1

Zn

0.490 **

1

Ni

1

Cr

0.422 **

1

Ti

1

Sr

1

S

1

Fe

0.700 **

0.602 **

0.566 **

1

** Correlation is significant at the 0.01 level (2-tailed).

for the heating period.

Analyzing the data in Table 7 reveals a general observation that the correlations between observed chemical elements in PM₁₀ during the non-heating period in the ambient air at TF were generally more robust compared to those between the same elements in the classroom, with a few exceptions. Furthermore, the correlations between concentrations of chemical elements present in the copper concentrate (As, Cd, Pb, Cu, Zn, S, and Fe) in the outdoor air at the TF were mostly strong (0.8 > r > 0.6) to moderate (0.6 > r > 0.4) and positive. In contrast, in the classroom, these correlations were mostly moderate and positive.

The notable disparity in correlation coefficient values between indoor and outdoor air can be attributed to the varying degrees of infiltration of certain elements from the external environment and the different sedimentation rates of these elements [36,37].

Analyzing the data presented in Table 8 leads to a general conclusion that the correlations between observed elements in PM₁₀ during heating periods in the ambient air at the TF were, in most cases, more robust compared to the correlations within the classroom, with a few exceptions. Additionally, the correlations between concentrations of elements present in the copper concentrate (As, Cd, Pb, Cu, Zn, S, and Fe) in the outdoor air at the TF were predominantly strong to moderate and positive, whereas in the classroom, these correlations were mostly moderate and positive.

The primary explanation for this pattern is the reduced ventilation of the classroom during the heating season, which significantly diminishes the infiltration of external particulate pollution into the interior space. Several findings regarding the correlations of chemical elements in PM₁₀ in outdoor air (shown in Table 7 and Table 8) align with the results obtained for the TP site (located less than 1 km from the TF site) in previously published papers [3,4].

4. Conclusions

This study is motivated by the imperative to spotlight air pollution’s significance through suspended particles in educational institutions across the Republic of Serbia and the latent health ramifications. Our work delves into the impact of smelting technology alterations and seasonal changes in Bor’s copper smelter on PM₁₀ levels and composition within a chosen classroom at the TF.

Average PM₁₀ levels indoors and outdoors at the TF remained beneath the national annual PM₁₀ limit (40 µg/m³). However, As levels exceeded the national annual target both indoors and outdoors, nearing double the target inside and 2.5 times outside. Chemical elements originating from the copper concentrate exhibited substantial enrichment factors, affirming their anthropogenic origins.

Heightened As concentrations were noted in the non-heating period, particularly when the old smelting technology was used in the copper smelter (2015). Over the entirety of the observed period, average concentrations of Pb, Cd, and Ni in PM₁₀ in the classroom and ambient air were below the prescribed limit and target values.

Correlation analysis unveiled robust positive correlations between chemical elements in PM₁₀ in outdoor air during heating and non-heating periods, surpassing those observed indoors. The presented results accentuate the need to reduce As content in PM₁₀ by constant waste gas emission control in the copper smelter and additional measures for particle infiltration prevention.

The presented results should contribute to safeguarding the health of students and citizens residing near the Technical Faculty in Bor.