Assessment of the Dust in Underground Coal Mine

Abstract

This paper considers extreme dusty conditions at workplaces in underground coal mine. These extreme conditions stem from various physical factors that affect employees’ performance. The extreme effect of the dust can significantly contribute to permanent health damage or even the death of employees. In this study, we present and discuss the results of measurements of airborne dust and respiratory dust taken during wall cutting in a coal mine and propose effective measures to reduce the burden on the life and health of employees and the environment.

1. Introduction

High concentrations of coal dust produced in surface or underground coal mines can lead to death or serious damage to the health of miners. It can also lead to coal dust explosions or even gas explosions, which can result in serious personal injury, environmental and economic losses. An explosion in a mine is usually caused by a sudden ignition and explosion of flammable gases (methane) or coal dust. Methane and air form an explosive mixture which is described by the lower explosive limit (LEL), about 5% by volume of methane, and the upper explosive limit (UEL), about 13% by volume.

Preventive measures for mines against explosions consist of a good supply of fresh air, which reduces the methane concentration below 5% by volume. More dangerous is the mixture of agitated coal dust with air. Coal dust explosions are prevented by humidification or pulverisation with stone dust.

Dust can be defined in different ways. Turekova states that “dust is defined as a collection of fine particulate matter of arbitrary size and composition that is generally dispersed in the air” [1]. In contrast is the statement of Weiss, who argues that “we consider dust to be a particulate matter of inorganic or organic origin that is dispersed in a gaseous environment and does not include smoke” [2].

Different definitions for the term dust can be found in different literature and professional sources and, in particular, different values for its maximum particle size. While according to STN 83 4501:1997-07 [3] dust particles do not exceed 75 µm in diameter, according to the World Health Organization (WHO) dust can be considered as particles with a maximum diameter of 100 µm. For the purpose of this research, however, the most relevant definitions of dust are those that consider dust particles to be between 1 µm and 500 µm in size, such as according to [3,4]. Combustible dust is dust containing particles of arbitrary structure and shape, with a maximum size of 500 µm, which at normal temperature and atmospheric pressure can be stirred up into the surrounding air and form explosive atmosphere. Due to gravity, they may settle to form a layer at least 1 mm high, which is then capable of explosion [5,6]. We deduce that dust can be considered as particles dispersed in a gaseous environment, most often air, which have a certain size and for their specificity we have to take into account their origin. The size of dust particles ranges from 1 μm to 100 μm. The size of dust particles larger than 30 μm is referred to as coarse dust and sediment very quickly in the environment [1]. According to its origin, dust is divided into organic (animal or plant) and inorganic dust (metallic, non-metallic and mixed).

For mining purposes, the Slovak Mining Authority Decree No. 21/1989 Coll. defines dust as solid particles found in the mining air [6]. According to its origin, mining dust can be divided into [2]: dust originating in the mining massif—the action of natural forces, for example, rock crushing during the movement of rock blocks and massifs; technological dust—the originators of dust are technological operations, for example, cutting, boring, drilling, blasting, and extraction of rubble; surface dust—intruded into the mining environment by the main ventilation stream directly from the surface.

Dust particles of various sizes are present in mining operations. According to the size of dust particles and the way they behave in the mining environment, dust can be divided into four basic groups [7]: respiratory dust—particle size ranges from 0.5 μm to 10 μm. It is part of the dust aerosol that penetrates into the alveoli of the lungs and is permanently deposited in them when inhaled; airborne dust—particle size ranges from 5 μm to 30 μm, it is part of the dust aerosol and is able to remain in the air for a longer period of time, it is usually carried by the ventilation system to greater distances from the source of formation; dust layers—particle size ranges from 30 μm to 100 μm, it is typical for its ability to settle quickly in the mine environment; stirred dust—particle size of this type of dust is identical to the particle size of dust layers, it is formed by stirring up of dust layers by walking of employees, by larger airflow or technological operations.

Exposure of the human body to mine dust over a prolonged period of time causes irreversible damage to the organism. The adverse effects of mine dust are often amplified by a number of other interacting factors—temperature, humidity, blasting fumes, exhaust fumes, vibration, and noise. Thus, in the long term, exposure to dust can bring diseases such as [8]: pulmonary fibrosis, mucosal inflammation, skin inflammation, eye inflammation, or ear infections.

The issue of occupational health and safety of employees is legislatively addressed by several regulations and decrees. The most important is Government Regulation 391/2006 Coll. on minimum and health requirements for the workplace [5]. Dust affecting employees in mining operations is dealt with in Slovak Mining Authority Decree No. 21/1989 Coll. Exposure to asbestos dust is dealt with in Slovak Government Decree No. 253/2006 Coll. on the protection of employees against risks related to exposure to asbestos at work [6,9].

When measuring and investigating the effects of dust on the human body, international regulations are used in practice, e.g., EN 481:1993:09 [10] and ISO 7708:1995 [11]. Standard EN 481:1993 defines sampling conventions for particle size fractions which are to be used in assessing the possible health effects resulting from inhalation of airborne particles in the workplace. They are derived from experimental data for healthy adults. Conventions are defined for the inhalable, thoracic and respirable fractions; extra thoracic and tracheobronchial conventions may be calculated from the defined conventions [10].

ISO 7708:1995 defines sampling conventions for particle size fractions for use in assessing possible health effects of airborne particles in the workplace and ambient environment. Defines conventions for the inhalable, thoracic and respirable fractions; extra thoracic and tracheobronchial conventions may be calculated from the defined conventions [11].

This paper aims to present and discuss the results of measurements taken in a coal mine and to propose effective measures to reduce the burden on the life and health of employees and the environment.

2. Materials and Methods

According to the current legislation, underground workplaces are categorized according to the Decree of the Slovak Mining Authority No. 21/1988 Coll. In accordance with the above-mentioned Decree, the Nováky mine is categorized according to §79 into the category of gassing mines, hazard class II. According to the Decree of the Ministry of Health of the Slovak Republic No 448/2008 Coll., the underground workplaces of the Nováky mine are categorized as category 3 within the categorization of works. Following the Act 355/2007 Coll. on the protection, promotion and development of public health and on the amendment of certain acts, Government Regulation 391/2006 Coll. on minimum and health requirements for the workplace and Decree of the Slovak Mining Authority 21/1989 Coll., the measurement of dust concentration in the Nováky Mine premises is subject to an annual plan [5,6,9].

Dust sampling is the responsibility of the employees of the Ventilation and Drainage Department, hereinafter referred to as the UVO. The samples obtained for internal purposes are evaluated within the company. Responsibility for the analysis of samples is bore by Nováky Mine Coal Testing Laboratory, hereinafter referred to as SLUBN. The Centre for Hygienic Laboratories Ostrava is responsible for determining the total proportion of crystalline silica (SiO₂) in the sample assessed at annual intervals. Sampling is carried out via direct data collection in the mining company Hornonitrianske bane Prievidza a.s. in the underground workplaces of the Nováky brown coal mine using accredited CIP 10 type instruments and stationary dust tables (Figure 1).

Measurements of airborne dust were carried out as part of an internal request by the Production Management Unit in February and March 2023. The measurements were carried out in the 11th mining field, in the main exhaust mine work 08 240 of the B airy pit at a stationing of 220 m. The standardized procedure of measuring using dust tables rests in placing the measuring station in the entire transverse axis of the mine at such a height that the bottom edge of the tables is located at half the height of the total height of the mine work. The size of the table is 150 × 200 mm stacked in a series to cover the entire transverse axis of the mine. The tables were made of hard plastic [7,12]. In the mine, the airflow ranged from 1400 m³/min to 1500 m³/min at the time of the measurements. This was related mainly with to the depressed weather, as the frequency of rotation of the main fan was set stable at the time, following the command of the head of ventilation. The airflow measurements were undertaken every day in the morning shift. Dust concentration was then calculated as the ratio of the total weight of dust captured on the mine tables and the measured airflow, converted to hourly dust concentration. The development of hourly dust concentration was, of course, also related to work operations that were carried out at all productive workplaces—cutting, boring, and heavy maintenance in the mine.

Measurements were made on stationary dust tables. Samples from these tables were collected at weekly intervals and submitted to SLUBN. Under SLUBN test laboratory conditions, the total specific dust mass in the exhaust corridors was collected and evaluated for a given calendar month. For the representativeness of measurements, the dust tables were placed in the profile of the mine taking into account the braking of the airflow. According to the area, which we calculate as the product of the height at the floor of the corridor visible to the naked eye, the height of the corridor from the ground to the highest point at the given stationing perpendicular to the corridor, and a coefficient of 0.82, which reflects that approximately 82% of the corridor is visible to the naked eye from the cross-section, 12% was the floor of rock that arose from the building of the corridor and dust deposits. The main ventilation exhaust works usually ranged from 10 m²–14 m² depending on the reinforcement, subsoil, or superstructure in which they were built.

The airflow volume is largely influenced by the mine’s ventilation system. Decree 21/1989 [6] states that ventilation of the mine must be ensured by at least one main intake mine work and at least one main exhalation mine work. At least one horizon of the intake mine work must be excavated at the level of the lowest mined point of the given mine. The lowest horizon of the main exhalation pit must be bored at the level of the uppermost excavated horizon of the given mine. This method ensures natural ventilation when the main fans fail. However, the decree further states that the ventilation of the mine for permanent operation must be continuous, induced artificially, and with negative pressure. It also states that on the main exhaust pit, this ventilation must be provided by a pair of main fans, where one is in operation and the other in backup, and they also have replacement fans of the same performance located in the same fan station. They must also be capable of switching automatically and ensuring the reversal of mine winds. This ventilation system provides the advantage that the induced vacuum of the main fans pulls gaseous pollutants from the rock massif or old mine work. With the failure of the main fans and the fact that the barometric pressure is higher in the mine, the barometric pressure in such a failure pushes gaseous pollutants into the rock massif or old mine works. The volume of airflow was measured roughly once every three days, and the measurement of airflow needed for the ventilation balance was done twice a month.

Respiratory dust measurements at the wall cutting were undertaken based on an internal request from the Production Management Unit and a request from the Occupational Health Service based in Bojnice in January 2023. The measurements were carried out on mechanized cutting of 111 021-95 of staff team X1 and the mechanized boring in the corridor 111 123-05 of staff team X2. Two CIP 10 devices were used for the measurements, attached to the 2 employees in the breathing zone. CIP 10 devices ATEX have CIP-R inlet and 10 L per min sampling flow rate. Dust exposure to employees of individual work operations or the total dust concentration of the workplace was measured with the given devices. These are devices that, based on a magnetic field, attract dust particles directly to the measuring zone of the device. The device captures these particles. The measurement output is the amount of captured dust particles per monitored time—in our case, this was a 9.5-h work shift in mine operation. Measurements were made over a long period of time and in continuous operation to capture all work operations at the given workplace. Therefore, we could express measurements for an average workplace occupation by individual employees. Subsequently, we took the captured amounts of dust from individual measurements from the devices, which were only quantitatively weighed and stored. After the measurement, there were calculated average amounts of dust captured on the dust meter, the airflow volume passing through the workplace, and the length of the work shift. After this evaluation, the samples were sent to testing laboratories to determine the siliceous components in the given sample [7,13,14].

Samples from the devices were collected at shift intervals and submitted to SLUBN. Under SLUBN test laboratory conditions, the total specific gravity of dust and its respiratory fraction were measured at the active workstations in question. The results found after seven measurements were sent by SLUBN to the Occupational Health Service in Bojnice for evaluation. Specific samples were subsequently sent to the chemical laboratories in Ostrava to detect the presence of SiO₂.

For the respiratory component and also for the measurement of total dust concentration, the measurements were given in units of mg·m⁻³. The measuring device was installed within the total ventilation stream behind the last active workplace without the possibility of passing from the given stationing to the next ventilation section. It was installed as close as possible to the exhalation pit of the given section. Together with the evaluation of the total dust concentration, the total humidity and temperature of the total exhalation airflow were also evaluated. The internal form of HBP a.s., was used to record the date of sampling, type of instrument, and work activity performed. These particular forms were also used to identify the most exposed work operations in terms of dust concentration assessment. Walking to and from the workplace was also considered a work activity [15,16].

Measurement with dust meters CIP 10 was used to measure the worker’s respiratory component at specific workplaces during specific work operations. The measurement with dust tables was used to determine the total dust concentration of individual mining fields at a certain number of workplaces.

3. Results

3.1. Measurement of Airborne Dust in the Mining Field

Using the described procedures, dust concentration values were calculated and appropriate types of disconnecting mechanisms were assessed for dust production when the mining section was loaded with a certain number and type of active workplaces.

For the month of February, the measured values are shown in

Table 1. Measurement of airborne dust in February 2023.

Sampling	Day of Sampling	Place of Sampling	Stationing of Sample (bm)	Total Concentration (mg∙m−3)
I.	8 February 2023	08 240	220	21
II.	15 February 2023	08 240	220	26
III.	22 February 2023	08 240	220	16
IV.	1 March 2023	08 240	220	24
Average (mg∙m−3)				21.75
Standard deviation (mg∙m−3)				4.35

.

The development of dust in the 11th mining field during I, II, and IV occurred mainly on the mechanized cutting 111 021-95 of the staff team X1 with mechanized support BMV—Mi with a total number of 42 pieces.

The mechanized boring of the opening corridors of the future wall cutting with the designation 111 023-95 was affected partially by the boring in corridors 111 123-05 by the staff team X2 and 111 223-05 by the contractor LGO. During this period, we can assess the airborne dust as excessive due to the occupation of the mining section by a larger number of teams.

In the third week of February, dust concentration levels were reduced due to the temporary suspension of work on the mechanized boring plant and the withdrawal of teams to other mining sections. At that time, there was only one active workplace in the mining section.

For the month of March, the measured values are shown in

Table 2. Measurement of airborne dust in March 2023.

Sampling	Day of Sampling	Place of Sampling	Stationing of Sample (bm)	Total Concentration (mg·m−3)
I.	8 March 2023	08 240	220	28
II.	15 March 2023	08 240	220	25
III.	22 March 2023	08 240	220	39
IV.	29 March 2023	08 240	220	38
V.	1 April 2023	08 240	220	7
Average (mg∙m−3)				27.4
Standard deviation (mg∙m−3)				12.93

.

In the first two weeks, only three active sites were located in the 11th mining field, there was one mechanized cutting and two mechanized borings. During this assessment period, mechanical rock disconnecting, breakdowns on the sprinkling mechanisms of the mining and boring machines and the transport of the rubble contributed to the development of the dust.

In the period of weeks III and IV in the 11th mining field, two active mechanized boring staff teams X3 and X4 were added in the boring of the future wall cutting marked 111 022-95 in the 111 122-05 and 111 222-05 corridors. During the evaluation period, the expansion of the wall breakthrough was proceeded with the subsequent installation of mechanized support in the 111 023-95 wall cutting.

After the last sampling was performed and evaluated, the total specific dust concentration from the 11th mining section was found to be 14 mg·m⁻³. In Table 2 we can see that this is the lowest calculated value. Taking into account the total exposure of the underlying measurement, 48 h, then, on average, the dust concentration per day at the end of March is 7 mg·m⁻³.

There was a significant influence on the total dust development in the period of the V., control sampling from running of the mechanized cutting 111 023-95 with mechanized support BMV—Mi with a total number of 34 pieces when the number of the boring staff teams in the given evaluation section was reduced from the previous 4 teams to 2 active teams. Dust development was again driven by the transport of rubble, mechanical disconnection of the coal seam, and failures in the sprinkling mechanisms. Temperature changes in March were also a significant contributor to the dust transport. Increasing daytime temperatures necessitated an increase in the power output of the main fans and hence an increase in the speed of the airflow [17].

3.2. Measurement of Respiratory Dust at the Wall Cutting

For specific measurements, we considered 9.5 h work shift at specific work sites. The results obtained for the measurement of in person collection of samples at the mechanized cutting 111 021-95 are shown in

Table 3. In person collection of respiratory dust at the wall cutting.

Date: 16 January 2023	Location: 111 021-95	Order of shift: 3.
Device: CIP 10	Work activity	Duration (minutes)
Shift: 9.5 h	Going underground to section	15
Temperature (°C): 26.5	Walking to workplace	23
Humidity (%): 68	Cleaning sections 1–5	52
Speed of airflow (m.s−1): 4.3	Moving sections 1–5	30
Concentration (mg·m−3): 28.3	Extrusion of the conveyor 5–17	20
Respiratory part (mg·m−3): 4.4	Loading trip	40
Temperature in laboratory (°C): 25	Cleaning sections 6–42	100
Humidity in laboratory (%): 54	Moving sections 6–42	45
	Dropping the roof–No. 2	92
	Burning–wooden corridor	40
	Gathering	45
	Brunch	30
	Walking from workplace	23
	Going to surface	15

.

In the case of personal sampling at this type of workplace, we have made one measurement, since the course of the process does not change and there is cyclicality. In mechanized coal mining, there is a single variable on this type of site. This is the discharge of coal mass from the roof. According to the length of the wall, it is divided into several parts. From a dust point of view, the designation of the overhead is not of major importance.

Mechanized mining is characterized by the disconnection of rocks over a large area, which is the cause of the development of large quantities of dust. At this particular type of site, dust concentration levels have been increased due to technical requirements for maintaining the safety of the work. Inadequate sprinkling of the disaggregated rock by the mining shearer also contributed to the increased dust concentration at the site [7,18].

We sent the obtained sample to the chemical laboratories in Ostrava to detect the presence of SiO₂. As the wall cutting did not pass through the bedrock of geological profile of the section, the sample was negative for SiO₂.

The results from the personal collection of samples for the dust concentration calculation generated during technological activities at the 111 123-05 mechanized boring sites are presented in

Table 4. In person collection of respiratory dust at the mechanized boring.

Date: 24 January 2023	Location: 111 123-05	Order of shift: 1.
Device: CIP 10	Work activity	Duration (minutes)
Shift: 9.5 h	Going underground to section	15
Temperature (°C): 30.1	Walking to workplace	42
Humidity (%): 84	Building range–1.2 m	97
Speed of airflow (m.s−1): 2.8	Arming range	78
Concentration (mg·m−3): 1.32	Prolonging 500	146
Respiratory part (mg·m−3): 0.4	Scratching–1.2 m	105
Temperature in laboratory (°C): 25	Brunch	30
Humidity in laboratory (%): 54	Walking from workplace	42
	Going to surface	15

.

During the personal sampling at this type of workplace, we performed eight measurements, as the overall process of mechanized boring of mine corridors is very flexible concerning the content of the work. We could not consider one measurement to be representative.

We calculated average values and standard deviations from the measurements of the dust concentration and respiratory component of the dust. In this way, we arrived at the result of 1.27 mg·m⁻³ (standard deviation 0.47 mg·m⁻³) of the concentration of dust at the given workplace and 0.32 mg·m⁻³ (standard deviation 0.11 mg·m⁻³) of the concentration of its respiratory component. When calculating the average values, we took into account the work shift, the temperature of the airflow before entering the workplace, at the workplace and when leaving the workplace, humidity and the speed of the airflow. In particular, the temperature was important from the point of view that some workplaces were ventilated with a separate ventilation stream, while others had continuous ventilation. Measurements were carried out over a period of 14 days in each shift. The work shift lasted 9.5 h, where the journey to and from the workplace was also considered work activity, i.e., the employee received the measuring device before entering the underground and the devices were taken from him only after exiting the underground. An employee of the ventilation and drainage department was also present at the workplace and recorded individual operations and their time consumption during the work shift.

Individual samples were sent to the chemical laboratories in Ostrava to test the presence of SiO₂. The mechanical excavation itself was carried out in the bedrock of the geological profile of the given section and also passed through old mining works. The average amount of SiO₂ reached a value of 1.4% of the weight of the assessed samples. Based on these results, we assess the given workplace as a place with an increased risk of acquiring occupational diseases.

4. Discussion

4.1. Discussion on the Results of Airborne Dust Measurement

In the overall assessment of dust development for the evaluated period of February and March, we found that the average concentration of measured airborne dust per work shift is 24.89 mg·m⁻³ with a standard deviation of 9.98 mg·m⁻³. We also concluded during the evaluation that the average number of active workplaces in the 11th mining field was 4 workplaces.

Through subsequent observations, we found that the largest share of the total dust concentration is borne by mechanized cuttings. In the workplaces, the coal massif is mechanically disaggregated over a large area, and therefore requires effective and reliable sprinkling system. The size of the usable area of the wall cutting is defined by the overall width of the cutting and the clear height of the coal seam. In total, we determined the size of the usable area of the mechanized cuttings based on the size of the support system, and the height of the seam as shown in

Table 5. Size of the usable area of the mechanized cuttings.

Staff Team	Workplace	Type of Support	Width of the Support (m)	Quantity (pcs.)	Average Height of Seam (m)	Usable Area (m2)
X1	111 021-95	BMV-Mi	1.5	42	2.8	117.6
X2	111 023-95	BMV-Mi	1.5	34	3	102

.

The sprinkling system of mining shearers was broken at these specific workplaces. (Figure 2) To directly eliminate the development of coal dust on the wall cuttings, we suggest re-functionalizing the given system. The high failure rate of the sprinkling mechanisms of the mining and boring shearers had a major impact on the airborne dust. In evaluating weekly measurements in March 2023, we found that the failure rate of the sprinkling mechanisms and active rock disconnection were responsible for the overall dust concentration. In addition, an insufficient sprinkling of disaggregated rock from the body of the rotation drum of the mining shearer contributed to the overall development of dust concentration at the workplace with mechanized boring [7,19,20,21,22,23].

In this regard, the supply hose of the pressure system is the faultiest. It is a non-reinforced PVC hose with a clear diameter of 1.2″. We suggest keeping the clear diameter of the supply hose, but replacing the PVC model with a reinforced model, which is more resistant to pressure effects [23,24].

As part of collective and personal protection against the effects of airborne dust, we suggest wearing respirators throughout the shift at active cutting and mining workplaces, as well as in the main exhalation ventilation stream. We suggest placing available respirators in the lamp room, where they will be available to every employee.

When searching for sources of dust, we came to the conclusion that the source of the second largest share in the development of dust was the belt transport of rubble, more specifically, mainly the overflow of rubble from one belt conveyor to another. The rubble falls on the next strip from a height of 0.5 m–0.7 m. During the inspection of the belt conveyors, not a single sprinkling device was installed.

4.2. Discussion on the Results of Respiratory Dust Measurement

Assessing the obtained samples, we concluded that the workers of the wall cuttings are subject to the greatest exposure to dust. At this type of workplace, rocks are disaggregated over a large usable area. Every single work activity on the wall cuttings is characterized by high dust generation.

The disaggregation itself is performed mechanically. Hence, the biggest contribution to the development of dust concentration is the insufficient rock sprinkling. We propose to carry out revisions of the sprinkling systems of mining shearers by employees of the machine and equipment maintenance. Performed revisions and detected deficiencies must be reported to the supervisor of the given workplace and the shift manager of the machine and equipment maintenance section.

The level of staff expertise at the workplace also contributes significantly to the overall dust concentration at the wall cuttings. As there is a very limited number of experienced and trained specialists, there is an increased probability of non-professional, improper setting of the mining bodies of the shearer which results in imperfect separation of the rock with subsequent above-average dust formation. In the case of non-professional handling of the mechanical support, there is insufficient roof coal removal of the wall cuttings, which also results in above-average dust generation at the workplace. We recommend further training of newly hired employees and increasing the periodicity of employee retraining in the field of operating mining shearers and mechanical support.

Assessing the samples from mechanical boring, we concluded that the workers of the given workplace experience the greatest exposure to dust during the mechanical disaggregation of the coal seam. The biggest dust producer is the very act of disaggregating the rock. The disaggregation of the coal seam is carried out by the rotary movement of the boring rosette. With greater strength of the coal seam, there is above-average friction between the rosette and the seam and a large amount of dust is generated. In such a case, the sprinkling system of the mining shearer plays a key role. Unfortunately, when collecting samples, we found out that in most cases the sprinkling system was not working. The lack of expertise of the employees also played a significant role in the development of dust concentration. We propose the same measures for mechanical boring as for mechanical mining. Namely, the revision of sprinkling systems with the obligation to report the facts to the shift leader of the machine and equipment maintenance and also the increase in the periodicity of retraining employees for the operation of mining shearers [25,26].

To protect employees from the impact of SiO₂ on the human body, we suggest conducting a more thorough geological survey of newly bored mine works. When verifying the thickness and structure of the layer, we suggest avoiding boring in the bedrock and in layers with a low energy value. We also recommend avoiding boring through old mine works, as these are already cut-off parts that are mostly covered with bedrock materials.

With previous measurements, we pointed out the importance of measuring the respiratory component of dust to protect the respiratory tract of employees. Not only the respiratory component but also dust layers and stirred-up coal dust directly threaten the lives and health of miners.

Coal dust, mainly due to its reactivity and small particle size, contributes to decrease of the lower explosive limit of flammable gases. By decreasing this limit, there is a direct threat to employees in the event of an explosion [27].

The leading place as the source of risk in any risk assessment would be occupied by dust layers. Due to the influence of the airflow, it is carried away from the source for long distances, where it settles on the lower part of the mining spaces when the speed of the airflow decreases. In such cases, it creates large piles of dust. When walking, it is dispersed and carried again over a long distance [28,29].

Dust layers are as a source of airborne dust concentration eliminated in mining conditions by sprinkling mining spaces. It is a method of washing coal dust from the sides and casing of the mine with a stream of water. Mine dust moistened in this way is loaded onto a belt conveyor and transported to the coal storage.

In the Nováky mine, sprinkling of mine spaces with inert dust is a less used method. It is a mixture of lime dust that easily absorbs moisture and thus its specific weight is higher than the specific weight of coal dust. In the event of a possible explosion, it does not swirl and leaves the coal dust unmoved under its layer.

The mine is also equipped with anti-explosion water barriers against the transmission of the effect of the pressure wave and the swirling of coal dust. It is a system of troughs located in the upper third of the mine on wobbly shelves [30,31]. Water is poured into the troughs. The amount of water required according to the cross-section of the mining tunnel is determined by Decree 21/1989 of the Slovak Mining Authority. The decree states that there must be at least 200 L of water per 1 m² of the mine section [6]. Anti-explosive water barriers are placed in front of objects that are protected in the range of 30 m–100 m in the length of the 30 m of water barrier.

5. Conclusions

Mining workplaces are included in the 3rd category within the categorization of work in the Slovak Republic. This categorization reflects the extreme conditions in which the employees have to work. Factors affecting their performance and health can significantly contribute to their permanent health damage, or even death.

With a more thorough description of the procedure and achieved results of measuring selected physical factors of the underground workplaces of the Nováky mine, we pointed out how extremely important the measurement and evaluation of these data is. The factors themselves must be assessed sensitively and subjectively for each employee, as there are no limit values for the mining environment in the legislation of the Slovak Republic. Due to the decline of lignite mining in Horná Nitra region, the last mine of this type in Slovakia is being closed. A number of risks come with this process.

Measures to be taken in order to reduce the risks should include more effective and reliable sprinkling. Next, reducing the number of boring staff teams from the current 8 to 4 teams can bring an increase in the membership base and expertise for individual teams and reduce dust concentration at workplaces. Reduction in the number of teams would decrease a large number of separate drafts in the mine and decrease the number of repairs to sprinkling devices on belt conveyors and mining shearers. Reduction in the number of work teams should be applied also at wall cutting, where the teams should be reduced from the current 3 to 2 teams. Consequently, with a reduction in total dust concentration, we expect a reduction in the cost of removing coal dust by sprinkling, or a reduction in the cost of purchasing inert dust. We also anticipate a reduction in the cost of procuring polystyrene troughs for anti-explosion water barriers. When summarizing the measures taken, regarding PPE, we suggest a better placement of containers with respirators and rubber earplugs directly at the entrance to the lamp room.

Finally, due to the attenuation of mining and the clearing of sections by passing through the old corridors, the microclimatic conditions at the workplaces are rapidly deteriorating. We suggest to put maximum effort to deviate from the closed mine works when planning the production. Unfortunately, a financial loss must be taken into account, as the wall cuttings will be shortened. On the other hand, the adopted measure will bring financial savings in the elimination of the reconstruction of the bored works due to the pressure and costs associated with additional boring.

After the adoption of the above measures, we expect a reduction in the risk of occupational disease, as the workload in exposure to dangerous factors of underground workplaces will be distributed among the increased membership of individual staff teams.