Air Pollution Associated with Total Suspended Particulate and Particulate Matter in Cement Grinding Plant in Vietnam

: Air pollution associated with suspended particles has become a signiﬁcant concern in Vietnam recently. The study aimed to (1) investigate dust sources; (2) measure concentration levels of Total Suspended Particulate (TSP), Particulate Matter (PM) fractions; (3) identify silica levels and the correlation with respirable particles at a cement grinding plant in Vietnam. A total of 312 samples (52 TSP, 160 PMs) at 13 processes were measured using the direct-reading dust meter. The silica composition was analyzed in a certiﬁed laboratory using the X-ray ﬂuorescence (XRF) technique. SPSS version 26 for Window was used to analyze the data. The operations of the cement grinding plant created multiple dust sources from the jetty to the cement dispatch process. The TSP levels ranged 0.06–38.24 mg m − 3 , and 40.38% ( n = 21) TSP samples exceeded the Permissible Exposure Limit (PEL) for an 8-h working shift. Besides that, there was a wide range and signiﬁcant concentration levels of PMs in the cement processes. The levels of PMs were PM 1 (0.00–0.06 mg m − 3 ), PM 2.5 (0.01–0.83 mg m − 3 ), PM 4 (0.02–4.59 mg m − 3 ), PM 7 (0.03–16.94 mg m − 3 ), and PM 10 (0.04–26.85 mg m − 3 ). The highest mean levels of PMs factions were measured at the pre-grinding process. The inefﬁcient operation of the dust collector contributed a signiﬁcant factor to the dust dispersion in this process. The silica’s mean (SD) composition in respirable dust was 20.4% (0.86) and was not signiﬁcantly different amongst the processes. There was a signiﬁcant correlation between the levels of respirable dust and silica exposure in the cement grinding plant (r = 0.99). The improvement of indoor air quality is needed to prevent health effects on cement workers.


Introduction
Air pollution is one of the most significant environmental risks to health globally, significantly affecting low and middle-income countries. The common indicators used for evaluating air quality and health effects are Total Suspended Particulate (TSP) and Particulate Matter (PM) [1]. According to the International Standardization Organization (ISO), "Dust is small solid particles, conventionally taken as those particles below 75 µm in diameter, which settle out under their own weight but which may remain suspended for some time" [2]. Dust is defined as small, dry, solid particles projected into the air by natural forces, such as wind, volcanic eruption, and by mechanical or artificial processes such as crushing, grinding, milling, drilling, demolition, shoveling, conveying, screening, bagging, and sweeping. Dust particles are usually in size range from about 1 to 100 µm in diameter, and they settle slowly under the influence of gravity [3]. Particulate matter is a mixture of solid and liquid particles in the air that is small enough not to settle onto the Earth's surface under the influence of gravity, classified by aerodynamic diameter [1]. Depending on the aerodynamic diameter, the airborne particle fractions are inhaled and deposited in the various regions of the respiratory system. Inhalable particulate fraction is that fraction of a dust cloud that can be breathed into the nose and mouth. Thoracic particulate fraction with the capacity of 500,000 tons per annual. The operation of the plant involves the participation of direct workers from the production, maintenance, and logistic department.
The main processes of the cement grinding plant consist of unloading raw materials at the jetty, raw materials storage and handling, pre-grinding with crusher, cement grinding in the ball mill, cement storage, and cement dispatch. The raw materials used in the cement grinding plant are clinker, limestone, gypsum, pozzolana, and slag. At the study site, two cement products are produced, which include cement bag 50 kg with clinker factor (56%), limestone (19%), gypsum (4%), other additives, and bulk cement used for industrial purposes with clinker factor (73-75%), limestone (12.5%), gypsum (4%), and other additives.

Study and Sampling Design
The authors surveyed the cement grinding processes under the support of the process engineer and departments representatives to identify dust sources and job group characteristics (daily tasks, number of exposed workers, and duration) in a working shift. The mean duration of working in each process was calculated based on the actual operation hours in 2020 of the crane at the jetty, ball mill, packing machine, and bulk dispatch. Based on the dust sources in the cement grinding plant, a sampling map of the TSP and PM samples was designed using Google Earth Pro and presented in Figure 1.
Atmosphere 2021, 12, x FOR PEER REVIEW 3 of 13 and humid climate year-round, with an average temperature of 27.5 °C. The city has two seasons which include the rainy season (May-November) and the dry season (December-April) [34]. The study site was established in 2003, and consists of 78 full-time employees with the capacity of 500,000 tons per annual. The operation of the plant involves the participation of direct workers from the production, maintenance, and logistic department. The main processes of the cement grinding plant consist of unloading raw materials at the jetty, raw materials storage and handling, pre-grinding with crusher, cement grinding in the ball mill, cement storage, and cement dispatch. The raw materials used in the cement grinding plant are clinker, limestone, gypsum, pozzolana, and slag. At the study site, two cement products are produced, which include cement bag 50 kg with clinker factor (56%), limestone (19%), gypsum (4%), other additives, and bulk cement used for industrial purposes with clinker factor (73-75%), limestone (12.5%), gypsum (4%), and other additives.

Study and Sampling Design
The authors surveyed the cement grinding processes under the support of the process engineer and departments representatives to identify dust sources and job group characteristics (daily tasks, number of exposed workers, and duration) in a working shift. The mean duration of working in each process was calculated based on the actual operation hours in 2020 of the crane at the jetty, ball mill, packing machine, and bulk dispatch. Based on the dust sources in the cement grinding plant, a sampling map of the TSP and PM samples was designed using Google Earth Pro and presented in Figure 1.

Dust Monitoring Procedure
A total of 52 TSP samples and 260 samples of PM1, PM2.5, PM4, PM7, and PM10 were measured in 13 sub-processes of the grinding plant during the working shifts. At each measurement point, the dust concentration level was measured at the worker's breathing zone during each of the four different working hours (7:30-9:30; 9:30-11:30; 12:30-14:30;

Dust Monitoring Procedure
A total of 52 TSP samples and 260 samples of PM 1 , PM 2.5 , PM 4 , PM 7 , and PM 10 were measured in 13 sub-processes of the grinding plant during the working shifts. At each measurement point, the dust concentration level was measured at the worker's breathing zone during each of the four different working hours (7:30-9:30; 9:30-11:30; 12:30-14:30; 14:30-16:30). These hours cover the duration of the working shift and represent the dust mass concentration level for all workers at the site, since operational processes are the same for all shifts. In order to measure the dust concentration in indoor workplaces, a direct-reading dust mass monitor (MET ONE AEROCET 531S (USA)) was used. The Aerocet 531S counts and sizes particles in different size ranges then use a proprietary algorithm to convert count data to mass measurement. The mass concentration precision of this equipment is 0.0001 mg m −3 . Before measuring, the dust meter was fully charged, performed zero tests in a clean environment, was calibrated by a qualified center, and was valid until April 2022. Next, the dust meter was set up in mass mode, and the PM 1 , PM 2.5 , PM 4 , PM 7 , PM 10 , and TSP mass concentration levels were measured and recorded on the paper sheet for all processes. The measurement process complied with the dust measurement protocol regulated by the Vietnamese standards (QCVN 02:2019/BYT) [37].
The time-weighted average (TWA) concentration for an eight-hour working shift was calculated by using the Formula (1) [38].
where: TWA (mg m −3 ) is the equivalent exposure limit for an eight-hour working shift; C n (mg m −3 ), (n = 4) is the dust mass concentration level represents for a period of time (T); T n (hours) is the duration in hours of the exposure in the concentration (C). The excessive TSP, PMs for an eight-hour working shift at the processes was identified by comparing to the Permissible Exposure Limit (PEL) regulated by the Vietnamese standard (QCVN 02:2019/BYT) [37].

Silica Analysis and Processing
In order to identify the silica concentration in respirable dust, five samples of accumulated dust that settled on the top surface of the machine or structure at the cement grinding, packing machine, truck dispatch, barge dispatch, and bulk dispatch were taken. Next, they were sealed in plastic bags and analyzed in the certified laboratory (Vilas 100). The ARL QUANT'X EDXRF analyzer (Thermo Scientific, Paisley, UK), operating using the X-ray fluorescence (XRF) technique was used to measure the composition of silica (silicon dioxide) with a minimum detection limit of 0.003%. Finally, the concentration of free silica in respirable dust was calculated using the Formula (2) [37].
where: C silica (mg m −3 ): concentration of free silica in respirable dust; C respirable dust (mg m −3 ): concentration of respirable dust (PM 4 ). SPSS version 26 for Windows was used to analyze the data, and Pearson analysis was used to identify the correlation between the levels of respirable concentration and silica exposure in the cement grinding processes.

Dust Sources and Exposed Job Groups in the Cement Grinding Processes
The operation of the cement grinding plant generates suspended particulates from all processes. The survey-based-process observation revealed the various dust sources in the processes where cement workers are exposed daily. In the jetty, the dust was generated in the opening space of the hopper and dispersed to the surrounding areas during unloading raw materials. The belt conveyor system is typically used to transfer raw materials to the storage or silos. However, the transferring of raw materials generated suspended dust at the changing-direction points, which raw materials fall freely to a lower position. In the pre-grinding process, the crusher was used to reduce raw materials' size before feeding the cement mill. In this process, dust came up at the changing-direction points underneath the feed bin connected to the belt conveyor. The efficient operational control of dust collectors in this process plays an essential role in controlling dust dispersion. After a long period of operation, raw materials shall build up inside the system that potentially reduces the airflow of the dust collector. In the cement grinding process, a closed system consists of ball mill, separator, primary bag filter, and air-slide was used. The main dust sources came from the primary bag filter's stack, which mainly emitted the ambient environment. In the final process, the primary dust sources were generated while operating the packing machine and loading cement bags to customer's truck or barge. While running the packing machine, cement leaked from the spout and fell freely to the ground, resulting in dust dispersion. The delivering of cement bags to a customer's truck or barge generated dust. Some dust sources in the cement grinding processes are present in Figure 2.
Atmosphere 2021, 12, 1707 5 of 13 before feeding the cement mill. In this process, dust came up at the changing-direction points underneath the feed bin connected to the belt conveyor. The efficient operational control of dust collectors in this process plays an essential role in controlling dust dispersion. After a long period of operation, raw materials shall build up inside the system that potentially reduces the airflow of the dust collector. In the cement grinding process, a closed system consists of ball mill, separator, primary bag filter, and air-slide was used. The main dust sources came from the primary bag filter's stack, which mainly emitted the ambient environment. In the final process, the primary dust sources were generated while operating the packing machine and loading cement bags to customer's truck or barge. While running the packing machine, cement leaked from the spout and fell freely to the ground, resulting in dust dispersion. The delivering of cement bags to a customer's truck or barge generated dust. Some dust sources in the cement grinding processes are present in Figure 2. In a basic working shift, cement workers in each job group are exposed to dust sources in the process. The operational control of the cement grinding processes from jetty to cement mill output was remotely performed by process operators in the central control room (CCR). During mill operation, process operators perform an interval inspection in the grinding processes every 60 min. In the dispatch process, the operation of the packing machine and delivery of cement bag to customer vehicles was manually performed by the operator and stevedore. The packer operator and stevedore were directly exposed to cement dust sources while operating the packing machine and the bag dispatch system. The job group characteristics and exposure duration in each process are summarized in Table  1.  In a basic working shift, cement workers in each job group are exposed to dust sources in the process. The operational control of the cement grinding processes from jetty to cement mill output was remotely performed by process operators in the central control room (CCR). During mill operation, process operators perform an interval inspection in the grinding processes every 60 min. In the dispatch process, the operation of the packing machine and delivery of cement bag to customer vehicles was manually performed by the operator and stevedore. The packer operator and stevedore were directly exposed to cement dust sources while operating the packing machine and the bag dispatch system. The job group characteristics and exposure duration in each process are summarized in Table 1.

Excessive Total Dust Concentration in the Cement Grinding Processes
The results of dust surveillance at the study site revealed that the TSP samples ( . The TSP at the study site is given in Table 2. The highest TSP concentration level was measured at the connecting point between the feed bin and the belt conveyor with 25.62 mg m −3 and was 6.4 times higher than the PEL for an 8-h working shift. The free-falling of raw materials from the feed-bin to the conveyor generated the pressure inside the belt conveyor. The dust collector is installed to balance the pressure inside the belt conveyor and control the dust emission. However, the inefficient operation of the dust collector contributed a significant factor to the dust dispersion in this process. The measurement in the bulk dispatch process recorded the lowest TSP concentration levels, with 0.15 mg m −3 .
The packing operator exposed excessive TSP levels for an eight-hour working shift limit at two sub-processes of the packing dispatch process. The measured TSP at the truck and barge loading process was 11.55 mg m −3 , 2.9 times higher than the PEL and 13.40 mg m −3 , 3.4 times higher than the PEL, respectively. The dispatch process was operating outdoors and had no dust collector because of the initial design. When operators dropped the cement bag from the loading chute to the truck or the barge, it generated and dispersed cement dust to the environment. The particulate size distribution, cement bag quality, and the height of free falling of the cement bag were the significant factors that caused the dust dispersion.
Previous studies reported the excessive TSP in some cement manufacturing processes. A study conducted in 2005 at a cement factory in Tanzania showed that the TSP at some processes exceeded the PEL, such as crane (38. [11]. The previous studies showed that the TSP at many processes in cement plants, such as cement mill, packing, raw mill, and crusher, exceeded the PEL, and these findings were comparable with the results in this study. In addition, the measurement results reported the TSP in other processes in a cement grinding plant such as jetty, material storage, pre-grinding, cement mill input and output, truck dispatch, and barge dispatch. The works provided the database of the TSP in all processes of the cement grinding plant for further research.

Wide Range Distribution of Particulate Matters in the Cement Grinding Process
Data obtained with this study showed a wide range distribution of particulate matter fractions in the workplace environment of the cement grinding plant. In addition, the measurements revealed the significant concentration levels of particulate matter in the whole process. The concentration levels of PMs were in the range of PM 1 Figure 3.
The measurement results divided the PMs into two principal groups: coarse particles, mostly larger than 2.5 µm in aerodynamic diameter, and fine particles, mostly smaller than 2.5 µm in aerodynamic diameter (PM 2.5 ).

Wide Range Distribution of Particulate Matters in the Cement Grinding Process
Data obtained with this study showed a wide range distribution of particulate matter fractions in the workplace environment of the cement grinding plant. In addition, the measurements revealed the significant concentration levels of particulate matter in the whole process. The concentration levels of PMs were in the range of PM1 (0.004-0.06 mg m −3 ), PM2.5 (0.01-0.83 mg m −3 ), PM4 (0.02-4.59 mg m −3 ), PM7 (0.03-16.94 mg m −3 ), and PM10 (0.04-26.85 mg m −3 ). The distribution of the particulate matter fractions at the study site is presented in Figure 3.  The measurement results divided the PMs into two principal groups: coarse particles, mostly larger than 2.5 µm in aerodynamic diameter, and fine particles, mostly smaller than 2.5 µm in aerodynamic diameter (PM2.5).
The mean (SD) levels of PM10-7-4 were 4.67 (7.04), 2.56 (3.91) and 0.66 (0.98) mg m −3 , respectively. The highest level of PM10-7-4 was measured at the connecting point between the feeding bin and the belt conveyor, a sub-process of pre-grinding process. The lowest The mean (SD) levels of PM 10-7-4 were 4.67 (7.04), 2.56 (3.91) and 0.66 (0.98) mg m −3 , respectively. The highest level of PM 10-7-4 was measured at the connecting point between the feeding bin and the belt conveyor, a sub-process of pre-grinding process. The lowest level of PM 10-7-4 was recorded at the bulk dispatch process. The mean (SD) PM 10-7-4 level in the cement grinding processes was in jetty (0.15 (0.07), 0.10 (0.05), 0.05 (0.02) mg m −3 , respectively); raw material storage and handling (6.36 (9.28) The peak mean levels of PMs factions were measured at the connecting point between the feeding bin and the belt conveyor, a sub-process of the pre-grinding process. The significant mean levels and wide range distribution of PM fractions were mainly measured at the pre-grinding, cement grinding, and cement dispatch process. In these processes, raw materials were crushed into small particles by the crusher and ball mill. In addition, the operation of the equipment in these processes generates heat, which reduces the moisture of the raw materials. These factors, in combination with the inefficient operation of the dust collectors, significantly contributed to the wide range distribution and levels of the PM fractions in cement grinding processes. There is currently no Vietnamese standard regulated for fine particles level in the indoor air environment.
The published works on the different fractions of particulate matter in the cement workplace environment are limited. Some researchers have reported the levels of PM 10 and PM 2.5 in the cement processes. A study reported the levels of PM 10-2.5 in cement mill (2.15, 0.03 mg m −3 , respectively), loading area (0.03, 0.02 mg m −3 , respectively), and crusher (0.25, 0.07 mg m −3 , respectively) [41]. Another researcher reported the level of PM 10-2.5 in the cement processes included cement mill (0.59, 0.87 mg m −3 , respectively), packing (0.51, 0.50 mg m −3 , respectively), and crusher (1.55, 1.80 mg m −3 , respectively) [42]. The levels of PM 10-2.5 in this study were significantly higher than in the reported studies. Other researchers reported the level of PM 10 , PM 2.5 in the ambient environment nearby the cement plants. The PM 2.5 level was 0.03 ± 0.01 mg m −3 and was in the range of 0.04-0.05 mg m −3 [43,44]. The PM 10 level was 0.03 mg m −3 and was in the range of 0.004-0.17 mg m −3 [45,46]. Therefore, the mentioned PM 10-2.5 levels in the surrounding areas of the cement plant were significantly lower than the PM 10-2.5 levels in the cement grinding processes in this study. For the PM in the ambient environment, a study reported the excessive PM 10-2.5 concentrations were in the range of 0.06-0.10, 0.02-0.04 mg m −3 , respectively, and were associated with health risks [47]. Other work reported the highest average monthly concentrations of PM 10-2.5 during dusty days were 0.3, 0.07 mg m −3 , respectively [48]. However, the concentration levels of PM 10-2.5 in this study were much higher than the mentioned reports.
Cement workers are exposed to a wide range and significant levels of PM fractions during the working shift. The coarse fraction with a dynamic diameter between 10 and 2.5 µm (PM 10-2.5 ) remains in the upper part of the respiratory tract, and the fine particles (PM 2.5 ) can penetrate deeper into the lung system and cause health effects [49]. Prolonged exposure to particulate matter, especially fine particles, has been proven to have negative health effects on the exposed group [46,[50][51][52]. Implementing an exposure reduction program to PM is needed to minimize the negative effects on workers' health.

The Correlation between Concentration Levels of Respirable Dust and Silica Exposure
The analysis results of the compositions and silica concentrations in the respirable dust samples at the study site are presented in Table 3. The measurement of silica (silicon dioxide) in respirable dust in the cement grinding processes showed that the range of silica content fluctuated between 19.35% and 21.29%, and the mean (SD) composition of silicon dioxide was 20.4% (0.86). This finding was comparable with other studies in which silica composition in cement fluctuated from 21-22%, and from 17-25% [53,54]. In this study, the silica composition was not significantly different between the processes. The highest silica content was measured at the bulk dispatch (21.29%), and followed by the truck loading (20.97%), packing machine (20.77%), cement mill (19.62%), and barge dispatch (19.35%). Because the silica composition is not significantly different amongst cement grinding processes or types of cement products, the level of silica exposure mainly depends on the concentration levels of respirable particles.
In this study, the respirable dust concentration level (PM 4 ) ranged between 0.02 and 4.59 mg m −3 . The measurement recorded the highest level of respirable particles at the feeding bin and belt conveyor, which exceeded the PEL for an eight-hour working shift. The respirable concentration level at other processes was below the regulated exposure limit. Other studies published high concentration levels of the respirable dust concentration, such as packing machine (4.5 mg m −3 ), loading (4.2 mg m −3 ), crushing (23 mg m −3 ), mill (6.7-7.1 mg m −3 ), and raw mill (43-47.6 mg m −3 ) [13,14]. A study conducted in a cement factory in Iran in 2019 figured out the range of respirable dust concentration was from 1.77 mg m −3 at cement mill to 6.12 mg m −3 at the crusher, packing and loading (3.4 mg m −3 ), raw mill (2.92 mg m −3 ) [55]. The respirable dust concentration levels in this study were lower than the same processes in mentioned studies. However, the concentration levels of respirable dust can be significantly fluctuated within cement grinding processes depending on the level of operational control.
The results of Pearson analysis showed that there was a positive and significant correlation between the concentration levels of respirable dust and silica exposure in the cement grinding plant (r = 0.99).

Conclusions
The study shows that the cement grinding plant has various dust sources that cement workers potentially expose daily. The TSP concentration in this study was in the range of 0.15 and 25.62 mg m −3 . The results of the TSP measurement reveal that four out of six processes in the cement grinding plant exceeded the PEL for an 8-h working shift. The excessive processes include the storage and handling of raw materials, pre-grinding, cement grinding, and cement dispatch processes. Besides that, the study reports a wide range and significant levels of particulate matter fractions in the cement grinding processes. The peak mean levels of PMs factions were measured at the connecting point between the feeding bin and the belt conveyor, a sub-process of the pre-grinding process. The inefficient operation of the dust collector significantly contributed to the PM dispersion in this process. In addition, the study reveals a significant correlation between the concentration levels of respirable dust and silica exposure in the cement grinding plant (r = 0.99). Prolonged exposure to respirable particles containing silica and fine particles caused negative health effects on cement workers [56,57]. Therefore, the improvement of indoor air quality is needed to prevent negative health effects on cement workers. Firstly, the maintenance of the dust collectors, ducting hoods, needs to be addressed to reduce the dispersion of particulate matter fractions in cement grinding processes. Secondly, the automatic bag applicator is recommended to reduce the manual work and dust exposure for packer operators and stevedores. Thirdly, a complete dust management plan should be implemented to enhance the air quality in the workplace environment and prevent health effects on cement workers. The elements of a dust management plant include surveillance of dust source, dust measurement and monitoring, health surveillance, maintenance of dust collector and relevant equipment, housekeeping, training, working procedures, and provision of personal protective equipment.