Occupational exposure to respirable crystalline silica in municipal household waste collection and road cleaning workers

ABSTRACT

Despite the increase in the number of cases among South Korean sanitation workers, lung cancer as a result of exposure to occupational carcinogen has not been sufficiently investigated. This study aimed to identify exposure levels of sanitation workers to respirable crystalline silica (RCS) for various tasks and factors that affect individual RCS exposure. Exposure to RCS was assessed for 90 sanitation workers from seven companies. The obtained geometric mean value of the RCS was 2.6 µg m–3, which is a similar level to recommendations set by California’s Office of Environmental Health Hazard Assessment's Recommended Exposure Limit. Meanwhile, coal briquette ash (CBA) collectors exhibited the highest RCS concentration (24 µg m–3), followed by road cleaning workers who used a blower, municipal household waste collectors, sweepers, and drivers (p < 0.05). Additionally, when the ANOVA was conducted, statistically significant differences were observed in RCS concentrations among various factors such as job task, season, employment type and city scale. Our study confirmed that sanitation workers who work outdoors could be exposed to RCS. Due to the possibility of exposure to high RCS concentrations, special attention should be paid to the collection of used CBA and road cleaning involving the use of a blower.

METHODS

Task description by companies

The investigated scenario was as follows: Companies A, B, and E collect the MHW. One team of two MHW workers and a driver collects waste while traveling in the specified area. The workers move by hanging onto the back of a truck and running for short distances. The collected waste is thrown to the press roll space at the rear end of the truck and moved into the truck container while being pressed and crushed. While the majority of waste is contained in plastic bags, CBA can also be found within a plastic bag adjacent to 4–6 units or stacked on the roadside without plastic bags. Once pressed, the CBA is crushed, and the dust is scattered into the air.

Company A and Company B collected approximately 100 used coal briquettes, while Company E collected 300 on the measurement day.

In the local government where Companies B and E are located, CBA is collected and buried with general waste. In contrast, in the local government where Company D is located, CBA is collected separately and sent to coal briquette recycling facilities where it is used as an industrial resource. Thus, in Company D, a team dedicated to the collection of CBA was operating and collected CBA contained in plastic bags using an ordinary truck without a press roll while visiting sites in the area for approximately four hours. They manually removed the CBA from the plastic bags in a designated location and placed this onto a conveyor belt to allow it to be stored in a container because only CBA can be inserted into the recycling facility. The task to transfer CBA into a container took approximately 40 min, during which time the CBA was crushed, and the resulting dust was scattered into the air in large quantities.

Street cleaners pick up trash using tongs and sweep foreign matters, such as sand, soil, and fallen leaves while walking alongside the road or sidewalk.

The tasks conducted at Company C include leaf blowing on the sidewalk using an air blower for 20–30 min/day, and then performing typical street cleaner tasks for the remainder of the day.

Company F is the street cleaner of a small local city. Due to the hilly topography, the company applies sand and calcium chloride to the road in early winter to prevent cars from skidding due to snow and rain. Then, sand and calcium chloride are removed using air blowers and sweeping for approximately 40 days in the spring when snowfall stops.

Company G also measured the “dedicated CBA collector”. The difference between Company D's “dedicated CBA collector” is that Company G manually loads (throws) briquettes on the roadside into truck. Then, the collected CBA are automatically removed from the truck into unused rice fields.

In contrast, in this study, there were as few as four dedicated CBA collectors. This is because many local governments do not operate dedicated CBA jobs, and CBA workers were difficult to recruit as research subjects. Table 1 shows the summary of study companies, job task, and number of RCS samples. Figure 1 illustrates the various tasks of the sanitation workers used in this study.

Exposure group selection and evaluation of the exposure levels of TD, RD, and RCS

Seven companies were selected as our study targets from various areas in South Korea. The detailed job tasks (occupations) were MHW workers, collection vehicle drivers, sweepers (fallen leaves and sand blowing), and CBA collectors.

The total dust (TD), respirable dust (RD), and RCS concentrations exposed to a worker during the daily working hours were evaluated based on NIOSH 0500, 0600, and 7500 methods, respectively. TD and RD were collected at the same time of day using the GilAir Plus personal pump and dual port manifold. TD was collected at a flow rate of 1.5 lpm using a three-piece cassette (SKC Cat No. 225-2LF) equipped with a PVC filter paper (SKC Cat No. 225-5-7) with a 37 mm diameter and a 5 µm-pore size. RD was also collected at 2.5 lpm using an aluminum cyclone (SKC Cat No. 225-01-02) equipped with a PVC filter paper, which was stored in a desiccator for 24 h before and after measurements to remove moisture. Then, the PVC filter was weighed three times using an electronic balance (Mettler-Toledo XP26, Resolution 0.001 mg)to calculate the average value (gravimetric analysis).

A PVC filter that collected RD following the gravimetric analysis was re-deposited in a silver filter in accordance with the NIOSH 7500 method, while X-ray diffraction (XRD) was used to analyze the RCS. For RCS, only quartz was detected, and the quartz concentration was quantified using the height of the peak rather than its area (to avoid interference from mullite and feldspar contained in the coal briquettes).The certified standard NISTSRM1878a was employed to obtain the calibration curve for the quartz concentration.The analysis took place at the Institute of Occupation and Environment, Korea Workers’ Compensation and Welfare Service, participating in the American Industrial Hygiene Association Proficiency Analytical Testing program. The analytical detection limit of quartz was 0.0025 mg/sample and the detection limit of the gravimetric analysis was 0.009 mg/sample.

Companies A and B collected RD using a disposable respirable PPI (parallel particle impactors, Cat No. 225-387), which collects air at a flow rate of 4 L/min, to increase the air sampling amount to improve the detection limit during silica analysis. Therefore, TD was not collected.

Statistical analysis

In the RCS sample, 34 of the total 90 samples were not detected. When probability plots were drawn using the time-weighted average (TWA) values of RCS, RD, and TD, they were determined as right-skewed. When normality was examined at the 5% significance level by conducting Kolmogorov–Smirnov analysis, none of the three factors exhibited a normal distribution. After converting all data into natural log for statistical analysis, excluding non-detected samples, the Kolmogorov–Smirnov analysis results showed that two factors exhibited a lognormal distribution and the RCS samples showed linearity in a log probability plot. In addition, the RCS content (%) of RD was calculated and the following analysis was performed.

The geometric mean (GM), geometric standard deviation (GSD), and minimum and maximum values were calculated using descriptive statistics. All results were classified into environmental and occupational variables, such as the job task (collector, driver, street cleaner, blower, and CBA collector), season (spring, autumn, and winter), employment type (directly employed vs. employed by subcontractors), and city size (large vs. small cities). Table 2 shows the sample size of RCS by different variables. In the analysis of variance (ANOVA) analysis, the undetected RCS samples were converted to 0.001 mg/m3, which is the background concentration of quartz in the air. An ANOVA and Turkey’s HSD post-hoc test were conducted to examine whether there was a significant difference in TD, RD and RCS mean concentration and RCS content depending on the occupational and environmental variables. In addition Welch's tests were conducted when the homogeneity assumption of variance was violated.

After converting all data into natural logs for Pearson correlation analysis, the results of the Shapiro–Wilk analysis showed that three factors, TD, RD, and RCS, exhibited a lognormal distribution. Pearson’s correlation analysis was conducted to examine the correlations among the RCS, RD, and TD exposure levels.

SPSS 23.0 software (IBM, Armonk, NY, USA) was used for statistical analysis, and figures were prepared using Origin 2016 software (OriginLab Co., Northampton, MA, USA).

Ethical consideration

An ethical review and approval was not required for this study as this study did not involve humans or animals. This manuscript is only a study on the characteristics of dust in the air exposed to workers, and does not contain personal information, such as the age and gender of workers.
RESULTS

RCS, RD, and TD exposure levels of sanitation workers

From various sanitation worker tasks, 90 RCS and RD samples together with 30 TD samples were collected. Table 3 shows the 8-h TWA concentrations of TD, RD, and RCS for each company. For RCS, 34 samples were less than the analytical detection limit. These samples were converted to 0.001 mg m–3, which is the value obtained by dividing the analytical detection limit of 0.0025 by 2.

For the TD samples, the concentration ranged from 0.014 to 13.460 mg m–3, and the GM was 0.278 mg m–3. For the RD samples, the concentration ranged from 0.004 to 0.499 mg m–3, and the GM was 0.053 mg m–3. For the RCS samples, the concentration ranged from 0.001 to 0.024 mg m–3, and the GM was 0.0026 mg m–3, which is approximately four times higher than the background concentration in the atmosphere.

When the ANOVA was conducted, statistically significant differences were observed in RCS concentrations among the companies (p < 0.001).

Relationships between RCS concentrations and various exposure factors

Table 4 shows the GM concentrations of TD, RD, and RCS according to the environmental and occupational factors of sanitation workers. CBA collectors exhibited the highest RCS GM concentration (0.0073 mg m–3), followed by workers who blow fallen leaves and sand using a blower (0.0051 mg m–3), collectors, street cleaners, and drivers (0.0026, 0.0017, and 0.0015 mg m–3) (p < 0.001). The GM concentration of the collectors was 2.6 µg m–3. However, 10 out of 21 samples exhibited values higher than 4 µg m–3, and the maximum concentration was 15 µg m–3, which was two-thirds the level of ACGIH TLV.

Figure 2 shows the RCS exposure levels of sanitation workers by work content in more detail than the results presented in Table 4. Collectors were subdivided into general, food, and recycling waste collectors. The blower task was divided into fallen leaves blowing and sand blowing (the mean concentration in Fig. 2 represents the arithmetic mean concentration). This indicates that the job task (work content) factor affected the levels of silica exposure experienced by sanitation workers.

The RCS GM concentration in winter was 0.0040 mg m–3, which was significantly higher than that in spring (0.0021 mg m–3) (p = 0.019). Workers employed by the subcontractors of local governments exhibited significantly greater exposure than those directly employed (0.0025 vs. 0.0031 mg m–3, p = 0.047). In the case of city size, small cities exhibited significantly higher exposure than larger cities (0.0019 vs. 0.0026 mg m–3, p < 0.001).

The RD GM concentrations of sanitation workers were affected by job task, season and employment type factors, but as for TD, the job task, season and city scale factor had a significant influence, while the employment type factor had no impact.

A more detailed description of ANOVA results by variable exposure factors is presented in the Supplementary information.

Correlations between the TD, RD, and RCS concentration

The RCS concentration exhibited significant correlations with the RD and TD concentrations. The Pearson correlation coefficient between RCS and TD was 0.655 (p < 0.01), which was a high value and higher than between RCS and RD, which was 0.213 (p < 0.05), which was at a low level. This implies that when the RCS concentration increases, the concentration of TD, which represents large particles, also increases (Table 5).
