Exposure profiles and source identifications for workers exposed to crystalline silica during a municipal waste incinerator relining period

ABSTRACT

In this study, respirable crystalline silica exposures to furnace relining workers of 7 exposure groups were assessed by conducting personal respirable dust samplings. All possible pollutant sources were identified for each exposure group through field observations, and bulk samples were randomly collected from each identified pollutant source. All collected samples were analyzed for their tridymite, cristobalite, and quartz contents by using the X-ray diffraction method. Results show that quartz was the only detectable crystalline silica content. We found that the resultant respirable quartz exposure levels presented in sequence for the 7 exposure groups (sand blasting > bottom ash cleaning > wall demolishing > relining > others > grid repairing > scaffold establishing) were different from that of the corresponding respirable dust exposure levels (bottom ash cleaning > wall demolishing > sand blasting > relining > grid repairing > scaffold establishing > others). 87.3–100% of workers’ respirable quartz exposures of the 7 exposure groups exceeded the TLV-TWA (0.025 mg m−3) indicating appropriate control strategies should be taken immediately. By comparing the fractions of quartz contained in personal respirable dust samples with that contained in all possible pollutant sources for each exposure group, this study identified main pollutant sources for each exposure group as: bottom ash cleaning and scaffold establishing: bottom ash; sand blasting: blasting sand; wall demolishing: refractory cement + wall ash; wall relining: refractory brick; grid repairing: wall ash + refractory cement; grid repairing: wall ash + refractory cement; others: blasting sand + bottom ash. Finally, effective control strategies were proposed for exposure reduction by using above information together with our field observations.

MATERIALS AND METHODS

Sampling strategy and sample analysis

The whole study was conducted in a municipal waste incinerator during its annual furnace relining period. All workers in each exposure group were selected for conducting personal respirable dust samplings. A total of 58 workers were selected from the 7 selected exposure groups, including the bottom ash cleaning (n = 7), scaffold establishing (n = 6), sand blasting (n = 8), wall demolishing (n = 8), relining (n = 9), grid repairing (n = 13), and others (n = 7). This task-based approach has been used to assess workers’ exposures for industries with dynamic occupational settings, such as constructions, geotechnical laboratory workers, slate industry, and quartz manufacturing. The sampling train consisted of a high flow sampling pump (GilAir/Clock. Part No. 800508, Gillian Instrument Co., MA, USA), and a respirable dust cyclone (Part No. 456243, MAS Inc., PA, USA) followed by a 37-mm filter cassettes (Cat. No. 225-1. SKC Inc., PA, USA) with a PVC filter (Cat. No. P-503700, Omega Specialty Instrument Co., MA, USA). The sampling flow rate was set at ∼1.7 L/min and was checked periodically throughout the entire sampling period (i.e., one work shift = ∼8 h).

In this study, all possible pollutant sources for each selected exposure group were identified based on our field observation, particularly the observation of their designated work tasks and previous work tasks conducted by the former exposure group. Table 1 shows all possible pollutant sources for the 7 selected exposure groups. For each identified possible pollutant source, bulk samples were randomly collected from the field. A total of 24 samples were collected, including the bottom ash (n = 3), blasting sand (n = 3), wall ash (n = 9; including upper wall ash (n = 3), middle wall ash (n = 3), and lower wall ash (n = 3), refractory brick (n = 3), refractory cement (n = 3), and fly ash (n = 3).

In this study, all personal samples were analyzed for determining their respirable dust concentration per NIOSH Method 0600. Both personal samples and bulk samples were analyzed for their crystalline silica contents (including tridymite, cristobalite, and quartz) by using the X-ray diffraction per NIOSH Method 7500. This study yields method of detection limits (MDLs) of 0.020, 0.018, and 0.008 mg for tridymite, cristobalite, and quartz, respectively.

Data analysis

Characterizing exposure profiles

In this study, the method adopted to characterize the exposure profile was based on the method recommended by the American Industrial Hygiene Association (AIHA) Exposure Assessment Strategies Committee. Therefore, the log-normality, the average exposure level and its corresponding 95% confidence interval for each selected exposure group were calculated. The log-normality of the exposure profile for each exposure group was examined by using the W-test as suggested by Gilbert. The arithmetic mean was used to describe the average exposure for a given exposure profile, since the value is directly related to its average and cumulative doses. The minimum variance unbiased estimate (MVUE) method was used to estimate the arithmetic mean (AMMVUE) and the resultant value was used to compare with the selected occupational exposure limit. This method is suitable for sample sizes from 5 to 500 with geometric standard deviations (GSDs) from 2 to 5. Detailed calculating procedures for both AMMVUE and its 95% confidence interval have been described by Attfield and Hewett. For each exposure profile, the point of estimate for the fraction of exposures exceeding the selected occupational exposure limit was calculated according to the method suggested by Hewett and Ganser.

Identification of main pollutant sources for each selected exposure group

In this study, the fractions of crystalline silica contained in all pollutant sources (by weight) were used to compare with that contained in personal respirable dust samples (by weight) to further determine the main pollutant sources for each exposure group to initiate effective control strategies for exposure reduction. Yet, it is true that the fraction of crystalline silica contained in the bulk samples of each possible pollutant source might not be exactly the same as that contained in the exposed respirable dusts, since we did not measure the respirable fraction of the collected bulk sample. But we assumed that the fraction of crystalline silica contained in the respirable dusts would be proportional to that contained in total dusts of each collected bulk sample. Based on this, we further assumed that the closer of the fraction of crystalline silica contained in a possible pollutant source to that contained in the exposed respirable dusts would had a greater contribution to the exposures of the given exposure group.

RESULTS

Exposure profiles for workers exposed to respirable dusts

Table 2 shows exposure profiles of the respirable dust for the 7 selected exposure groups. We found that all resultant exposure profiles were log-normally distributed. The magnitude of the resultant respirable dust exposure levels for the 7 selected exposure groups in sequence were: (1) bottom ash cleaning 9.21 mg m−3, (2) wall demolishing 2.72 mg m−3, (3) sand blasting 1.84 mg m−3, (4) wall relining 1.21 mg m−3, (5) grid repairing 0.934 mg m−3, (6) scaffold establishing 0.840 mg m−3, and (7) others 0.726 mg m−3, respectively. Among them, the top three were significantly higher than the rest four exposure groups. We also found that the exposure profiles of the respirable dust for the above 7 selected exposure groups were consistently in the form of a log-normal distribution with GSDs ranging from 2.25 to 3.06.

Exposure profiles for workers exposed to crystalline silica

Table 3 shows respirable crystalline silica exposure profiles for the 7 selected exposure groups. We found that quartz was the only crystalline silica material containing in all collected samples. The magnitude of the resultant respirable quartz exposure levels in sequence for the 7 selected exposure groups were: (1) sand blasting 0.587 mg m−3, (2) bottom ash cleaning 0.386 mg m−3, (3) wall demolishing 0.116 mg m−3, (4) others 0.082 mg m−3, (5) grid repairing 0.042 mg m−3, (6) wall relining 0.041 mg m−3, and (7) scaffold establishing 0.040 mg m−3, respectively. Among them, the top two were significantly higher than the rest four exposure groups. Again, we also found that the exposure profiles of the respirable quartz for the 7 selected exposure groups were all in the form of a log-normal distribution with GSDs ranging from 2.55 to 3.34. Table 4 shows that 87.3–100% respirable quartz exposures in all selected exposure groups exceeded the current time-weighted average threshold limit value (TLV-TWA = 0.025 mg m−3) set by American Conference for Governmental Industrial Hygienists (ACGIH). The above results suggest that respirable quartz exposures of furnace relining workers were quite severe.

Crystalline silica contents in respirable dusts and possible pollutant sources

Table 5 shows the fractions of quartz contained in respirable dusts for the 7 selected exposure groups. Among them, both exposure groups of the sand blasting (32.4%) and others (12.6%) were significantly higher than the other selected exposure groups (scaffold establishing 4.83%; grid repairing 4.55%; wall demolition 4.42%; bottom cleaning 4.24%, and wall relining 3.38%). The above results suggest workers of different exposure groups were exposed to different pollutant sources.

On the other hand, we also found that quartz was the only detectable crystalline silica in all collected bulk samples. Table 6 shows the fractions of quartz contained in bulk samples collected from all possible pollutant sources. The fractions of the quartz contained in blasting sand (58.9%), bottom ash 11.1% and refractory brick (9.79%) were significantly higher than those contained in fly ash (6.98%), wall ash (5.76%) and refractory cement (3.09%). The above results suggest the existence of intrinsic differences among possible pollutant sources.
