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

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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.

Introduction

Workers in a variety of industries are being excessively exposed to respirable crystalline silica because of many materials containing it. The US National Institute for Occupational Safety and Health (NIOSH) indicates that more than 1.7 million US workers are potentially exposed to respirable crystalline silica [1]. In Taiwan, although the actual number of workers exposed to respirable crystalline silica remains unknown, the silicosis is rated the most prevalent occupational disease among all industries. With the exception for the mining industry, the refractory material manufacturing industry has the second largest number of workers with silicosis in all industries [2]. An epidemiological study conducted on 1022 male refractory brick workers employed for at least 6 months between 1954 and 1977 yielded a standard mortality ratio (SMR) of 1.51 (95% CI = 1.04–2.12) for respiratory tract cancers [3]. A study conducted in China on 6266 silicotic and nonsilicotic refractory brick workers employed before 1962 and followed for mortality from 1963 to 1985 found a standardized rate ratio (SRR) of 2.1 (no 95% CI was provided) for silicotic refractory brick workers [4]. Most importantly, crystalline silica has been confirmed as human carcinogen by IARC [5].

It is known that the refractory materials have been widely used in the metallurgical, foundry, and municipal waste incinerators for furnace lining purposes. Therefore, it is expected that furnace relining workers might be highly exposed to crystalline silica. But to the best of our knowledge only two case reports can be found in the literature. The first one was conducted on two metallurgical furnace relining workers, one mainly used a jackhammer for removing the old refractory lining material and the other conducted less jackhammering but focused more on collecting and dumping the pieces and chunks [6]. Results show that both workers’ exposure levels were 1.23 and 2.52 times in magnitude of the US OSHA time-weighted average permissible exposure limit (PEL-TWA = 10 mg/m3/(%crystalline silica + 2)) for respirable crystalline silica, respectively. The second study was conducted on one foundry worker while conducting the pneumatic chipping and mixing of the refractory materials for relining ladles [7]. Result shows that his respirable crystalline silica exposure level was ∼2.74 times in magnitude of the US OSHA PEL-TWA. The above two case studies clearly indicate that furnace relining workers’ respirable crystalline silica exposures could be very significant.

In Taiwan, the governmental labor statistics reveals that there are ∼500 furnace relining workers currently being employed by 3 main contractors. Furnace relining workers can be divided into 7 exposure groups according to their work tasks. These include the bottom ash cleaning (for manually grabbing and tossing the bottom ash into a dumpster), scaffold establishing (for setting up the scaffold inside the furnace for the convenience of conducting following work tasks), sand blasting (for removing ash coated on the furnace wall by using the blasting sand and the cleaning of the fly ash from air pollution control devices), wall demolishing (by using a jackhammer for removing damaged refractory material), relining (including the mixing and patching the new refractory materials on the damaged furnace wall), grid repairing (for replacing or repairing the damaged furnace grids by welding), and others (for supervising the whole relining process). Although workers worked multiple projects concurrently, workers of the same exposure groups were specified to perform the same work tasks in different projects. On average all relining workers conducted furnace relining related work tasks for ∼250 workdays per year as reported by the three contractors.

The first objective of this study was set out to assess exposure levels of furnace relining workers of different exposure groups of different work tasks. Considering workers of different exposure groups might be exposed to several pollutant sources simultaneously, the second objective of this study was to identify their main pollutant sources and to propose effective control strategies for exposure reductions for each exposure group.

Section snippets

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

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

Discussion

In Table 2 we found that the respirable dust exposure profiles for the 7 selected exposure groups were all log-normally distributed. The above results suggest that workers of each exposure group might have experienced to a very similar exposure scenario. The above inference can be confirmed through field observations (i.e., workers of each individual exposure group did conduct the same work tasks and were exposed to the same pollutant sources). For the same reason, it is not so surprising to

Conclusions

In this study, we found that workers in each selected exposure group did share a very similar exposure scenario, and hence their respirable dust and respirable quartz exposure levels can be characterized by using a log-normal distribution. Workers of different exposure groups exposed to respirable dusts with different exposure levels could be explained by the intrinsic difference in their involved work tasks. But the differences in their respirable quartz exposure levels were not only resulting

Acknowledgment

The authors wish to thank the Institute of Occupational Safety and Health (IOSH) of the Council of Labor Affairs in Taiwan for funding this research project.

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