Study Measures Potential Exposures From Sweepers
Following the publication of the OSHA respirable crystalline silica (RCS) standard in March 2016, whether powered industrial sweepers would be allowed in foundries that use silica sand was unclear. Although these are basically vacuums, they generally use brushes to lift particles and push them into the path of the vacuum. This raises the question of how the housekeeping provision of the rule would apply:
(h) Housekeeping: (1) The employer shall not allow dry sweeping or dry brushing where such activity could contribute to employee exposure to respirable crystalline silica unless wet sweeping, HEPA filtered vacuuming or other methods that minimize the likelihood of exposure are not feasible. (29 CFR 1910.1053 (h)(1).
In order to determine whether powered industrial sweepers could contribute to employee exposure, AFS sponsored a research project to measure exposures associated with use of sweepers and the results were shared with OSHA staff. In early October, OSHA agreed to allow the use of industrial sweepers not equipped with HEPA filters, as long as the employer follows other applicable requirements in the standard (e.g., the hierarchy of controls to reduce and maintain exposures below the permissible exposure limit) and maintains the equipment in accordance with manufacturer recommendations. OSHA encourages employers to acquire industrial sweepers equipped with HEPA filters when their existing sweepers need to be replaced.
Several observations and conclusions can be drawn from the study and from committee discussion:
HEPA Filtered Sweepers
OSHA has indicated acceptance of HEPA filtered sweepers, considering them in the same category as HEPA filtered vacuums.
Overall, the study data show a 17% reduction with HEPA filtered equipment. However, the data also demonstrate substantial variability.
Maintenance is a key to the successful operation of HEPA filtered sweepers. To protect the HEPA filter from quickly overloading, a staged filter system is used, requiring frequent attention.
Cleaning the filter can be a dusty process. If compressed air is used, the operation should be performed in conjunction with a ventilation system to capture the dust.
Wet sweepers are allowed by the standard.
Only one set of tests was performed with wet sweeping and the data are not considered to be reflective of wet sweeper performance due to background activities and measurement questions.
Maintenance is important for the water supply and to direct the mist to the proper area of the brush. Too much or too little water can be problematic. Droplet or mist size may also be important to maintain. Some users have found ionized water, such as waste ozonated water from pollution control systems, can help water bind to dust particles.
OSHA has indicated the use of sweepers not equipped with HEPA filters is considered an “other [housekeeping] method” and they are not prohibited by the standard’s housekeeping provisions, as long as they are operated and maintained properly.
Although results vary considerably, data for the 4-hour periods before and after dry sweeping for all sites show that RCS exposures are slightly lower (about 9%) after sweeping.
Poorly maintained sweepers may increase RCS concentrations. Sweepers have filters that are effective at trapping some respirable dust particles, although not as efficiently as a HEPA filter. However, filters can become damaged, seals can become compromised, or hoppers can fill up and release dust. This may be indicated by a visible dust cloud as the sweeper travels.
Industrial hygienists experienced in foundry operations have found sweeper operator exposures to be low compared to other foundry activities. We recommend that sweeper operator exposure levels and visual review of dust generation be used as indicators of sweeper fitness. Foundries that use dry sweepers should measure operator exposure, adhere to manufacturer recommended maintenance practices, and observe sweepers for signs of visible dust generation.
The project involved six foundries with four measurement sites each. Dry sweeping was performed at each site. In addition to testing dry sweeping at all sites, an extra day of wet sweeping was tested at one site and an extra day of HEPA sweeping was tested at three sites. The three major sweeper manufacturers were represented. Testing methods included both traditional time averaged samples and real time measurements.
The exposure of interest was the non-sweeper employee exposure, those employees in the work area whose exposure would be increased or decreased if sweepers were used. The primary research question was whether sweeping, by removing dust loading from the floor, reduced the amount of RCS that would be generated by forklift and pedestrian traffic.
Results were widely variable and dependent on numerous process and operational factors and confounders. As demonstrated by the real time respirable dust exposure data from Site C, the foundry is a dynamic environment with many potential dust sources in the background that can mask the effect of sweeping. It is difficult to tease out the contribution of fork lift traffic traveling over a dirty/clean floor from the contribution of other dust producing activities.
The difficulty of precisely identifying the sweeper effect is demonstrated in the chart in Figure 1, from Site C of the study. It is possible process-related dust-producing activities may account for the slight increase in average dust level in the four hours before and four hours after sweeping (28.5 vs 26.9 µg/m3). A qualitative review of the dust data reveals upward trending of dust levels throughout the shift except for sharp reductions corresponding to the 8:30 a.m. break time for grinders and the 11 a.m. lunch break. The slightly higher concentration for the post-sweeping period could be due to the normal increase in levels throughout the day rather than to the sweeper use. Similarly, a qualitative review of data for the 13-minute period of sweeping activity shows a decline in dust levels from beginning to end of the sweeper period. Nevertheless, the overall quantitative average for that period (27.5 µg/m3) is slightly higher than average for the pre-sweeping period. As this example shows, it is difficult to extract the contribution of the sweeper effects from the overall exposure data.
Despite the difficulty in extracting sweeper related effects from general dust data, it is possible to make some limited overall observations.
Dry Sweeping With Powered Industrial Sweepers
Table 1 shows the data for the 4-hour periods before and after dry sweeping for all sites. Although results vary considerably, the RCS exposures are slightly lower (about 9%) after sweeping.
In addition, some important qualitative observations can be made. Poorly maintained sweepers may increase RCS concentrations. For example, the sweeper at Site F had some problems, not only as demonstrated by the data, but also as apparent to the people on site who observed a visible dust cloud as the sweeper travelled. Sweepers have filters that are effective at trapping some respirable dust particles, although not at the 99-plus % level as a HEPA filter. Indeed, the dirtier the filter the more effective is the fine particle capture efficiency. However, filters can become damaged, seals can become compromised, or hoppers can fill up and release dust.
While not the focus of, nor measured in this research project, industrial hygienists experienced in foundry operations have found sweeper operator exposures to be low compared to other foundry activities. We recommend that sweeper operator exposure and visual review of dust generation be used as indicators of sweeper fitness. Foundries who use dry sweepers should measure operator exposure, adhere to manufacturer recommended maintenance practices, and observe sweepers for signs of visible dust generation.
HEPA Filtered Sweepers
Three test sites deployed HEPA filtered sweepers in an additional round of testing. The results for HEPA equipped sweepers are shown in Table 2. Overall, the data show a 17% reduction with HEPA filtered equipment.
However, the data also demonstrate variability. Some of the factors that may account for variability, in addition to the process issues discussed above, are the low background levels at some of the sites. For example, the pre-sweeping level at site A location was below the detection limit, so a value of ½ the detection limit (i.e. 6 µg/m3) was used for calculations. The post sweeping measurement resulted in a 287% increase over pre-sweeping. However, it is important to note that the increase may have been due to background process related build up during the day, and that level was still below the Action Level.
By way of qualitative observation, it is important to note that maintenance is also key to the successful operation of HEPA filtered sweepers. To protect the HEPA filter from quickly overloading, a staged filter system is used, requiring frequent attention. Cleaning the filter can be a dusty process. If compressed air is used, the operation should be performed in conjunction with a ventilation system to capture the dust.
Only one set of tests was performed with wet sweeping. The data in Table 3 show a 13% increase in dust exposure after wet sweeping, but that result is not reflective of wet sweeper performance. A review of the real time data shows a cyclic pattern of background events with one event in the pre-sweeping period and three other events in the post-sweeping period, likely accounting for the data results. In addition, the possibility should be noted that the real time monitor readings may have been affected by fine water particles.
As with dry and HEPA filtered sweepers, some qualitative observations can be made about wet sweepers. Maintenance is important to maintain the water supply and direct the mist to the proper area of the brush. Droplet or mist size may also be important to maintain. Some users have found that ionized water, such as waste ozonated water from pollution control systems, can help water bind to dust particles.
For all powered industrial sweepers (HEPA or non-HEPA), the employer using the sweeper must ensure compliance with all applicable provisions of the silica standard, including the PEL. Thus, the employer must assess the exposures of employees operating or working in the vicinity of the sweeper in accordance with paragraph (d) of the standard. And if any of those employees are exposed to silica levels above the PEL, the employer must use feasible engineering and work practice controls to reduce and maintain each employee’s exposure to or below the PEL in accordance with paragraphs (c) and (f)(1) of the standard. Such controls could include appropriate modifications to the sweepers (e.g., installing a shroud around the bottom of the equipment to limit escaping dust) or establishing new, appropriate work practices (e.g., limiting the speed of operation). If feasible engineering and work practice controls are not sufficient to reduce exposures to or below the PEL, the employer must use them to reduce employee exposure to the lowest feasible level and provide appropriate respiratory protection. See 29 C.F.R. § 1910.1053(f)(1).
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