National Institute for Occupational Safety and Health
IN-DEPTH SURVEY REPORT OF A LOCAL EXHAUST VENTILATION DEVICE FOR SUPPRESSING RESPIRABLE AND CRYSTALLINE SILICA DUST FROM POWERED SAWS
REPORT WRITTEN BY:
REPORT DATE: May, 2006
REPORT NO: EPHB 317-12a
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
SIC CODE: 1761
SURVEY DATE: July 26-28, 2005
SURVEY CONDUCTED BY:
EMPLOYER REPRESENTATIVE CONTACTED:
EMPLOYEE REPRESENTATIVE CONTACTED:
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.
Respirable dust from the trials ranged from 0.13 to 6.59 mg/m3 with an average of 0.84 mg/m3 for roofers and from 0.45 to 3.82 mg/m3 with an average of 2.01 mg/m3 for cutters/roofers. The respirable dust exposures for all cutters/roofers indicated concentrations exceeding the OSHA PEL; it was also exceeded for some of the roofers. The respirable silica concentrations ranged from 0.04 to 0.15 mg/m3 with an average of 0.09 mg/m3 for roofers, and from 0.13 to 1.21 mg/m3 with an average of 0.48 mg/m3 for cutters/roofers. As with respirable dust, the respirable silica exposures to cutters/roofers were higher than the exposures for roofers.
In general, higher respirable dust and respirable silica exposures were observed for the saw operators than for other roofers. However, all workers were overexposed to respirable dust and respirable silica for at least one monitored shift. The use of the engineering control alone is not sufficient for reducing exposures below acceptable occupational exposure levels. Redesign options for the control should be considered. In the meantime, work practice modifications, administrative controls, and a comprehensive respiratory protection program should be implemented in order to control respirable dust and respirable silica exposures.
The Engineering and Physical Hazards Branch (EPHB) of the Division of Applied Research and Technology (DART) has been given the lead within NIOSH to study and develop engineering controls, and assess their impact on reducing occupational illness. Since 1976, EPHB (and its predecessor, the Engineering Control Technology Branch) has conducted a large number of studies to evaluate engineering control technology based upon industry, process, or control technique. The objective of each of these studies has been to evaluate and document control techniques and to determine their effectiveness in reducing potential health hazards in an industry or for a specific process.
The goal of this project was to quantify the exposure to crystalline and respirable silica when using a powered saw with aftermarket local exhaust ventilation while cutting concrete roofing tiles. In this case, the local exhaust ventilation consisted of a shroud attached to the cutting section; the shroud was connected to a small electric axial fan through flexible hoses. The dust was then collected in a dust bag for removal. The system was permanently attached to the saw.
Occupational Exposure to Crystalline Silica
When proper practices are not followed or controls are not maintained, respirable crystalline silica exposures can exceed the NIOSH Recommended Exposure Limit (REL), the OSHA Permissible Exposure Limit (PEL), or the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) [NIOSH 2002, ACGIH 2001]. NIOSH recommends an exposure limit of 0.05 mg/m3 to reduce the risk of developing silicosis, lung cancer, and other adverse health effects.
If respirable dust contains more than 1% silica, the OSHA PEL for respirable dust exposures in general industry can be calculated as shown in the equation below:
where % silica refers to the percentage of silica by mass contained in an 8-hour sample [29 CFR 1910.1000]. By this equation, the respirable dust PEL is dependent upon the percentage silica contained in the dust. The PEL ranges from 5 mg/m3 if the dust contains no silica, to 0.1 mg/m3 if the dust is 100% silica.
The current OSHA permissible exposure limit (PEL) for respirable dust containing crystalline silica (quartz) for the construction industry is expressed in millions of particles per cubic foot (mppcf) and is calculated using the following formula [29 CFR 1926.55]:
Since the PELs were adopted, the impinger sampling method that was used to evaluate silica exposures in mppcf has been rendered obsolete by gravimetric sampling [OSHA 1996]. OSHA is not aware of any government agencies or employers in this country that are currently using impinger sampling to assess worker exposure to dust containing crystalline silica, and impinger samples are generally recognized as being less reliable than gravimetric samples [OSHA 1996]. OSHA currently instructs its compliance officers to apply a conversion factor of 0.1 mg/m3 per mppcf when converting between gravimetric sampling and particle count standard when characterizing construction operation exposures [OSHA 2001].
The ACGIH® TLV®s for cristobalite, quartz, and tridymite are all 0.05 mg/m3 [ACGIH 2004]. The ACGIH® has published a notice of their intent to change the TLV® for α-quartz and cristobalite (respirable fraction) to 0.025 mg/m3 and to withdraw the documentation and adopted TLV® for tridymite [ACGIH 2004].
Construction sites were identified through contacts between NIOSH and the inventor of the engineering control, a roofing contractor. A site was selected if a minimum of 4 hours of activities was established for any set day. A typical sampling day was usually 8 hours of sampling, including 30 minutes for a lunch break. Subject participation was voluntary.
In order to avoid overloading the filter media, the full shift sampling was split into two samples per worker per shift; one sample was collected during the morning and a second during the afternoon. To allow comparison to OSHA, NIOSH, and ACGIH standards, the results of the morning and afternoon were combined to generate an 8-hour time weighted average (TWA) exposure to respirable dust or respirable silica. The average percent quartz of the two samples was calculated to determine the appropriate OSHA PEL using the following equation:
as recommended by OSHA, where Quartz1 and Quartz2 are the mass of quartz in samples 1 and 2, respectively, and Dust1 and Dust2 are the total mass of respirable dust in samples 1 and 2 respectively [OSHA 1996].
Gravimetric analysis for respirable particulate was carried out with the following modifications to NIOSH Method 0600: 1) the filters and backup pads were stored in an environmentally controlled room (20±1 ºC and 50±5% relative humidity) and were subjected to the room conditions for at least two hours for stabilization prior to tare and gross weighing, and 2) two weighings of the tare weight and gross weight were performed [NIOSH 1994]. The difference between the average gross weight and the average tare weight was the result of the analysis. The limit of detection for this method was 0.02 mg.
Crystalline silica analysis of filter and bulk samples was performed using X-ray diffraction. NIOSH Method 7500 was used with the following modifications: 1) filters were dissolved in tetrahydrofuran rather than being ashed in a furnace; and 2) standards and samples were run concurrently and an external calibration curve was prepared from the integrated intensities rather than using the suggested normalization procedure [NIOSH 1994]. These samples were analyzed for quartz and cristobalite. The limits of detection for quartz and cristobalite on filters were 0.01 and 0.02 mg, respectively. The limit of quantitation is 0.03 mg for both quartz and cristobalite. The limits of detection in bulk samples were 0.8% for quartz and 1% for cristobalite. The limit of quantitation was 2% for both forms of crystalline silica in bulk samples.
For the two roofers who were mainly responsible for cutting the tiles, personal breathing zone respirable dust sampling was conducted concurrently using an active sampling portable laser photometer (MIE personal DataRAM model pDR1200, Thermo Electron Corporation, Waltham, MA). The direct reading instruments were zeroed at the beginning of each sampling period according to the manufacturer’s recommendations. By fitting the Personal DataRAM (pDR) with a BGI4L metal cyclone (BGI Inc. Waltham, MA) dust pre-selector, the photometer was used to determine the concentration of respirable dust.
Downstream from the photometric sensing stage, a 37 mm PVC filter with 5 micrometer pore size was also fitted to the photometer and connected via Tygon tubing to a sampling pump calibrated to a flow rate of 2.2 liters/minute to provide simultaneous collection of the analyzed dust. Real-time sampling was stopped during the workers’ lunch break to limit the analysis to time spent performing roofing activities. The filter samples from the pDRs were analyzed for respirable dust and for silica using the modified NIOSH analytical methods previously discussed. The results of the PDR filter samples were used to compare real-time and filter-based exposure estimates and to calculate the respirable silica content of the real-time exposures.
The pDRs measured respirable dust concentrations once per second. Because of data storage limitations of the pDRs, the respirable dust concentration readings were averaged over a 5 second interval and logged. The logged data was used to calculate 8-hour TWA respirable dust exposures. The real-time data was also overlayed onto the video recordings of the roofers cutting and laying tiles. This technique, Video Exposure Monitoring (VEM), provided a visual representation of the respirable dust concentration during roofing activities in the form of a bar on the side of the image. The VEM was then used to identify the tasks or work practices that resulted in high exposures to respirable dust.
In order to isolate exposures from the use of the saw, the logged pDR data was also paired with the video recordings of the work shift and used to determine task-based exposure levels. A taskbased TWA exposure was calculated by excluding times that the task was not performed. This process effectively removed potential confounding exposures to provide a better description of the effectiveness of the local exhaust ventilation (LEV) installed on the circular powered saw.
Description of controls
The fan flow rate was obtained in laboratory trials; by using a flow meter and mass flow sensor installed (Sierra Flo-Box series 900 model 904M flow meter and Sierra model 730-N5-1 sensor, Sierra Instruments Inc., Monterey, CA) at the outlet of the fan (with the collection bag removed) following a 1-1/2 to 2-in flexible coupling and a 2-ft length of 2-in diameter pipe. The connection between the fan outlet and flexible coupling was wrapped in parafilm to make an airtight seal. The fan was capable of pulling an average flow rate of 1056 liters/minute or 37.3 cubic feet per minute (cfm), with the saw on, no blade and no load on the system. When an attempt was made to repeat this measurement in the field at the end of the work day, dust from the roofing tiles in the air stream prohibited its completion.
The system was activated automatically when the trigger switch on the saw was depressed, and shut down when the trigger was released. The local exhaust ventilation was installed as a permanent attachment which added to the weight of the tool, possibly compromising the manufacturer’s design specifications. In addition, the installation of the dust collector required the saw to be used by the worker pulling the saw toward themselves, necessitating the removal of the blade guard.
Wind and weather measurements
RESULTS AND DISCUSSION
A total of thirty-five samples were collected at three different sampling locations over a three day period. During the installation of the concrete roofing tiles, employees cut the tiles to accommodate the shape of the roof structure (e.g., peaks, hips and valleys). Employees used an electric Skil® saw with the aftermarket LEV control installed to cut the tiles. During cutting activities, exposures to respirable crystalline silica exceeded occupational exposure limits such as the OSHA PEL, NIOSH REL, and ACGIH TLV. The two individuals cutting the tiles had higher levels of exposure than those laying tiles. A leaf blower was used to clean the tiles to prevent staining from water or moisture. This process created a considerable amount, of airborne concrete dust, comparable or even greater than the dust created when cutting the tiles.
Respirable Crystalline Silica and Respirable Dust Exposures
Table 3 illustrates the concentrations of silica dust for roofers and cutters/roofers. Respirable silica concentration 8-hour TWA exposures ranged from 0.02 mg/m3 to 0.51 mg/m3, with an average exposure of 0.28 mg/m3. The exposures to cutters/roofers were higher with an average 8-hour TWA quartz exposure of 0.38 mg/m3, compared to an average 8-hour TWA respirable quartz exposure of 0.07 mg/m3 for the tile layers. All PBZ TWA samples for silica but one exceeded the NIOSH/ACGIH criteria.
The 8-hour TWA exposures to respirable dust were compared to the OSHA PELs, and the respirable quartz 8-hour TWA exposures were compared to the NIOSH REL and ACGIH TLV. A summary of the comparisons is shown in Table 4.
The real-time respirable dust data were also used to calculate 8-hour TWA exposures, which ranged from 0.84 mg/m3 to 2.5 mg/m3. The results of the direct reading sampling are summarized in Table 5.
One hundred percent of the 8-hour TWAs calculated from the direct reading data exceeded exposure limits enforced by OSHA. In general, the TWAs calculated from the direct reading instruments were higher than the corresponding gravimetric sampling results. However, there was no significant difference (p=0.4818) between the average TWA respirable dust exposures calculated from the gravimetric data and the real-time sampling data.
The video exposure monitoring data indicate that the two main respirable dust exposures occurred during tile cutting and tile cleaning tasks. The respirable dust exposure during cutting ranged from 6.04 mg/m3 to 17.40 mg/m3 with a TWA exposure of 11.95 mg/m3. Each cut was approximately 10 to 20 seconds in duration. On average, cutting tasks accounted for approximately 60 minutes (12.5%) of a full, 8-hour work shift. Respirable dust exposures while cleaning the tiles with the leaf blower ranged from 2.16 mg/m3 to 5.38 mg/m3. The TWA respirable dust exposure during cleaning was 3.44 mg/m3. On average, the two saw operators spent 10 minutes using the leaf blower each shift. The mean respirable dust TWA exposures were significantly higher during cutting than during tile cleaning and were significantly higher than the overall TWA respirable dust exposure. Table 6 presents a summary of task-based respirable dust results from real-time monitoring.
The video exposure monitoring images displayed more visible dust during tile cleaning than tile cutting, but the real-time monitoring indicated that higher respirable dust concentrations were observed during tile cutting. These results suggest that most of the dust re-aerosolized by the blower is above the respirable size range.
The control technology examined in this report was evaluated previously at other roofing sites by OSHA (on two occasions), Pinnacol Assurance and Colorado State University (CSU). The results of the personal exposure monitoring performed during these evaluations are reported in Table 7. The OSHA investigations found that the exposure of one of the three employees sampled exceeded the OSHA PEL for silica in construction. Two of the OSHA-measured exposures exceeded the NIOSH REL and ACGIH TLV for crystalline silica as an 8-hr TWA. Neither OSHA report describes the amount of time the employees spent cutting tiles. The OSHA report of the July 31, 2001 investigation does note that the sampling train used to evaluate the employee sampled for 446 minutes was disconnected from the sampling pump for about and hour and fifteen minutes, so the reported results may underestimate that employee’s exposure. The other employee evaluated on that occasion was reported by OSHA to have cut more tiles. Pinnacol Assurance reported that neither of the roofers’ exposures they evaluated exceeded the OSHA PEL, but one employee’s silica exposure did exceed the ACGIH TLV (and NIOSH REL). The Pinnacol Assurance report does not provide any information on work practices, such as the amount of time spent cutting tiles. The evaluation by CSU found that neither of the two employees evaluated were exposed to silica in excess of the OSHA PEL. However, one of their exposures was slightly in excess of the NOISH REL and one was slightly below. The CSU report notes that cutting tile was estimated to represent less than 25% of the roofers’ activities. One explanation for the differences among these results and between these results and the NIOSH evaluation is that the amount of time spent cutting tile (or using the leaf blower) varied from job to job.
Bulk crystalline silica sampling results
Wind and weather results
Conclusions and recommendations
As mentioned in the results and discussion section, a leaf blower was used to remove dust from the tiles. This process should be carefully reviewed as it creates an unnecessary exposure to all employees, whether they cut or lay the tiles. Researchers recommend investigating the use of alternative means of cleaning the tiles without making particles airborne. The use of manual cutters has proven to reduce exposures below the OSHA PEL and the NIOSH REL. Researchers recommend the further investigation of manual cutters or improved local exhaust ventilation. The results indicate that, in general, workers will need to use respiratory protection when they are using the saw and/or the leaf blower.
Tile cutting activities produced higher TWA concentrations of respirable dust than leaf blowing activities. Therefore, work practice modifications and administrative controls should target these two activities. Work practice modifications could include a greater awareness of positioning when cutting tiles; workers should be encouraged to stay upwind of the cutting and cleaning operations whenever possible.
All of the workers were exposed to respirable crystalline silica in excess of the NIOSH REL. While some of the tile layers’ exposures to crystalline silica can be attributed to cleaning activities, working in close proximity to tile cutting also contributed to their exposures. At times, both of the tile saws were operating in the same area, providing a dual source of exposure to the roofers. Work practice changes such as distancing the saw operators from each other and from other roofers may also reduce exposures.
Both of the ventilated saw operators were overexposed to respirable dust and respirable silica during each shift observed. All of the roofers, including roofers with mainly tile laying responsibilities, were sometimes overexposed to silica. The use of the tile saw equipped with local exhaust ventilation did not reduce exposures to respirable dust and silica below occupational acceptable levels. The local exhaust ventilation control should be modified to effectively control exposures to dust, or the company should seek other engineering controls proven to reduce exposures below occupational limits. In the meantime, work practice modifications, administrative controls, and a comprehensive respiratory protection program should be implemented in order to control respirable dust and respirable silica exposures.
Table 1: Experimental Design
B05-498 Excluded: filter cassette disconnected from respirable dust pre-separator during sampling
Table 3: Respirable Silica Calculations
+ = Exceeded Criteria
Table 4: Comparison of 8-hour TWA Exposures to Standards
Table 5: 8-hour TWA Respirable Dust Exposure Calculated from Direct Reading Data
a The 8-hour TWA respirable dust exposure for Saw Operator #2 on 07/28/2005 only includes afternoon sampling data. Due to a pump malfunction, the PDR operated in a passive sampling capacity during the morning collection period. This morning collection period was excluded from the analysis.
Table 6: Summary of Task-Based Respirable Dust Results from Real-Time Monitoring
Table 7: Summary of Previous Saw Evaluations
Mention of company names or products does not constitute endorsement by the Centers for Disease Control and Prevention. The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.
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