National Institute for Occupational Safety and Health
RESULTS OF A PILOT STUDY OF DUST CONTROL TECHNOLOGY FOR ASPHALT MILLING
Payne & Dolan, Inc.
With assistance from
REPORT WRITTEN BY:
REPORT DATE: October 25, 2004
REPORT NO: EPHB282-11b
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
SIC CODE: 1611 (Highway and Street Construction)
SURVEY DATE: October 7-9,2003
SURVEY CONDUCTED BY:
EMPLOYER REPRESENTATIVE CONTACTED:
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 primary aim of this project is to determine if the engineering controls supplied with new milling machines and operated according to the manufacturers' recommendations are adequate to control worker exposures to respirable dust and respirable crystalline silica (in the front of quartz). The long term goal of this project is to reduce worker exposures to silica by providing data to support the development of a set of best practice guidelines for the equipment if the engineering controls are adequate, or to develop a set of recommendations to improve the performance of controls if they are not adequate.
Many construction tasks have been associated with overexposure to crystalline silica (Rappaport et al. 2003). Among these tasks are tuck pointing, concrete sawing, concrete grinding, and abrasive blasting (NIOSH 2000, Thorpe et aL 1999, Akbar-Kanadeh and Brillhart 2002, Glindmeyer and Hammad 1988). Road milling has also been shown to result in overexposures to respirable crystalline silica (Linch 2002, Rappaport et aL 2003, NJ Dept. of Health and Senior Services 2001). However, all three of the milling studies are limited because they do not provide enough information about the operating parameters and engineering controls present on the milling machines to determine if the . overexposures were due to a lack of effective controls or poor work practices. This study will attempt to fill that knowledge gap.
A variety of machinery and work practices are employed in asphalt pavement recycling, including cold-planers, heater planers, cold-millers, and heater-scarifiers (public Works 1995). Cold-milling, which uses a toothed, rotating drum to grind and remove the pavement to be recycled, is primarily used to remove surface deterioration on both asphalt and Portland cement concrete road surfaces (public Works 1995). The milling machines used in cold milling are the focus of this investigation.
The cold milling work observed during this pilot study was part of the US 12 reconstruction project from CTH KP to STH 19 west in Wisconsin. This was a section of old US 12 that was rehabilitated and will be transferred to the local township. There was a 20 ft. wide concrete pavement placed in the early 1900's. Above that was a layer of crushed aggregate base course of varying thickness; about 0 inches to 10 inches. Above that was varying asphalt pavement thickness of about 8 to more than 12 inches which had been placed at various times in about the last 60 years. The asphaltic pavement was rutted and cracked to varying degrees. The portion of old US 12 from Simpson Road to the south where it meets the new US 12 was re-graded on new alignment; this pavement removal was specified by a Common Excavation item The Common Excavation item specification enables the contractor to remove the pavement in any way they see fit. In this case, the contractor chose to mill the pavement because it was of value as a recycled product. The existing concrete was also removed and disposed of. In the area from Simpson to the north, this section was not re-graded, but the entire existing asphaltic pavement was removed under the item of Removing Asphaltic Surface, This specifies the removal of only the asphaltic pavement. Again, the contractor chose to salvage this pavement and recycle it (Neuhauser 2004).
This study was facilitated by a partnership in cooperation with the National Asphalt Pavement Association that includes milling machine manufacturers, contractors, employee representatives, NIOSH, and other interested parties. One of the milling machine manufacturers, manufacturer A, had initially arranged to perform the pilot study on a 2003 milling machine model with the latest, "state-of-the-ar" water spraying system produced by the manufacturer. The mill is a half-lane milling machine rated at 560HP with a water spray system capable of l5gpm at 200psI. This system was put into production in 2001 - before initial discussions on testing airborne dust concentration.
About one week before the pilot study was to be performed; manufacturer A learned that the 2003 mil would not be available for the test. Instead, an older machine was substituted. The replacement machine was a half-lane milling machine rated at 800 HP with an eight-foot cutter and an older model water system designed for 10gpm at 50psi.
Manufacturer A knew that the lower water flow at much lower pressure would result in higher airborne dust content, hut Manufacturer A was still confident that the dust suppression system even on the older model machines should still be very effective if properly maintained and operated. Therefore Manufacturer A agreed to perform the test (even with the last minute changes) as a means of getting at least a good baseline for this type of airborne dust test.
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.
The OSHA PEL for respirable dust containing 1 % quartz or more in general industry is expressed as an equation (29 CFR 1910.1000):
If, for example, the dust contain no crystalline silica, the PEL is 5 mg/m3, and if the dust is 100% crystalline silica, the PEL is 0.1 mg/m3. For tridymite and cristobalite, OSHA uses half the value calculated using the formula for quartz (29 CPR 1910.1000).
The current OSHA permissible .exposure limit (PEL) for respirable dust containing crystalline silica (quartz) for the construction industry is measured by impinger sampling. The PEL 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 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 has determined that sampling procedures in the construction industry should be the same as in general industry, and that the mppcf PEL in 29 CFR 1926.55(a) is equivalent to the mg/m3 PEL in 29 CFR 1910.1000 (OSHA 1996).
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 a-quartz and cristobalite (respirable fraction) to 0.025 mg/m3, and to withdraw the documentation and adopted TLV® for tridymite (ACGIH 2004).
Descriptive data about the milling machine were gathered during the afternoon and evening prior to the fist day of sampling, while the machine was undergoing repairs at the Payne & Dolan shop. Information was collected about the number, type, condition, and placement of water spray nozzles on the drum and at the conveyor belt transitions; the cutter drum rotation rate (measured using a non-contact tachometer (TAC2K, Dwyer Instruments, Inc., Michigan City, Indiana) while the mill was rug); the hours on the machine; the cutter drum condition; and the cutter bits, including their spacing, condition, make, and model number. NIOSH personnel worked with Payne & Dolan staff and the manufacturer's representative to install water flow meters and pressure gauges, install new water spray nozzles and cutter bits, and restore the water spray nozzles at the secondary conveyor transition to the manufacturer's specification. This tie was also used to learn about the operation of the machine and safe work practices.
Water flow rate was measured using two water flow meters (Conflowmeter II, Conflow, Inc., Washington,PA) installed in the water supply lies on the mill. One meter was installed in the line between the water pump and the cutter drum spray bar. The second meter was installed in the line between the water pump and the conveyor transition sprays (Figure 1). Water pressure was measured using pressure gauges attached to tee-fittings installed in the water line supplying the cutter drum spray bar (Figure 2), in the water line supplyig.the spray bar for the first conveyor belt transition, and in the water line supplying the nozzles on the secondary conveyor transition. The readings on these meters and gauges were observed and recorded periodically throughout both days of milling.
Vehicle speed and direction of travel was measured using a Trimble GeoXT handheld data-logging global positioning system (GPS) unit (Trimble Navigation Ltd., Sunnyvale, CA) receiver on the second day of milling. The GPS unit was placed on padding on the top of the milling machine in an empty fire extinguisher bracket (the extinguisher had been relocated previously) in front of the operator's station (Figure 3). Speed was also recorded during both days of milling by a NIOSH researcher observing and recording the foot speed reading on the instrument panel of the mil every two minutes. As a measure of productivity, the time was recorded when each dump truck was loaded and pulled away from the milling machine.
Depth of cut was measured every fifteen minutes during both milling days using a tape measure held at the edge of the cut pavement. The width of the cut was measured as well. Bulk samples of the milled pavement were collected on a periodic basis from material left in or next to the cut by the milling machine. Wind speed and direction and temperature were recorded using a data-logging weather station (MultiLog Weather Station, Fourier Systems, Inc., Atlanta, GA). A hand-held multi-directional impeller wind meter was also used during the survey (Skywatch Meteos, JDC Electronic SA, Yverdon-Ies-Bains, Switzerland).
The work practices and use of personal protective equipment were recorded for each worker sampled, including the worker's position and distance relative to the milling machine (e.g., walking alongside, following behind, riding). Information obtained from conversations with workers to determine if the sampling days were typical of the nominal work load helped to place the sampling results in proper perspective. Data were recorded describing other operations nearby that generated dust, including the process, its location relative to the milling machine, and whether it was upwind or downwind of the milling machine.
Dust and silica Sampling Methods
On both days of sampling, personal breathing zone samples on the three members of the milling crew were collected at a flow rate of 4.2 liters per minute (L/min) using a battery operated sampling pump at the employee's waist connected via flexible tubing to a: preweighed, 37-mm diameter, 5-micron (~m) pore-size polyvinyl chloride filter supported by a backup pad in a three-piece filter cassette sealed with a cellulose shrink band in accordance with NIOSH Methods 0600 and 7500, and a cyclone (GK 2.69 Respirable/Thoracic Cyclone, BGI Inc., Waltham, MA) placed in the employee's breathing zone (NIOSH 1994, HSE 1997).
Area samples were collected on both days of sampling at six locations on the milling machine using an array of instruments mounted on a metal frame (Figure 4). The locations included the dashboard on the operator's platform, near the level controls on both sides of the mil, near the cutter drum on both sides of the mil, and on the right side near the transition from the primary conveyor to the loading conveyor (Figure 5). The sampling instruments in each array included a Iight-scattering aerosol photometer (pDR, Thermo Electron Corp., Franklin, MA) with a 10 millimeter (mm) nylon cyclone connected to the inlet via flexible tubing. The pDR was in turn connected via flexible tubing to a battery-operated sampling pump calibrated at a flow rate of 1.7 L/min. A preweighed 5 µm pore-size polyvinyl chloride filter supported by a backup pad in a two piece filter cassette sealed with a cellulose shrink band was placed in line between the pDR and the pump. Also included in each sampling array were two battery-operated sampling pumps, both connected though flexible tubing to a 10-mm nylon cyclone and a pre-weighed, 37-mm diameter, 5-¡.m pore-size polyvinyl chloride filter supported by a backup pad in a two-piece filter cassette sealed with a cellulose shrink band, in accordance with NIOSH Methods 0600 and 7500.
On the second day of sampling, additional area samples were collected at the six locations described above at a flow rate of 4.2 liters/minute using a battery-operated sampling pump connected via flexible tubing to a pre-weighed, 37-mm diameter, 5- micron (¡.m) pore-size polyvinyl chloride filter supported by a backup pad in a three piece filter cassette sealed with a cellulose shrink band in accordance with NIOSH Methods 0600 and 7500, and a cyclone (OK 2.69 Respirable/Thoracic Cyclone, BGI Inc., Waltham, MA) attached to the metal frame.
Gravimetric analysis for respirable particulate was cared out with the following modifications to NIOSH Method 0600: 1) the filters and backup pads were stored in an environmentally controlled room (20:1 1°C and 50:i5% 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 the hìgher-flow filter and all 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 lower-flow filter samples were not analyzed for silica.
The silica content of the pavement was determined though testing of bulk samples collected from milled pavement left in or next to the cut by the milling machine. The limit of detection for quartz in bulk samples was 0.8%. The limit of quantitation was 2%.
Initial activities on site, such as installing new water spray nozzles and new cutter-drum teeth, were devoted to returning the mil to the manufacturer's specifications. During the first day of sampling, samples were collected over thee sampling periods during a typical milling job. The second day of sampling was divided into four 2-hour periods. During the first period, the manufacturer-specified nozzles (UniJet® model 11005 SS, Spraying Systems Co., Wheaton, IL) rated at 0.5 gallons per minute (gpm) at 40 pounds per square inch (psi) were used. For the second and third period, higher-flow nozzles (UniJet® model 11008, Spraying System Co., Wheaton, IL) rated at 0.8 gpm at 40 psi were installed on the cutter-dr spray bar. during the final period, the manufacturer-specified nozzles were used again. This design was used instead of a randomized design because of the length of time and effort required to change the nozzles in the field. The spray nozzles for the primary conveyor and the material transfer conveyor were not changed. Personal and high-flow area samples for respirable dust and respirable crystalline silica were changed approximately every two hours during both sampling days. The lower-flow area samples were collected for the full shift on both days.
RESULTS AND DISCUSSION
Description of the Mill and the Controls
The 8 ft 6 in wide cutter drum held 193 bits arranged in a helical coil around the drum. The drum and holders were in factory spec condition, with 50 hrs on the holders. New bits were installed on the evening before the first sampling day, and were changed several times during the next two days. The dr rotation was measured and found to agree with the manufacturer's specification of92.8 rpm.
Personal protective equipment and work practices
Respirable dust and crystalline silica sampling results
Respirable quartz results from personal samples ranged from below the limit of to detection 0.1 1 inglm3. The TW A respirable quartz exposures for October 8 were 0.013 mg/m3 for the operator, 0.064 mg/m3 for the foreman, and 0.030 mg/m3 for the third milling Crew member. The TWAs were also calculated using the value of LOD/.. for quartz results less than the LOD of 0.01 mg. Their 8-hour quartz TWAs were 0.014 mg/m3. 0.066 mg/m3, and 0.027 mg/m3. respectively. On October 8. the foreman's TWA and 8-hour TWA exposure exceeded the NIOSH quartz REL of 0.05 mg/m3.
Table 2 lists the results of the milling crew's respirable dust and respirable quartz samples on October 9. Calculating TWA respirable dust exposures by nozzle type and employee results in TWAs of2.1 mg/m3 for the operator, 0.62 mg/m3 for the foreman, and 1.1 mg/m3 for the rear control operator for the UniJet® 11005 nozzle, versus 1.4 mg/m3, 0.65 mg/m3, and 0.89 mg/m , respectively, for the UniJet® 11008 nozzle. TWA quartz results for the UniJet® 11005 nozzle were 0.12 mg/m3 for the operator, 0.041 mg/m3 for the foreman, and 0.050 mg/in3 for the worker operating the rear controls. 8 TW A quartz results for the UníJetlIII008 nozzle were 0.081 mg/m3 for the operator, 0.041 mg/m3 for the foreman, and 0.043 mg/m3 for the third crewman. In contrast to October 8, the operator received the highest exposures on the second sampling day, regardless of the nozzle type. Inspection of Table 3 shows that the highest emissions were associated with the left side of the mil and the upper conveyor. Unfortunately, the weather station did not log data, so the effect of the wind direction on those results is unknown.
Table 4 reports the results of the area samples collected on October 8 with the lower-flow respirable dust samplers (1.7 L/min) and the pdR direct-reading instruments. Those results indicate that for both sampling methods, that exposures were lowest in the operator's station and highest at the upper conveyor (the material transfer conveyor). This table also lists the gravimetric/pDR ratios calculated for each sampling location. These ratios were used to adjust pDR short term sampling concentrations, which are provided later in this report. Table 5 presents the results for the lower-flow respirable dust samples and the pdR samples for October 9, including the results from all nozzle types. The upper conveyor produced the highest exposure on that day as welL. Perhaps the operator's personal exposures differ from the area sample at his work station because he spends most of the time on one or the other side of the mm, while the sampler was in the center of his work station (Figure 7). Table 5, which provides the results of the lower-flow and pDR area samples, shows that, as in Table 3, the highest emissions were associated with the left side of the mill and the upper conveyor. The mill removed material from the right side of the road, so the left side of the mill was facing the paved edge of the road, while the right side faced the previous cut. This orientation may explain in par why the exposures were higher on the left side of the milL. The manufacturer's representative attributed this result to passing truck traffic, Table 6 relates area sampling results to productivity, water flow, and water spray pressure while Table 7 describes the work done on October 9.
When reviewing the results of sampling conducted on October 9, for the purpose of this study it is useful to compare the results obtained when the different water spray nozzles were used. For the purpose of this analysis, the results were converted to the natural log (In) scale because on the log scale the variability of the data did not vary with concentration. From the In scale analysis of the respirable dust results (Tables 8~ II), the ratio of the UniJet® 11005 nozzle to the UniJet® 11008 nozzle. results is the ratio of their geometric means, 1.2. This indicates that the respirable dust measurements obtained while the UniJet® 11005 nozzle was used are about 20% higher, though the result is not statistically significant, even at the lO% significance leveL. Results for the pDRs are similar. Those results are also about 20% higher for the UniJet® 11005 and are also not statistically significant at the 10% leveL. Examination of the tables indicates that for most groupings, the UniJetlIl1005 nozzle does give higher results. Review of all of the respirable dust data from October 9 shows that 59% of the UniJet® 11005 nozzle results exceeded the PEL, while 50% of the UniJet!I 11008 nozzle results did so. For personal breathing zone samples, half of the results while the UniJet® 11005 was in use exceeded the PEL; one third of the UniJet® 11008 results exceeded the PEL.
From the In scale analysis of the quartz sampling results (Tables 12-15), the ratio of the UniJet® 11005 nozzle to the UniJet® 11008 nozzle results is the ratio of their geometric means, 1.16. Thus, for respirable quartz, the results while UniJet® 11005 nozzles were installed on the cutter drum were about 16% higher, though the result is not statistically significant, .even at the 10% significance leveL. Examination of the tables indicates that the quartz results are not as consistent as those for respirable dust. For data from both days, 7 respirable quartz results were less than the LOD and 13 values were between LOD and LOQ. For the seven quartz values' below the LOD, the value Lan/.. was substituted.
Quartz in bulk samples
CONCLUSIONS AND RECOMMENDATIONS
Conclusions from this pilot study regarding the effect of increasing the water flow to the spray nozzles on dust and quart exposures are limited by several factors.. First, the higher flow nozzles were only installed on the cutter drum and the cutter drum extension. Second, the nozzles selected represent a difference of only 30% more flow when the higher-flow nozzles were installed. Nevertheless, the respirable dust measurements obtained while the UniJet® 11005 nozzles were used were about 20% higher than when the higher-flow UniJet®11008 nozzles were used. The respirable quartz results while UniJet® 11005 nozzles were installed on the cutter drum were about 16% higher. Neither the reductions in respirable dust concentrations nor those for the quartz results were statistically significant, even at the lO% significance leveL. Statistical significance mean that the difference is not due to chance alone.
Research on dust controls in mining has shown that water sprays have two roles, wetting of broken material being transported, and airborne capture (NIOSH 2003). Uniformly wetting the broken material during the breakage process is far more effective, and ensures that dust particles stay attached to the material (NQSH 2003). Increasing the water flow rate and increasing the number of sprays have both been shown to be effective; it is too early in this project to recommend an approach. In one instance, dust from a shearer dr was reduced by 60% when the 46 smaller orifice nozzles were substituted for the originall7 nozzles, with the pressure and flow held constant (NIOSH 2003). The shape of the spray is another important factor in its effectiveness.
Future studies might test the water system on a newer-model machine to begin to explore the relationship between water flow and pressure and airborne dust concentrations. Variables related to the site and job might obscure this relationship, but it is hoped that a trend might emerge.
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Table 1: Results of Personal Breathing Zone Samples
ND indicates a result less than the limit of detection for this method of 1 O¡.g of quartz per sample. Results in parentheses indicate a semi-quantitative value between the limit of detection and the limit of quantitation.
Table 2: Results of Personal Breathing Zone Samples
ND indicates a result less than the limit of detection for this method of 10 µg of quartz per sample.
Table 3: High~Flow Area Sample Results
ND indicates a result less than the lit of detection for this method of 10µg of quartz per sample.
Table 4: Low-Flow and pDR Area Sample Results
Table 5: Low-Flow and pDR Area Sample Results
Table 7: Descriptions of Milling Machine Cuts
Elapsed times are actual operating times.
Table 8: Respirable Dust Mean by UniJet® Nozzle Type
Table 9: Respirable Dust Means and Geometric Mean by Area or Personal Sample
Table 10: Respirable Dust Mean and Geometric Mean by Worker
Table 11: Respirable Dust Mean and Geometric Mean by Location
Table 12: Respirable Quartz Means by UniJet® Nozzle Type
Table 13: Respirable Quartz Means and Geometric Means by Area or Personal Sample Payne and Dolan October 8 and 9,2003
Table 14: Respirable Quartz Means and Geometric Mean by Worker
Table 15: Respirable Quartz Means and Geometric Means by Location
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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