Respirable Concrete Dust—Silicosis Hazard in the Construction Industry

ABSTRACT

Concrete is an extremely important part of the infrastructure of modern life and must be replaced as it ages. Many of the methods of removing, repairing, or altering existing concrete structures have the potential for producing vast quantities of respirable dust. Since crystalline silica in the form of quartz is a major component of concrete, airborne respirable quartz dust may be produced during construction work involving the disturbance of concrete, thereby producing a silicosis hazard for exposed workers. Silicosis is a debilitating and sometimes fatal lung disease resulting from breathing microscopic particles of crystalline silica. Between 1992 and 1998, the National Institute for Occupational Safety and Health (NIOSH) made visits to construction projects where concrete was being mechanically disturbed in order to obtain data concerning respirable crystalline silica dust exposures. The construction activities studied included: abrasive blasting, concrete pavement sawing and drilling, and asphalt/concrete milling. Air samples of respirable dust were obtained using 10-mm nylon cyclone pre-separators, 37-mm polyvinyl chloride (PVC) filters, and constant-flow pumps calibrated at 1.7 L/min. In addition, high-volume respirable dust samples were obtained 0 0 on 37-mm PVC filters using 1/2" metal cyclones (Sensidyne model 18) and constant-flow pumps calibrated at 9.0 L/min. Air sample analysis included total weight gain by gravimetric analysis according to NIOSH Analytical Method 600 and respirable crystalline silica (quartz and cristobalite) using x-ray diffraction, as per NIOSH Analytical Method 7500. For abrasive blasting of concrete structures, the respirable crystalline silica (quartz) concentration ranged up to 14.0 mg/m 3 for a 96-minute sample resulting in an eight-hour time-weighted average ( TWA ) of 2.8 mg/m 3. For drilling concrete highway pavement the respirable quartz concentrations ranged up to 4.4 mg/m 3 for a 358-minute sample, resulting in an eight-hour TWA of 3.3 mg/m 3. For concrete wall grinding during new building construction the respirable quartz measurements ranged up to 0.66 mg/m 3 for a 191-minute sample, resulting in an eight-hour TWA of 0.26 mg/m 3. The air sampling results for concrete sawing ranged up to 14.0 mg/m 3 for a 350-minute sample resulting in an eight-hour TWA of 10.0 mg/m 3. During the milling of asphalt from concrete highway pavement, the sampling indicated a respirable quartz concentration ranging up to 0.34 mg/m 3 for a 504-minute sample, resulting in an eight hour TWA of 0.36 mg/m 3. The results of this work indicate the potential for respirable quartz concentrations involving disturbance of concrete to range up to 280 times the NIOSH Recommended Exposure Limit (REL) of 0.05 mg/m 3 assuming exposure for an eight- to ten-hour workday. Considering the aging of the concrete infrastructure in the United States, these results pose a challenge to all who have an interest in preventing silica exposures and the associated disease silicosis.


METHODS

Air samples were taken using 10-mm nylon cyclone preseparators, 37-mm PVC filters, and constant-flow pumps calibrated at 1.7 L/min. High-volume air samples were obtained on 00 37-mm PVC filters using a 1/2" metal cyclone (Sensidyne model 18) and constant-flow pumps calibrated at 9.0 L/min.

The air sample analysis included total weight gain by gravimetric analysis according to NIOSH Analytical Method 600. The limit of detection (LOD) for this procedure is 0.02 mg. The air samples were then analyzed for respirable crystalline silica (quartz and cristobalite) using x-ray diffraction, as per NIOSH Analytical Method 7500.  The LOD for quartz on the filters is 0.01 mg and the limit of quantification (LOQ) is 0.03 mg. The LOD for cristobalite on the filters is 0.015 mg and the LOQ is 0.03 mg.

The concentration of respirable dust and respirable quartz were then computed using the formula:

The percent quartz content of a sample may be computed by dividing the quartz mass by the gravimetric mass.

RESULTS

The form of crystalline silica found in the air samples for this study was quartz. None of the air samples involved in this work resulted in the detection of cristobalite. The respirable quartz data is found in Tables I through VI. The quartz concentration measured during the particular work activity (sample period) is presented along with the resulting eight-hour TWA. To calculate the eight-hour TWA it was assumed that no exposure occurred during any nonsampled portion of the shift.

Abrasive Blasting of Concrete

Table I summarizes the concrete abrasive blasting samples for this study. Samples 1–3 were obtained during sandblasting of a concrete wall outdoors. Samples 4–7 and 15–22 were obtained during sandblasting of prestressed concrete building structures inside a ventilated blasting shed. Samples 8–14 were obtained during abrasive blasting of the steel structure under a bridge. Coal slag was used as the abrasive in an unventilated curtained enclosure to abrasive blast the steel structure and presumably the surrounding concrete. Samples 8–14 in Table I indicate a potential for quartz concentrations to exceed the assigned protection factor (APF=25) for the continuous flow type CE respirator being used.

Concrete Drilling

The dust samples in Table II represent concrete drilling work during interstate highway repair. To repair cracked concrete pavement, large blocks of the pavement are sawed so that the damaged portion can be removed. For small jobs, a jackhammer may be used to break up the heavy block of sawed concrete; for larger jobs a clamshell-type device may be used with a backhoe to lift the block out. Another method is to drill holes through the block and use a crane with a friction type attachment in the holes to lift the concrete block. After the broken pavement is removed, steel reinforcement is installed before new concrete is placed into the hole. To key the new concrete into the surrounding pavement, horizontal holes are drilled into the surrounding concrete so that steel rods can be anchored in these holes with epoxy. After an additional steel reinforcement bar is tied off, the new concrete is placed. A typical rig to drill the holes is a set of three pneumatic drills in a carriage that is used as an attachment to a backhoe.

Table II presents data from two interstate highway construction sites (samples 1–8 and 9–22) where this concrete drilling procedure was being performed. In Figure 2, a laborer is seen stationed at the pneumatic drills. A heavy equipment operator is stationed in the cab of the backhoe (not seen, but off to the right in Figure 2). Typically, this drilling is done dry without any dust control. Clouds of dust can be seen drifting from these types of operations. The laborer at the controls of the drill works in higher dust concentrations than the operator of the backhoe; however, the laborer can, at times, move out of the direction that the dust is moving, while the operator has to sit in the cab. Often these cabs are not enclosed or, if enclosed, do not provide adequate protection from respirable dust.

Figure 3 shows two pneumatic drills mounted on a piece of mobile equipment so that vertical holes can be drilled into blocks of concrete pavement. The holes were used to attach a gripping device so that a small crane could lift the concrete block out of the pavement.

Concrete Grinding

Tables III and IV present data from a construction site where the concrete walls of a large building were being smoothed using an electric angle grinder equipped with a 4" diamond-impregnated steel disc. The average quartz content of the airborne respirable dust collected was 6.3 +- 1.6%.

Samples 1–6 in Table III were obtained during concrete grinding without dust control. The personal breathing zone samples indicated a respirable quartz dust concentration of 0.66 mg/m3 for the 191-minute sample duration. Due to worker breaks and interruptions the estimated actual grinding time was 147 minutes. This concentration represents a potential exposure of over 13 times the NIOSH REL of 0.05 mg/m 3 for respirable quartz if this procedure were employed for a full shift. If the worker had no other crystalline silica exposure for the remainder of the shift, the exposure would still have been over five times the NIOSH REL.

Samples 7–14 in Table III were obtained during wet grinding. Separate personal breathing zone samples were obtained for the grinder and for the helper. The helper used a new, clean, pesticide-type spray can filled with tap water to wet down the surface of the concrete wall just ahead of where the worker using the grinder was working (see Figure 4). For the grinder, the 342-minute personal breathing zone sample indicated a respirable dust concentration of 0.36 mg/m 3 and a respirable quartz concentration of 0.02 mg/m 3 , which was between the limit of detection and limit of quantification for the sample. For the helper, the personal breathing zone sample indicated a respirable dust concentration of 0.16 mg/m 3 for the 225-minute sample duration. Respirable quartz dust was not detected. To be detected and quantified, at least 0.01 mg and 0.03 mg of quartz, respectively, must have been on the filter. The two high-volume area samples (9.0 L/min.) indicated respirable dust concentrations of 0.16 and 0.26 mg/m 3 and respirable quartz concentrations of not detectable and 0.02 mg/m 3, respectively. The data indicate that the respirable quartz concentrations were low but measurable during full-shift sampling. Sample time for the grinder operator personal sample was 342 minutes; however, due to worker breaks and tool problems the estimated actual grinding time was 270 minutes. If grinding occurred during a greater portion of the sampling time, quartz may have been measured in these samples.

Three different local exhaust dust control systems were used (Micro mini-vac, Maxi-vac, and WAP) (see Figure 5). Micro mini-vac and Maxi-vac are two vacuum-type dust collection systems that were made by the same manufacturer. Table IV summarizes the exhaust control system sampling data.

Samples 1–6 in Table IV were obtained while using a dust collection system sold by SawTec named the Micro mini-vac. The system weighs approximately 46 pounds, is 14"W x 52"L x 16-1/2"H and produces 760 CFM air flow at 8.5" maximum static pressure according to SawTec. The vacuum has one 4" inlet for a flexible tube. The system uses a bag to collect the suctioned dust. Information with the equipment states that the bag “filters concrete dust to 0.5 micron” (see Figure 6). A metal shroud was attached to the grinder, and a flexible tube connected the vacuum to the shroud. Neither respirable quartz nor respirable dust were detected by the 127-minute personal breathing zone sample. Due to tool and/or system problems the estimated grinding time was 77 minutes. The high-volume area sample (9.0 L/min.) indicated a respirable dust sample concentration of 0.66 mg/m 3 and a respirable quartz sample concentration of 0.04 mg/m 3 . The four side-by-side area samples (1.7 L/min.) did not detect respirable quartz, but averaged 0.54 mg/m 3 of respirable dust. The data indicate that short-term sampling may not collect enough dust to permit a quartz measurement. Occasional dust leaks at connections of the air handling system were observed.

Samples 7–12 in Table IV were obtained while using a dust collection system sold by SawTec named the Maxi-vac. According to information available, the system weighs 140 pounds, is 21"W x 36"L x 78"H and produces 900 CFM air flow at 8.5" maximum static pressure. The vacuum can be arranged for up to two 4" inlets or one 6" inlet. The Maxi-vac uses the same bag material as the Micro mini-vac. The Maxi-vac was connected to a shroud on the grinder in a similar fashion as with the Micro mini-vac. The shroud used for this system had a cut-out to allow the disc to reach into corners. The personal breathing zone sample indicated a respirable dust concentration of 1.4 mg/m 3 and a respirable quartz dust concentration of 0.13 mg/m 3 for the 150-minute sample. Due to worker breaks and interruptions the estimated actual grinding time was 96 minutes. This quartz concentration, if maintained for an 8- to 10-hour shift, would be over 2-1/2 times the NIOSH REL. Even if no exposure is assumed for the remainder of the shift, the 8-hour TWA would be 0.04 mg/m 3. The high-volume area sample (9.0 L/min.) indicated a respirable dust concentration of 0.61 mg/m 3 and a respirable quartz concentration of 0.03 mg/m 3 during the 150-minute sample. The four side-by-side area samples did not detect respirable quartz, but found an average of 0.53 mg/m 3 of respirable dust. Occasional dust leaks at connections of the air handling system were observed.

Samples 13–18 in Table IV were obtained while using a WAP dust collection system, which is a type of canister vacuum. The WAP vacuum has a 10-gallon capacity and uses 1600 watts with 13 amps or 1000 watts with 8.3 amps. The system uses disposable paper filter bags. A flexible tube was used to connect it to a shroud fitted to the grinder. The 117-minute personal breathing zone sample resulted in a respirable dust concentration of 0.30 mg/m 3 . Respirable quartz was not detected. Due to worker breaks and interruptions the estimated actual grinding time was 100 minutes. The high-volume area sample (9.0 L/min.) indicated a respirable dust concentration of 0.26 mg/m 3 and a respirable quartz concentration between the limits of detection and quantification of 0.02 mg/m 3. None of the four side-by-side area samples detected respirable dust.

Concrete Sawing

Table V summarizes the concrete sawing data. Samples 1–4 were obtained at an office building remodeling project. A worker used a gasoline-powered handheld masonry saw (see Figure 7) to saw indentations in the concrete floor on each of the 16 floors of the building so that restroom floor drains could be installed. The concrete cutting was done by a plumber. The saw was used without water being applied to the blade or using a dust collector. The only attempt to control the dust was with the use of a floor stand fan directed toward an open window. The investigators found a respirable concentration of quartz for the concrete cutter operator of 14.0 mg/m 3 during a 350-minute sample. The area samples for this operation were found to be 4.1, 3.4, and 3.2 mg/m 3 respirable quartz. Three bulk samples of the concrete dust were found to be 38, 47, and 43 percent silica.

Samples 5–12 were taken during highway construction. This site consisted of a four-lane highway being constructed, during which expansion joints were being sawed in the fresh concrete within six hours of being placed. Three workers were involved in the operation. Two of the workers operated commercial-type walk-behind concrete saws and the third operated a water truck, which provided water to the saws. Hoses transported water from the water truck to the saws so that the diamond-tipped saw blades could be cooled while in use. Neither the saw operators nor the water truck driver used respiratory protection. The water truck driver spent almost all of his time in the cab of the water truck while the saws were in use.

Samples 13–15 were obtained during reconstruction of an interstate highway bridge where blocks of the old concrete were sawed completely through so the old pavement could be removed. This operation consisted of three workers operating concrete saws to saw through the decking of an existing bridge. Each of the saws was equipped with a water supply that provided water to the diamond-impregnated saw blades. Water was obtained from tanks mounted on the back of trucks, and was transported by gasoline pumps to the saws. General area air samples were collected to determine the potential exposure concentrations of respirable silica dust. The duration of the air sampling was approximately three hours, due to inclement weather.

Samples 16–24 were obtained during repair of an interstate highway. Blocks of the old concrete pavement that needed to be replaced were sawed through and lifted out so that new concrete could be poured (see Figure 8). This operation consisted of two workers operating two concrete saws to saw blocks of concrete pavement. Each of the saws was equipped with a water supply that provided water to the diamond-tipped saw blades. For each saw, water was obtained from 725-gallon water tanks mounted on trucks. The water source for the two saws was under pressure from 3.5 and 4.0 horsepower water pumps. The workers did not use respirators.

Asphalt/Concrete Milling

Table VI summarizes data obtained during two shifts of asphalt milling. This operation consisted of three workers operating an asphalt mill to remove old asphalt from an interstate highway (see Figure 9). This machine operates similarly to the continuous mining machines used in coal mining with a horizontal drum studded with carbide bits and a water spray system to wet down dust. The mill is set to remove a predetermined depth of asphalt or concrete. For the operation reported here, the entire thickness of asphalt was removed from the underlying concrete pavement, thereby abrading the concrete surface in the process. The operator spent almost all of his time on top of the mill in the open. A laborer spent his time walking alongside the machine, as did the foreman. None of the workers used respirators.

Although respirable dust was found in the breathing zone of the mill operator during both shifts, quartz was not detected. On both days, elevated levels of respirable quartz were found in the breathing zone of the laborer. During the first shift (samples 1–6), the 8-hour TWA for respirable quartz dust was 0.36 mg/m 3 . This concentration is over 7 times the NIOSH REL of 0.05 mg/m 3. During the second shift (samples 7 – 11), the 8-hour TWA for respirable quartz dust was 0.10 mg/m 3, or twice the NIOSH REL. During the first shift, the investigators found 8-hour TWA respirable quartz concentrations ranging from the REL to over 4 times the REL at Area 1 above the conveyor belt. During the second shift, quartz was detected in the Area 1 samples, although at levels too low to quantify. The Area 2 sample during the first shift (sample 6) located on the top of machine (front end) also detected quartz at a level too low to quantify.

The results show that the laborer walking beside the machine was being exposed to respirable quartz at levels above the NIOSH REL and the OSHA PEL. The mill was provided with water from a water truck. During these types of operations maintenance of the water spray system is essential to assure that it is working as intended. Possible problems may include spray nozzle misalignment or water not being provided at high enough pressure. Dust may leak out the sides of the machine, and therefore may require additional water sprays. An additional water spray may be needed at the conveyor belt. Care should be taken to insure that water is always provided to the drum while in operation. The mill should be shut down if it runs out of water before the water truck can return with a new load.
