LEARNING OBJECTIVES
Upon completion of this module, the student will be able to:
1. Describe the hazards associated with silica and the typical construction site materials which may present this hazard
2. Review common construction site operations where this hazard is most prevalent.
3. Review mitigation techniques to reduce this hazard while performing and using standard construction site tools and materials
4. Identify the proper respirator to use when performing construction site operations
5. Discuss best work practices to minimize inhalation risks on sites with potential silica exposure
Introduction
This section addresses the control of exposures to respirable dust containing crystalline silica, which is known to cause silicosis, a serious lung disease, as well as increase the risk of lung cancer and other systemic diseases. The following provides information on the effectiveness of various engineering control approaches for several kinds of construction operations and equipment, and contains recommendations for work practices and respiratory protection, as appropriate.
BACKGROUND
Quartz is the most common form of crystalline silica. In fact, it is the second most common surface material accounting for almost 12% by volume of the earth’s crust. Quartz is present in many materials in the construction industry, such as brick and mortar, concrete, slate, dimensional stone (granite, sandstone), stone aggregate, tile, and sand used for blasting. Other construction materials that contain crystalline silica are asphalt filler, roofing granules, plastic composites, soils, and to a lesser extent, some wallboard joint compounds, paint, plaster, caulking and putty. Cristobalite, a less common form of crystalline silica, is formed at high temperatures (>1,470°C) in nature and by industrial processes. The ceramic and brick lining of boilers and vessels, some ceramic tiles, and volcanic ash contain cristobalite.
Of course, potential silica exposure levels will depend on the concentration of silica in materials at construction sites, as well as factors in the work environment (such as enclosed, semi-enclosed, or open spaces and/or multiple operations generating silica dust) as well as environmental conditions (such as wind direction and speed). Therefore, employers are encouraged to conduct periodic exposure monitoring to confirm that engineering and work practice controls are effective and that appropriate respiratory protection is being used where necessary. Controls continue to evolve and it is encouraged that equipment suppliers and contractors to work with industrial hygienists to evaluate new designs and products to obtain objective information that can be used to evaluate performance and support informed decisions on use.
This section is divided into nine sections that cover different construction operations. Eight are for specific equipment or operations: Stationary Masonry Saws, Handheld Masonry Saws, Hand- Operated Grinders, Tuckpointing/Mortar Removal, Jackhammers, Rotary Hammers and Similar Tools, Vehicle-Mounted Rock Drilling Rigs, and Drywall Finishing.
MASONRY SAWS: STATIONARY & HANDHELD
Exposure to fine particles of silica has been shown to cause silicosis, a serious and sometimes fatal lung disease. Construction employees who inhale fine particles of silica may be at risk of developing this disease. Employees produce dusts containing silica when they cut, grind, crush, or drill construction materials such as concrete, masonry, tile and rock. The small particles easily become suspended in the air and, when inhaled, penetrate deep into employees’ lungs.
Studies show that using a stationary masonry saw to cut bricks, concrete blocks and similar materials can result in hazardous levels of airborne silica if measures are not taken to reduce dust emissions. Stationary saws should always be used with dust control measures. At worksites without dust controls for these tools, studies have found that employee silica exposures can be as high as 20 times the Occupational Safety and Health Administration’s (OSHA) benchmark
SILICA DUST CONTROL MEASURES
Wet Cutting
Most stationary saws come equipped with a water basin that typically holds several gallons of water and a pump for recycling water for wet cutting. If a saw’s water supply system is not currently operating, the manufacturer may be able to supply the necessary accessories to reactivate wet cutting capability. Most suppliers stock these accessories since water cooling prolongs the life of the saw blade and tool.
Wet cutting is the most effective method for controlling silica dust generated during sawing because it controls the exposure at its source. Dust that is wet is less able to become or remain airborne.
Maintenance
To minimize dust emissions from saws equipped for wet cutting, keep pumps, hoses and nozzles in excellent operating condition. Regular saw maintenance reduces silica exposures and ensures optimal operation of the equipment. Saws and dust control devices should be on a routine maintenance schedule.
Vacuum Dust Collection Systems
When wet methods cannot be implemented, one alternative is the use of vacuum dust collection (VDC) systems. Stationary masonry saws with VDC systems are commercially available and have the ability to capture a substantial amount of dust. With these systems, a vacuum pulls dust from the cutting point through special fittings connected directly to the saw (fixed-blade saws) or, alternatively, through a dust collection device connected to the back of the saw (plunge-cut saws). A dust collector (exterior hood) mounted to the back of a saw requires a high exhaust airflow to ensure good dust capture between the saw blade and dust collector.
VDC systems can be purchased as a kit. These kits should include a dust collector (exterior hood), vacuum, vacuum hose, and filter(s). The components of a VDC system are discussed below.
• Dust collector (exterior hood): Be sure to use the appropriate sized dust collector for the wheel and if it is a retrofit on the saw, be sure to follow the manufacturer’s instructions when installing the device.
• Vacuum: Choose a vacuum with the appropriate power and capacity for your job. Obtaining a flow rate on a VDC system of 80 CFM or better will give the best results while performing mortar removal.
• Vacuum hose: A flow rate of 80 CFM is best maintained with a 1.- to 2-inch diameter hose. If the diameter is larger, the airflow velocity will be reduced. If the diameter is smaller, airflow resistance will be higher. Airflow resistance also increases with hose length; excessively long hoses should be avoided.
• Filters: Double filtration is important. The use of a high-efficiency particulate air (HEPA) filter is critical to prevent the escape of respirable silica dust from the vacuum exhaust. HEPA filters are at least 99.97 percent efficient in removing fine dust particles from the air. A prefilter or cyclonic separator in addition to a HEPA filter will improve vacuum efficiency and extend the service life of the more costly HEPA filter. A cyclonic separator removes large particles that are capable of overloading or clogging the filter.
• Systematic cleaning: Choose a vacuum equipped with a back-pulse filter cleaning cycle. Such auto-cleaning mechanisms will reduce the time required for vacuum maintenance and improve the overall efficiency of the dust collection system. If the vacuum does not have an auto-cleaning mechanism, the employee can periodically turn the vacuum cleaner on and off. This allows the bag to collapse and causes the prefilter cake to dislodge from the filter.
• Monitoring VDC efficiency: Purchasing a dust collection system equipped with a static pressure gauge allows the employee to monitor the system’s efficiency. Systems lacking a static pressure gauge can be monitored visually. If a dust plume increases and becomes more visible where the dust collector meets the working surface, the system is not working efficiently.
Ventilation Booths
A booth (with fan) erected around a saw can help reduce dust, but may require some experimentation to provide adequate effectiveness. The following are tips for design an effective booth.
• Minimize the size of the operator opening to reduce the chance of dust escaping into the operator’s breathing area.
• Use a fan large enough to provide an average of 250 feet per minute air velocity across the face of the operator opening.
• Do not let the saw blade protrude beyond the open face of the booth.
• Build a trapdoor into the lower back of the booth to access the interior for cleaning and to remove debris.
• Always position the booth so that the exhaust fan does not blow dusty air on other employees. When possible, have the booth exhaust downwind.
Fans
Fans are not effective dust control devices when used as the sole control method and should not be used as the primary method for managing dust. Fans can, however, be useful as a supplement to other control methods. Use fans in enclosed areas, such as bathrooms, where dust would build up due to poor air circulation.
For the best effect, set an exhaust fan (the bigger, the better) in an open window or external doorway. Position the saw nearby, so that the fan captures dust and blows it outside. Avoid positioning employees between the saw and the fan. Also, avoid positioning employees near the exhausted air. An exhaust fan works best if a second window or door across the room is open to allow fresh air to enter.
| Note: The use of compressed air to clean surfaces or clothing is strongly discouraged. Using compressed air to clean work surfaces or clothing can significantly increase employee exposure, especially in enclosed and semi-enclosed spaces. Cleaning should be performed with a HEPA-filtered vacuum or by wet methods. |
GRINDING & TUCKPOINTING
Employees produce dusts containing silica when they grind on concrete and similar materials. The grinders’ abrasive action generates fine particles that easily become suspended in the air and, when inhaled, penetrate deep into employees’ lungs.
In fact, on average, grinder operators’ silica exposures (along with those of tuckpointers) are among the highest in the construction industry.
SILICA DUST CONTROL MEASURES
Vacuum Dust Collection Systems
Vacuum dust collection (VDC) systems for grinders include a shroud, which surrounds the grinding wheel, hose, filters and a vacuum to pull air through the shroud. Many manufacturers offer grinders with dust collection options. Employers may also purchase equipment to retrofit grinders for vacuum dust collection. The effectiveness of vacuum systems depends on several factors, including the user’s technique, the surfaces being finished, and the efficiency of the dust collection system.
Wet Grinding
Water provides excellent dust control during tasks involving abrasive action on concrete. When applied at the point where dust is generated, water wets the dust particles before they can become airborne. Water-fed equipment is regularly used to control dust during granite and concrete grinding and polishing operations, as well as during concrete and masonry cutting with abrasive wheels. These tools include a nozzle or spout that provides a stream of water to the grinding wheel. For example, some equipment provides water through a hole in a hollow shaft or a nozzle at the edge of the wheel.
Adjustments in Work Methods
Employee Positioning
Where possible, exposures can be reduced if employees work at a greater distance from the grinding point. These reductions have been demonstrated for employees grinding on ceilings and for employees sanding drywall. Dust falls on employees who stand directly below the grinding point. If the grinder is attached to an adequately supported pole, the employee can manipulate the grinder at a distance from one side where the dust is less concentrated. While this method does not eliminate exposure, it can help reduce the amount of dust in the employee’s breathing area.
Grinding Wheel Size
A study comparing construction employees’ respirable silica exposure at nine construction sites found that short-term exposure levels were about 30 percent higher for employees operating grinders with 7-inch wheels than for operators grinding with 4.5-inch wheels. Additionally, diamond wheels used for rougher, more aggressive grinding were associated with exposure levels approximately 60 percent higher than those associated with abrasive wheels used for fine finishing. Therefore, whenever possible, use a smaller rather than a larger wheel, and use the least aggressive tool that will do the job.
Construction Work Methods
Where practical, employers can reduce employees’ silica exposures by utilizing construction methods and techniques that minimize the amount of grinding required. Examples include taking steps to minimize pouring/casting flaws and defects by ensuring tighter fitting forms, improved finishing, grinding on pre-cast panels outdoors before installation inside, or using factory installed chase and grooves on pre-cast structural. Silica exposures may also be reduced if grinding is done while the concrete is still “green”. Additionally, for a given amount of material removed from a surface, less airborne dust will be generated if some of the material can be removed as larger chips instead of finely ground particles. An employee might use a hammer and chisel or power chipping equipment to remove most of the mass before using a grinder to smooth the surface.
JACKHAMMERS
Workers produce dusts containing silica when they use breaker hammers (commonly known as jackhammers) to chip and break rocks or concrete. The hammer’s crushing action generates small particles that easily become suspended in the air and, when inhaled, penetrate deep into employees’ lungs.
SILICA DUST CONTROL MEASURES
Wet Methods
Wet methods reduce dust by wetting the material at the impact point, before the dust gets into the air. Wet particles are heavier and more likely to stick to each other than dry particles and tend to settle more quickly. Thus, wet methods decrease the amount of particulate matter suspended in the air. This form of dust suppression is effective for both respirable and visible dust.
The ideal wet method of dust control uses the minimum amount of water to get the maximum result. Spray directed at the point of impact is optimal. The spray must not be too fine otherwise the air motion around the jackhammer will not allow the spray to contact dust at the impact point. For example, employees operating 90-pound jackhammers reduced their silica exposure between 50 and 98 percent using just 1/8 gallon of water per minute as a spray.
Water for dust suppression can be applied manually, or using a semi-automated water-feed device.
Manual spraying
In the simplest method for suppressing dust, a dedicated helper directs a constant spray of water at the impact point, while another employee operates the jackhammer. The helper can use a hose with a garden-style nozzle to maintain a steady and carefully directed spray at the impact point where material is broken and crushed.
An experienced helper will be able to adjust the water flow to achieve the maximum dust suppression using the minimum amount of water, thus reducing water run-off.
Periodically picking up a hose and spraying the general area is not effective. Simply pre-wetting the concrete or asphalt prior to breaking the surface is also ineffective. Because the jackhammer continues to break through silica-containing material, dust is constantly produced. To be effective, spray application must be continuous and directed at the point of impact.
Water Spray System
This alternative uses the same principle as manual spraying, but eliminates the need for a helper to hold the hose.
Jackhammers retrofitted with a spray nozzle aimed at the tip of the tool offer a dramatic decrease in silica exposure. Although water-fed jackhammers are not commercially available, it is neither expensive nor difficult to retrofit equipment and parts are available at well-stocked hardware stores
ROTARY HAMMERS AND SIMILARTOOLS
Using rotary hammers or similar tools to drill small-diameter holes in concrete, bricks, masonry blocks, tiles and similar materials can expose employees to hazardous levels of airborne silica if measures are not taken to suppress dust emissions.
SILICA DUST CONTROL MEASURES
Vacuum Dust Collection Systems
Vacuum dust collection (VDC) systems are commercially available for handheld drills, usually as add-on systems. The systems enclose the drill bit in a suction ring (dust entrance), which includes a port for attaching a vacuum to collect dust and concrete particles generated during drilling.
Dust Barriers
An employee who drills only an occasional small hole in the course of a day may have relatively low silica exposure. It is a good idea to minimize exposure to even small amounts of silica dust, so you might want to experiment with techniques for capturing dust from a single small hole initially developed in the asbestos abatement industry.
One simple dust control method involves inserting the drill bit through a barrier, which is then pressed against the working surface during drilling. The dust exiting the hole collects against the barrier. If the barrier is damp, it forms a better seal against the working surface and also moistens the dust, thus capturing more dust and reducing the amount that can escape when the employee For example, employees sometimes drill through shaving cream in an upside-down waxed paper cup or through a damp sponge to minimize exposure to asbestos. These materials compress and are held in place by the pressure of the advancing drill. Assuming the barrier material can make a good connection with the surface, this method is appropriate for most materials that an employee might drill.
Tips for Devising a Dust Barrier for Occasional Drilling.
For optimal results, the following measures are recommended:
• Insert the drill bit through the barrier until the tip is just visible, and then set the tip against the working surface in the correct position.
• Ensure that there are no gaps between the working surface and the barrier through which small particles can escape and become airborne.
• Withdraw the drill bit by pulling it through the barrier, so that the barrier collects any debris drawn out with the bit.
• Dispose of dust and debris after completing each hole. Handle the barrier carefully to minimize dust release.
• Add a moist material to the barrier to wet dust and minimize release during disposal.
• When using a cup, use waxed paper, which will compress under pressure, rather than Styrofoam, which will crack.
• Do not allow the barrier to become overloaded. For deeper holes, periodically check under the barrier; it may be necessary to clean or empty it before the hole is complete.
Wet Methods
Wet methods are generally not appropriate for use with electric rotary hammers unless the tools are designed for use in damp environments.
Pneumatic drills, however, can be used for wet drilling, and some come equipped with a water feed capability. While designed primarily for use in explosive atmospheres, water-fed pneumatic drills can also be used to control silica exposures.
Wet methods are usually the most effective way to control silica dust generated during construction activities because wet dust is less able to become or remain airborne. Although few specific data are available regarding wet methods for drilling small holes, studies have shown that drilling with water-fed bits or water spray at the bit-rock interface can substantially reduce respirable dust generated by rock drilling rigs.
VEHICLE-MOUNTED ROCK DRILLING RIGS
Workers produce dusts containing silica when they use rock-drilling rigs mounted on trucks, crawlers or other vehicles to drill into rock, concrete, or soil. Studies have shown that drilling into rock, concrete, or soil may produce hazardous levels of respirable silica if measures are not taken to limit and control dust emissions.
SILICA DUST CONTROL MEASURES
Dust Collection Systems
Various types of dust collection systems are available for earth drills. Commonly used equipment incorporates a movable suction duct attached to a shroud (a flexible rubber skirt) that encloses the drill hole opening and captures the cuttings coming through the hole. Drilling equipment that does not include these controls can be retrofitted by the manufacturer or a mechanical shop.
Dusty air pulled from the shroud enclosure usually passes through a flexible duct leading to a primary dust separator and a secondary filter system. The dust separator often includes a self-cleaning “back-pulse” feature that discharges the collected particles to the ground. Some secondary filter systems are also self-cleaning. Finally, the exhaust air is discharged to the atmosphere.
Wet Methods
The proper use of wet methods requires a skilled operator. In wet drilling, too much water can create mud slurry at the bottom of the hole that can entrap the bit, coupling and steel extensions. Too little water will not effectively control dust emissions. Studies indicate that the optimal water flow rate is best achieved by slowly increasing the water to the point where visible dust emissions are eliminated.
While water injection methods work well for percussion, drag and button bits, special consideration is required to protect bits with rollers (tri-cone bits) from excess water on moving parts.
Operator Isolation
Drill operators using rigs with enclosed cabs can reduce their potential silica exposure by spending as much time as possible inside the vehicle cab while drilling is in progress. To be effective, the cab must be well sealed and ventilated. Door jams, window grooves, power line entries and other joints should be tightly sealed. Provide a slight positive pressure, using filtered air, to prevent dust from leaking into the cab. For the best dust control, use a high-efficiency particulate air (HEPA) filter. Some equipment permits the operation of the drill from inside the cab.
An exposure survey found that if operators spend time inside a fully enclosed cab and use wet drilling together with a dust collection system, dust exposures can be reduced up to 76 percent when compared to wet drilling alone. While the use of enclosed cabs substantially reduces silica exposures, operators might be unwilling to keep windows and doors closed if the cab is not air conditioned. Equipment might be upgraded by installing aftermarket ventilation and air conditioning systems. Even in a sealed cab, dust already inside the cab can become airborne. Clean cabs daily to remove dust tracked in on boots or settled on surfaces.
DRYWALL FINISHING
Even when dust does not contain silica, employees performing dusty jobs may be at risk. Excessive exposure to airborne dust can contribute to tissue injury in the eyes, ears and respiratory passages.
When sanding drywall joint compound, employees generate a substantial amount of airborne dust. The smallest dust particles – the respirable particles – are hazardous because they are deposited deep in the lungs. Dust that contains silica presents a particularly dangerous hazard, but exposure to high levels of dust, whether or not it contains silica, can also be harmful to health. To avoid potentially hazardous exposures, employers should implement effective dust control measures during all drywall finishing activities.
The primary method for avoiding silica exposure, and thereby eliminating the risk of developing silicosis, is to use only silica-free joint compounds. Drywall finishers can also reduce their dust exposure by using vacuum dust collection equipment or wet sanding methods.
SILICA DUST CONTROL MEASURES
Silica-Free Joint Compounds
Many manufacturers offer joint compounds that contain little or no silica. In a study of six brands of joint compound purchased at retail stores, no crystalline silica was detected in three brands. The silica present in a sample of one of the six products, however, was substantially different from the percentage listed on the material safety data sheet (MSDS) for that product. Nonetheless, OSHA recommends that employers rely on manufacturers’ information and use proper methods to minimize employees’ dust exposures, rather than testing joint compounds themselves.
Vacuum Dust Collection Systems
Vacuum dust collection (VDC) systems for drywall sanding equipment are commercially available, and studies show that they significantly reduce total dust concentrations. Vacuum dust collection systems typically consist of a sanding screen and a head, with a hose port to connect a portable wet or dry vacuum. Vacuum sanders can be handheld or pole-mounted.
Wet Sponge Method
Wet methods are often the most effective means of controlling dust because particles never have a chance to become airborne. Drywall compound manufacturers often recommend using wet finishing methods for dust control.
The wet sanding method for drywall finishing uses a sponge to wet the drywall joint compound and remove residues. For wet sanding, saturate a sponge with clean lukewarm water and wring it out to prevent dripping. Then gently rub the high spots using as few strokes as possible to avoid grooving the joints. The sponge should be cleaned frequently.
In addition to reducing employee exposures, wet finishing methods offer other advantages. For example, wet methods often require less cleanup, the wallboard face is not scuffed during finishing, and joints are easier to conceal with paint than joints that are dry sanded. Wet finishing can be more complicated on poorly finished joints because employees may find it difficult to remove large amounts of joint compound with this method. Therefore, employees should apply joint compound smoothly so that little finishing is required.
Some contractors are concerned about the increased drying time associated with wet methods. All wet-sanded areas must dry thoroughly before applying additional coats of joint compound or decorating. Some employees, however, already use heat guns or space heaters to shorten joint compound drying times; these methods allow painting to begin sooner, even after wet sanding. Further, the time spent drying the joint compound might be offset by the time it would otherwise take to remove dust particles from the walls before painting.
TIPS FOR USING DRYWALL SPONGES
• Use quality application techniques to minimize excess joint compound on the surface.
• Use as few sanding strokes as possible to avoid grooving the surface.
• Use drying aids to shorten drying time.
• Make sure that sponges and water buckets are thoroughly cleaned after each use to prevent dust from drying on the equipment and becoming airborne.
• Consult manufacturers’ recommendations for wet sanding drywall compounds.
RESPIRATORY PROTECTION
Overall, effective wet methods are invaluable in keeping silica levels below hazardous levels.
However, in situations where wet methods may not be appropriate or feasible, and VDC systems are used as an alternative control option. It is necessary to supplement the VDC system with the worker wearing a properly fitted, NIOSH-approved half-face-piece or disposable respirator equipped with an N-, R- or P- 95 filter to ensure exposure is below hazardous levels.
In situations, where neither method (Wet, VDC) can be incorporated, it is necessary for the worker to wear a full-face-piece respirator equipped with an N-, R- or P-95 filter to ensure exposure is below hazardous levels.
Other employees in close proximity to the work operations where silica dust is generated may also need respiratory protection if effective controls are not implemented. The level of respiratory protection is dependent on the employee’s silica exposure, which varies depending on factors in the work environment (such as enclosed, semi-enclosed, or open spaces and/or multiple operations generating silica dust), environmental conditions (such as wind direction and speed), and the percentage of silica found in the material.
WORK PRACTICES
Common sense work practices can help employees limit their exposure to silica. Examples include:
• Clean up spills and waste before dust can spread.
• Wear a rubber apron to keep wet dust off clothing. When it dries, the dust can become airborne.
• Whenever possible, work upwind of any dust sources. This can be as simple as working from the other side of the pile when shoveling debris.
• Keep roadways damp at sites where the surface includes high silica aggregate or crushed concrete.
• Wet down silica-containing debris and rock spoil piles prior to removal or disturbance. Encourage employees to watch for dust sources containing silica and make adjustments or use dust control methods to reduce their silica exposure.
Dumping or Pouring Materials
The farther objects fall when dropped, the more dust they will generate on impact. When dumping or pouring materials (for example, debris into a dumpster or raw materials into a mixer), minimize drop distances by releasing materials close to their destination level. Support the bag, bin, or barrow just above the top of the pile and slowly add materials onto the pile. When a long drop is unavoidable, use enclosed disposal chutes or slides.
• Use wheelbarrow ramps of appropriate height (not too tall for a small dump pile).
• Moisten the dumpster contents, floors and walls prior to adding any debris to reduce dust released upon impact.
• Spray the debris stream with water mist to help suppress dust.
Sweeping
• Take steps to limit the use of dry sweeping. Reduce the quantity of debris and the distance and frequency of sweeping. Use a vacuum or wet mop, or moisten the material and scrape it into position.
• Collect and transport debris by bucket or wheelbarrow from smaller local piles rather than pushing it for longer distances to a central pile.
• Avoid dry sweeping debris with sweeping compounds that contain quartz sand (crystalline silica) as the grit.
Removing Debris from Slots or Uneven Surfaces
• Use a vacuum instead of a blower. Use vacuum hose attachments sized for the situation. For example, remove tailings from handheld drill holes using a HEPA-filtered vacuum.
• Flush cracks with water instead of using compressed air.
Vacuums
• Use vacuums with self-cleaning features (back-pulse). Make sure that employees are fully trained in vacuum operation.
• Handle vacuum bags carefully and have a disposal receptacle nearby.
• Avoid overfilling vacuum canisters or bags. The extra weight makes bags difficult to handle and subject to tearing.
• Avoid shaking or jarring the vacuum. Follow the manufacturer’s instructions for recommended handling.
• Avoid depositing or storing collected debris where it will be disturbed or run over and become a source of dust exposure for another employee.
REFERENCES
Controlling Silica Exposure in Construction: