Controlling silica dust

Silica dust can be controlled very effectively with tools and methods that are widely available. Water, well-designed vacuum dust collection and good planning can reduce exposure by 90 per cent or more for most common tasks. This lesson works through the hierarchy of controls for silica, explains how wet methods and local exhaust ventilation work, shows how to size a dust collector, and describes housekeeping, administrative controls and the place of respirators. HSE-240 covers respirator selection and fit testing; this lesson only explains how respirators fit into a silica control plan.

The hierarchy of controls applied to silica

Controls at the top of the hierarchy are more reliable because they do not depend on people remembering to use them. In practice, most silica tasks need a combination: an engineering control at the source, supported by work practices and housekeeping, with respirators where the remaining exposure is still too high.

Elimination and substitution

The most effective control is to avoid creating silica dust at all. This often has to be decided at the design or planning stage, long before workers arrive on site.

• Design out cutting and drilling. Cast-in anchors, sleeves and openings in concrete avoid later coring and drilling. Ordering block, pavers and stone cut to size at a controlled facility moves the dust to a place where it can be controlled better.

• Choose methods that make less dust. Splitting pavers with a block splitter instead of cutting them; using powder-actuated fasteners or adhesive instead of drilling; bursting or crushing concrete instead of jackhammering where feasible.

• Substitute materials. Use non-silica abrasives for blasting, such as steel grit, garnet with low free silica, crushed glass or dry ice. Choose lower-silica products where they meet the design need, such as porcelain or sintered surfaces in place of high-silica engineered stone, and check their silica content on the safety data sheet.

Wet methods

Water suppresses dust by wetting particles at the point where they are created, making them heavier so that they fall rather than become airborne. Wet methods are simple, cheap and effective when done properly. Studies of wet cutting with masonry saws commonly report reductions in respirable silica of 80 to over 90 per cent compared with dry cutting.

For wet methods to work:

• Water must reach the point of cutting or grinding continuously, through an integrated water feed on the tool or a nozzle aimed at the blade or bit, not by occasionally spraying the surface.

• The flow must be enough to wet the dust throughout the task. Follow the tool manufacturer's recommended flow rate; too little water produces a fine mist of dusty droplets that can dry and become airborne.

• Slurry must be cleaned up before it dries, because dried slurry turns back into respirable dust.

• Electrical safety must be managed with ground-fault protection and tools designed for wet use. Water also creates slip hazards and, in cold weather, ice.

Local exhaust ventilation and vacuum dust collection

Local exhaust ventilation (LEV) captures dust at the source before it reaches the worker's breathing zone. For hand tools, this means a shroud around the blade, wheel or bit connected to a vacuum dust collector. For stationary equipment, it may mean a hood connected to a fan and filter. An effective tool-mounted system has four parts working together:

Sizing the vacuum

Recognized guidance for hand-held grinders used for mortar removal and surface grinding calls for a vacuum providing at least 25 cubic feet per minute of airflow per inch of wheel diameter, about 11.8 litres per second for each 25 millimetres. The figure comes from OSHA's Table 1 and is widely used in Canadian guidance and by tool manufacturers. A 125 mm (5-inch) grinder therefore needs about 59 L/s, and a 180 mm (7-inch) grinder about 83 L/s. Vacuums are often rated by their maximum airflow with no hose or filter load, so the airflow delivered at the shroud is usually lower than the label suggests.

CFM_PER_INCH = 25          # recognized airflow guidance for grinders
LPS_PER_CFM = 0.4719       # 1 cfm = 0.4719 L/s

def required_airflow(wheel_inches):
    cfm = CFM_PER_INCH * wheel_inches
    return cfm, cfm * LPS_PER_CFM

for wheel_in, wheel_mm in ((4.5, 115), (5, 125), (7, 180), (9, 230)):
    cfm, lps = required_airflow(wheel_in)
    print(f"{wheel_mm} mm ({wheel_in} in) wheel: at least {cfm:5.1f} cfm = {lps:5.1f} L/s")

# Effect of losses: a vacuum rated at 150 cfm open-flow may deliver far less.
rated = 150
for loss, cause in ((0.25, "long hose"), (0.40, "long hose and partly loaded filter")):
    delivered = rated * (1 - loss)
    ok = "enough" if delivered >= 175 else "not enough"
    print(f"rated {rated} cfm, {cause}: about {delivered:.0f} cfm delivered, {ok} for a 7 in grinder")

115 mm (4.5 in) wheel: at least 112.5 cfm =  53.1 L/s
125 mm (5 in) wheel: at least 125.0 cfm =  59.0 L/s
180 mm (7 in) wheel: at least 175.0 cfm =  82.6 L/s
230 mm (9 in) wheel: at least 225.0 cfm = 106.2 L/s
rated 150 cfm, long hose: about 112 cfm delivered, not enough for a 7 in grinder
rated 150 cfm, long hose and partly loaded filter: about 90 cfm delivered, not enough for a 7 in grinder

The loss figures in the script are illustrative, but the point is real: check the tool and vacuum manufacturers' combined specifications, keep hoses short, empty the pre-separator and clean or change filters as instructed. Some vacuums include an airflow indicator or alarm that warns when airflow falls too low; use one where you can.

Capture velocity and hood placement

For fixed hoods on benches and stationary equipment, what matters is the air speed at the point where dust is released, called the capture velocity. Air speed falls off very quickly with distance from the hood opening; for a simple open hood, the speed at one hood-diameter away is only about a tenth of the speed at the face. Hoods must be placed as close as possible to the source and enclose it where practical. A hood a metre away from a grinding wheel is almost useless.

Enclosed cabs and control rooms

On heavy equipment such as drill rigs, crushers, excavators breaking concrete and milling machines, an enclosed cab with filtered, pressurized air can protect the operator. The cab must be sealed, with doors and windows closed during dusty work, a filtration system with high-efficiency filters, positive pressure to keep dust out, and heating and cooling so that operators do not open windows. Cabs must be kept clean; a dusty cab floor becomes a source of exposure.

Combining controls

Controls work together. If one control removes 90 per cent of the dust and a second removes 50 per cent of what remains, the combined reduction is 95 per cent, not 140 per cent. Each control acts on what the previous one left behind, so the fractions remaining multiply.

uncontrolled = 1200   # ug/m3, illustrative dry-cutting exposure during the task

controls = [
    ("integrated water feed", 0.90),
    ("cutting outdoors, worker upwind", 0.30),
    ("reduced cutting time by pre-ordering sizes", 0.50),
]

remaining = uncontrolled
for name, reduction in controls:
    remaining *= (1 - reduction)
    print(f"after {name:42s} {remaining:7.1f} ug/m3")

overall = 1 - remaining / uncontrolled
print(f"overall reduction {overall:.1%}")

for apf, resp in ((10, "half-facepiece, APF 10"), (25, "loose-fitting PAPR, APF 25")):
    print(f"inside {resp}: about {remaining / apf:.1f} ug/m3 if worn and fitted correctly")

after integrated water feed                        120.0 ug/m3
after cutting outdoors, worker upwind               84.0 ug/m3
after reduced cutting time by pre-ordering sizes    42.0 ug/m3
overall reduction 96.5%
inside half-facepiece, APF 10: about 4.2 ug/m3 if worn and fitted correctly
inside loose-fitting PAPR, APF 25: about 1.7 ug/m3 if worn and fitted correctly

Even after a 96 per cent reduction, a very dusty uncontrolled task can still leave exposure above the limit during the task, which is why respirators are still needed for some tasks. The respirator calculation depends on the assigned protection factors and fit testing covered in HSE-240.

Administrative controls and work practices

Administrative controls change how work is organized rather than the equipment used. For silica they include:

• Planning and sequencing so that dusty tasks happen when fewer people are nearby, for example cutting before other trades arrive.

• Restricting access to areas where dusty work is under way, with signs, barriers or a designated cutting area set up downwind and away from air intakes.

• Positioning workers upwind of outdoor cutting and away from the dust plume.

• Maintenance schedules for water feeds, shrouds, hoses and filters.

• Training and supervision so that workers know how to set up controls and supervisors check they are used.

Job rotation, moving workers between dusty and clean tasks to reduce each person's exposure, is generally not recommended for carcinogens like silica. It spreads the risk across more people rather than reducing the total.

Housekeeping

Settled dust is a reservoir of future exposure. Clean up with wet methods, such as wetting the dust and scooping or mopping it up, or with a vacuum fitted with a HEPA filter. Do not dry sweep, dry brush or use compressed air on silica dust, as Lesson 3 explained. Clean up slurry before it dries. Dispose of collected dust in sealed bags or containers so that it does not become airborne again.

Respirators in the silica control plan

Respirators are needed where engineering and administrative controls cannot keep exposure low enough, during the time it takes to install other controls, and in emergencies. For silica, typical choices are filtering facepiece or elastomeric half-facepiece respirators with particulate filters, full-facepiece respirators and powered air-purifying respirators, chosen according to the expected exposure and the assigned protection factor. A respirator program with medical evaluation, fit testing, training and maintenance must be in place, as described in HSE-240. Respirators should never be the only control for a task where engineering controls are available.