18.7 Abrasive Blasting and Surface Preparation
Key Takeaways
- Silica sand as an abrasive is the classic preventable exposure; NIOSH recommends substitution with lower-silica media such as steel grit, garnet, or staurolite.
- The substrate matters as much as the abrasive: blasting painted steel can release lead, cadmium, or chromium regardless of how clean the medium is.
- OSHA 29 CFR 1910.94(a) requires blast-cleaning enclosures to be exhaust ventilated so that a continuous inward flow of air is maintained at all openings.
- Abrasive-blasting operators must use a NIOSH-approved type CE supplied-air respirator with a hood or helmet, not an air-purifying respirator.
- Blasting generates noise routinely exceeding 105 dBA, heat stress inside the hood, and injection injuries from the abrasive stream, so the hazard assessment must be multi-agent.
Abrasive Blasting and Surface Preparation
Abrasive blasting appears in the BGC Work Environments and Industrial Processes subject area alongside welding, spray painting, and vapour degreasing. It is worth studying carefully because it is a multi-agent operation: a single task simultaneously produces respirable silica, toxic metals, extreme noise, heat stress, and a physical injection hazard, and each requires a different control.
1. Two Independent Sources of Exposure
The most common analytical error in blasting assessment is attributing all exposure to the abrasive.
ABRASIVE MEDIUM SUBSTRATE COATING
(what you shoot) (what you shoot at)
| |
Silica, metals, Lead, cadmium,
trace contaminants chromium, zinc,
| coal tar, PCBs
| |
+------------> AIRBORNE <------------+
DUST
Blasting a lead-painted bridge with a silica-free abrasive still produces a lead exposure, frequently far above the PEL, and the resulting spent media is likely a toxicity-characteristic hazardous waste. The exposure assessment must always cover both the medium and the coating.
2. Abrasive Media
| Medium | Silica content | Notes |
|---|---|---|
| Silica sand | Very high | The classic cause of accelerated silicosis in blasters; banned or heavily restricted in many jurisdictions and strongly discouraged by NIOSH |
| Coal slag / boiler slag | Low crystalline silica | Widely used, but can contain beryllium, arsenic, and other trace metals; requires its own hazard evaluation |
| Garnet | Low | Good performance, low free silica |
| Staurolite | Low | Low free silica alternative |
| Steel shot and grit | None | Recyclable, low dust generation, but heavy and requires recovery equipment |
| Aluminium oxide, silicon carbide | None (silicon carbide is not crystalline silica) | Aggressive cutting, higher cost |
| Sodium bicarbonate, dry ice, plastic media | None | Low-aggression options for sensitive substrates; dry ice adds a CO₂ asphyxiation hazard in confined areas |
NIOSH's long-standing recommendation is substitution away from silica sand, which is the highest-order control available for the medium. Note the exam trap: silicon carbide is not crystalline silica, and a low-silica medium does not address the coating hazard at all.
Wet and vacuum blasting methods suppress dust at the point of generation and are a process-change control that reduces both worker exposure and site contamination.
3. Enclosure and Ventilation Requirements
OSHA 29 CFR 1910.94(a) governs abrasive blasting ventilation, and the core requirement is directional:
The blast-cleaning enclosure shall be exhaust ventilated in such a way that a continuous inward flow of air will be maintained at all openings of the enclosure during the blasting operation.
Associated requirements:
- Doors must be flanged and tight when closed; doors on the ends of blast enclosures containing conveyors must overlap the opening.
- Exhaust ducts must not discharge into the workroom; the dust collector must be located outside the blasting enclosure area where practicable.
- Visibility inside the enclosure must be maintained by adequate ventilation, since a blaster who cannot see is both less productive and less safe.
- Blast cleaning rooms that operators enter require sufficient air changes and a supply of make-up air.
- Static electricity must be controlled by bonding and grounding, since dry abrasive flowing through a hose generates substantial charge.
The pressure logic is the mirror image of the operator booth discussed under isolation and containment: the blast enclosure is negative relative to the room, while any operator enclosure is positive relative to the room.
4. Respiratory Protection: Type CE Is Mandatory
This is the most frequently tested single fact about abrasive blasting.
Operators performing abrasive blasting must wear a NIOSH-approved type CE abrasive-blasting respirator — a supplied-air respirator with a hood or helmet that also protects the head and neck from rebounding abrasive. Type CE respirators are approved in continuous-flow and pressure-demand configurations.
An air-purifying respirator is not acceptable for the blasting operator, for several converging reasons:
- Dust concentrations inside a blast hood routinely reach hundreds of times the exposure limit, exceeding any air-purifying respirator's maximum use concentration.
- The blast stream carries no warning properties and no reliable end-of-service indicator.
- A hood is required for impact protection from rebounding abrasive regardless of the respiratory question.
Breathing air must meet CGA Grade D or better: 19.5 to 23.5% oxygen, carbon monoxide no more than 10 ppm, carbon dioxide no more than 1,000 ppm, condensed hydrocarbons no more than 5 mg/m³, and no objectionable odour. Where an oil-lubricated compressor supplies the air, a carbon monoxide alarm or high-temperature alarm is required, because compressor overheating produces CO directly at the intake side of the system. Compressor intakes must be sited away from vehicle exhaust and other combustion sources.
Helpers and adjacent workers who are not inside the blast stream still require respiratory protection selected on the basis of their own measured exposure — commonly a half-mask or full facepiece with a P100 filter, but that determination must be made from data.
5. The Other Agents
Noise. Abrasive blasting is among the loudest routine industrial operations, commonly exceeding 105 dBA at the operator and often higher inside an enclosure where reverberation adds several decibels. Dual hearing protection is frequently necessary, and the blast hood itself provides little attenuation. A worker at 105 dBA reaches 100% OSHA dose in under an hour.
Heat stress. The hood, the protective suit, and the supplied air combine to impair evaporative cooling. Supplied air can be conditioned with a vortex tube or air-conditioning device, which is a genuine engineering heat-stress control rather than a comfort item. Work/rest scheduling and hydration must be planned before the job, not improvised.
Physical injury. The abrasive stream can cause severe injection injury and subcutaneous emphysema on contact with skin. Controls include a dead-man control valve on the nozzle that stops flow when released, nozzle-hose coupling restraints and whip checks to prevent hose whipping if a coupling fails, and prohibiting the practice of directing the nozzle at another person.
Confined space. Blasting inside tanks, vessels, and ship hulls combines all of the above with a permit-required confined space: the operation itself generates the atmospheric hazard, so continuous monitoring, forced ventilation, an attendant, and rescue provisions are required.
6. Waste and Environmental Considerations
Spent abrasive contains both the medium and everything removed from the substrate. Where the coating contained lead, cadmium, or chromium, the spent media frequently fails the Toxicity Characteristic Leaching Procedure and must be managed as hazardous waste. Outdoor blasting also requires containment — shrouding, negative-pressure enclosures, or ground sheeting — to prevent site and off-site contamination and to protect adjacent workers and the community.
A contractor removes lead-based paint from a steel bridge using a garnet abrasive containing less than 1% free silica. Which statement best describes the exposure hazard?
What respiratory protection is required for the operator performing abrasive blasting, and why?
Under 29 CFR 1910.94(a), how must a blast-cleaning enclosure be ventilated?
An abrasive blasting job inside a large storage tank is being planned. Which combination of hazards must the plan address beyond respirable dust and metals?