Silica dust at work: exposure, monitoring and controls

Short answer: the limit for respirable crystalline silica is 0,1 mg/m³, and it is a maximum limit, so exposure must be kept as far below it as is reasonably practicable. It applies to the respirable fraction of quartz, cristobalite, tridymite and tripoli. Where employees may be exposed, an approved inspection authority (AIA) must measure their exposure at least every 24 months.

The limit is in Table 2 of Annexure 2 to the Regulations for Hazardous Chemical Agents, 2021, published as Government Notice R. 280 in Government Gazette 44348 on 29 March 2021. They repealed the Regulations for Hazardous Chemical Substances, 1995, and their exposure limit tables took effect 18 months after promulgation. Mines are outside their scope: section 1(3) of the Occupational Health and Safety Act, 1993 excludes a mine and a mining area.

Where silica dust comes from

Crystalline silica is found in sand, stone, concrete, brick, block and mortar. The dust that matters is the respirable fraction: particles at least 100 times smaller than ordinary sand. Typical sources are:

• Construction: cutting, sawing, drilling and grinding concrete, brick, block and mortar.

• Stone and building products: cutting or crushing stone, and manufacturing stone countertops, bricks, concrete blocks and ceramic products.

• Foundries and blasting: foundry work, and abrasive blasting with sand.

Workers who inhale these particles are at increased risk of silicosis, an incurable lung disease, and of lung cancer, chronic obstructive pulmonary disease and kidney disease.

The limit, as the regulations state it

Table 2 lists "Silica, crystalline" in four forms, all in mg/m³, where (R) means the respirable fraction:

• Cristobalite (CAS 14464-46-1): 0,1 (R), notation CARC.

• Quartz (CAS 14808-60-7): 0,1 (R), notation CARC.

• Tridymite (CAS 15468-32-3): 0,1 (R).

• Tripoli (CAS 1317-95-9): 0,1 (R).

CARC marks a carcinogen in GHS category 1A or 1B. Because Table 2 sets maximum limits, regulation 10(1)(b) regards control as adequate only if exposure is "at a level as low as is reasonably practicable below that maximum limit". That has two practical consequences:

• Below 0,1 mg/m³ is not automatically enough. If a further reduction is reasonably practicable, the regulations expect it.

• Respirators do not count as adequate control. Regulation 10(1)(a) lets a restricted limit be met with respirators in a zoned area once other controls have done what is reasonably practicable. Regulation 10(1)(b) has no such provision for maximum limits, only conditions for temporary excursions.

Occupational exposure limits explained covers both types of limit.

How silica exposure is measured

• By an AIA, at least every 24 months: regulation 6 requires monitoring by an AIA that is representative of employees' exposure, after the health and safety representative or committee has been informed and given a chance to comment.

• Personal samples: a pump draws air through a cyclone worn in the breathing zone, which collects the respirable fraction on a filter over the shift.

• Laboratory analysis: the filter is analysed for crystalline silica, usually by X-ray diffraction or infrared spectrometry.

• A silica result, not just a dust result: the report should give respirable crystalline silica for each sample. A respirable dust figure alone does not show the silica exposure.

• Records: keep assessment and monitoring records for at least 30 years (regulation 9(e)).

Controls that work

Regulation 10(2) lists the measures an employer must use where reasonably practicable. For silica, the main controls are:

• Wet methods: water suppression on saws, grinders and drills; the regulations name "use of wet methods" (regulation 10(2)(e)(iii)).

• Extraction: on-tool extraction, or local extraction ventilation at fixed processes (regulation 10(2)(e)(ii)). It must be examined and tested by an AIA at intervals not exceeding 24 months; see what the LEV examination checks.

• Enclosure and separation: enclose or automate dusty processes and keep them apart from other work (regulation 10(2)(e)(i) and (iv)).

• Substitution: for example, a blasting abrasive that does not contain crystalline silica (regulation 10(2)(d)).

• Clean-up without compressed air: regulation 13(a) prohibits, as far as reasonably practicable, using compressed air to remove particles of a hazardous chemical agent from any surface or person. Use a suitable vacuum cleaner or wet cleaning instead.

• Respirators: where other controls are not enough, respirators must be capable of controlling exposure to below the limit, correctly selected and properly used (regulation 11(2)).

A quick self-check

• List every task that cuts, grinds, drills, crushes or blasts sand, stone, concrete or brick, and who does it.

• Find your last AIA survey: is it less than 24 months old, and does it report respirable crystalline silica rather than only dust?

• If results are below 0,1 mg/m³, ask whether they could reasonably be lower.

• Check that saws and grinders have water or extraction fitted, and that nobody uses compressed air to clean down.

• Ask your occupational health practitioner whether silica-exposed employees need medical surveillance under regulation 7(1)(b).

How Qualitas can help

Qualitas Occupational Consulting is a Department of Employment and Labour Approved Inspection Authority and a SANAS-accredited (ISO/IEC 17020) inspection body for asbestos, lead, noise and hazardous chemical agents. Our hazardous chemical agents surveys measure respirable crystalline silica with personal samplers during normal work, compare the laboratory results with the Table 2 limit and recommend practical controls, in a report written for management, SHEQ teams and auditors.

Where extraction controls the dust, our local exhaust ventilation surveys check that it works. For intervals and triggers, see how often hazardous chemical agent monitoring must be done.

Not sure what your site needs? Start with our free Compliance Gap Analysis, request a quote, or call or WhatsApp 082 649 6599.

This article summarises the Regulations for Hazardous Chemical Agents, 2021 (Government Notice R. 280, Government Gazette 44348, 29 March 2021) for general guidance. It is not legal advice; refer to the regulations for the full requirements.

Christo de Wet is a SAIOH-registered occupational hygienist with 15 years' experience and the founder of Qualitas Occupational Consulting.