Respirable silica: the hidden carcinogen in drilling mud, cement and bentonite

Crystalline silica (quartz, CAS 14808-60-7, and cristobalite, CAS 14464-46-1) is found in sand, rock and many mineral products. When these products are handled as dry powders, they release fine respirable dust that can reach deep into the lungs. Under the GHS, respirable crystalline silica is classified as Carcinogenicity Category 1A and Specific Target Organ Toxicity, Repeated Exposure, Category 1 (lungs).

Where it turns up on site. Chemical inventories on Ugandan oil and gas and construction projects commonly include silica in:

• Portland and oil-well cements, which may also contain hexavalent chromium, another Category 1A carcinogen;
• bentonite and activated clays used in drilling fluids;
• drilling mud additives and gelling agents;
• some lubricants and fillers.

Health effects

• Silicosis: irreversible scarring of the lungs, which can disable or kill.

• Lung cancer.

• Chronic obstructive pulmonary disease (COPD) and a higher risk of tuberculosis.
• Short-term irritation of the eyes, skin and airways. Wet cement also causes alkaline burns.

The damage builds up over years and often appears after a worker has moved on, which is why it is easy to underestimate.

A typical risk picture. Workers mixing silica-containing powders by hand for about four hours a shift, in semi-open areas, with variable respirator use, face a high risk. That combines a Category 1A carcinogen with frequent, uncontrolled exposure.

Controls, in order of effectiveness

1. Substitute low-silica or pre-mixed liquid products where they perform adequately.
2. Engineer out the dust: closed or bulk transfer systems, bag-slitting stations with extraction, local exhaust ventilation and wet methods.
3. Organise the work: fewer people in the mixing area, job rotation, no dry sweeping, and good housekeeping with vacuums fitted with HEPA filters.
4. Protect the worker: FFP3 or P3 respirators, fit-tested and correctly worn, with goggles and coveralls.
5. Watch for effects: air monitoring, and health surveillance including lung function tests and chest X-rays for exposed workers.

What to do

• Check SDS Section 3 of every powder for quartz or cristobalite.
• Run personal air sampling for respirable crystalline silica during mixing tasks.
• Treat "no recent air monitoring" as a gap in itself, not as evidence that exposure is acceptable.

How INRC can help

INRC can screen dust levels in mixing areas with our particulate monitors, design a personal sampling programme for respirable crystalline silica with laboratory analysis, and advise on controls. See our occupational health and safety and environmental monitoring services. Contact us to discuss your site.

This article is general guidance, not legal advice. Check current legislation and regulations for your specific project.