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All about silica dust

Anyone working at a construction site or quarry will be exposed to dust from wood or stone. They can breathe in this dust, which is known as silica. Find out why this can be harmful, along with advice on how to manage exposure. Our guidance is for business owners, managers, workers and health and safety professionals.

What is silica?

Silica is a natural substance found in stone, rocks, sand and clay in its crystalline form. It is also found in bricks, tiles, concrete and some plastics. When these materials are worked on, for example when being cut or drilled into, crystalline silica is released as a very fine dust that can be breathed in. This dust causes significant ill-health effects, including silicosis and cancers.

There are other forms of silica that do not have a crystal structure and these are referred to as amorphous silica. This can be found in diatomaceous earth, silica gel and synthetic amorphous silica.

Graphic showing the amount of silica in different substances. Sandstone 70 to 90 per cent. Plastic composites 19 to 90 per cent. Concrete and mortar 25 to 70 per cent. Shale 40 to 60 per cent. China stone, up to 50 per cent. Granite 20 to 45 per cent. Tile 30 to 45 per cent. Slate 20 to 40 per cent. Brick, up to 30%. Ironstone, up to 15 per cent. Basalt and dolerite, up to 5 per cent. Limestone, up to 2 per cent. Marble, up to 2 per cent.

Why is silica harmful?

Silica dust is only harmful when it’s inhaled deep into the lungs, where oxygen is transitioned into the blood. Silica can be found in sand but sitting on a sandy beach will not cause any respiratory harm because any sand particles breathed in will generally be too big to go beyond the nose or upper airways. It is the very fine airborne silica dust that can be harmful. This is known as the respirable fraction.

Respirable particles are typically less than around 5 micrometres (μm) in size. Compare this size to the ‘full stop/period’ at the end of the sentence, which is around 200-300 μm in diameter, and the finest sand on the beach, which is about 50-70 μm. Individual silica dust particles are so small that they are invisible to the naked eye in normal light. This results in relatively high airborne concentrations with no visible or physical awareness that people are inhaling the dust.

In 1996, the International Agency for Research on Cancer (IARC) reviewed the scientific evidence and concluded that crystalline silica dust is carcinogenic to humans. It is classified as a Group 1 carcinogen, meaning it is a definite cause of cancer in humans.

Exactly how silica dust causes lung cancer is unclear. The IARC suggests the most likely cause is when silica dust deposits in the lungs, the toxicity makes it difficult for the body’s natural defence cells to remove it. The dust remains in the lungs, causing persistent inflammation. This inflammation can damage deoxyribonucleic acid (DNA) in the lung cells and lead, in some people, to lung cancer.