RESPIRABLE CRYSTALLINE SILICA:
WHAT IS IT?
Crystalline silica is a natural substance found in stone, rocks, sand and clay, as well as products like bricks, tiles, concrete and some plastic composites. When these materials are worked on, for example by cutting or drilling, the crystalline silica is released as a very fine dust which can be breathed in. This dust is one of the oldest workplace hazards – and it still causes hundreds of thousands of deaths across the world every year.
Silica dust is only harmful when it’s inhaled deep into your lungs, where oxygen is taken up into the blood. Sitting on a sandy beach won’t cause any respiratory harm because any sand particles breathed in will generally be much too big to go beyond your nose or upper airways. But as a very fine airborne dust, silica can be dangerous. It’s the respirable fraction that is hazardous.
Respirable particles are typically less than around 5 micrometers in size. Compare this to the full stop at the end of the sentence, which is around 200–300 micrometers in diameter, and the finest sand on that beach, which is about 50–70 micrometers. Individual silica dust particles are so small that they are invisible to the naked eye in normal light – so you can have relatively high airborne concentrations without being aware that the dust is being inhaled.
WHO IS AFFECTED?
Exposure to silica dust occurs in many industries. Common scenarios where people may be exposed include: - breaking, crushing, grinding or milling silica-containing material such as concrete, aggregate or mortar - drilling, cutting, chiselling or sanding silica-containing material - dealing with cement - moving earth, eg excavating, mining, quarrying or tunnelling - abrasive blasting or sandblasting - laying, maintaining or replacing ballast - handling, mixing or shovelling dry materials that include silica - using silica, sand or silica-containing products in the manufacturing process of glass and other non-metallic mineral products - using sand as a moulding medium in foundries - using silica flour (a finely ground form of crystalline silica) - dry sweeping up after a task where silica dust has been created.
Long-term exposure to silica dust – caused by tasks like cutting, drilling, grinding or polishing materials containing silica, or even just sweeping up after a task – can cause silicosis (an irreversible chronic respiratory illness), lung cancer and a number of other serious diseases, including a chronic obstructive pulmonary disease such as emphysema.
Silica dust is one of the most significant causes of work-related disease around the world. Exactly how silica dust causes lung cancer is unclear – the International Agency for Research on Cancer suggests the most likely cause is when the dust deposits in the lungs its toxicity makes it difficult for the body’s natural defence cells to remove and so it stays there, causing persistent inflammation. This constant inflammation can damage the DNA in the lung cells and lead, in some people, to lung cancer.
Toxicology studies with crystalline silica have shown that the biological response is largely determined by the surface properties of the dust particles. Adding a chemical coating to the surface of a quartz particle in a cellular experiment dramatically reduces the inflammatory response. Similarly, quartz particles coated with clay or other materials show less toxicity than freshly crushed rock. Some scientists have suggested that for there to be a risk for lung cancer it’s first necessary to develop silicosis. But the evidence for this is weak, and research shows increased cancer risks among workers who don’t have silicosis. Of course, decreasing exposure to silica will reduce the risk for both diseases.
The human epidemiological evidence shows an increased risk of lung cancer in a wide range of occupations exposed to silica dust. Increased risks are seen at very low levels of exposure – at or below exposure for a working lifetime at the current limit in Britain of 0.1 mg/m3 . This is reflected in reviews like the one recently published by the US’s Occupational Safety and Health Administration. Many countries have moved to reduce the workplace exposure limit to 0.05 mg/m3 or less.
Research from Imperial College London suggests that around 900 new cases of lung cancer each year in Britain can be attributed to past exposure to silica dust in construction, granite and stone industries, and various industrial processes.
Lung cancer is difficult to treat and most of those diagnosed with the disease will die within a few years – only one in 20 will live 10 or more years. It’s estimated that nearly 800 people die a year from lung cancer caused by silica exposure at work. There are about half a million people exposed to silica dust at work in the UK, and probably around 5 million in the European Union.
In Europe, the majority of these people are either employed in the construction sector (81 per cent) or in making products used in construction such as bricks, glass or cement (10 per cent). The people employed in these sectors are mostly in small companies – around 80 per cent are in organisations employing fewer than 10 people.
WHAT YOU NEED TO DO
The main aim should be to stop silica dust getting into the air in the first place. You may be able to select a process that avoids or cuts down the dust being released, for example, taking into account silica dust control at the design stage of a construction project by planning buildings with pre-built recesses for plumbing, gas and electric wiring so there’s less need to cut or drill masonry and concrete, or getting materials cut to size off-site in a facility where it’s easier to control dust exposure.
Try to remove or substitute materials containing crystalline silica from the work process – it may be possible to use a different material, for example substituting olivine or another safer material for silica sand in abrasive blasting. If it’s not possible to protect operatives from silica dust at the design stage or by changing processes or materials, then your first step is to monitor or assess the exposure and identify the jobs and tasks that need better controls.
A survey by the UK’s Health and Safety Laboratory shows that when employers do this they are more likely to put in appropriate controls.
Select from a range of options available to control exposure to silica dust at work:
• In a factory or workshop environment, the best strategy is to use engineering controls like enclosures or hoods and local exhaust ventilation to extract the contaminated air at the point it’s produced, or to use water suppression on fixed machinery.
• Where work with hand-held power tools generates dust, for example on construction sites, the best strategies are to use localised ventilation on the tool or suppress the dust using water spray systems. When you buy or rent new equipment, make sure the controls are appropriate. High-value plant and equipment, for example the rock-drilling machines used in tunnelling or the crushing equipment operating at recycling plants, are now likely to have dust suppression integrated into the design. Small items may not have these controls as standard, although you can often buy an add-on control sleeve to extract the dust.
In all situations, if it’s not possible to adequately control airborne silica dust using engineering solutions, and air sampling data shows that there is still an unacceptable level of dust, then operatives should use personal respiratory protection.
As well as getting all the relevant controls in place, and checking that they’re working properly in practice, you should give people who could be at risk from silica dust exposure information about the possible risks and how exposure can be cut down. And you need to instruct or train affected employees too, including on how to use the control measures in place to protect them.
Where there is a reasonable chance that workers may develop silicosis you should consider introducing health surveillance (legally required in Britain). This may be as simple as the health professional asking an individual about symptoms and making a record of the job and the exposure they receive. In some cases it may be appropriate for the employee to have a chest x-ray and lung function tests, which could help detect chronic obstructive pulmonary disease or silicosis. There is no effective way of detecting lung cancer as part of a surveillance procedure, which underlines the importance of stringently controlling exposure to silica dust to minimise risk.
Respiratory protection is generally considered the ‘last resort’, and health and safety legislation in countries such as Britain positions it in that way. But the reality is that respirators are often necessary to achieve adequate control, either because of the difficulty of controlling exposure using another method, or because it would be impractical and costly to use engineering controls in some circumstances. In addition, respirators are often a very effective way of reducing exposure. For high-risk tasks, you shouldn’t rely on respiratory protection alone – it may not give enough protection, it only protects the person wearing it, and people make mistakes with masks, for example wearing them at the wrong time or for too long.
MEASURING EXPOSURE TO SILICA DUST
We can measure exposure to silica using conventional occupational hygiene sampling gear. To measure exposure an occupational hygienist uses equipment including a battery-operated sampling pump, polythene tubing, cyclone sampling head, pre-weighed PVC filters, a flowmeter to measure the amount of air flowing through the sampler, clips and belts. The equipment is worn throughout the working shift with the sampling head close to the operative’s nose/mouth. After the shift, the filter is sent to a specialist laboratory for analysis for crystalline silica.
The result is expressed as a concentration – milligrams of crystalline silica per cubic metre of air sampled (mg/m3 ). You can also ask the laboratory to reweigh the filter to give the respirable dust concentration and the percentage of silica on your sample. Your data can then be compared with the occupational exposure limit. In Britain this is known as the workplace exposure limit (WEL) and the silica WEL is 0.1 mg/m3 , averaged over eight hours. This type of monitoring should be carried out by a professional with the right level of competence.
This article is a condensed version of IOSH 'No Time To Lose' Fact Sheet ( RESPIRABLE CRYSTALLINE SILICA: THE FACTS)
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