Silica is found in many sands, stones and rocks, and in the products containing them. The mineral is abundant, one of the most common on earth. The form of silica of concern to health is more properly known as crystalline silica which is molecules of SiO2 ordered in a crystalline structure. It is sometimes called free crystalline silica because “silica” is largely just SiO2, thus free of other elements.
The most common form of silica is quartz, and it is encountered very widely in nature being a component of desert and beach sands, farm soils and the country rock from which minerals are mined (e.g. gold and coal mines). Consequently, whenever the earth is disturbed or sands and stones are worked, such as in mining, quarrying, tunnelling, drilling and digging foundations or trenches, silica is likely to be encountered. Because it is abundant, accessible and relatively cheap, silica has found application in many industrial processes: for example in moulding sands in foundries; in abrasive materials for abrasive blasting, in construction materials such as concrete, tiles and bricks; and as a primary constituent of ceramics and glass.
Products made from natural silica containing materials are commonplace, for example tomb stones, billiard tables, cladding on buildings and granite counter tops. Whenever these materials are cut, polished or ground, silica dust is generated. Because of silica’s versatility and abundance many millions of people are exposed worldwide. The USA’s Occupational Safety and Health Administration (OHSA) estimated that slightly over a million workers were exposed in the USA in 2013. (Incidentally, in 2016 OHSA published a substantial document on silica called Occupational Exposure to Respirable Crystalline Silica. A Rule by the Occupational Safety and Health Administration. It is a comprehensive reference on exposures and diseases and is available from https://www.osha.gov/silica.)
In poorer more populous countries the numbers are even higher, India and China in particular. In South Africa, the number of exposed workers is unknown, but will be substantial because many of the silica-generating industries operate here, mining and construction in particular. Combining numbers exposed and the likelihood of silica exposure, the most important locally mined commodities are probably gold, coal, aggregate and sand, and diamonds. Platinum mines typically have country rock low in silica, but silicosis has been found in platinum miners without documented exposure to other sources of silica, albeit rarely. Silica is one of the toxins that cause many diseases; probably the best known of which is silicosis – the scarring of the lung formerly known as phthisis. Silicosis is incurable; no treatment has been shown to reduce the scarring or to stop the disease getting worse. Of substantial concern for individual workers but also for public health is that silica dust causes an increased risk of tuberculosis – silica exposed people are about three times more likely to get the infection than their unexposed counterparts.
Because so many miners got tuberculosis and passed it on to others, the industry contributed to the epidemic in southern Africa. Other serious diseases due to the mineral are chronic obstructive pulmonary disease (COPD) – the same disease that smokers get - and lung cancer. Silica is a less potent cancer causing substance (carcinogen) than agents such as tobacco, asbestos and arsenic, but the evidence that it causes lung cancer is nevertheless strong. Less common, but serious, diseases are due to silica’s effect on the immune system, immune-linked skin and kidney diseases are associated with exposure. With the exception of tuberculosis, the silica associated diseases typically arise from long-term (chronic) exposure, usually 20 or more years, unless exposure levels were high in which case much shorter duration exposure causes the diseases.
It is important to know that silica diseases can present for the first time long after exposure has ceased and that silicosis progresses in the majority of people, even in those with cessation of exposure. These factors mean that the burden of disease in current employees underestimates the eventual impact of exposure, typically to a large extent, and that access to diagnostic services is required for the lifetime of former workers. Silica diseases are caused by breathing the very fine dust (termed respirable dust) that can reach the deep parts of the lungs where gas is exchanged. Activities that generate freshly produced, very fine dry dust un-associated with clay – drilling or cutting rocks or concrete for example - are particularly dangerous.
To determine whether there is a risk of disease the respirable dust in the workers’ breathing zones has to be measured, and then the silica in the respirable dust has to be quantified. The techniques for measuring respirable dust and the silica in it are well established, but because very small amounts of respirable silica have to be measured great care has to be paid to doing it properly and quality controlled laboratories should determine silica concentrations. To capture the respirable dust, workers’ wear a filter attached to a small pump which draws a known volume of air through the filter so that the fine dust is deposited onto it.
Because an individual worker’s exposure is being determined, this is known as a personal sample.
The dust on the filter is then weighed and the amount of silica, usually in micrograms, is ascertained.
The concentration of the respirable dust in the air is expressed as milligrams or micrograms per m3 of air, i.e. per 1 000 litres. Workplace air standards have been set for respirable silica: the South African standard (known as an occupational exposure limit – OEL) is 0.1 mg/m3 or 100 µg/m3. Measuring 0.1 mg (a 10th of 1000th of a 5th of a teaspoon of dust) in a 1 000 litres of air clearly takes care and expertise. Our OEL is not stringent enough to prevent silica associated diseases in everyone.
The USA has recently lowered its standard to 0.05 mg/m3 and a widely cited standard which is thought to protect everyone is a quarter of our OEL at 0.025 mg/m3.
Controlling respirable dust is fundamental to preventing silica associated diseases and should be the focus of occupational health services, workplace management and employees, and the inspectorates. Large companies generally rely on occupational hygienists and engineers to plan and oversee programmes to reduce dust, but enterprises and practitioners can access information from internet tools developed by occupational health agencies.
These tools are designed to assist in the selection of practical controls based on practices that have worked in similar settings. The UK’s Health and Safety Executive’s COSHH Essentials [www.hse.gov.uk/pubns/guidance] and the Netherlands’ Stoffenmanager [https://stoffenmanger.nl/] are useful sites; as is South Africa’s MOSH Learning Hub site established by the Chamber of Mines www.mosh.co.za.
The USA’s National Institute for Occupational Safety and Health (NIOSH) has a range of publications on hazard control in mining including a handbook on dust control in minerals mining and processing available at http://www.cdc.gov/niosh/mining/works/coversheet1765.html# The n for Diseases control and Prevention Smoking and silica exposure probably increase the risks of lung cancer and COPD by more than the sum of the risk from each exposure, Also, smoking increases the chances of developing active tuberculosis.
Smoking cessation programmes should, therefore, be an integral part of interventions to prevent disease in workplaces with silica exposure. It is generally recommended that workers with silicosis should be removed from further exposure to silica. This recommendation is based on the reasonable assumption that the risks of disease progression and tuberculosis will be increased by additional exposure. The evidence for increased risks is, however, scant, and removal from exposure may mean job loss. The health effects of unemployment and lack of income can be profound. Consequently, a considered approach is required which balances the impacts of various interventions. It should also be borne in mind that there is no evidence that removal from exposure is urgent. The silica associated diseases are serious but entirely preventable through dust control.
The mining industry’s milestone to eliminate silicosis (and by inference the other diseases) is therefore laudable. The milestone formulated at the 2014 Occupational Health and Safety Summit Milestones is: To eliminate Silicosis: By December 2024, 95% of all exposure measurement results will be below the milestone level for respirable crystalline silica of 0.05 mg/m3 (these results are individual readings and not average results).
Using present diagnostic techniques, no new cases of silicosis will occur amongst previously unexposed individuals. (“previously unexposed individual” are those unexposed to mining dust prior to December 2008 i.e. equivalent to a new persons who entered the industry in 2009)
David Rees Head: Occupational Medicine and Epidemiology, National Institute for Occupational Health, NHLS. Professor of Occupational Health, University of the Witwatersrand, Johannesburg.
