Respirable Crystalline Silica

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John Cherrie, Institute of Occupational Medicine, Edinburgh

Institute of Biological Chemistry, Biophysics and Bioengineering, Heriot Watt University

Introduction

Crystalline silica is the name for a group of naturally occurring minerals found in many types of rock. It can be released into the air when the rock or articles made from the rock are crushed, cut, or worked in some other way. It is the fine fraction of the dust, the respirable fraction, that is harmful to health when inhaled. Respirable crystalline silica is often abbreviated as RCS. Exposure to RCS over many years is harmful to health, causing silicosis and increasing the risk of lung cancer and other serious diseases amongst workers. Restricting exposure to very low levels (around 0.05 mg/m3 of RCS, averaged over each working day) will minimise the risk of harm. Methods of exposure control include eliminating crystalline silica from the process, adapting the process to reduce emission into the workroom, e.g. by using water to keep dust from becoming airborne, use of local ventilation and personal respiratory protection.

Silica minerals

Key points: crystalline silica includes quartz and other related materials. Quartz is found in most rocks and consequently in many building materials. For example, building sand predominantly contains quartz.

The minerals quartz, cristobalite and tridymite are different forms of crystalline silica (silica is silicon dioxide or SiO2); they are crystalline silica polymorphs. In a crystalline mineral the atoms are arranged in a 3-dimensional repeating patterns, and it is this structure that gives the mineral many of its physical and chemical properties. Crystalline polymorphs differ in the crystalline structure although they all have the same composition, in this case SiO2. There are also many minerals made from silicon dioxide that are in a non-crystalline or amorphous form, for example diatomaceous earth, silica gel, and synthetic amorphous silica (or SAS). There are also a group of minerals known as silicates, which are composed of silicon, oxygen and at least one other chemical element. Silicates are the most abundant type of mineral on Earth and include olivine, micas, and clays. It’s important not to confuse amorphous silica or silicate minerals with crystalline silica, which is generally more hazardous than these other materials.

Quartz is found in most rocks, but particularly in sandstone (70- 90% crystalline silica) and granite (typically around 30% crystalline silica); quartz is just yellow sand. Some minerals contain only a small amount of quartz, for example limestone and marble both have around 2% crystalline silica content. Many building products contain crystalline quartz – bricks up to 30% and concrete and mortar between 25 and 70%.

Cristobalite and tridymite occur naturally in lava formations and these minerals can be formed when silica is heated to very high temperatures. For example, diatomaceous earth is a sedimentary mineral deposit of mostly amorphous silica diatom skeletons. For some industrial uses the materials is heated to around 1,000 oC, which converts the amorphous silica to crystalline silica, mostly as cristobalite[1]. Diatomaceous earth is used in a variety of industrial applications, including filtration and in agriculture.

Jobs and industries where exposure occurs

Key points: Exposure to RCS occurs in construction, foundries, steel production, potteries, and other industries. Daily average exposure to RCS is typically around 0.05 to 0.5 mg/m3.

Exposure to RCS can occur in many occupations, for example: abrasive blasting workers, bricklayers, concrete workers, construction workers cutting stone or drilling in stone, crushing and grinding operators, demolition workers, digger drivers, foundry workers, furnace men, glass manufacture workers, kiln operators, machinists, mining machine operators, moulding and casting operators, pottery workers, quarry workers, rock drillers, sandblasters, steelworkers, stonemasons, tunnel workers, welders, and workers grinding, abrading, buffing or polishing.

High levels of RCS exposure are found in particular in mining and quarrying, manufacture of coke and other fuels, manufacture of mineral products, manufacture of metals and machinery, electricity and gas supply, and in construction. Exposure also occurs in brickmaking, stonemasonry, potteries, quarrying and several other industries. Tasks that involve the use of power tools to cut or grind mortar, bricks or mineral products can produce very high airborne levels of dust and RCS.

Exposure to RCS is measured in units of mg/m3, with daily average exposures typically around 0.05 to 0.5 mg/m3. Many EU Member States have national exposure limits for RCS and, in the proposed amendments to the Carcinogens or Mutagens Directive, proposed by the Commission in 2016, a Limit Value for occupational exposure of 0.1 mg/m3 (measured or calculated in relation to a reference period of eight hours) has been recommended.

You cannot rely on visualising airborne dust to assess the extent of the crystalline silica risk because the fine respirable size particles are not easily seen. If dust containing crystalline silica is visible in the air, or the workplace is heavily contaminated with settled dust, then the airborne dust concentration is likely to be greater than any of the existing occupational exposure limits for RCS in the European Union and is potentially harmful to health.

Health hazards from crystalline silica

Key points: Exposure to RCS over many years can cause silicosis and increase the risk of lung cancer amongst workers.

Exposure to RCS at work can cause silicosis, an irreversible chronic respiratory illness, lung cancer and a number of other serious diseases (see below: “Other serious diseases caused by exposure to RCS”). It is one of the most important causes of occupational disease around the world.

Crystalline silica particles can be inhaled and deposit in the narrow lung airways and air sacs (alveoli) where oxygen is taken up into the body. The particles are persistent in the lungs and are toxic to their natural defence cells (macrophages) causing an inflammatory response and subsequent deposition of fibrotic or scar tissue in the lung. As this scarring gets worse it becomes more and more difficult for the worker to breathe, and this worsening can continue after the exposure to RCS stops. The early stages of the disease are described as simple or nodular silicosis, but if there is sufficient exposure the disease may go to what is called ‘progressive massive fibrosis’ (PMF), where the fibrotic nodules (lumps) combine into larger lumps of 1 cm or more in diameter. Silicosis can be fatal if the lungs stop working effectively (respiratory failure), or there are serious complications such as pneumonia, tuberculosis or other infections.

The symptoms of silicosis are likely to occur after many years of exposure, and can include:

  • Shortness of breath when walking or doing other physical activities;
  • Severe persistent cough;
  • Fatigue.

However, many people can have the early stages of silicosis without obvious symptoms. Health surveillance is therefore an important part of preventing serious disease. In Europe the number of fatal cases of silicosis has been decreasing steadily for at least the last 40 years and there are now around 1 in 100,000 of the population who die from this cause each year[2].

Lung cancer is more common amongst people diagnosed with silicosis than amongst other worker exposure to RCS. However, epidemiological studies, mostly in the diatomaceous earth industry, industrial sand production, pottery, granite production and mining, have also shown that there is a clear increased risk of lung cancer amongst those workers who have prolonged exposure to RCS, regardless of whether they do or do not have silicosis.

The overall relative risk of lung cancer from cohort studies of RCS exposed workers is around 1.3, i.e. around a 30% increase in risk, and in case-control studies the odds ratio is around 1.4, i.e. 40% increase. In cohort studies of silicotics the relative risk is 1.7. In a number of studies where exposure measurements were available it has been possible to investigate the exposure-response relationship for RCS and lung cancer. In pooled analyses of data from the key informative studies there is considerable heterogeneity, but there is an overall statistically significant relationship that shows increased risks down to the equivalent of 0.04 mg/m3 exposure to RCS over a working lifetime (45-years). It has been suggested that differences between the dusts, such as the freshness of the particle cleavage surfaces and the presence of contaminant coating on the RCS surface could explain some of the differences in outcome between studies. Although the lung cancer risk is apparently greater for silicotics than for non-silicotic workers there is evidence from a large epidemiological study carried out in China and another in the USA that having silicosis is not a perquisite for a lung cancer risk. So, preventing silicosis by health surveillance will not necessarily prevent workers from lung cancer from RCS exposure.

When the RCS has been recently produced by high-energy work processes such as blasting, drilling or chipping the toxicity is greatest. As particles “age” in the air, the surface reactivity and the toxicity decrease. In addition, if there are other particles present and these adhere to the surface of the RCS the toxicity may also be reduced. For example, controlled toxicological experiments have demonstrated decreased toxicity of RCS when aluminium or aluminosilicate clay particles coat the RCS particles. There is little evidence that exposure to RCS in desert regions of the world, where the particles are “aged”, results in silicosis in the general population.

In a number of epidemiological studies exposure to RCS has also been associated with chronic renal disease resulting in premature death, with the risk increasing as cumulative RCS exposure increases. Epidemiological data also supports an association between RCS exposure and autoimmune diseases such as scleroderma, rheumatoid arthritis and systemic lupus erythematosus. Although, there was no clear exposure-response relationship evident for these diseases. The biological mechanisms linking RCS exposure to kidney or autoimmune diseases are unclear.

It is important for people to be aware of the potential symptoms of lung cancer, especially if they are or have been a smoker or have worked with RCS. The key things to look out for are:

  • Having a persistent cough for more than a few weeks or a change in a cough you have had for some time;
  • Coughing up phlegm with spots of blood in it;
  • Shortness of breath;
  • A pain in the chest or shoulder that won’t go away;
  • Loss of appetite;
  • Fatigue;
  • Loss of weight.

How many people are harmed by RCS exposure?

Key points: Around 5.5 million workers in the EU are regularly exposed to RCS, and each year around 7,000 die from past exposure to RCS. With stringent controls in the future we could reduce the number of deaths in Europe from silica-related lung cancer to around 400 per year.

In the EU there are probably around approximately 5.5 million workers potentially exposed to RCS, with around three-quarters of them employed in the construction industry; most of the exposed workers are employed by SMEs. Average exposures across all industries have most probably been decreasing over the last 20 or more years – perhaps by between around 5 and 15% per annum, although clearly in specific worksites or industry sectors the trends may differ (and the trend is less strong at times over the period)[3]. Using the available epidemiological data and details of the work activities it has been estimated that each year in the EU there are about 7,000 deaths from lung cancer and 8,000 new cases of the disease attributable to past exposure to RCS. This corresponds to about 2.5% of all lung cancer deaths amongst the exposed workers.

If no new initiatives are taken to reduce exposure to RCS, based on the assumption that current trends in employment and reductions in exposure are maintained, the predicted numbers of lung cancer deaths in 2070 attributable to RCS would be slightly lower than today at around 6,000 per annum. The lung cancers attributable to RCS would have reduced to 1.3% of all lung cancer deaths in the exposed population.

With stringent efforts to control RCS exposures in the future the number of predicted lung cancer deaths in 2070 could drop to under 400 per year, which over the next 50 years there might be more than 100,000 lives saved. The majority of the companies that would be affected by efforts to control RCS exposure are SMEs, and the costs of achieving stringent control may be very high for a large proportion of the affected organisations, and could adversely affect their financial viability. However, for society as a whole the human costs of inaction are very large and overall monetised benefits to society outweigh the financial costs of tighter control.

Prevention and control

Key points: Effective techniques to control RCS exposure are available, e.g. elimination of crystalline silica from some processes, process modification to reduce emission into the workplace, local ventilation and respiratory protection. Well-designed systems can reduce exposure by more than a factor of ten.

Control of exposure to dusts containing crystalline silica requires a coordinated strategy to reduce dust emission, to minimise spread around the workroom and to protect the worker, where appropriate. See, for example, SLIC guidance on RCS on construction sites[4]. It should involve:

  • Design and operation of work processes to minimise emission of RCS into the air;
  • Control exposure by measures that are proportionate to the health risk, i.e. taking dust control very seriously;
  • Choose the most effective and reliable control options that minimise the escape and spread of RCS;
  • Use a combination of engineering and other control measures, along with suitable respiratory protection;
  • Regularly check that control measures continue to be effective;
  • Inform and train all employees on the hazards and risks from RCS, and how to use the available control measures;
  • Ensure that the introduction of new control measures does not increase the overall risk to worker health and safety.

The Chemical Agents Directive, establishes a hierarchy of control, with elimination of hazards at the top of the list and use of personal protective equipment at the bottom. Elimination would involve completely removing the source of crystalline silica from the process; for example, in an abrasive blasting process it is possible to choose an alternative to crystalline silica from a wide range of alternative materials such as slags or glass beads. The use of silica containing abrasive blast material is forbidden in most European countries or its use requires specific permission from the authorities. In making efforts to eliminate or substitute materials it is always important to ensure that the risk is reduced, i.e. that the hazard is less and the level of exposure is not increased.

It is possible to adapt the process to reduce the emission of RCS into the workroom, e.g. by reducing the amount of energy being dissipated into the crystalline silica material or by using water or controlling the moisture content of powders. For example, when cutting kerbstones and paving it is recommended that the work is done with water suppression using a hose directly attached to the tool delivering at least 0.5 litres per minute of water onto the cutting surface. In addition, for this type of work it is necessary for the workers involved to wear a respirator, either a disposable respirator certified to FFP3 or similar. Healy and colleagues (2014) in their research on Irish stonemasons showed that wet cutting resulted in exposure to RCS that was about 35 times lower than during dry cutting, which underlines the potential impact that can come from relatively simple changes to working methods.

Control of airborne dust is often achieved through the use of local ventilation, particularly in fixed worksites such as factories. On mobile work sites the use of on-tool local extraction may be an attractive alternative to the use of water. Research in the USA has shown that the use of local ventilation in grinding may have a greater impact on worker exposure to RCS in such situations that wet working, with the water reducing exposure on average between 7 and about 50 times, depending on the grinding equipment used. However, the use of local on-tool ventilation was more effective, reducing average exposures by between 50 and around 150 times[5] [6]. A second study investigated the impact of local ventilation on concrete-cutting hammer drills[7]. In this study the local ventilation reduced respirable dust levels from around 0.3 mg/m3 to less than 0.03 mg/m3.

The main issue is that engineering controls, such as local ventilation, are not as widely used as they should be and where there are such controls they are often not fully effective or properly maintained[8].

If it is not possible to adequately control airborne RCS using engineering solutions it is necessary to protect workers using personal respiratory protection. Respirators are designed to cover the nose and mouth, and filter the air to remove specific contaminants before they can enter the lungs. When the worker inhales, the air will mostly either pass through the filter or through leaks in the seal between the respirator and the face. The overall effectiveness of a respirator depends on the “total inward leakage” of RCS. We express the effectiveness of respirators as the ratio of the concentration in the workers breathing zone outside the mask and the concentration inside the facepiece; this is the protection factor or PF. A protection factor of 10 means that the respirator reduces the concentration by ten times, i.e. only 10% of the original challenge would “leak” into the facepiece. A protection factor of 100 means only 1% of a contaminant should leak into the mask.

There are several different designs of respirator, from a half-mask that just covers the nose and mouth through to full-face respirators. Many half-masks are now designed to be disposable and these are often referred to as filtering facepiece respirators because these are designed so that the whole of the mask is made from the filter material. Respiratory protection is tested and certified to European Standards (European Norms – EN), for example filtering facepiece respirators are certified to EN 149.

When selecting a respirator it is important to ensure it is both adequate and suitable. A device will be adequate if it can provide sufficient reduction in exposure to protect the workers’ health and it will be suitable if it is right for the wearer, for example in terms of fit to the face and comfort. In many countries respirators are given an Assigned Protection Factor (APF), a numerical rating that indicates how much exposure should be reduced when the device is properly work. For example, if a device has an APF of 10 then the exposure should be reduced by at least ten times in use, i.e. if the worker would have been exposed to 0.5 mg/m3 of RCS if they had not worn the mask then when wearing it their exposure should be less than 0.05 mg/m3.

There are three types of filtering facepiece respirator identified in the standard: FFP1 are the simplest types of device and they offer an APF of 4, FFP2 are of slightly more sophisticated design offering an APF of 10, while FFP3 masks offer the greatest level of protection with an APF of 20.

All respirator wearers should have a “fit test” to ensure the mask they are being asked to wear is suitable and the device will protect them. Ideally the fit tests should be carried out by a trained and experienced technician. There are a number of sources of guidance on respiratory protection. [4] [9] [10]

An important development in Europe to try to raise standards of control in many industries where RCS exposure is present is the social partner agreement between fifteen trade associations and the European trade unions – this is the European Network on Silica or NEPSi[11]. The agreement aims to protect the health of employees in the industries covered by the agreement, by minimizing exposure to RCS through the application of good practice guidance, which is freely available on the organisation website. However, it is notable that the construction industry is not currently a party to the NEPSi agreement (although the SLIC guidance provides a valuable alternative). A key part of the initiative is to regularly monitor RCS exposure, and to provide worker training plus health surveillance for silicosis.

Research in Finland has shown that the introduction of the NEPSi good practice agreement coincided with a strong decrease in the exposure to RCS . In the years following the agreement there was more than a 10-fold decrease in the average exposures to RCS[12]. Before the NEPSI agreement, more than half of Finnish workplace had exposures above 0.2 mg/m3, but by 2013 only around 10% of the measurements were above the Finish occupational exposure limit (0.05 mg/m3). Data from the rest of Europe shows a steady decline in exposure in sites covered by the NEPSi agreement[13], with the average exposure around 0.02 mg/m3 and most measurements less than 0.1 mg/m3.

Legislation and guidance

Key points: RCS is regulated Europe under the provisions of the Chemical Agents Directive, although there are no European indicative or binding limit values. Individual Member States have their own limit values.

RCS in the form or either quartz or cristobalite is classified by the International Agency for Research on Cancer (IARC) as a human carcinogen (Group 1). Crystalline silica is not regulated under the REACH Regulations in Europe, although in some member states it is classified as a carcinogen.

Exposure to RCS is regulated under the provisions of the Chemical Agents Directive (Directive 98/24/EC - CAD)

There are no EU indicative or binding Occupational Exposure Limit Values (OELVs) for RCS. National limits in Member States vary from 0.05- 0.15 mg/m3, although most countries have an OELV of 0.1 mg/m3, 8-hour average. Current OELVs for RCS in different jurisdictions can be sourced through the GESTIS - Substance Database[14]. There is evidence to suggest there are important risks at or below 0.1 mg/m3 RCS, and so it is important to encourage employers to control exposure to as low a level as possible, ideally below 0.05 mg/m3.

Monitoring exposure

Key points: Air sampling to measure exposure to RCS can be undertaken using established occupational hygiene methods with specialist laboratory analysis. High volume sampling may be required to measure low exposure levels.

RCS exposure can be measured using conventional occupational hygiene sampling gear using a battery operated sampling pump, with a cyclone sampling head or similar to select the respirable fraction of the aerosol. The dust sample is collected on pre-weighed PVC filters and the crystalline silica can be analysed by x-ray diffractometry or infrared spectrophotometry. There are standard methods published by many national competent authorities, e.g. Sweden: SIS[15]; UK: HSE[16]; France: AFNOR[17]; International: ISO[18].

General guidance on sampling airborne dust in the workplace is available:

  • EN 689: Workplace atmospheres - Guidance for the assessment of exposure by inhalation to chemical agents for comparison with limit values and measurement strategy, 1995, CEN.
  • EN 481: Workplace atmospheres - Size fraction definitions for measurement of airborne particles, 1993, CEN.
  • EN 1232: Workplace atmospheres - Pumps for personal sampling of chemical agents- Requirements and test methods, 1997. CEN

Personal samples should be collected throughout the working shift with the sampling head located close to the worker’s nose / mouth in the breathing zone. The filters need to be sent to a specialist laboratory for analysis for crystalline silica. The final result is expressed as a concentration – milligrams of RCS per cubic metre of air sampled (mg/m3). The filter can also be reweighed to allow a calculation of the respirable dust concentration and the percentage of RCS on the sample. The data can then be compared with the relevant OELV.

Measurement of low level exposure, i.e. <0.1 mg/m3, or exposure during tasks within a work day may require specialised high-volume sampling systems[19].

It is necessary to have appropriate competency to undertake this type of occupational hygiene monitoring work.

Health surveillance

Key points: Health surveillance to protect workers from silicosis should be undertaken for all workers exposed to RCS above 0.05 mg/m3. There is no appropriate health surveillance for the early detection of lung cancer.

Where there is a chance that workers may develop silicosis it is appropriate for an employer to consider introducing health surveillance. This may be as simple as the health professional asking about symptoms and making a record of the job and the exposure the individual receives. However, in some cases it may be appropriate for the worker to have a chest x-ray and/or tests of their lung function.

Unfortunately, there is no effective way of detecting lung cancer as part of the surveillance procedures, which underlines the importance of stringently controlling exposure to RCS to minimize future risk.

An example of an appropriate health surveillance regime for workers regularly exposed to RCS above 0.05 mg/m3 might include the following:

  • Completion of respiratory symptoms questionnaire (at first employment and annually thereafter);
  • Lung function testing to measure the forced expiratory volume in one second (FEV1) and the forced vital capacity (FVC) (annually); and
  • A chest X-ray at first employment for comparison with X-ray taken after 15 years of work.

Respirable dust

Particles from less than around 0.1mm (100 μm) can be inhaled into the nose or mouth. However, as the air passes down into the lungs the larger particles are progressively deposited in the upper airways and it is only the smallest particles (<10 μm) that can reach the gas-exchange region of the lung where they can contribute to the risk of disease.

The size, shape and density of a particle determines how it behaves in air. We use the particle’s aerodynamic diameter to account for the effects of all of these variables, where aerodynamic diameter is the diameter of a spherical particle of density 1000kg/m3 that has the same falling speed in air as the particle in question. The aerodynamic diameter of a particle is critical in determining how it will behave when it inhaled.

Airborne dust concentration can be measured by drawing a sample of contaminated air through a filter and then weighing the amount of dust collected. Inhalable dust represents the size fraction of the aerosol that can be inhaled into the nose or mouth and thoracic dust the fraction that those materials that are hazardous when deposited anywhere within the lung airways including the gas-exchange region. For dusts like crystalline silica, which mainly have their effect in the deepest parts of the lung, i.e. the alveoli, we measure the respirable fraction, which is defined in relation to the aerodynamic diameter of the dust, see Figure 1.

Figure 1: Size (aerodynamic diameter) criteria for measuring dust

RCS figure.jpg

Other serious diseases caused by exposure to RCS

In addition to causing silicosis and lung cancer, exposure to RCS can also cause the following:

  • Infections, usually in addition to silicosis, such as tuberculosis (pulmonary and extra-pulmonary) or other mycobacterial, fungal, and bacterial lung infections;
  • Chronic obstructive pulmonary disease (COPD);
  • Autoimmune disease such as scleroderma and rheumatoid arthritis;
  • Chronic kidney disease.

Further reading

US OSHA Factsheet on crystalline silica https://www.osha.gov/OshDoc/data_General_Facts/crystalline-factsheet.pdf

The British HSE has a range of guidance and resources relevant to  silica and its health risks:

Respirable Crystalline Silica in the Workplace: New Occupational Safety and Health Administration (OSHA) Standards – a report. https://www.aiha.org/government-affairs/Documents/CRS%20Silica%20Report-04-16.pdf

ILO web resources on silicosis. http://www.ilo.org/safework/areasofwork/occupational-health/WCMS_108566/lang--en/index.htm

Canada’s Worksafe BC film explains how silica damages the lungs: https://www.youtube.com/watch?v=AHAygJEZNTc

The IARC monograph on RCS. https://monographs.iarc.fr/ENG/Monographs/vol100C/mono100C-14.pdf

Global Burden of Disease visualisation portal. http://vizhub.healthdata.org/cod/

A guide to controlling construction dust with on-tool extraction. http://www.hse.gov.uk/pubns/cis69.pdf

US OSHA guide on controlling silica exposures in construction. https://www.osha.gov/Publications/3362silica-exposures.pdf

US OSHA Factsheet on Protecting Workers from the Hazards of Abrasive Blasting Materials. https://www.osha.gov/Publications/OSHA3697.pdf

Information and practical resources, including an online planning tool, from the Work Safely with Silica website, run by the USA’s Center for Construction Research and Training. http://www.silica-safe.org/

General information to help employers control dust exposure. http://www.hse.gov.uk/dust/index.htm?ebul=hsegen&cr=5/06-jul-15

Information on respiratory protection. http://www.perosh.eu/wp-content/uploads/2013/06/sheet-1-WP-Protection-factors-for-RPD1.pdf

Research into the effects of stubble growth on respiratory protective equipment. http://www.hse.gov.uk/research/rrhtm/rr1052.htm

SLIC Guidance. https://osha.europa.eu/en/guidance-national-labour-inspectors-on-addressing-risks-from-worker-exposure-to-respirable-crystalline-silica

NEPSI. http://www.nepsi.eu/sites/nepsi.eu/files/content/editor/good_practice_guide_-_english_original_additional_task_sheets_-251006_modified_16072012-.pdf

INRS. http://www.inrs.fr/risques/silice-cristalline/ce-qu-il-faut-retenir.html

USA, https://www.cdc.gov/niosh/topics/silica/

References

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  2. Global Burden of Disease Cancer Collaboration, Fitzmaurice, C., Allen, C., Barber, R. M., Barregard, L., Bhutta, Z. A., et al. (2016). Global, Regional, and National Cancer Incidence, Mortality, Years of Life Lost, Years Lived With Disability, and Disability-Adjusted Life-years for 32 Cancer Groups, 1990 to 2015: A Systematic Analysis for the Global Burden of Disease Study. JAMA Oncology. http://doi.org/10.1001/jamaoncol.2016.5688
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  5. Akbar-Khanzadeh, F. et al., 2007. Crystalline Silica Dust and Respirable Particulate Matter During Indoor Concrete Grinding—Wet Grinding and Ventilated Grinding Compared with Uncontrolled Conventional Grinding. Journal of occupational and environmental medicine, 4(10), pp.770–779
  6. Akbar-Khanzadeh, F. et al., 2010. Effectiveness of Dust Control Methods for Crystalline Silica and Respirable Suspended Particulate Matter Exposure During Manual Concrete Surface Grinding. Journal of occupational and environmental medicine, 7(12), pp.700–711.
  7. Shepherd, S., Woskie, S. R., Holcroft, C., & Ellenbecker, M. (2008). Reducing Silica and Dust Exposures in Construction During Use of Powered Concrete-Cutting Hand Tools: Efficacy of Local Exhaust Ventilation on Hammer Drills. Journal of Occupational and Environmental Medicine / American College of Occupational and Environmental Medicine, 6(1), 42–51. 
  8. Easterbrook A. (2009). Silica baseline survey: Main report (RR689). Sudbury: HSE Books
  9. DGUV Regel 112-190: Benutzung von Atemschutzgeräten http://publikationen.dguv.de/dguv/pdf/10002/r-190.pdf
  10. Respiratory protective equipment (RPE) www.hse.gov.uk/respiratory-protective-equipment/index.htmhttp://www.hse.gov.uk/respiratory-protective-equipment/index.htm
  11. The European Network on Silica. http://www.nepsi.eu/
  12. Tuomi T, et al. (2014). Application of good practices as described by the NEPSI agreement coincides with a strong decline in the exposure to respiratory crystalline silica in Finnish workplaces. The Annals of Occupational Hygiene, 58(7): 806–817
  13. Komhout H 2013 Inhaled Particles presentation. Cited in: Peckham 2016 Exploring the future of occupational health. http://www.sacramentoasse.com/media/Trevor%20Peckham.pdf (accessed 30 May 2017)
  14. GESTIS International Limit Values Available at: http://limitvalue.ifa.dguv.de
  15. SIS  http://www.sis.se/en/external-categories/water-and-environmental-technology-astm-vol-11/atmospheric-analysis-astm-vol-1107/astm-d7948-14
  16. HSE http://www.hse.gov.uk/pubns/mdhs/pdfs/mdhs101.pdf
  17. AFNOR https://www.boutique.afnor.org/norme/xp-x43-243/air-des-lieux-de-travail-dosage-par-spectrometrie-infrarouge-a-transformee-de-fourier-de-la-silice-cristalline-echantillonna/article/737251/fa122129
  18. ISO http://www.iso.org/obp/ui/#iso:std:iso:16258:-1:ed-1:v1:en
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