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The Continuing Persistence of Silicosis

Categories: Respiratory Health

the dust which is stirred and beaten up by digging penetrates into the windpipe and lungs and produces difficulty in breathing. (Agricola, 1556)

worker kicking up dust with a machineCrystalline silica (silicon dioxide) has long been recognized as an occupational hazard. The Occupational Safety and Health Administration (OSHA) estimated in 2003 that over 2 million workers were potentially exposed to crystalline silica dust in general industry, construction and maritime industries. Based on OSHA compliance inspection data, Yassin et al estimated that about 119,000 of such workers were exposed. Inhalation of crystalline silica can cause silicosis, a preventable but incurable type of lung fibrosis. At current U.S. levels of exposure, chronic inhalation generally takes a decade or longer to cause disease. However, high levels of exposure can cause disease more quickly. Severe cases can be disabling or even fatal. Breathing silica dust is also associated with tuberculosis, lung cancer, and chronic obstructive pulmonary disease (COPD). Exposure to silica dust may also cause various autoimmune diseases and chronic renal (kidney) disease.

Silica is the most abundant compound in the earth’s continental crust. It is found in crystalline and non-crystalline (amorphous) forms. Examples of crystalline silica include quartz, tridymite, and cristobalite. Quartz is a major component of soil and rocks. Many occupations and industries create quartz-containing dust through activities like drilling, tunneling, or quarrying, or by cutting, breaking or crushing materials that contain quartz. Sandblasting, a practice outlawed in many countries, is especially hazardous. Cristobalite and tridymite are found in rocks and soil and also produced by heating quartz or amorphous silica (as in foundry work and in manufacturing brick and ceramics). Occupations with hazardous exposures to crystalline silica continue to emerge and occupational exposures that exceed Occupational Safety and Health Administration (OSHA) and Mine Safety and Health Administration (MSHA) permissible exposure limits (PELs) continue to be regularly documented.

There are no surveillance data in the U.S. that permit us to estimate accurately the number of individuals with silicosis. The true extent of the problem is probably greater than indicated by available data. Undercounting of silicosis occurs because there are no national medical monitoring surveillance programs, and there can be a failure to diagnose silicosis or record it as a cause of death on a death certificate. Silicosis often presents long after workers have left causative jobs. Such cases may not be detected in Bureau of Labor statistics as occupational disease and will not be detected if disease presents after retirement. Even so, mortality surveillance shows that an average of 162 individuals died annually from or with silicosis in the U.S. over the period 2000-2005. According to formulas published by Rosenman et al., the 162 silicosis deaths per year would predict about 1,975 newly-recognized living silicosis cases per year. On average, about 29 of the 162 deaths occurred before the age of 65. Mazurek et al. reported that deaths in younger individuals aged 15-44 accounted for 37% of silicosis-related years of potential life lost in the US before age 65 over the period 2000-2005.

In 2002, the National Institute for Occupational Safety and Health (NIOSH) published an extensive review of the Health Effects of Occupational Exposure to Respirable Crystalline Silica. It found “a significant risk of chronic silicosis for workers exposed to respirable crystalline silica over a working lifetime” at current OSHA and MSHA PELs and at the NIOSH recommended exposure limit (REL). It recommended: “Until improved sampling and analytical methods are developed for respirable crystalline silica, NIOSH will continue to recommend an exposure limit of 0.05 mg/m3 as a time-weighted average (TWA) for up to a 10-hr workday during a 40-hr workweek. NIOSH also recommends substituting less hazardous materials for crystalline silica when feasible, using appropriate respiratory protection when source controls cannot keep exposures below the REL, and making medical examinations available to exposed workers.”

People only get silicosis from inhaling respirable crystalline silica dust into their lungs. Thus, every case represents failure to prevent excessive exposure. Sixteen years ago, Wagner published an editorial titled The Inexcusable Persistence of Silicosis. In it, he noted that the scientific bases for preventing silicosis had long been known. He called for a comprehensive workplace standard to eliminate silicosis. Such standards are now on the regulatory agendas of OSHA and MSHA. Although there has already been some controversy about these rulemaking efforts, all should agree with their motivation. Hazardous silica exposures and the diseases they cause should be, and can be, made a thing of the past in the U.S. and elsewhere in the world.

We invite your feedback on chronic work-related diseases like silicosis that take years to develop, often showing up after retirement. Should more be done to determine how often they occur? Or to prevent them and help the people who have them? We look forward to your comments.

David Weissman, MD, and Paul Schulte, PhD

Dr. Weissman is Director of the NIOSH Division of Respiratory Disease Studies.

Dr. Schulte is Director of the NIOSH Education and Information Division.

Web pages of interest

Selected readings

  1. Cohen RA, Patel A, Green FH. Lung disease caused by exposure to coal mine and silica dust. Semin Respir Crit Care Med. 2008 Dec;29(6):651-61.
  2. Mazurek JM, Wood JM. Silicosis-related years of potential life lost before age 65 years–United States, 1968-2005. MMWR Morb Mortal Wkly Rep. 2008 Jul 18;57(28):771-5.
  3. NIOSH. Health effects of occupational exposure to respirable crystalline silica. DHHS (NIOSH) Publication Number 2002-129 (2002). [http://www.cdc.gov/niosh/docs/2002-129/, accessed 9/22/11].
  4. Rosenman KD, Reilly MJ, Henneberger PK. Estimating the total number of newly-recognized silicosis cases in the United States. Am J Ind Med. 2003 Aug;44(2):141-7.
  5. Rosenstock L, Cullen M, Fingerhut M. Occupational Health. In: Jamison DT, Breman JG, Measham AR, Alleyne G, Claeson M, Evans DB, Jha P, Mills A, Musgrove P, editors. Disease Control Priorities in Developing Countries. 2nd edition. Washington (DC): World Bank; 2006. Chapter 60.
  6. Steenland K. One agent, many diseases: exposure-response data and comparative risks of different outcomes following silica exposure. Am J Ind Med. 2005 Jul;48(1):16-23.
  7. Straif K, Benbrahim-Tallaa L, Baan R, Grosse Y, Secretan B, El Ghissassi F, Bouvard V, Guha N, Freeman C, Galichet L, Cogliano V; WHO International Agency for Research on Cancer Monograph Working Group. A review of human carcinogens–part C: metals, arsenic, dusts, and fibres. Lancet Oncol. 2009 May;10(5):453-4.
  8. Wagner GR. The inexcusable persistence of silicosis. Am J Public Health. 1995 Oct;85(10):1346-7.
  9. Yassin A, Yebesi F, Tingle R. Occupational exposure to crystalline silica dust in the United States, 1988-2003. Environ Health Perspect. 2005 Mar;113(3):255-60.

Public Comments

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

  1. October 18, 2011 at 9:43 pm ET  -   Louis R. Janis

    No mention of dremil rotorary hand tools associated with hand crafting of quartz newsletter, without proper protection of eyes, nose, mouth (inhalation.

    Link to this comment

    • AUTHOR COMMENT November 7, 2011 at 9:42 am ET  -   David Weissman and Paul Schulte

      Thank you for your comment. This is an excellent illustration of the wide range of occupations where exposure to respirable crystalline silica can be a hazard.

      Link to this comment

  2. October 18, 2011 at 11:38 pm ET  -   Jagdish Patel

    Thank you for the post. Here, India, Silicosis is still a huge problem though we have no reliable estimates. There are references from 3 million to 10 million workers at risk of silicosis. How many actually are now living with silicosis and how many of them die each year is a mystery and will remain mystery for a long time to come. In my State -Gujarat- we have silica flour (quartz crushing) industry in 2 small towns. We find workers who inform, they have not even worked for 6 months and they got silicosis.

    I wonder, if malnutrition and/or some other factors , too, might be responsible for such acute cases. In 1982 or so, NIOH carried out a study in this industry and reported high dust levels. The report read, “… dust levels are so high that even if some one work for 6 month, one will get silicosis…” The then factory inspector, after retirement, informed me in an interview that he located several hundred cases in villages around the town, who had history of working in these units. No actions were taken or what ever actions were taken were not effective. In year 2000, I received complaints from other part of the state, near this town. My visit to the villages confirmed that they all had worked in this industry and they said 500 have already died and 500 more are suffering. NO studies on the number was ever carried out. In 2007 we started receiving complaints from neighboring state Madhya Pradesh that huge numbers of tribal workers migrated to work in this industry and over 1000 have died! From other parts of Gujarat,too we received complaints in this period and we carried out a survey to identify those who have worked in these units and then requested Government to organize diagnosis. They found some 35 cases.Large number of them have died. IN yet another town where we are active and run weekly clinic we have screened over 400 workers to find 176 confirmed cases out of which 63 have died. This tale of only two industries in the state.In our State glass, ceramic,mining and foundry, too exist on big scale and no one knows the status there.

    I know of the countries in Asia like Cambodia or Bangladesh or Pakistan where the situation is much worse.

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    • AUTHOR COMMENT November 7, 2011 at 9:43 am ET  -   David Weissman and Paul Schulte

      Thank you for your posting. It is a powerful reminder that silica exposure and silicosis continue to be widespread, serious public health problems in many parts of the world.

      Link to this comment

  3. October 19, 2011 at 2:09 pm ET  -   james celenza

    part of our cleaner construction campaign:- Expanding the reach of green building to include air toxics.

    The fundamental goal of this project is to encourage institutions, developers, designers, and professional organizations to adopt a Green during construction pledge to the benefit of the surrounding community and workers and visitors on the site by reducing both particulate matter, dust and silicates, and toxic gases, like CO.

    Construction can be dirty work but we have an opportunity to make it cleaner. The green building movement focuses on constructing energy efficient buildings and using less toxic building materials.

    Organizations such as Green Global and LEED have developed certification paradigms that are gaining adherents and advocates among building investors, designers, developers, and architects. Investors are increasingly requiring designers and builders to meet these certifications. And many firms, banks, design firms, stipulate that they will only rent space in a “green certified” building.

    While there are obvious benefits to this green building movement what is left out of this approach is air quality while structures are being built.

    Air Quality Concerns.

    Carbon monoxide, fine and ultrafine particles, polycyclic aromatic hydrocarbons (PAHs), sulfur and nitrogen oxides, benzene are emitted in significant amounts by diesel and gasoline-powered vehicles and equipment on construction sites. These toxins have been linked to cardiovascular disease, respiratory symptoms, decline in lung function, exacerbation of asthma, and cancer.

    During construction soil, granite, and concrete will be dug, drilled crushed, impacted, abraded. Soil, granite, and concrete contain crystalline silica (sand). Occupational exposure to silica produces silicosis, a chronic, disabling lung disease characterized by nodules of scar tissue in the lungs. Each year nearly 300 workers die from silicosis in the US, hundreds more are disabled. Between 3000-7000 new cases occur each year. In addition, crystalline silica is carcinogenic.

    Construction, more than any other industry, leads in premature mortality (years of potential life lost) from silicosis.

    Why not Green During the Construction Phase?

    This is an opportunity to directly address the health and environmental impact of stationary equipment and motor vehicles, of dust and silicate exposures, and to integrate worker and community health into a seamless package.

    RICOSH in coordination with the American Lung Association of RI, and with the official support of the Air Resources Unit of RI DEM and the US OSHA Providence office have begun to develop protocols that integrate air pollution issues into the Green Building approach. The fundamental goal of this project is to encourage institutions, developers, designers, and professional organizations to adopt a Green during construction pledge and include key parameters of all three metrics in bid and contract specs for construction projects.

    v Restrict idling of gasoline and diesel vehicles.

    v Apply dust suppression controls

    v Reduce exhaust emissions

    This would benefit the surrounding community and workers and visitors on the site by reducing both particulate matter, dust and silicates, and toxic gases, like CO. In addition this approach will achieve reductions in greenhouse gas emissions. For more information contact us at RICOSH.

    Link to this comment

    • AUTHOR COMMENT November 7, 2011 at 9:44 am ET  -   David Weissman and Paul Schulte

      We couldn’t agree more. “Green” construction should not trade off occupational safety and health against environmental health. “Green construction” should, by definition, benefit construction workers, occupants and the environment.

      Link to this comment

  4. October 24, 2011 at 8:28 pm ET  -   Dr. Michel H Nazaire

    Occupational Lung Diseases involving exposure in the workplace,in developed or developing countries,have become matters of great importance:silicosis,asbestosis,black lung,pneumoconiosis etc…

    The pulmonary response to inorganic dust is thought to be proportional to the amoumt and duration of exposure that contributes to the retention of dust in the distal air spaces.The histologic tissue reaction to retained dust involves alterations that initiate and perpetuate pulmonary inflammation-associated with high susceptibility to infectious diseases- leading to fibrosis,pulmonary disability,exhibiting potential causes of biological,radiological and functional test changes or irreversible damage and cardiac consequences.

    Awareness of such potential health hazards increases concerns about identification,elimination of risks,reducing exposure and need for education,communication with the working man about cause and prevention of accidents and illnesses.

    Besides environmental monitoring (engineering controls,ventilation),in order to protect the respiratory system of workers,prevention programs include complete employee protection(personal protective equipment) such as respiratory protective devices and medical surveillance.Industrial respirator is a criterion of an effective industrial hygiene and respiratory program,requiring development and knowledge..

    General guidelines for selecting protective eqipment are being used referrable to the adverse working environment,the user’s characteristics and to national and international standards:

    1) Evaluation of the respirator user under controlled conditions:medcal examination of personal required to use the respirators(preemployment physical examination,post exposure medical examination-chest xray,pulmonary function tests..

    2)Available types of respirators(Which respirators can the worker wear?):Air purifying respirators,non powered and powered,Athmosphere supplying respirators(self contained breathing apparatus-open and closed circuit,Supply air respirators Utilization of respirators become critical in accordance to specific approval requirements and an agreed physiological value,implementing comprehensive safety procedures with particular reference to revising the respirator program :Quality of medical surveillance,required pulmonary function tests,discern dgree of contra-indication to respirator wear: -visual,hearing,respiratory,cardiovascular,neurondocrinal,and miscellaneous or current health condition.-.

    Some experimental approaches have been also attempted for the preventon and treatment of pulmonary dust diseases using aerosol ,hormonal therapy,corticosteroid and other chemotherapy….

    Link to this comment

    • AUTHOR COMMENT November 7, 2011 at 9:45 am ET  -   David Weissman and Paul Schulte

      Thank you for your comment. As you suggest, the entire industrial hygiene prevention hierarchy—elimination, substitution, engineering controls, administrative controls, and personal protective equipment all have a role to play in reducing inhalation of silica dust. Worker training and medical surveillance are also key components of a comprehensive approach to the problem.

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  5. October 25, 2011 at 12:13 pm ET  -   Guillemin

    “Sandblasting, a practice outlawed in many countries, is especially hazardous.” This quotation of the David Weissman and Paul Schulte’s introduction raises the question : Why the USA has not forbidden this practice ?

    An answer has been given by Gerald Markowitz and David Rosner in their paper “The Limits of Thresholdds : Silica and the Politics of Science, 1935 to 1990″ (Am J Public Health 85: 253-263,1995). The NIOSH recommendation to ban the use of sand blasting provoked a storm of protest and a massive lobbying of the industry (sand blasting equipments manufacturers, painting contractors and sans producers)to avoid this ban.

    This means that part of the victims of silica in the USA is due to this unethical conduct.

    Link to this comment

    • AUTHOR COMMENT November 7, 2011 at 9:45 am ET  -   David Weissman and Paul Schulte

      In 1992, NIOSH published the Alert Preventing Silicosis and Deaths from Sandblasting. Among its recommendations is the following: “Prohibit silica sand (or other substances containing more than 1% crystalline silica) as an abrasive blasting material and substitute less hazardous materials.” NIOSH continues to make this recommendation.

      Link to this comment

  6. October 25, 2011 at 1:39 pm ET  -   Mike

    In the blog, Drs. Weissman and Schulte point out that “Until improved sampling and analytical methods are developed for respirable crystalline silica, NIOSH will continue to recommend an exposure limit of 0.05 mg/m3 as a time-weighted average (TWA) for up to a 10-hr workday during a 40-hr workweek.”

    Why would NIOSH have to wait for improved sampling and analytical methods before establishing a lower REL? Why is it that NIOSH RELs seem to be limited to single-shift sampling? Why doesn’t NIOSH, in the case of silica, consider establishing a lower REL based on composite sampling, wherein airborne dust from multiple workshifts are sampled onto the same filter? Wouldn’t that allow for a lower REL?

    A lower NIOSH REL will do nothing to help workers in India and other Asian countries mentioned by Dr. Patel in his comment (above). But it could help protect the health of U.S. workers employed in jobs where resources are available to do better than a limit of 0.05 mg/m3 at controlling silica dust exposure.

    A NIOSH BLOG on composite sampling in the context of occupational health protection would be of interest. It could discuss potential benefits of composite sampling, problems with composite sampling, and legislative and other impediments to basing RELs/PELs on composite sampling.

    Link to this comment

    • AUTHOR COMMENT November 7, 2011 at 9:47 am ET  -   David Weissman and Paul Schulte

      A policy in effect since 1995 notes that “NIOSH recommended exposure limits (REL) will be based on risk evaluations using human or animal health effects data, and on an assessment of what levels can be feasibly achieved by engineering controls and measured by analytical techniques.” Thus, the REL is a practical occupational exposure limit and RELs are not proposed if they cannot be reliably measured.

      You also suggest that the sensitivity of silica exposure assessment might be improved by pooling samples collected over several shifts and analyzing the composite sample rather than collecting and analyzing samples collected over a single shift. This is clearly an interesting suggestion. However, before recommending it as a standard practice, research would be needed to establish effectiveness across a range of occupations and industries with potentially hazardous silica exposures, including very dusty settings where several collection devices might be needed to sample even a single shift. Potential benefits would need to be weighed against cost, loss of information about variability between shifts, etc.

      Those interested in research on silica sampling and analytical methods may wish to submit an abstract to the Second ASTM D22 Symposium on Silica and Associated Respirable Mineral Particles to be held October 25-26, 2012 at the Hyatt Regency Atlanta in Atlanta, GA, USA.

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  7. November 2, 2011 at 5:37 pm ET  -   jack h

    Question: Just how bad are the filter aid diatomaceous earth products used on swim pool filters?

    Link to this comment

    • AUTHOR COMMENT November 3, 2011 at 12:17 pm ET  -   David Weissman and Paul Schulte

      Health Canada has posted an excellent discussion of diatomaceous earth (DE) on its website. It notes that untreated DE contains around 1% crystalline silica. However, DE which has been “calcined” (processed using high pressures and/or heat) can have much higher crystalline silica content, ranging anywhere from 1% to 75% cristobalite. Toxicity of a particular DE sample is strongly related to crystalline silica content. The web page cites an OSHA memorandum stating that, “there is no requirement under the HCS [Health Communication Standard] to state a definitive finding of carcinogenicity on the label or MSDS [material safety data sheet] for diatomaceous earth products containing less than 1% crystalline silica…”

      Link to this comment

  8. January 27, 2012 at 6:31 am ET  -   samuel gray

    I appreciate your Crystalline silica related useful entry information.Many thanks for sharing this great site information.

    Samuel

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  9. February 20, 2012 at 1:23 pm ET  -   Matthew Evans

    At (company name removed), LLC Las Vegas, NV we mandate the use of NIOSH Respirators on any project with chemical fumes, and concrete dust. I does cost extra money, but the health and well being of our employees is paramount to a couple of bucks.

    Link to this comment

    • February 23, 2012 at 11:48 am ET  -   David David Weissman and Paul Schulte

      Thank you for sharing this comment. We appreciate your commitment to protecting employees’ health. Of course, respirators should be used in combination with other measures. Whenever possible, engineering controls such as wet cutting and ventilation should be used to minimize concentrations of airborne contaminants. Also, administrative controls can be used to limit exposures, such as limiting access into potentially hazardous areas to only those truly needing to be present. In some situations, respirators may also need to be used. In order to provide optimal protection, respirators must function properly and must fit the wearer properly. To help in choosing the most appropriate type of respirator, NIOSH has published the document NIOSH Respirator Selection Logic. http://www.cdc.gov/niosh/docs/2005-100/ NIOSH has also published information that users may find helpful on its Respirator Trusted-Source Information Page. http://www.cdc.gov/niosh/npptl/topics/respirators/disp_part/RespSource.html

      Link to this comment

  10. January 6, 2014 at 3:07 pm ET  -   Saimon

    Thanks for this informative post. I am a regular visitor of your site.

    Thanks once again
    Saimon

    Link to this comment

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