Silica: The Deadly Dust

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University of New Mexico Health Sciences Center , East Tennessee State University

Summary Statement

A presentation on what silica is and the illnesses that result from exposure. Includes pre and post test and questions and answers.

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Silica (Silicon Dioxide - SiO2)

  • Crystalline silica occurs naturally in the earth’s crust
  • Earth’s most abundant mineral
  • Three most common forms
    • Cristobalite
    • Tridymite
    • Quartz – most abundant component of soil & rock
  • Respirable particles of silica (<5 µ in diameter) produced when crystalline silica-containing rock and sand is used or processed
    • Mining, milling, and stone work
    • Quarrying and tunnel operations
    • Foundry and boiler work
    • Sandblasting and drilling
    • Pottery and glass making
  • Occupational exposure associated with respiratory diseases
    • Silicosis (chronic, accelerated, acute)
    • Progressive pulmonary fibrosis
    • Chronic obstructive pulmonary diseases
    • Lung cancer
    • Increased risk for TB
  • Occupational exposure associated with other diseases
    • Systemic autoimmune diseases
      • Rheumatoid arthritis, SLE, scleroderma, small vessel vasculitides
    • Renal Disease
      • Glomerulonephritis, nephrotic syndrome, end-stage renal disease

Silica – Historical Overview

  • Silicosis
    • First reported by ancient Greeks
  • Prevalence
    • Peaked in the industrial countries in the last half of the 19th century
    • Disease still prevalent in the developing world and not eliminated from the developed world
  • Largest industrial disaster in US history
    • Gauley Bridge, WV – 1930-1932
  • Tunnel construction
    • Silica content of the rock - >90%
  • >475 workers died
  • 1,500 were disabled from chronic silicosis

View of Gauley Bridge, WV circa 1930
Source: The Hawk’s Nest Incident, Cherniak M, 1986

View of Hawk’s Nest tunnel interior, March 13, 1932
Source: The Hawk’s Nest Incident, Cherniak M, 1986

Silicosis: Crude mortality rates by state, U.S. residents age 15 and over, 1987-1996


  • A pulmonary disease caused by inhalation of dust particles of respirable size
  • Three presentations and severity of the disease
    • Classic silicosis
    • Accelerated silicosis
    • Acute silicosis
  • Chronic (classic) silicosis
    • 20+ years of exposure to low-medium dust levels
  • Accelerated silicosis
    • 5-10 years of higher dust exposure
  • Acute silicosis
    • <1-3 years exposure to extremely high levels of free crystalline silica


Pathogenesis: Chronic Silicosis (Classic and Accelerated)
Source: ACCP Pathology Slide Set, No. 36-2

Pathogenesis: Acute Silicosis
Source: Silicosis. Weber and Banks. In: Textbook of Clinical Occupational & Environmental Medicine. Rosenstock & Cullen, eds., 1994, page 270.

Chronic Classic Silicosis

  • Simple silicosis
    • Development predominantly in the upper lobes
    • Rounded opacities < 1cm in diameter seen on chest x-ray
    • Enlarged hilar lymph nodes with peripheral calcification - eggshell calcification

  • Progressive Massive Fibrosis
    • A massing of small rounded opacities in upper lobes
    • Large opacities > 1 cm
    • Hila retract upward & lower zones become hyper-inflated & appear emphysematous

Source: Pathology of Occupational Lung Disease, 2nd ed. A Churg, FHY Green, 1998, pg. 163

Accelerated Silicosis

  • Characterized by same features as chronic classic silicosis
  • Time from initial exposure and development of radiographic findings and symptoms and change in pulmonary function much shorter
  • Rapid progression to PMF with severe respiratory impairment

Acute Silicosis

  • Radiographic Evidence
    • Diffuse alveolar infiltrate
    • Air bronchograms
    • Ground glass appearance
    • Hilar & mediastinal lymphadenopathy
    • Bullae formation
    • Air trapping
    • Volume loss
    • Cavity formation

      Source: Environmental & Occupational Medicine, 2nd ed., Rom W, 1992, pg. 353.

      Source: Diagnosis of Diseases of the Chest, Fraser & Pare, 1970, pg. 923.


Chronic silicosis

Simple silicosis

  • Symptoms
    • Often no symptoms
    • Chronic productive cough may be due to industrial bronchitis from dust exposure
  • Physical exam
    • Normal breath sounds
    • Course breath sounds with co-existing bronchitis

Progressive Massive Fibrosis

  • Symptoms
    • Range from chronic productive cough to exertional dyspnea
  • Physical exam
    • Diminished breath sounds
    • Prolonged expiration
    • Clubbing rare

Acute silicosis

  • Symptoms
    • Irritative cough - sometimes productive
    • Weight loss
    • Fatigue
    • Dyspnea
  • Physical exam
    • Crackles heard on auscultation


Chronic silicosis

Pulmonary Function Testing

  • Simple silicosis
    • Normal lung function
  • PMF
    • Severe restriction
    • Mixed obstructive/restrictive defect
    • Loss of pulmonary compliance
    • Hypoxemia


The Occupational History

  • What kind of work do you do?
  • Do you think your health problems are related to your work?
  • Are your symptoms better or worse when you are at home or at work?
  • Are you now or have you previously been exposed to dust, fumes, chemicals, radiation or loud noises?
  • History of silica exposure
  • Radiographic evidence consistent with silicosis
  • Absence of other illnesses that mimic silicosis
    (LS Newman. N Engl J Med 1995; 333:1129)


  • Prevalence
    • 5.3% in workers with x-ray evidence of silicosis
    • 25% in workers with acute or accelerated silicosis
    • As high as 75% among South African gold miners
  • Diagnosis difficult
    • TB infection can be walled off in the lung by the silica induced fibrosis
    • False negative acid-fast-staining sputum smear may occur
  • Radiographic changes seen with TB infections can mimic advanced cases of silicosis
  • Diagnosis
    • Presence of chest x-ray changes of a worker with silicosis over a short period of time indicates superimposed TB infection until proven otherwise
    • Annual PPD
  • If results become positive without clinical evidence of active TB, the patient should be treated with 1 year of INH

Silica Exposure & Cancer

  • Crystalline silica deposited in lungs causes epithelial & macrophage injury and activation and persistent inflammation
  • Human subjects exposed to dust containing crystalline silica showed an increase in the levels of sister chromatid exchange and chromosomal aberrations in peripheral blood lymphocytes
  • Animal studies have shown gene mutations and tumor formation as a result of marked and persistent inflammation and epithelial proliferation
  • IARC
    • Crystalline silica inhaled in the form of quartz or cristobalite from occupational sources is carcinogenic to humans (Group 1)

Silica Exposure & Autoimmune Disease

  • First described in 1953 by Caplan
    • Unusual radiologic changes in the lungs of Welsh coal miners who had pneumoconiosis
  • Since then the autoimmune disease linked with crystalline silica exposure
    • Rheumatoid arthritis, scleroderma, SLE, some small vessel vasculitides
  • Inhalation of crystalline silica particles leads to chronic immune activity and fibrosis
  • Studies have shown that crystalline silica can be mobilized from lungs to other organs - lymph nodes, spleen, and kidney
  • Silicosis has been linked to an increase in autoantibodies, immune complexes, and excess production of immunoglobulins, even in the absence of a specific autoimmune disease
  • Possible Mechanisms
    • May be result of adjuvant (a substance that enhances an immune response to an antigen) effect on antibody production
    • Cell death by necrosis and apotosis (an active process involved in gene regulation)
    • Host susceptibility and genetic differences may explain why all workers exposed to silica do not develop immune disorders may

Silica Exposure & Renal Disease

  • Epi studies
    • Statistical significance between silica exposure and several renal diseases
    • An increasing standardized rate ratio for acute and chronic renal disease with increasing cumulative crystalline silica exposure and an excess of end-stage renal disease incidence (highest for glomerulonephritis)
  • Intensity of exposure to silica dust may be more important than cumulative exposure or duration in the development of autoimmune diseases
  • Study crystalline silica exposure most strongly associated with ESRD and median exposure was below the OSHA permissible exposures levels


  • Prevention/ Prevention/Prevention
  • Workers at risk for progression of disease and TB infection
  • Yearly chest x-ray and PPD Flu and pneumococcal vaccine
  • Aggressive treatment of TB infections
  • Dyspnea treated with inhaled bronchodilators
  • O2 for cor pulmonale, hypoxemia, pulmonary hypertension


  • Occupational Health Surveillance
    • Gather information on cases of occupational illness and injury and workplace exposures
    • Condense, refine, and analyze the data
    • Disseminate analyzed data to workers, unions, employers, governmental agencies, public
    • Plan and execute interventions - primary prevention - based on the analyzed data
  • Occupational Sentinel Health Event
    • "A disease, disability, or untimely death which is occupationally related and whose occurrence may: 1) provide the impetus for epidemiologic or industrial hygiene studies; or 2) serve as a warning signal that materials substitution, engineering control, personal protection, or medical care may be required"
  • State-based Surveillance
    • Sentinel Event Notification Systems for Occupational Risk (SENSOR)
    • Many state based silicosis surveillance projects
    • May help in case investigations
    • Many states have a legal requirement to report a case of silicosis to the appropriate state agency
  • Hierarchy of Controls
  • Engineering
    • Substitution, control hazard at source (wet process), improved ventilation
  • Administrative
    • Rotating workers
  • Personal Protective Equipment
    • Respirators

Prevention - Regulation

  • Federal Coal Mine Health and Safety Act of 1969 (Coal Act)
  • Federal Mine Safety and Health Act of 1977 (Mine Act)
    • MSHA
  • Occupational Safety and Health Act of 1970
    • NIOSH
    • OSHA
  • Current OSHA PEL for respirable silica
    • 10 mg/m3 / %SiO2 + 2 for 8-hour TWA
  • Current NIOSH REL for respirable silica
    • 50 µg/m3 TWA for up to 10 hours/day during a 40 hour workweek

SIlica: The Deadly Dust

Any Questions?

"The way to dusty death. Out, out, brief candle!
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more."

- Shakespeare, Macbeth, Act V, Scene V

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