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Karen B. Mulloy, DO, MSCH
Program in Occupational and Environmental Health
University of New Mexico Health Sciences Center

Ken Silver, PhD
Department of Environmental Health
East Tennessee State University

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

Silicosis

• 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

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
    
    
    
    

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

Diagnosis

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)

Silicotuberculosis

• 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

Treatment

• 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

Prevention

• 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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