AUTHOR AND EDITOR INFORMATION

Section 1 of 8  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

INTRODUCTION

Section 2 of 8  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

Background

The word silicosis comes from the Greek word silex, meaning flint. It is perhaps the oldest known occupational disease; Hippocrates and Pliny referred to this disease.¹

Silica is a naturally occurring mineral that is mainly composed of silicon dioxide (SiO₂). It exists in a crystalline and amorphous state. Quartz, cristobalite, and tridymite are the 3 most common forms of crystalline silica, which causes silicosis. Amorphous silica is not toxic. Quartz exists in alpha and beta forms, with alpha being more common than beta. Alpha quartz makes up 12% of the earth's crust by weight, and it is a major component of igneous rocks, such as granite and pegmatite. It also occurs in sandstone, slate, and shale. Quartz is the most common form of inhaled silica.

Silicosis, asbestosis, and coal-workers' pneumoconiosis (CWP) all belong to a group called pneumoconioses. The exact translation of this word is lung dust. These pulmonary diseases are characterized by nonneoplastic granulomatous and fibrotic changes of the lungs after the inhalation of inorganic substances, such as coal dust, asbestos, or crystalline SiO₂ (silica).² Of these pneumoconioses, silicosis is the most common in the United States and most often occurs in people working in fields involving high exposure to dust. Such people include miners, construction workers, ceramics workers, tunnel drillers, sandblasters, quarry workers, and stone carvers.

Pathophysiology

Pathophysiologic processes

The pathology of most pneumoconioses is defined as pulmonary fibrosis resulting from a chronic inflammation (alveolitis). The inflammatory process involves phagocytosis of the inhaled silica dust by alveolar macrophages and its deposition in the lung interstitium. Deposited silica damage macrophages and epithelial cells, which release various enzymes and inflammatory cytokines, such as tumor necrosis factor–alpha (TNF)-alpha and interleukin-1 (IL-1). These inflammatory mediators begin destroying the lung parenchyma by attracting other inflammatory cells (macrophages, neutrophils, and lymphocytes), resulting in alveolitis.

The inflammation is quickly followed by repairing the damage done. Various growth factors stimulate fibroblast and type II pneumocyte activity, including TNF, IL-1, fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), and insulinlike growth factor (IGF). Collagen and fibronectin production rapidly increases and eventually leads to fibrosis. Animal models have shown that, even after the exposure to silica ceases, dust-laden macrophages continue to produce inflammatory mediators, such as IL-1 beta and TNF-alpha, propagating the inflammation-fibrosis cycle.³

Some have suggested that initial low-dose exposures to silica dust may initially cause fibrosis of hilar lymph nodes (eggshell calcification), which in turn diminishes the person's ability to clear the lungs of any additional inhaled dust.⁴ Therefore, subsequent exposure to silica dust can cause severe acute disease in the case of high dust exposure or pulmonary fibrosis in the case of prolonged, low dust exposure. Of surprise, such hilar fibrosis is not common in CWP.

Histologic features

Histologic examination of the fibrotic nodules characteristic of classic (chronic and accelerated) silicosis reveals mature collagen arranged as an onion skin (concentric layering) surrounded by inflammatory cells. The interior of the lesions contains dust-laden macrophages and lymphoid cells. In addition, birefringent silica crystals may also be seen with polarized light. With time, the interior of the lesions becomes acellular and hyalinized, with possible granulomatous inflammation.³

These nodules rarely occur around pulmonary arteries; they are typically found surrounding respiratory bronchioles in the upper lung zones. With time, they can coalesce to form large lesions, the hallmark of progressive massive fibrosis (PMF) that eventually destroys the lung architecture, producing severe restriction, decreased diffusion capacity, debilitating dyspnea, and hypoxemia.⁵

In acute silicosis, severe alveolitis is accompanied by an alveolar lipoproteinosis with little or no nodularity or fibrosis. Silicoproteinosis is a form of acute disease and the most severe form of silicosis. It occurs over months to a year usually after exposure to freshly fractured quartz (crystalline silica). The rapid course is due to exposure to an extremely high concentration of silica dust. Patients often present with a rapidly progressing dyspnea, and most die within 1 year. Although the pathogenesis of silicoproteinosis is not well defined, case studies implicate either type II pneumocyte hyperplasia with subsequent surfactant overproduction or the inability of the pulmonary macrophages to effectively clear the normally produced surfactant. The final effect is the deposition of proteinaceous material in the alveoli. Collagen deposition and fibrosis are rarely seen in silicoproteinosis.⁶

CWP versus silicosis

The pathology of CWP is distinct from that of silicosis.⁷ Although they both tend to surround respiratory bronchioles, no collagen deposition or hyalinization is observed in CWP lesions. A typical finding is the deposition of coal dust, which forms a densely pigmented area around the small airways. Also present is focal emphysema; these pigmented areas are usually surrounded by local fibrosis. PMF rarely occurs in CWP and is probably related to amount of quartz in the coal dust.

Frequency

United States

Approximately 2 million workers are exposed to crystalline silica each year. Nearly 100,000 (5%) of these workers are exposed to levels above the permissible exposure level (PEL) of 0.10 mg/m³. Despite efforts over the past 30 years to reduce occupational exposure levels for silica and other toxic dusts, these diseases continue to be prevalent in industrial countries and pose a serious problem in developing nations.⁸ Certain occupations are especially risky, and workers performing those types of work continue to develop pneumoconiosis despite laws that mandate lowering levels of exposure wearing of protective gear. These occupations include sandblasting and processing of silica flour, scouring powders, or diatomaceous earth.

The data are similar for CWP.⁹ Despite the Coal Mine Health and Safety Act of 1969, which established limits on permissible dust exposures in underground coal mines and which instituted a radiographic screening program for underground coal miners, the disease still occurs in miners who started work after the new limits were in effect. Although the overall trend of CWP for miners working more than 20 years markedly decreased between 1970 and 1995, the trend is unclear for those working less than 20 years. These results raise questions about possible excessive exposure levels in certain states, at small mines, and at some surface and contract miner facilities.

Mortality/Morbidity

People with chronic silica inhalation have a 3-fold greater risk of mycobacterial infections than that of their healthy counterparts. The radiologic diagnosis of silicosis may be difficult because tuberculosis may obscure the silicosis. The infecting organisms are Mycobacterium tuberculosis, Mycobacterium kansasii, and Mycobacterium avium-intracellulare complex. The explanation of this revolves around the malfunctioning alveolar macrophages in individuals with silica inhalation.

• Silica dust inhalation has been implicated in a rapidly proliferating pleural disease that resembles mesothelioma and in lung cancer (silicosis, not silica inhalation).⁷ Exposure to silica, independent of silicosis, may be responsible for the chronic obstructive pulmonary disease (COPD, primarily emphysema) that occurs in these patients. This is at least partly attributed to airflow obstruction caused by the silica dust itself. Peribronchiolar scarring and hypertrophy and scarring of peribronchial lymph nodes may cause airway compression and hyperinflation.¹⁰
• Patients with pneumoconiosis are at increased risk for certain systemic diseases. Rheumatoid arthritis (Caplan syndrome), scleroderma (Erasmus syndrome), synovitis, interstitial granulomatous dermatitis (Ackerman syndrome), and renal disease are reported to occur more frequently in these patients than in healthy people.¹¹ Of note, exposure to fine silica dust, such as that which dental technicians encounter, is crucial for the development of the cumulative trauma disorders (CTDs). Dental technicians are at risk for exposure to this type of silica flour.⁸
• Patients with CWP are predisposed to chronic bronchitis, PMF, focal emphysema, and mycobacterial infections. In addition, diffuse interstitial pulmonary fibrosis (DIPF) has been found in lungs of coal miners with an overall incidence of approximately 18%. Coal workers with rheumatoid disease are predisposed to the rapid development of pulmonary nodules even after low exposures to dust (Caplan syndrome). Caplan syndrome, or rheumatoid pneumoconiosis, consists of round discrete rheumatoid nodules 0.5-5 cm in diameter that are superimposed on the opacities of pneumoconiosis.¹²

Race

No predilection for race is observed.

Sex

No predilection for sex is observed.

Age

Age is not a factor. The time and intensity of exposure determine the likelihood of disease.

Anatomy

Silica granulomas have crystalline particles contained within giant cells. The silica crystals, when examined under polarized light, are doubly refractile. Silica granulomas develop progressive fibroblastic proliferation, making a dense, collagenous encapsulation.

The minute nodules become a relatively acellular focus of concentric layers of hyaline-appearing connective tissue. This tissue enlarges and coalesces into a large, fibrous scar that may become so extensive that it incorporates an entire pulmonary lobe. Fine clefts may be evident between collagenous lamellae, which harbor silica that can be identified by using polarized light.

Clinical Details

Diagnosis

Chronic silicosis is a diagnosis based on the patient's history (ie, a history of probable exposure to silica dust combined with a proper temporal relationship of exposure to disease) and radiologic evaluation (ie, a chest radiograph demonstrating characteristic lesions).⁵

Inspiratory crackles and results of pulmonary function tests occasionally indicate restrictive disease and may support the diagnosis, but these findings are not considered diagnostic. Patients are usually asymptomatic, but they may present with dry or productive cough, exertional dyspnea, and sexual dysfunction.

The insidious onset of these diseases causes many physicians to attribute these symptoms to regular aging. The first sign of the disease may be found on a routine radiologic study.

Patients with advanced disease may have signs of pulmonary failure on physical examination; these include clubbing, cyanosis, and right heart failure.

Acute and accelerated silicosis has a relatively rapid course. Patients often have signs and symptoms of pulmonary disease even before radiologic studies are performed. This is especially true in acute silicosis (silicoproteinosis), in which patients have incapacitating dyspnea and body wasting. This disease is often fatal.

Tissue biopsy should not be performed as a diagnostic tool unless the history is unclear or questionable.

Clinical presentations and progression

Silicosis can be divided into 4 distinct clinical presentations⁷:

• The first, acute silicoproteinosis, occurs after intense exposure to silica and is rapidly progressing and aggressive and often fatal.
• The second, accelerated silicosis, occurs with the inhalation of high concentrations of silica over a short period. Symptoms may occur in 2-5 years. Patients have pulmonary insufficiency due to pulmonary fibrosis and nodularity.
• The third, simple chronic nodular silicosis, is an indolent disease that requires more than 10 years of low-dose silica inhalation. Pulmonary nodules tend to be well circumscribed and are usually 1-3 mm in diameter. Patients are often asymptomatic, with normal pulmonary function. The diagnosis is usually discovered incidentally on radiologic study. Patients have an increased risk of tuberculosis due to pulmonary macrophage dysfunction.
• The fourth, complicated chronic nodular silicosis, is characterized by the coalescence of the small nodules into large conglomerate opacities (>10 mm) that may obstruct airflow.

There are 2 main factors that affect the rate of progression and severity of the disease:

1. First is the amount of silica that is inhaled, with regard to the intensity and duration of exposure.
2. Second is whether other minerals are present in the inhaled dust that may affect the toxicity of the quartz. A high percentage of free silica usually results in a high degree of pulmonary fibrosis.³

Coal-workers' pneumoconiosis

CWP occurs after exposure to coal dust for longer than 20 years. The severity of the disease depends on the amount of coal inhaled, the duration of exposure, and the carbon content (or rank) of the coal dust. A high rank of coal dust, as with anthracite, increases the risk CWP.⁷ Federal regulation of permitted occupational exposure to coal dust has substantially decreased the occurrence of CWP over the past 30 years.

Because these potentially fatal and debilitating diseases are currently incurable, it is crucial to identify people at risk in the earliest possible stages by means of radiologic screening and then to provide supportive care. Also, effort must be made to strictly control the amount of exposure in the workplace to prevent pneumoconiosis.⁵

The presentation and treatment of CWP are similar to those of silicosis.

Preferred Examination

Without the proper history, occupational lung disease cannot be diagnosed on the basis of radiologic findings. When the history is questionable, tissue biopsy should be performed to diagnose the disease.

International Labour Office (ILO) International Classification of Radiographs of Pneumoconioses

In 1980, the ILO International Classification of Radiographs of Pneumoconioses created standard radiographs that allowed for accurate diagnoses of pneumoconiosis and other interstitial lung diseases. This system uses a step-by-step method to describe the lesion. The shape, size, location, and abundance of opacities are considered. The radiographs are classified after they are compared with the standard radiographs, and all results are recorded in a systematic fashion.

However, this guideline has not been changed in more than 20 years, and though a digital version is being developed, no such classification exists for CT scans at the present time. This lack creates a serious dilemma because CT is considered to be better than radiography as the imaging modality for diagnosing early signs of silicosis and CWP.

Chest radiography

The chest radiograph is a relatively insensitive and nonspecific tool for diagnosing pneumoconiosis because silicosis and CWP are virtually indistinguishable on radiologic studies. In addition, the results may cause underestimation or overestimation the extent of disease. Normal chest radiographs do not rule out interstitial fibrosis. However, the ease of performing the study, combined with its cost-efficiency, makes radiography almost indispensable.

CT and HRCT

CT scanning is more sensitive and specific than chest radiography, and it is more useful in finding other lung diseases such as cancer, emphysema, and atelectasis, which may coexist in patients at risk for pneumoconiosis.⁷ CT can depict nodules that chest radiography cannot, though HRCT is best for detecting nodules smaller than 3 mm.

Both CT and HRCT are useful in identifying large, confluent lesions that a chest radiograph may miss. HRCT can show fine parenchymal details, and it is the tool of choice for patients with normal chest radiographs and/or CT and for those with pulmonary lesions smaller than 1.5 mm. Emphysema is also best visualized on HRCT. The major downfall of CT and HRCT is the lack of standardization of technique and scoring. Therefore, for someone who may have silicosis or a borderline chest radiograph, chest CT should be done and supplemented with HRCT at several levels of the middle and upper lung.⁷

DIFFERENTIALS

Section 3 of 8  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

Other Problems to be Considered

COPD
Idiopathic pulmonary fibrosis
Alveolar proteinosis
Chronic alveolitis
Hypersensitivity pneumonitis
Berylliosis

Chronic beryllium disease is a granulomatous disorder characterized by a cell-mediated immune response to beryllium. It produces pulmonary and noncutaneous immunologic findings and represents an occupational chronic beryllium disease.¹³

RADIOGRAPH

Section 4 of 8  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

Findings

On chest radiographs and CT scans, the appearances of silicosis and CWP are almost identical. However, the nodules seem to be small on chest radiographs.

Approximately 20 years of exposure is required to result in a positive chest radiograph. Chest radiography is approximately 80% sensitive, and the usual findings are multiple, small (<1 cm) lung opacities. These opacities tend to occur in the upper and posterior regions of the lungs. They are usually round and well circumscribed. Their size and opacity vary little. These nodules are calcified in 10-20% of patients. With the progression of the disease, the nodules can merge to form large opacities. This change is indicative of PMF, and it occurs more frequently in silicosis than in CWP.

The lesions must be at least 1 cm to be classified as PMF. Enlarged, calcified (eggshell calcification) lymph nodes are usually in the hila and mediastinum. PMF has a distinct appearance on chest radiography. PMF is usually symmetrical, but it may be unilateral. It appears as irregular, masslike or sausage-shaped opacities that are typically seen in the posterior upper lobes with associated hilar retraction.⁷ These lesions are usually lenticular instead of round and therefore appear less dense than expected on frontal images.

PMF has an angel's-wing appearance on chest radiographs. They are large lesions kept apart from the pleura by aerated lung. Sequential imaging shows that these lesions tend to migrate toward the hila, leaving behind peripheral areas of emphysema. The emphysema and lung volume loss help distinguish unilateral PMF from lung cancer. Cavitation may be a complication, with ischemic necrosis or concomitant tuberculous infection. Punctate, linear, or massive calcifications of the PMF lesions may be noted.¹²

CT SCAN

Section 5 of 8  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

Findings

CT findings

On CT scans, these diseases appear as small, discrete nodules that have a predilection for the posterior portions of the upper lobes. The size of the nodules should be well correlated on CT scans and on chest radiographs.

On CT, PMF typically appears as irregular, lens-shaped, bilateral, large (>10 mm) attenuations in the posterior portions of the upper lobes. These lesions are often well circumscribed, calcified, and surrounded by cicatricial emphysema. Thickening of the adjacent extrapleural fat is common. Masses larger than 4 cm in diameter may exhibit central necrosis. Cavitation is infrequent. If necrosis or cavitation is seen in PMF, mycobacterial infection should always be considered. Pulmonary massive fibrosis can be misdiagnosed as bronchogenic carcinoma when pathognomonic characteristics, such as bilateral lens-shaped attenuations, well-defined borders, irregular calcifications, and lung nodularity, are missing.

HRCT findings

The typical appearance of pneumoconiosis on HRCT is branching and nonbranching centrilobular nodules that represent bronchiolar lesions. These lesions can be divided into 2 patterns: (1) ill-defined, fine, branching lines or nodules and (2) well-defined, discrete nodules. If present, interstitial fibrosis manifests as traction bronchiectasis, honeycombing, or large attenuations.¹²

On HRCT, lesions classified as p in the ILO criteria appear as small, branching structures or a group of small dots. They are usually associated with centrilobular emphysema. Lesions designated as q or r appear as either discrete, well-circumscribed, round nodules or as irregularly shaped, contracted nodules. Fusion of the small nodules can produce a large confluent lesion.⁷

On HRCT, silicoproteinosis appears as a ground-glass or alveolar pattern; no nodules are observed.

CWP appears as focal emphysema and nodularity of the posterior portions of the upper lobes. Nodules smaller than 7 mm are micronodules, those 7-20 mm are macronodules, and those larger than 20 mm are PMF. According to the ILO criteria, round nodules smaller than 1.5 mm are p, those 1.3-3 mm are q, and those 3-10 mm are r. Lesions of CWP also become cavitated and calcified.

On HRCT, CWP with p lesion appears as little, branching lines or ill-defined, punctate attenuations. In some cases, small areas of low attenuation with a central dot can be seen. These areas are thought to represent either irregular fibrosis surrounding respiratory bronchioles or dust macules on dilated respiratory bronchioles. Lesions classified as q and r types are well-circumscribed, round, or contracted nodules.¹²

Subpleural micronodules can be seen on HRCT. These lesions can coalescence into large pseudoplaques in CWP.

The appearance of PMF is similar to that of silicosis.

INTERVENTION

Section 6 of 8  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

Treatment consists of halting the progression of the disease, ameliorating symptoms, preventing comorbidities, providing supportive therapy, and helping patients maintain good health.

In addition, patients have chronic pulmonary disease and are in need of pneumococcal pneumonia and influenza vaccination, mucolytic and expectorant treatment, bronchodilation therapy, nutritional counseling, oxygen therapy, and regular exercise. Current smokers should quit immediately.

Experimental whole-lung lavage is being investigated as a possible treatment for slowing the progression of silicosis. The lavage aims at removing some of the dust load, macrophages, and inflammatory mediators from the lungs. Lung transplantation is used as a last resort for patients with end-stage disease.⁸

MULTIMEDIA

Section 7 of 8  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

Media file 1:  Reticular and small nodular opacities are present in the middle and upper lung zones. Also present are large, round opacities predominantly on the right; these are consistent with conglomerate nodules. Eggshell calcification of the mediastinal lymph nodes is also apparent, most notably in the regions of the right paratracheal and aortopulmonary window.
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Media type:  X-RAY

Media file 2:  Silicosis with progressive massive fibrosis. Image shows large, conglomerate nodules in both the middle and upper lung zones. Peripheral hyperlucency represents emphysematous lung tissue secondary to central migration of the large nodules. Also shown is evidence of volume loss in both upper lobes.
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Media type:  X-RAY

Media file 3:  Simple silicosis is depicted as widespread nodules measuring 2-5 mm in diameter, with a predominance in the middle and upper lung zones.
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Media type:  X-RAY

Media file 4:  Images show small nodules predominantly in the middle-to-upper lung zones, several of which are calcified. The large, round opacity overlying the posterior aspect of the fifth rib and the anterior aspect of the second rib on the right side is consistent with a conglomerate nodule.
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Media type:  X-RAY

Media file 5:  Image shows small nodules predominantly in the middle-to-upper lung zones, several of which are calcified. Eggshell calcification is present in the bilateral hilar lymph nodes. The large mass in the infrahilar region of the right lung is consistent with progressive massive fibrosis.
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Media type:  X-RAY

Media file 6:  High-resolution CT images of advanced coal-worker's pneumoconiosis with parenchymal nodules, calcifications, and progressive and massive fibrosis. Advanced-stage silicosis is indistinguishable from this condition.
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Media type:  X-RAY

Media file 7:  High-resolution CT images of advanced coal-worker's pneumoconiosis with parenchymal nodules, calcifications, and progressive and massive fibrosis. Advanced-stage silicosis is indistinguishable from this condition.
	View Full Size Image
Media type:  X-RAY

REFERENCES

Section 8 of 8

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• Intervention
• Multimedia
• References

1. Sherson D. Silicosis in the twenty first century. Occup Environ Med. Nov 2002;59(11):721-2. [Medline].
2. Kuschner WG, Stark P. Occupational lung disease. Part 2. Discovering the cause of diffuse parenchymal lung disease. Postgrad Med. Apr 2003;113(4):81-8. [Medline].
3. Fujimura N. Pathology and pathophysiology of pneumoconiosis. Curr Opin Pulm Med. Mar 2000;6(2):140-4. [Medline].
4. Seaton A, Cherrie JW. Quartz exposures and severe silicosis: a role for the hilar nodes. Occup Environ Med. Jun 1998;55(6):383-6. [Medline].
5. Wagner GR. Asbestosis and silicosis. Lancet. May 3 1997;349(9061):1311-5. [Medline].
6. Marchiori E, Ferreira A, Muller NL. Silicoproteinosis: high-resolution CT and histologic findings. J Thorac Imaging. Apr 2001;16(2):127-9. [Medline].
7. Kim JS, Lynch DA. Imaging of nonmalignant occupational lung disease. J Thorac Imaging. Oct 2002;17(4):238-60. [Medline].
8. De Vuyst P, Camus P. The past and present of pneumoconioses. Curr Opin Pulm Med. Mar 2000;6(2):151-6. [Medline].
9. Centers for Disease Control and Prevention. Pneumoconiosis prevalence among working coal miners examined in federal chest radiograph surveillance programs--United States, 1996-2002. Morb Mortal Wkly Rep. Apr 18 2003;52(15):336-40. [Medline].
10. Ooi GC, Tsang KW, Cheung TF, et al. Silicosis in 76 men: qualitative and quantitative CTevaluation--clinical-radiologic correlation study. Radiology. Sep 2003;228(3):816-25. [Medline].
11. Kroesen S, Itin PH, Hasler P. Arthritis and interstitial granulomatous dermatitis (Ackerman syndrome) with pulmonary silicosis. Semin Arthritis Rheum. Apr 2003;32(5):334-40. [Medline].
12. Akira M. High-resolution CT in the evaluation of occupational and environmental disease. Radiol Clin North Am. Jan 2002;40(1):43-59. [Medline].
13. Berlin JM, Taylor JS, Sigel JE, et al. Beryllium dermatitis. J Am Acad Dermatol. Nov 2003;49(5):939-41. [Medline].
14. Filipchenko LL, Slinchenko MZ, Sydorenko OM, Iertseva NO. Significance of immunoenzyme analysis in early diagnosis of dust bronchitis [in Ukrainian]. Lik Sprava. 2003;(2):91-3. [Medline].
15. Garg K, Lynch DA. Imaging of thoracic occupational and environmental malignancies. J Thorac Imaging. Jul 2002;17(3):198-210. [Medline].
16. Goodwin SS, Stanbury M, Wang ML, et al. Previously undetected silicosis in New Jersey decedents. Am J Ind Med. Sep 2003;44(3):304-11.
17. Hnizdo E, Vallyathan V. Chronic obstructive pulmonary disease due to occupational exposure tosilica dust: a review of epidemiological and pathological evidence. Occup Environ Med. Apr 2003;60(4):237-43. [Medline].
18. Ho JC, Lam WK, Ooi GC, et al. Lymphoepithelioma-like carcinoma of the lung in a patient with silicosis. Eur Respir J. Aug 2003;22(2):383-6. [Medline].
19. Ogawa S, Imai H, Ikeda M. Mortality due to silico-tuberculosis and lung cancer among 200 whetstonecutters. Ind Health. Jul 2003;41(3):231-5. [Medline].
20. Advanced pneumoconiosis among working underground coal miners--Eastern Kentucky and Southwestern Virginia, 2006. MMWR Morb Mortal Wkly Rep. Jul 6 2007;56(26):652-5. [Medline].
21. Smith DR, Leggat PA. 24 years of pneumoconiosis mortality surveillance in Australia. J Occup Health. Sep 2006;48(5):309-13. [Medline].
22. Chong S, Lee KS, Chung MJ, Han J, Kwon OJ, Kim TS. Pneumoconiosis: comparison of imaging and pathologic findings. Radiographics. Jan-Feb 2006;26(1):59-77. [Medline].

Article Last Updated: Jul 24, 2007