Lung Pathology in U.S. Coal Workers with Rapidly Progressive Pneumoconiosis Implicates Silica and Silicates

Abstract

Rationale: Recent reports of progressive massive fibrosis and rapidly progressive pneumoconiosis in U.S. coal miners have raised concerns about excessive exposures to coal mine dust, despite reports of declining dust levels.

Objectives: To evaluate the histologic abnormalities and retained dust particles in available coal miner lung pathology specimens, and to compare these findings with those derived from corresponding chest radiographs.

Methods: Miners with severe disease and available lung tissue were identified through investigator outreach. Demographic as well as smoking and work history information was obtained. Chest radiographs were interpreted according to the International Labor Organization classification scheme to determine if criteria for rapidly progressive pneumoconiosis were confirmed. Pathology slides were scored by three expert pulmonary pathologists using a standardized nomenclature and scoring system.

Measurements and main results: Thirteen cases were reviewed, many of which had features of accelerated silicosis and mixed dust lesions. Twelve had progressive massive fibrosis, and 11 had silicosis. Only four had classic lesions of simple coal workers' pneumoconiosis. Four had diffuse interstitial fibrosis with chronic inflammation, and two had focal alveolar proteinosis. Polarized light microscopy revealed large amounts of birefringent mineral dust particles consistent with silica and silicates; carbonaceous coal dust was less prominent. On the basis of chest imaging studies, specimens with features of silicosis were significantly associated (P = 0.047) with rounded (type p, q, or r) opacities, whereas grade 3 interstitial fibrosis was associated (P = 0.02) with the presence of irregular (type s, t, or u) opacities.

Conclusions: Our findings suggest that rapidly progressive pneumoconiosis in these miners was associated with exposure to coal mine dust containing high concentrations of respirable silica and silicates.

Keywords: anthracosis; coal mining; pathology; pneumoconiosis; silicosis.

Figure 1.

Flow diagram of study participants. RPP = rapidly progressive pneumoconiosis.

Figure 2.

Posteroanterior radiographs of a coal miner with rapidly progressive pneumoconiosis who underwent a bilateral lung transplant at age 60 years (see pathology in Figure 3). He had 35 years of coal-mining experience, of which 28 years were at the face of the mine, and no history of smoking. (A) Chest radiograph obtained after 24 years of coal mine employment showing category 3 simple pneumoconiosis with q- and r-type opacities. (B and C) Chest radiographs obtained after 32 and 35 years of coal-mining employment, respectively. Multiple, large, mass-like lesions and nodules in the bilateral upper lung fields, which coalesce further and enlarge as time progresses, are shown.

Figure 3.

Explanted left lung from the miner whose radiographs are shown in Figure 2. (A) Photograph of the lung explant. The upper lobe is completely replaced by progressive massive fibrosis (PMF). Pale nodular areas can be seen within the PMF, indicative of silicosis. The apical segment of the lower lobe is also involved with PMF. Elsewhere the lung shows nodular lesions of pneumoconiosis. (B) Low-magnification view of section from area of PMF (left) and area involved with simple coal workers’ pneumoconiosis (right). Both areas show predominantly silicotic lesions. (C) Close-up of upper boxed area shown in B. A silicotic nodule can be seen on the pleural surface (pleural pearl), together with subpleural, semiconfluent, silicotic nodules. There is a marked lymphoid reaction in the pleura. (D) Close-up of lower boxed area in B showing pleural fibrosis with shallow underlying interstitial fibrosis as well as lymphoid aggregates.