SilicosisSilicosis is a lung disease caused by inhalation of crystalline silicon dioxide (silica). Currently, it is the most prevalent occupational disease in the world. Silicosis is a slowly progressive disease, usually presenting after decades of exposure as slowly progressive nodular fibrosing pneumoconiosis. Silica occurs in both crystalline and amorphous forms but crystalline forms are much more fibrogenic. The crystalline form are quartz, crystobalite. Silicosis is characterized in its early stages by nodules in the upper zones of the lung. As the disease progresses these nodules may coalesce into hard collagenous scars. Fibrotic lesions occur in the hilar lymph node and pleura. Sometimes thin sheets of calcification occur in the lymph nodes and are seen radiographically as egg shell calcification i.e. (calcium surrounding a zone lacking calcification. It the disease continues to progress, expansion and coalescence of lesions produce progressive massive fibrosis. Histologically the lesions of silicosis consists of concentric layers of hyalinized collagen surrounded by a dense capsule of more condensed collagen. Examination of the nodules by polarized microscopy reveals the birefringent silica particles. Silicosis is associated with an increased susceptibility to TB. It is postulated that silicosis results in depression of cell mediated immunity and the crystalline silica may inhibit the ability of pulmonary macrophages to kill phagocytosed mycobacteria. Nodules of silicotuberculosis often display a central zone of caseation.