All of the following statements about silicosis are true except

With long-term, less intense exposure, small rounded opacities in the upper lobes may appear on the chest radiograph after 15-20 years of exposure, usually without associated impairment of lung function (simple silicosis). Calcification of hilar nodes may occur in as many as 20% of cases and produces a characteristic "eggshell" pattern. Because silica is cytotoxic to alveolar macrophages, patients with silicosis are at greater risk of acquiring lung infections that involve these cells as a primary defense (Mycobacterium tuberculosis, atypical mycobacteria and fungi). Because of the increased risk of active tuber-culosis, the recommended treatment of latent tuberculosis in these patients is longer. Ref Harrison 19th edition pg 1690