A radiographic study of inhalational lung diseases is conducted. One pattern of involvement is seen in persons whose total lung capacity, diffusing capacity, and compliance is decreased. This pattern consists of numerous bilateral nodular opacifications on chest radiographs. Polarizable needlelike crystals are seen on microscopic examination of these nodules. What inhaled substance is most likely to produce these findings?

Silicotic nodules form when the silica crystals ingested by macrophages elicit a fibrogenic response as cytokines, such as tumor necrosis factor, are released. The nodules may become confluent with progressive massive fibrosis. Pneumoconioses such as silicosis lead to restrictive lung disease. Cigarette smoke contributes to loss of lung parenchyma with emphysema. Mold spores tend to elicit a hypersensitivity pneumonitis that rarely goes on to extensive restrictive lung disease. Sulfur dioxide in polluted air tends to drive chronic obstructive lung disease. Wood dust tends to elicit an asthmatic response.