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. 2016 Jan 1;124(1):A13–A18. doi: 10.1289/ehp.124-A13

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graphic file with name ehp.124-A13.g002.jpg — A section of lung shows the ravages of progressive massive fibrosis (PMF). The disease is characterized by large, dense masses of fibrous tissue that often appear in the upper lungs. The lung itself can appear black due to the slow buildup of coal dust particles over the years.

© Biophoto Associates/Getty