This figure is an Adverse Outcome Pathway, or an overview of one way chemicals may cause pulmonary (lung) fibrosis. Pulmonary fibrosis is the development of scar tissue (similar to a scab on the skin) within the lung. This makes it difficult for people to breathe by preventing the lungs from expanding to fill with air. As a result, people can die from a lack of oxygen in their blood. Ordinarily, if chemicals caused some type of mild lung injury, PPAR-gamma would prevent the development of pulmonary fibrosis. Here, we show that it is possible for chemicals to drive the development of pulmonary fibrosis by inactivating the PPAR-gamma receptor. This allows TGF-beta to stimulate pulmonary fibrosis, as a result of lung injury either by the chemical or some other cause (e.g., inhaled sand, smoke, or other chemicals) that may irritate the lung. Unfortunately, once pulmonary fibrosis has developed, it is not reversible ⸺ the scar tissue remains forever.