Pneumonitis is a general term for inflammation of lung tissue. Chronic inflammation of lung tissue can lead to irreversible scarring (pulmonary fibrosis). Pneumonitis is not a specific disease but a sign of an underlying problem.Acute chemical pneumonitis causes swelling of the lung tissue, movement of fluid into the air spaces in the lung and reduced ability to absorb oxygen and remove carbon dioxide. In severe cases, death may result from hypoxia.Chronic pneumonitis may follow low levels of exposure to the irritant over long periods of time, causing inflammation which may lead to fibrosis, resulting in decreased gas exchange and stiffening of the lung, and ultimately leading to respiratory failure and death.