A focal lung pneumatosis is an enclosed pocket of air or gas in the lung and includes blebs, bullae, pulmonary cysts, and lung cavities. Blebs and bullae can be classified by their wall thickness.

The terms above, when referring to sites other than the lungs, often imply fluid content.

Lung cysts are seen in about 8% of the general population, with an increased prevalence in older people, and are not associated with emphysema. They may be part of the aging changes of the lungs, and cause a slight decrease in their diffusing capacity. The presence of multiple pulmonary cysts may indicate a need to evaluate the possibility of bullous or cystic lung diseases. Cavitation indicates workup for serious infection or lung cancer.

The most common disease causing blebs or bullae is paraseptal emphysema though centrilobular emphysema may sometimes be involved.

Other conditions associated with lung bullae are:

A pulmonary cyst is not necessarily the same type of cyst seen in many cystic lung diseases. The cyst for example in pneumocystis pneumonia is not the same as the pulmonary cyst.^{[citation needed]}

Cystic lung diseases include:

A focal lung pneumatosis that is an incidental imaging finding such as on a CT scan, without suspicious findings (such as findings indicating any of the diseases listed above), generally does not indicate further follow-up.