The cause is unknown. People 30 to 40 years old are affected most often.
Cigarette smoking is a risk factor for pulmonary histiocytosis.
Spontaneous pneumothorax occurs frequently in this condition. Histiocytosis occurs in about 1 out of 10,000 people. Histiocytosis may also be manifest in childhood. The skull is a frequent site of involvement. Tumors in the weight bearing bones such as the legs or spine may fracture spontaneously.