The exact prevalence of idiopathic pulmonary fibrosis is unknown, but it has been estimated at 3 to 6 cases per 100,000 persons. A more recent study, however, placed the prevalence at 20 cases per 100,000 for males and 13 cases per 100,000 for females. The same study estimated the incidence to be 10.7 cases per 100,000 per year for males and 7.4 cases per 100,000 per year for females, although this was less clearly defined. UIP is the most common idiopathic interstitial lung disease, accounting for over 60% of these cases. It most often occurs in people between the ages of 40 and 70 years, but it may happen at any time. The incidence increases with increasing age, so that approximately two-thirds of people are over age 60 at the time they are diagnosed. Men are affected twice as often as women. The disease has no apparent geographic or ethnic predilection. Cigarette smoking has been identified as a potential risk factor for UIP, increasing the risk 2 to 3 times above nonsmokers. Although the majority of affected individuals smoke cigarettes, UIP appears to occur randomly and spontaneously and not as the result of any other disease or known exposure. Any associated exposure or disease would have to raise the possibility that the individual has one of the many other interstitial lung diseases (Table 1).

These characteristics of UIP can be contrasted with the other forms of idiopathic pulmonary fibrosis. Nonspecific interstitial pneumonia (NSIP) is second in frequency to UIP. NSIP also predominantly affects middle-aged adults at an average age of 49 years. However, unlike UIP, NSIP can affect children and has also been reported in the elderly. Also unlike UIP, NSIP affects females slightly more often than males.

Respiratory bronchiolitis interstitial lung disease (RBILD) and desquamative interstitial lung disease (DIP) are believed to be two different forms of the same disease with RBILD being less severe than DIP. The average age at onset of RBILD is 36 years old, which is younger than the other forms of idiopathic interstitial lung disease. There is a slight male predominance. The most striking feature of RBILD is that every affected person is a cigarette smoker.

DIP and acute interstitial pneumonia (AIP) are relatively rare. The average age at onset of DIP is 42 years old, which is younger than UIP or NSIP but older than RBILD. Like UIP, men are affected with DIP nearly twice as often as women.

Finally, AIP is diagnosed at an average age of 49, but both children and elderly have been affected. Males and females are affected equally. A notable feature is that most affected individuals experienced an acute viral-like illness just prior to the onset of AIP.

Only about 5% of cases of IPF appear to run in families, which means familial pulmonary fibrosis (FPF) occurs in perhaps only two people out of every million. The age at which symptoms start varies significantly, ranging anywhere from infancy up to age 80. However, as with IPF, symptoms generally begin in middle age.

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References

• Baumgartner KB, Samet J, Stidley CA, Colby TV, Waldron JA, and the Collaborating Centers. Cigarette smoking: a risk factor for idiopathic pulmonary fibrosis. American Journal of Respiratory and Critical Care Medicine. 155:242-248, 1997.