What Is Idiopathic Pulmonary Fibrosis (IPF)?

"IPF is a specific form of chronic fibrosing interstitial pneumonia, limited to the lung, and associated with the histologic appearance of UIP [Usual Interstitial Pneumonia] on surgical (thoracoscopic or open) lung biopsy. The aetiology is unknown. The definite diagnosis of IPF in the presence of a surgical biopsy showing UIP includes the following. 1. Exclusion of other known causes of interstitial lung disease such as drug toxicities, environmental exposures, and collagen vascular diseases. 2. Abnormal pulmonary function studies that include evidence of restriction and/or impaired gas exchange. 3. Abnormalities on conventional chest radiographs or high-resolution computed tomography (HRCT). . . . All IPF is UIP, but on the other hand, not all UIP is IPF." ¹ IPF, as it was defined in the past, is the most common nonoccupational cause of interstitial lung disease--about 25% to 40% of the cases. [Harber, p. 216]

 

Is IPF an Occupational Disease?

No, IPF is not an occupational disease; it is an idiopathic disease, meaning that the cause is unknown.

 

Has the Diagnosis of IPF Changed in Recent Years?

Yes, the definition of IPF was much broader 20 years ago. "UIP is the histopathological pattern that identifies patients with IPF. Other patterns such as DIP, respiratory bronchiolitis-associated interstitial lung disease (RBILD), NSIP, LIP, AIP, and idiopathic bronchiolitis obliterans organising pneumonia (idiopathic BOOP) are considered separate entities and are to be excluded from the group of patients with IPF." A surgical lung biopsy is now recommended for most patients.¹

 

What Are the Most Common Causes of Occupational Interstitial Lung Disease?

The most common causes are silica, asbestos, coal, beryllium, and hard-metal cobalt. Other minerals and metals are listed in Haz-Map linked to benign and other pneumoconioses. Also, moldy organic mists and dusts cause hypersensitivity pneumonitis in a variety of occupations, including workers exposed to farm dusts, wood dusts, birds, and metalworking fluids (MWFs). See Table 1. Also see Interstitial Lung Disease in Haz-Map at the NLM website. Hypersensitivity pneumonitis in the chronic stage causes interstitial fibrosis. Precipitating antibodies are neither sensitive nor specific, and their presence is no longer considered a hallmark of HP. [Murray, p. 1783-99] Symptoms of asbestosis are slower in developing, and patients with asbestosis have less respiratory impairment than patients with IPF. [Rom, p. 299]

 

Do Case Control Studies Prove that IPF Is Caused by Metal and Wood Dusts?

Case control studies show that patients with IPF, as it was broadly defined in the past, are more likely to report occupational dust exposures than people in the control group. ^2,3,4,5 One of the weaknesses of case control studies is recall bias, meaning that the validity of recall about past exposures may vary between people who have a chronic lung disease and those who don't. Since these case control studies used the old case definition of IPF, it is possible that many of the patients were misdiagnosed. For example, in the Baumgartner et al paper², negative serum precipitin tests were used to rule out hypersensitivity pneumonitis, but it is now known that these tests are not reliable. As noted in a another recent paper, "Hypersensitivity pneumonitis due to exposure to MWFs is under-recognized by health care providers, and current surveillance systems are inadequate to provide a true estimate of its occurrence."⁶ As noted by Taskar and Coultas, "Given the low utilization of the gold standard for diagnosing IPF and the complexity of diagnosis, there is high likelihood for misdiagnosis."³

References

1. International consensus statement on idiopathic pulmonary fibrosis. Full text at: http://www.erj.ersjournals.com/cgi/content/full/17/2/163
2. Occupational and environmental risk factors for idiopathic pulmonary fibrosis: a multicenter case-control study. Full text at:  http://aje.oxfordjournals.org/cgi/reprint/152/4/307
3. Is idiopathic pulmonary fibrosis an environmental disease? Full text at: http://pats.atsjournals.org/cgi/content/full/3/4/293
4. Occupational and environmental factors and idiopathic pulmonary fibrosis in Japan. Full text at: http://annhyg.oxfordjournals.org/cgi/content/full/49/3/259
5. Occupational exposure and severe pulmonary fibrosis. 
6. Hypersensitivity pneumonitis due to metal working fluids: Sporadic or under reported?

brownjay@haz-map.com