What Causes Fibrosis of the Lung?
Pulmonary fibrosis results from a diverse array of causes including connective tissue diseases, chronic hypersensitivity pneumonitis, occupational/environmental exposures, medications, genetic mutations, and idiopathic processes—with idiopathic pulmonary fibrosis (IPF) being the most common form when no cause is identified 1, 2.
Major Categories of Causes
Connective Tissue Disease-Related ILD
Systemic autoimmune conditions frequently cause pulmonary fibrosis, including:
- Systemic sclerosis (SSc)
- Rheumatoid arthritis (RA)
- Polymyositis (PM)
- Mixed connective tissue disease (MCTD)
- Systemic lupus erythematosus (SLE)
These conditions produce fibrotic patterns including usual interstitial pneumonia (UIP), fibrotic nonspecific interstitial pneumonia (F-NSIP), and other patterns 1.
Hypersensitivity Pneumonitis (HP)
Fibrotic HP represents a critical diagnostic consideration that must be distinguished from IPF because management differs fundamentally—antigen remediation and immunosuppression are key in fHP, whereas these approaches are harmful in IPF 2. The fibrotic element may manifest as UIP, F-NSIP, or bronchiolocentric fibrosis patterns 1.
Occupational and Environmental Exposures
Multiple inhalational exposures cause pulmonary fibrosis 1, 3:
- Metal dust exposure
- Wood dust exposure
- Air pollution
- Asbestos (causing asbestosis)
- Silica (causing silicosis)
- Other pneumoconioses
Environmental causes appear to interact with genetic susceptibility and may also trigger acute exacerbations in established disease 3.
Medications and Drug Toxicity
Certain commonly prescribed drugs can induce pulmonary fibrosis, though specific agents are not detailed in the guideline evidence provided 1, 4.
Genetic Causes
Telomere-related gene (TRG) mutations represent the most common genetic cause of familial pulmonary fibrosis 5. These mutations can produce:
- Syndromic telomeropathies with hematological and hepatic manifestations
- Variable penetrance across family members
Surfactant-related gene mutations typically manifest with isolated pulmonary involvement 5.
The MUC5B promoter polymorphism represents a significant genetic risk factor, though only some individuals with this variant develop pulmonary fibrosis, suggesting gene-environment interactions 6.
Other Specific Entities That Progress to Fibrosis
The 2022 ATS/ERS/JRS/ALAT guidelines identify multiple fibrotic lung diseases 1:
- Idiopathic fibrotic NSIP
- Pleuroparenchymal fibroelastosis (PPFE)
- Fibrosing organizing pneumonia (FOP)
- Fibrotic sarcoidosis
- Fibrotic Langerhans cell histiocytosis
Idiopathic Pulmonary Fibrosis (IPF)
When comprehensive evaluation excludes all identifiable causes, the diagnosis becomes IPF—defined as fibrotic ILD limited to the lungs with UIP pattern on imaging or histology 2, 4. Risk factors include:
- Aging (most prominent risk factor)
- Male gender
- Cigarette smoking
- Gastroesophageal reflux disease
- Diabetes mellitus
Critical Diagnostic Pitfall
The distinction between IPF and fibrotic HP represents one of the most challenging diagnostic dilemmas because these entities have overlapping characteristics but require fundamentally different management approaches 2. This differentiation requires multidisciplinary discussion integrating clinical history (particularly antigen exposure), radiologic patterns, and when available, pathologic findings. Missing a diagnosis of fHP and treating as IPF denies the patient antigen avoidance and appropriate immunosuppression; conversely, misdiagnosing IPF as fHP and using immunosuppression causes harm 2.
Emerging Mechanistic Understanding
Recent evidence suggests pulmonary fibrosis may represent a self-sustaining pathological tissue-repair state rather than continuous injury-dependent process 7. Failure of alveolar epithelial regeneration produces persistent cellular senescence and chronic distress signaling, creating feedback loops involving immune mis-repair, stromal stabilization, and mechanical reinforcement through matrix stiffening. This framework explains why fibrosis often progresses after the initiating insult resolves and why anti-inflammatory therapies show limited benefit in IPF 7.