Pulmonary fibrosis develops when repeated injury to lung tissue triggers an abnormal healing response, replacing flexible lung tissue with thick, stiff scar tissue. In many cases, a specific cause can be identified: workplace dust exposure, medication side effects, autoimmune disease, or radiation therapy. But in a large number of cases, no cause is ever found, a condition called idiopathic pulmonary fibrosis (IPF), which carries a median survival of 2 to 5 years after diagnosis.
How Lung Scarring Actually Happens
Healthy lungs repair small injuries all the time. In pulmonary fibrosis, that repair process goes haywire. After an injury, the body releases a signaling molecule called TGF-beta, which activates specialized repair cells called myofibroblasts. These cells produce collagen, the structural protein that forms scar tissue. Normally, once the wound heals, the process shuts down. In pulmonary fibrosis, it doesn’t.
TGF-beta keeps firing regardless of what originally caused the damage. The myofibroblasts keep producing collagen and simultaneously block the enzymes that would normally break excess collagen down. The result is a self-reinforcing loop: scar tissue accumulates, the lungs stiffen, and the tiny air sacs where oxygen enters the bloodstream are gradually destroyed. This is why pulmonary fibrosis is progressive and, once established, difficult to reverse.
When No Cause Can Be Found
Idiopathic pulmonary fibrosis is the most common and most serious form. “Idiopathic” simply means the cause is unknown. IPF primarily affects adults over 60 and is more common in men. The pooled global incidence is roughly 5.8 per 100,000 people, with North America seeing higher rates (about 9.0 per 100,000) compared to Europe (5.1) and Asia (4.4).
Although the trigger remains unidentified, IPF is not random. Aging and smoking are the two strongest risk factors. Both diseases manifest mainly after age 60, and smoking history is present in a majority of IPF patients. Gastroesophageal reflux disease (GERD) and past viral infections also appear to play a role, possibly by causing chronic, low-level injury to the lung lining that eventually tips the repair process into overdrive.
Genetic Risk Factors
Pulmonary fibrosis runs in some families. When two or more relatives develop the disease, it’s classified as familial pulmonary fibrosis (FPF). Researchers have identified rare gene variants in about 25% of FPF cases, clustered in two biological pathways: one involved in producing surfactant (the slippery coating that keeps air sacs open) and one involved in maintaining telomeres (the protective caps on chromosomes that shorten with age).
Variants in the TERT gene, which helps maintain telomere length, account for 8% to 15% of all familial cases. PARN and RTEL1 variants each make up another 5% to 10%. Even in non-familial IPF, the most strongly associated genetic variant sits in the promoter region of the MUC5B gene. This variant leads to excess mucus protein in the small airways, impairing the lungs’ ability to clear inhaled particles and pathogens, which may set the stage for chronic damage.
Workplace and Environmental Exposures
Inhaling certain dusts and fibers over months or years is one of the most well-established causes. The particles lodge deep in the lungs and provoke the kind of chronic inflammation that leads to scarring. Some of the strongest associations involve:
- Silica dust from mining, stone cutting, and sandblasting, which raises the risk of fibrosis roughly 1.7 times. Silica particles found in lung biopsies of IPF patients have been linked to faster decline in lung function.
- Asbestos fibers from construction, shipyard work, and insulation, which cause asbestosis, a well-known form of pulmonary fibrosis along with increased risk of lung cancer.
- Metal dust (nickel, iron, aluminum) from welding and machining, associated with a 1.4 to 2.0 times increased risk.
- Agricultural and wood dust, with risk increases of 1.6 to 2.35 times and 1.3 to 2.0 times, respectively.
- Coal mine dust, which causes coal workers’ pneumoconiosis, commonly known as black lung disease.
Air pollution also contributes. The collapse of the World Trade Center towers released a complex mix of heavy metals, silica, asbestos, wood dust, and chemical compounds. First responders and nearby residents have shown elevated rates of diffuse lung scarring consistent with pulmonary fibrosis. You don’t need a single catastrophic exposure; years of breathing moderately polluted air can accumulate enough damage to trigger the fibrotic process.
Autoimmune and Connective Tissue Diseases
Your immune system can also be the source of the lung injury. Several autoimmune conditions cause chronic inflammation in lung tissue that progresses to fibrosis. In one study of 110 patients with connective tissue disease and interstitial lung disease, about 25% progressed to a pattern of worsening, irreversible fibrosis.
Rheumatoid arthritis and systemic sclerosis (scleroderma) together account for more than 70% of progressive fibrotic lung disease among autoimmune patients. Among those who progressed, rheumatoid arthritis was the most frequent underlying condition at 33%, followed by Sjögren’s disease (30%), scleroderma (18%), and inflammatory muscle disease (15%). The fibrosis in these cases behaves similarly to IPF, progressively stiffening the lungs over time, though treatment strategies differ because the underlying autoimmune disease can sometimes be controlled.
Medications That Can Cause Lung Scarring
Dozens of medications have been linked to pulmonary fibrosis as a rare but serious side effect. The risk is highest with certain cancer treatments and immune-modifying drugs. Among cancer and immune therapies reported to cause fibrosis, the most frequently implicated include rituximab, methotrexate, etanercept, leflunomide, and adalimumab. Bleomycin, a chemotherapy drug, has long been recognized as one of the most potent triggers of drug-induced lung fibrosis.
Heart and blood pressure medications can also be involved. Amiodarone, a drug used for irregular heart rhythms, is one of the best-known cardiovascular culprits. Certain blood pressure medications in the angiotensin receptor blocker and ACE inhibitor classes, along with some diuretics and beta blockers, have also appeared in reports. The fibrosis typically develops gradually over weeks to months of use, and in some cases, stopping the medication can slow or halt progression if caught early.
Radiation Therapy
Radiation treatment for cancers in or near the chest, particularly lung cancer and breast cancer, can damage healthy lung tissue in the radiation field. The fibrotic phase typically begins 6 to 9 months after treatment, following an earlier inflammatory phase. Imaging studies show that visible lung changes can appear even in areas receiving relatively low doses, around 5 to 6 Gy, though the most pronounced scarring occurs in areas receiving 20 Gy or more.
Radiation-induced fibrosis is generally limited to the area of the lung that was directly exposed, which distinguishes it from conditions like IPF that affect both lungs diffusely. The severity depends on the total dose, the volume of lung tissue treated, and whether the patient received concurrent chemotherapy, which can amplify the damage.
How Pulmonary Fibrosis Is Identified
High-resolution CT scanning is the primary tool for identifying pulmonary fibrosis. Doctors look for a specific pattern called usual interstitial pneumonia (UIP), which is the hallmark of IPF. The defining features on a scan are a honeycomb pattern of small cysts clustered at the base and outer edges of the lungs, along with irregular thickening of lung tissue and widening of the airways from traction as scar tissue pulls on them.
Certain scan findings make IPF less likely and point toward other causes: scarring concentrated in the upper lungs rather than the base, widespread hazy “ground glass” areas suggesting active inflammation rather than established scar, or scattered tiny nodules. When the scan shows a clear UIP pattern in someone with no identifiable exposure, autoimmune disease, or medication trigger, a diagnosis of IPF can often be made without a lung biopsy. When the picture is less clear, additional testing, including blood work for autoimmune markers and sometimes a surgical biopsy, helps narrow the cause.