Pulmonary fibrosis develops when repeated injury to the lungs triggers an abnormal healing response, replacing healthy tissue with thick, stiff scar tissue. In roughly half of all cases, no specific cause is ever identified, a condition called idiopathic pulmonary fibrosis (IPF). In the other half, the scarring can be traced to environmental exposures, autoimmune diseases, medications, radiation, or genetic predisposition. Often, multiple factors overlap.
How Lung Scarring Actually Happens
Healthy lungs repair minor damage all the time. In pulmonary fibrosis, that repair process goes haywire. The current understanding centers on repetitive injury to the thin lining of the air sacs. Each round of damage releases chemical signals that cause repair cells called fibroblasts to multiply and transform into a more aggressive type that produces large amounts of collagen and other structural proteins. A key driver of this process is a signaling molecule called TGF-beta, which promotes scar tissue production and blocks the body’s normal cleanup systems that would otherwise keep things in check.
Over time, the excess collagen stiffens the lung tissue, making the walls of the air sacs thicker and less flexible. Oxygen has a harder time passing from the lungs into the bloodstream. The scarring is progressive and, in most forms, irreversible.
Idiopathic Pulmonary Fibrosis
IPF is the most common and most studied form of the disease. The global incidence is about 5.8 cases per 100,000 people per year, with North America seeing higher rates (around 9 per 100,000) compared to Europe (5.1) and Asia (4.4). The typical patient is male, over 60, and has a history of smoking. On imaging, IPF shows a distinctive pattern of scarring concentrated in the lower and outer portions of the lungs, often with a honeycomb-like appearance that distinguishes it from other lung diseases.
The “idiopathic” label means no occupational exposure, medication, or autoimmune condition can be identified as the trigger. But “unknown cause” doesn’t mean “no cause.” Researchers believe IPF results from a combination of genetic susceptibility, aging, and cumulative low-level exposures that individually wouldn’t cause disease but together push the lungs past a tipping point.
Occupational and Environmental Exposures
Breathing in certain dusts over months or years is one of the most well-established causes of lung fibrosis. The three classic culprits are silica dust, asbestos fibers, and coal dust. Silica exposure affects workers in mining, sandblasting, stonecutting, and construction. Asbestos, though now heavily regulated, still causes disease in people exposed decades ago in shipbuilding, insulation work, and textile manufacturing. Coal miners develop their own pattern of scarring, sometimes with an unusual form of lung destruction where the scarring itself is relatively mild but the surrounding tissue breaks down.
Less common but still documented triggers include haematite dust (iron ore mining), carbon dust (electrode manufacturing), and certain glass fibers. The common thread is that these tiny particles lodge deep in the lungs, where the body’s attempts to wall them off with scar tissue become the disease itself.
Organic Dusts and Mold
Not all dangerous inhaled particles come from industrial settings. Hypersensitivity pneumonitis is a form of lung inflammation triggered by organic materials like bird droppings, feathers, mold spores, and bacteria found in hay or grain. Bird-related hypersensitivity pneumonitis is the most common type and occurs in people who keep pigeons, parrots, or other birds, or who work in poultry farming. With low-level, continuous exposure, the condition can quietly progress to a chronic form characterized by permanent fibrotic scarring inside the lungs. Unlike the acute form, which clears up when exposure stops, chronic hypersensitivity pneumonitis is only sometimes reversible.
Autoimmune and Connective Tissue Diseases
The immune system can turn against the lungs in several autoimmune conditions. Systemic sclerosis (scleroderma) is the autoimmune disease most frequently accompanied by interstitial lung disease, but when it comes to progressive, worsening fibrosis specifically, rheumatoid arthritis is actually the leading cause. In one study of patients whose connective tissue disease progressed to fibrotic lung disease, rheumatoid arthritis accounted for 33% of cases, followed by Sjögren’s disease at 30%, scleroderma at 18%, and inflammatory muscle disease at 15%. Together, rheumatoid arthritis and scleroderma account for more than 70% of progressive fibrotic lung disease linked to autoimmune conditions.
The mechanism is similar across these diseases: the immune system’s chronic attack on the body’s own tissues generates ongoing inflammation in the lungs, which eventually triggers the same fibroblast-driven scarring seen in other forms of pulmonary fibrosis.
Medications That Can Scar the Lungs
Dozens of medications have been linked to lung toxicity, but certain drugs carry a well-documented fibrosis risk. One of the most studied is amiodarone, a heart rhythm medication widely prescribed for irregular heartbeats. Amiodarone can damage lung tissue both directly, through a toxic effect on cells, and indirectly, by triggering an immune reaction. Pulmonary fibrosis develops in 5% to 7% of patients who first experience amiodarone-related lung inflammation. In some cases, the scarring appears without a noticeable inflammatory phase beforehand.
Several chemotherapy drugs, certain antibiotics, and some anti-inflammatory medications also carry fibrosis risk. The challenge with drug-induced fibrosis is that symptoms can mimic the underlying disease being treated, delaying recognition that the medication itself is causing harm.
Radiation Therapy
Radiation treatment for lung, breast, or other chest cancers can cause fibrosis in the exposed lung tissue. The scarring can develop months or even years after treatment ends, sometimes as a late consequence of earlier acute inflammation. Risk increases with higher radiation doses to the lung, but patient factors matter too. People with pre-existing interstitial lung changes on imaging face dramatically higher risk: one study found that 26% of these patients developed severe radiation-related lung injury, compared to just 3% of those with normal lungs at baseline.
Older age, tumors located in the lower lungs, and concurrent chemotherapy (particularly with certain drug classes) also raise the odds. The same TGF-beta signaling molecule that drives fibrosis in other contexts plays a central role here. Patients whose blood levels of this molecule remain elevated after radiation are significantly more likely to develop lung damage.
Genetic Risk Factors
Pulmonary fibrosis sometimes clusters in families, and specific genetic variants help explain why some people develop the disease while others with similar exposures do not. One of the strongest known genetic risk factors is a variant in a gene called MUC5B, which is involved in mucus production in the airways. Carrying this variant raises the odds of developing IPF roughly fourfold.
Telomere length is another important genetic factor. Telomeres are the protective caps on the ends of chromosomes that shorten each time a cell divides. People with mutations in genes that maintain telomeres (like TERC and PARN) have abnormally short telomeres, which accelerates cellular aging in the lungs and is associated with higher risk of fibrosis, more severe disease, and worse outcomes. Telomere shortening isn’t purely genetic, though. Smoking, poor nutrition, and lower socioeconomic status all contribute to faster telomere erosion, illustrating how genes and environment interact.
Researchers now believe the most common path to pulmonary fibrosis involves someone with a genetic predisposition, such as the MUC5B variant or short telomeres, accumulating environmental hits over a lifetime until the lungs’ repair capacity is overwhelmed.
Acid Reflux and Microaspiration
Gastroesophageal reflux disease (GERD) is remarkably common in people with pulmonary fibrosis, and growing evidence suggests it may be more than a coincidence. The proposed mechanism is microaspiration: tiny amounts of stomach acid and bile repeatedly reach the lungs, causing low-grade chemical injury to the air sac lining. Studies in mice show that stomach contents in the lungs trigger collagen buildup, thickening of the air sac walls, and activation of the same pro-scarring pathways (including TGF-beta) seen in other forms of fibrosis. In humans, pepsin and acid have been found in fluid washed from the lungs of IPF patients, supporting the idea that reflux reaches the lower airways.
The relationship may also work in reverse. As the lungs stiffen and lose elasticity, the increased negative pressure in the chest pulls on the esophagus and weakens its upper valve, making reflux episodes more frequent. This creates a vicious cycle where fibrosis worsens reflux, and reflux worsens fibrosis.
Smoking
Cigarette smoking is a risk factor that cuts across nearly every category. It directly damages the air sac lining, shortens telomeres, increases susceptibility to environmental exposures, and is one of the strongest clinical predictors of IPF. Smoking also worsens outcomes in people who develop fibrosis from other causes. The combination of smoking history with male sex and age over 60 is considered strong enough to support an IPF diagnosis on imaging alone, without a lung biopsy.