What Is Pulmonary Fibrosis? — Thickening Paper Windows, Scars on the Wall Oxygen Crosses

Breath That Shows First on the Stairs

My father, past seventy, keeps stopping as he climbs the apartment stairs. A dry cough has also continued for months. My father said it was 'because of old age,' and the family thought so too. Then, in the doctor's office, he hears the term pulmonary fibrosis.

Shortness of breath upon exertion is a common way that diseases troubling the walls inside the lungs first reveal themselves, and about three in ten people complain of a cough (PMID 38648021). Pulmonary fibrosis is a disease in which scars build up on the thin walls between the air sacs inside the lungs, causing those walls to thicken and stiffen. The reason breathing becomes difficult, and the target of treatments that slow progression, is this wall.

Starting from that wall, this article follows why scars accumulate, how they appear and are diagnosed, and their course and treatment. There is also a section for anyone who has heard the term "interstitial change" on a health checkup CT scan.

The Thin Wall That Oxygen Crosses

When you inhale, air travels deep into the lungs to tiny pockets. Alveoli are tiny air sacs hanging like grape clusters at the ends of air passages, where oxygen and carbon dioxide are exchanged with the blood. The outside of each alveolus is wrapped in tiny blood vessels like a net. Capillaries are the thinnest vessels connecting arteries and veins, where oxygen and substances actually move back and forth between blood and tissues. In the lungs, that counterpart is the air inside the alveolus.

Between the air in the alveolus and the blood in the capillary lies only a thin wall. This wall is made of the cell layer covering the alveolus, the wall of the capillary, and the narrow gap between the two (PMID 16738195). The interstitium is the tissue filling the gap within that thin wall between the alveoli and capillaries. This shares only its sound in Korean with the old name for epilepsy ("ganjil"), which is a completely different word. Oxygen crosses this wall, filled with the interstitium, from the air in the alveolus to enter the blood in the capillary. Diffusion is the process by which a substance naturally spreads from an area of higher concentration to an area of lower concentration. Diffusion handles oxygen crossing the wall, and the thinner the wall, the easier it crosses. The path that breath takes to the alveoli is followed in "How Do We Breathe? — An Upside-Down Tree, and the Leaves Where Air Meets Blood".

This wall resembles traditional Korean window paper pasted onto a frame. Thin window paper lets in light and breeze. If you layer paper on top of paper repeatedly, less light gets through, and the paper stiffens and sways less in the wind. In pulmonary fibrosis, scars pile up layer after layer on this thin wall.

There are also ways the analogy breaks down. Window paper is layered on from the outside by someone else, but what causes scars to settle on the lung wall is the body itself trying to fix a damaged wall. Why that repair becomes excessive is explored in the next section. Also, window paper can be torn off and repassed, but the drugs currently used for pulmonary fibrosis of unknown cause only slow the progression and cannot remove scars that have already settled (PMID 41762087).

What brings about scars is injury, and injury is commonly accompanied by inflammation. Inflammation is a reaction where the body responds to stimuli such as injury, infection, or substances entering from outside by gathering immune cells, causing the area to swell. Fibrosis is the process where organs harden as scar tissue accumulates to fill in long-standing injury and inflammation.

The diseases that afflict this lung wall are not singular. Interstitial lung disease is an umbrella term for various lung conditions where inflammation, fibrosis, or both develop in the interstitium. There are over 200 named conditions alone, and in most of them, gas exchange goes awry due to inflammation or scarring of the wall, making one short of breath and exercise difficult. The clinical course varies greatly from disease to disease, with some cases spontaneously reversing or stabilizing (PMID 35964592). The most common is idiopathic pulmonary fibrosis, which accounts for roughly one-third of the total according to one review's compilation (PMID 38648021).

Idiopathic pulmonary fibrosis is an interstitial lung disease in which scars continue to accumulate in the lungs even though no cause can be found. "Idiopathic" means the cause is unknown. It mainly occurs in older adults (PMID 30501130) and is considered the archetype of lung diseases characterized by accumulating scars (PMID 41762087).

Pulmonary fibrosis is a disease in which scars accumulate in the interstitium within the thin wall that oxygen crosses; interstitial lung disease is the name grouping several such diseases, and idiopathic pulmonary fibrosis—whose cause is unknown—is its archetype.

Why Scars Accumulate — Repeated Minor Injuries and Misguided Healing

If you scrape your knee, a scab forms, and in time, fresh skin grows underneath. The walls of the alveoli are also repaired when injured. In idiopathic pulmonary fibrosis, this repair process is thought to go awry. The cells lining the inner surface of the alveoli, where they meet the air, suffer repeated minor injuries, and the subsequent healing strays off course, leading to an excessive buildup of scar material (PMID 29052582, PMID 33359599). This process is believed to occur in bodies aging with a genetic susceptibility to the disease, though many questions remain about how to determine who is vulnerable (PMID 29052582).

Scars breed scars. When repeated injury combines with immune responses and misguided tissue repair, the tissue is repeatedly re-woven, creating a state where the scar perpetuates itself (PMID 37156412). Cells that build scars gather and pour out excessive amounts of substances like collagen, which destroys the architecture of the lungs (PMID 21719092). As a result, the lungs become less stretchy and the exchange of gases is disrupted (PMID 28365056). This scar tissue is just like layers of paste building up on traditional Korean paper screen doors.

The place of inflammation has shifted. This disease was once thought to be a long-standing inflammation, but current evidence points to the abnormal activation of alveolar wall cells as the starting point of scarring (PMID 21719092). How important inflammation is as a cause remains debated, and large studies testing anti-inflammatory drugs that failed played a part in this shift in perspective. However, new studies on the role of immune cells are slowly changing that view again (PMID 30704705). It is still unknown what first pulls the trigger on scarring in idiopathic pulmonary fibrosis, and what keeps that process turned on (PMID 39264073).

Even without knowing the cause, several factors associated with this disease are known. The disease occurs more often in people over 60, in men, and in people who smoke or used to smoke (PMID 25362769), and the number of new cases increases steeply with advancing age (PMID 29052582). While smoking is recognized as a well-established risk factor for developing the disease, how it affects the course after the disease has started has been unclear (PMID 42339520).

Family history also plays a role. Several gene variants that cause pulmonary fibrosis have been identified, and in some cases, multiple people in a single family suffer from the disease. Although genetic testing for such families has already been adopted into medical practice in some countries, there is no international consensus yet (PMID 36549714).

The air at workplaces and at home is also a clue. Work-related diseases may not be distinguishable from non-work-related ones by symptoms, lung function, or imaging, which makes taking a detailed work history important. Removing oneself from the causative exposure is the core of treatment, and doing so can bring substantial improvement (PMID 37535448).

Hypersensitivity pneumonitis is an interstitial lung disease caused by the body's immune system reacting excessively to substances inhaled into the lungs. According to the same statistics, about 15% of interstitial lung diseases are this condition (PMID 38648021). Those substances are usually proteins from birds and molds, and long-standing cases mostly stem from prolonged, low-level exposure to birds or mold at home. Completely avoiding the causative agent is the center of treatment, but some patients with scarring may continue to get worse even after all exposure is cut off (PMID 32764620).

The body's immunity is also a cause. Autoimmunity is when the immune system, which protects the body, mistakes its own cells for enemies and attacks them. Conditions like rheumatoid arthritis and other autoimmune diseases accompany about a quarter of interstitial lung diseases (PMID 38648021), and some of those cases progress toward a continuous accumulation of scars (PMID 30578335).

Idiopathic pulmonary fibrosis is thought to be a disease where the repair of repeated minor injuries goes wrong, causing scars to accumulate.

How It Appears — Shortness of Breath, a Dry Cough, and Sounds Heard During an Examination

Idiopathic pulmonary fibrosis appears with a dry cough, fatigue, and shortness of breath that progressively worsens with exertion (PMID 35001525). Both coughing and shortness of breath also reduce quality of life (PMID 29052582).

There are also findings that doctors look for during an examination. Velcro crackles are sounds that are heard like tiny popping noises while inhaling when a stethoscope is placed on the chest. In pulmonary fibrosis, they are often heard near the end of inhalation at the lower parts of the lungs, sounding like peeling apart Velcro. Clubbing is a change where the fingertips swell up bluntly and the fingernails curve roundly. Clubbing is seen in about one-third of adults with interstitial lung disease, and at a similar rate in other conditions such as bowel disease or liver disease (PMID 35067513). Not having clubbing does not mean a person does not have this disease, and having it does not mean they do. Crackles can also be heard in other conditions, so neither finding alone determines the disease.

Diagnosis brings together a detailed medical history, physical examination, blood tests, pulmonary function tests, chest CT, and sometimes even a tissue biopsy (PMID 30884853).

A pulmonary function test has a person take a deep breath in and blow it out as hard as possible, measuring the volume and speed of the exhaled air. FVC is the total volume of air that comes out when a person exhales as hard as possible to the end in that manner. When the lungs stiffen, they stretch less, the effort required for breathing increases, and function changes in the direction of reducing the size of the lungs that hold breath (PMID 39264073, PMID 16738195). Interstitial lung diseases usually show this change along with a decrease in the ability of gases to cross the walls (PMID 16738195). In this disease, pulmonary function tests also provide information for gauging the future. A decrease in FVC over the span of a year is associated with an increased risk of death (PMID 38648021).

In the disease covered in "What Is COPD? — A Balloon That Won't Empty, and the Breath That Leaves in the First Second", the problem was that breath could not empty out fully. In pulmonary fibrosis, the problem is that the lungs stiffen and fail to inflate properly. How this shape is read on a report, and why lung volume tests are used separately to confirm whether the lungs have really become smaller, is covered in "Reading a Spirometry Report — One Point Among Your Peers, Two Graphs Drawn by a Breath".

This disease appears with shortness of breath upon exertion and a dry cough, and pulmonary function tests show changes toward the lungs stiffening and failing to inflate properly.

Diagnosis — Sliced CT and the Judgment of Multiple Experts

At the center of diagnosis is an image taken by slicing the chest very thinly. A high-resolution CT is a CT that slices the chest to a very thin thickness, making fine structures inside the lungs visible. This test forms the backbone of diagnosing interstitial lung disease (PMID 38056824). The patterns commonly seen in idiopathic pulmonary fibrosis are net-like patterns clustered at the outer edges and lower parts of both lungs, fine airways widened by being pulled by scars, and small empty spaces gathered like a honeycomb (PMID 29052582).

If these patterns are clear, a path opens up to diagnose without taking out lung tissue. The 2018 diagnostic guidelines jointly issued by four academic societies, including the American Thoracic Society (ATS) and the European Respiratory Society (ERS), strongly recommended against performing a surgical lung tissue biopsy for new patients whose CT shows a typical pattern. For cases where the pattern is ambiguous or points to another disease, a tissue biopsy was conditionally recommended (PMID 30168753).

The name "idiopathic" is attached only after filtering out other interstitial lung diseases whose causes can be found (PMID 29052582). This is why doctors ask about work history, home environment, and clues to autoimmune diseases.

A multidisciplinary discussion is a process where doctors from various fields put together a single patient's symptoms, CT, and sometimes tissue biopsy results to arrive at a diagnosis. This discussion is currently the standard for diagnosing interstitial lung disease and leads to a confident diagnosis in the majority of patients (PMID 40590695). Even so, when diagnostic criteria are applied strictly, some patients remain who cannot be classified into either side (PMID 29471816). A diagnosis of idiopathic pulmonary fibrosis made without tissue can change, so it is re-examined periodically (PMID 29154106).

Diagnosis is reached by gathering the patterns on high-resolution CT, the process of ruling out other causes, and the judgment of multiple experts, and that diagnosis is also re-examined over time.

Course — A Path That Varies for Everyone

There is a number frequently cited when talking about the course of the disease. A 2022 review noted that, based on untreated patients with idiopathic pulmonary fibrosis, the median survival time after the onset of symptoms is 3 to 5 years, while adding that the natural course of this disease varies from person to person and is difficult to predict (PMID 35659420). A 2026 review also cited an average of 3 to 5 years after diagnosis, adding that the two current drugs merely slow the progression (PMID 41762087). Treatment has changed in the meantime, too. A 2017 review recorded that in less than the preceding decade, two drugs that alter the course of the disease were approved worldwide (PMID 28365056), and several studies testing new treatments are still underway today (PMID 37156412).

Because these numbers are values that summarize a group (such as a middle value or an average), they do not tell us how long any single individual will live. The course of this disease varies widely, ranging from people who remain stable without symptoms, to those who gradually run short of breath, to those who suddenly worsen (PMID 30884853).

More than 90% of idiopathic pulmonary fibrosis worsens over time (PMID 40358445). Even in interstitial lung diseases that are not idiopathic pulmonary fibrosis but involve scarring, a significant proportion follow a path of worsening similar to that of idiopathic pulmonary fibrosis (PMID 42276742). Progressive pulmonary fibrosis is the name given when interstitial lung diseases other than idiopathic pulmonary fibrosis worsen as scarring continues to increase. This is a case where progression continues even though the patient receives treatment tailored to the underlying disease (PMID 42276742). A 2022 international guideline defined this name as cases where two or more of symptoms, CT scans, and lung function have worsened over the past year with no other explanation (PMID 35486072).

An acute exacerbation is when the breathing of someone with an interstitial lung disease worsens significantly, usually in less than a month, and a CT scan shows new damage spreading across both lungs. A 2018 review proposed defining it this way (PMID 30578331). A 2026 international working group document notes that although acute exacerbations make the illness severe and carry a high risk of death, there is no proven effective treatment (PMID 42447235). Looking across studies that followed patients with idiopathic pulmonary fibrosis, about 9% experienced an acute exacerbation within 1 year, and about 19% within 3 years (PMID 39256473).

There are several causes for a sudden worsening of breathing other than acute exacerbations. Heart failure or a state where the body is excessively filled with water can look similar and must be ruled out first (PMID 30884853), and cases where breathing worsens due to other causes like pneumonia or bronchitis are grouped and handled separately. However, severe pneumonia can also trigger an acute exacerbation (PMID 42447235). Treatment for acute exacerbations relies on steroids despite the low quality of evidence, supplemented by supportive care and palliative care. Even so, the course is often poor (PMID 37615152).

Idiopathic pulmonary fibrosis generally progresses, but the rate varies from person to person, and survival time numbers are values that summarize a group.

What We Do — Slowing Down Drugs, Moving Practice, Oxygen, Transplantation

Two drugs are widely used for idiopathic pulmonary fibrosis today. Antifibrotic drugs are medications that slow the rate at which scars accumulate in the lungs. There are two: pirfenidone and nintedanib, and both slow the rate at which lung function declines. Even so, the course remains poor (PMID 37156412). These drugs only slow the progression; they do not stop it or reverse the scars that have already formed (PMID 39264073, PMID 41762087). For this reason, review articles suggest that starting treatment as early as possible is very important (PMID 39264073).

The extent of the slowdown is as follows. In idiopathic pulmonary fibrosis, interstitial lung disease associated with autoimmune diseases where the skin and organs harden, and progressive pulmonary fibrosis regardless of the cause, these drugs reduce the annual rate of FVC decline by roughly 44 to 57% (PMID 38648021). In an analysis combining randomized trials and observational studies of patients with idiopathic pulmonary fibrosis, taking the drug was associated with a risk of death about 0.55 times that of not taking it, and acute exacerbations were also fewer. Although differences between studies were substantial, the results favoring reduced mortality remained consistent when separating randomized trials and observational studies. The reduction in acute exacerbations was consistent for one drug but not the other (PMID 34217681). Looking strictly at progressive pulmonary fibrosis other than idiopathic pulmonary fibrosis, there was insufficient data to be confident that the drugs reduce mortality (PMID 34895203).

Drugs also come with burdens. Both drugs can be hard to tolerate in some cases (PMID 33359599), and side effects involving the skin and stomach have been reported with one drug (PMID 25362769). There are still many uncertainties regarding which drug to choose, when to start, and whether to use them together (PMID 29471816). For progressive pulmonary fibrosis, international guidelines from 2022 gave a conditional recommendation for one of the two drugs and stated that the more research is needed for the other (PMID 35486072). In 2025, results also emerged showing that a drug from a different class further reduced the progression of progressive pulmonary fibrosis (PMID 42276742).

Interstitial lung disease with a known cause begins by addressing that cause. If occupational exposure is the cause, removing oneself from that exposure is the core of treatment (PMID 37535448). In interstitial lung disease associated with autoimmune diseases, medications that regulate the immune system can slow the decline of lung function or even improve it (PMID 38648021). Avoiding the causative agent is central to hypersensitivity pneumonitis, and while immune-suppressing drugs are commonly used, they have not been shown to slow the increase in scarring (PMID 35140104).

Pulmonary rehabilitation is a program that combines exercise training, education, and self-management to help breathless people move their bodies again. In a systematic review pooling studies on interstitial lung disease, pulmonary rehabilitation appeared to increase the distance walked in 6 minutes by an average of about 40m and improve breathlessness and quality of life. The findings were similar when looking solely at idiopathic pulmonary fibrosis, and no adverse effects from rehabilitation were reported. However, the certainty of the evidence ranged from low to moderate, and it remains uncertain whether it helps people live longer (PMID 34559419). The 2023 guidelines from the American Thoracic Society strongly recommended pulmonary rehabilitation for adults with interstitial lung disease and rated the quality of evidence as moderate (PMID 37581410). In idiopathic pulmonary fibrosis, walking capacity gained through rehabilitation tends to decline over time, whereas improvements in symptoms and quality of life tend to last longer. The authors of a review on rehabilitation for this disease supported including pulmonary rehabilitation in standard care and starting it early in the disease course so that the effects last longer (PMID 38958566).

Oxygen is also used. For patients with interstitial lung disease whose blood oxygen levels drop below the threshold during the 6-minute walk test, oxygen reduces symptoms and improves quality of life (PMID 38648021). Guidelines and medical staff determine where the baseline is and who should use oxygen.

Lung transplantation is surgery to replace diseased lungs with donated ones. If drugs work by slowing progression, transplantation occupies a different place. A 2026 review noted that lung transplantation remains the only treatment capable of significantly extending both lifespan and quality of life in idiopathic pulmonary fibrosis (PMID 41762087). The median survival time for patients who received a transplant for end-stage interstitial lung disease was 5.2 to 6.7 years, whereas advanced patients who did not receive a transplant survived less than 2 years (PMID 38648021). Because the two groups were not assigned by a draw, this difference cannot be entirely attributed to transplantation. Transplantation is a challenging treatment that requires carefully selecting suitable candidates and having a donor lung available (PMID 39264073), and some people cannot realistically receive it due to accessibility or health status (PMID 36517177).

Smoking is cited as a modifiable risk factor involved in the development and worsening of interstitial lung disease, leading some reviews to prioritize smoking cessation. However, quitting is not easy. Nicotine addiction, mental state, and surrounding influences are intertwined, requiring personalized help and steady support (PMID 39015486). In an analysis pooling studies of patients with idiopathic pulmonary fibrosis, current smokers were associated with about a 1.88 times higher risk of developing lung cancer compared to non-current smokers (those who had quit and those who had never smoked). Total mortality showed no difference depending on smoking status, though it varied by region, and no association was seen with acute exacerbations (PMID 42339520). Among other tobacco-related interstitial lung diseases, while there is no robust evidence for management, smoking cessation is the primary measure, and after quitting, most cases follow a mild course, though some progress to prolonged respiratory failure (PMID 39423337).

All of these elements are brought together for an individual. Nutrition, mental state, physical stamina, and coexisting conditions are monitored, and as the disease advances, lung transplantation and palliative care are considered together (PMID 35659420). The goal is to live as well and as long as possible even after diagnosis (PMID 31253376).

Treatment begins with removing oneself from exposure if a cause exists, and combines drugs that slow progression, pulmonary rehabilitation, oxygen, lung transplantation, smoking cessation, and supportive care for life.

While Climbing a Flight of Stairs

We return to the first scene. When the father starts climbing stairs, his leg muscles use more oxygen, and his breath and heart speed up. Blood passes the alveoli faster, and the time for oxygen to cross the wall shortens. If the wall is thin, that short time is still plenty.

One of the reasons oxygen runs short when moving in this disease lies right here. Because of scars, the wall between the alveoli and capillaries is damaged, and the ability of gases to cross that wall decreases. When the lungs stiffen, they stretch less, and the effort required to breathe increases (PMID 39264073). It is like how less light gets through layers of pasted traditional Korean paper. A drop in blood oxygen during exertion is considered a hallmark of interstitial lung disease (PMID 34559419).

As time passes, another link is added. When gas exchange breaks down and lung stiffening overlap, body tissues lack oxygen, muscles weaken, and the condition progresses toward an increasing shortage of breath (PMID 39264073).

What treatment does in this scene is different from returning the wall to being thin. Medications only slow the rate at which scars accumulate (PMID 39264073), what rehabilitation has been reported to improve is walking distance, shortness of breath, and quality of life (PMID 34559419), and oxygen improved symptoms and quality of life for people whose blood oxygen dropped below a threshold during a walking test (PMID 38648021). Adding oxygen during exercise is commonly used, but more evidence is needed (PMID 38958566).

A wall damaged by scars transfers less gas, stiff lungs demand more breathing effort, and in this disease, a drop in blood oxygen during movement is counted as a hallmark.

If You Heard About 'Interstitial Changes' on a Health Checkup CT

Some people are told that a faint net-like pattern was seen at the edges of their lungs on a chest CT taken for lung cancer screening or for another reason. Interstitial lung abnormalities are changes that can look similar to interstitial lung disease and are seen incidentally on CT in people not suspected of having interstitial lung disease. Low-dose CT is a chest CT scan adjusted to use less radiation than a standard CT. As these scans, which are used for lung cancer screening, increase, finding interstitial lung abnormalities is also expected to increase (PMID 32649920). How screening CTs are read is covered in "Lung Cancer and Low-Dose CT Screening — Reading a Small Spot in the Picture Over Time".

In an analysis pooling multiple studies, interstitial lung abnormalities were found in about 7% of people who underwent lung cancer screening and the general population, respectively, and in about 26% of family groups with a history of pulmonary fibrosis. They were more common in older individuals, males, and those with a lower FVC (PMID 37534937).

Interstitial lung abnormalities are associated with progression on imaging, increased mortality, and complications from medical treatments such as chemotherapy or surgery (PMID 32649920). In the previous analysis as well, interstitial lung abnormalities were associated with higher mortality (PMID 37534937). However, progression occurs in only some of them (PMID 38692806), and although review articles suggest that a significant proportion progress (PMID 40590695), the criteria for identifying who is most likely to progress are still being refined (PMID 37534937). The type shaped like a scar on the outer edges of the lungs is more likely to progress and is also more associated with mortality (PMID 32649920).

The 2025 document from the American Thoracic Society distinguishes between interstitial lung abnormalities and interstitial lung disease based on symptoms such as shortness of breath and cough, abnormalities or declines in lung function, and imaging showing scarring or progression. It also suggests initially checking the symptoms and lung function of individuals with interstitial lung abnormalities, and monitoring them with chest CT every 2 to 3 years. The document also includes suggestions to look for these changes in lung cancer screening images, and to check for them in adults with autoimmune diseases and close family members in families with pulmonary fibrosis (PMID 40387336).

An incidentally found interstitial lung abnormality progresses in some cases, but who will progress has not yet been fully identified, and the 2025 American Thoracic Society document suggests monitoring it over time.

When It Occurs Together with Emphysema

On the CT scans of long-term smokers, the upper parts of the lungs are sometimes broken down while the lower parts are stiffened. Emphysema is a condition where the walls of the air sacs break down, causing the small air pockets to lose their shape. Having emphysema in the upper lungs and fibrosis in the lower lungs in the same person is grouped and discussed together (PMID 32279289).

In such individuals, the lung function numbers measuring the volume of breath appear relatively well preserved, but the capacity for gases to cross the walls is greatly reduced. Rises in lung blood pressure, acute exacerbations, and lung cancer frequently overlap, leading to a poor clinical course, and there are as yet no established guidelines for handling this (PMID 32279289). Mortality when these two diseases—which share risk factors of smoking, being male, and advanced age—overlap is higher than in people with chronic obstructive pulmonary disease alone, but results compared with those who have idiopathic pulmonary fibrosis alone are mixed. Quitting smoking and regular pulmonary function tests are recommended, and pulmonary rehabilitation can be helpful (PMID 35385639). The emphysema side of the disease is covered in "What Is COPD? — A Balloon That Won't Empty, and the Breath That Leaves in the First Second".

When emphysema and fibrosis exist together in one lung, the volume of breath may appear less diminished, but the capacity for gases to cross can be severely reduced, and complications tend to overlap easily.

Where to Look Further

The principle by which oxygen crosses in the alveoli and capillaries begins in "How Do We Breathe? — An Upside-Down Tree, and the Leaves Where Air Meets Blood". The disease on the side where breath cannot fully escape and emphysema are covered by "What Is COPD? — A Balloon That Won't Empty, and the Breath That Leaves in the First Second", while the task of reading a small shadow found on a screening CT over time is handled by "Lung Cancer and Low-Dose CT Screening — Reading a Small Spot in the Picture Over Time". Pneumonia, which can be confused with an acute exacerbation, is in "How Do a Cold, the Flu, and Pneumonia Differ? — An Infection That Stays in the Entryway, and One That Reaches the Inner Room".

How to translate the statement that the risk of death in the group that used the drug was "0.55 times" into the story of how many people is found in "If the Risk Was Cut in Half, How Many Fewer People Got Sick? — Relative and Absolute Risk".

In idiopathic pulmonary fibrosis, what first pulls the scar, why recovery goes awry only in certain people, and among incidentally found changes, who progresses to disease are questions noted as not yet answered by the review articles this piece relies on.

Source Papers

• "Interstitial Lung Disease: A Review." (2024) — PMID 38648021 — Interstitial lung disease presents with shortness of breath on exertion, about 30% have a cough, idiopathic pulmonary fibrosis accounts for about 1/3, connective tissue disease for 25%, and hypersensitivity pneumonitis for 15%; a 5% decline in FVC over 12 months is associated with about a 2-fold increase in mortality; antifibrotic drugs slow the annual FVC decline by about 44% to 57% in idiopathic pulmonary fibrosis, systemic sclerosis-associated ILD, and progressive pulmonary fibrosis; immunomodulatory treatment is used for connective tissue disease-associated ILD; oxygen improves symptoms and quality of life for individuals whose oxygen levels drop on a walking test; and post-transplant median survival is 5.2 to 6.7 years compared to under 2 years for non-transplanted progressing patients.
• "Interstitial lung diseases." (2022) — PMID 35964592 — There are over 200 interstitial lung diseases, mostly caused by inflammation or fibrosis of the interstitium leading to impaired gas exchange, shortness of breath, and reduced exercise tolerance; the clinical course varies greatly by disease; some resolve spontaneously or remain stable, while progressive pulmonary fibrosis leads to respiratory failure and death.
• "Idiopathic Pulmonary Fibrosis: An Overview." (2026) — PMID 41762087 — Idiopathic pulmonary fibrosis is the prototype of fibrotic interstitial lung disease, resulting in death on average 3 to 5 years after diagnosis; two treatments approved in the UK only slow progression (the ideal treatment being to halt and reverse it), and lung transplantation is the only treatment that significantly increases lifespan and quality of life.
• "Idiopathic Pulmonary Fibrosis: Epidemiology, Natural History, Phenotypes." (2018) — PMID 30501130 — Idiopathic pulmonary fibrosis has an unknown cause, occurs primarily in older adults, has increasing prevalence and incidence, is associated with genetic and environmental factors, has a variable natural course, and aims to improve the current median post-diagnosis survival of 3 to 4 years.
• "Idiopathic pulmonary fibrosis." (2017) — PMID 29052582 — Symptoms include cough, shortness of breath, and reduced quality of life, with incidence rising sharply with age; exclusion of other ILDs and a usual interstitial pneumonia pattern on high-resolution computed tomography (bilateral, peripheral, basilar reticular markings, traction bronchiectasis, and subpleural cystic spaces) are key; it involves abnormal repair following repeated alveolar epithelial injury in aging individuals with genetic susceptibility, though susceptibility definitions remain complex; and two drugs slow its progression.
• "Idiopathic pulmonary fibrosis: Disease mechanisms and drug development." (2021) — PMID 33359599 — It involves abnormal repair accompanied by excessive collagen deposition following repeated microinjuries to the alveolar epithelium in individuals with genetic predisposition; two drugs slow functional decline but are not curative and carry challenging side effects.
• "Idiopathic pulmonary fibrosis." (2023) — PMID 37156412 — Repeated epithelial injury, fibroblast accumulation, and matrix deposition combine with mismatched immune responses and wound healing to drive self-sustaining fibrosis; multidisciplinary diagnosis is used; two drugs reduce the decline in lung function but only slow progression, and the prognosis remains poor, with many new treatment trials underway.
• "Idiopathic pulmonary fibrosis." (2011) — PMID 21719092 — Once considered a chronic inflammatory process, evidence now indicates that abnormally activated alveolar epithelial cells drive fibrosis, where fibroblast foci hypersecrete matrix components like collagen, causing scarring and destruction of lung architecture, though the connection mechanisms between aging, epithelial activation remain unknown.
• "Idiopathic pulmonary fibrosis." (2017) — PMID 28365056 — Normal tissue is replaced by deformed matrix and alveolar architecture is destroyed, leading to reduced lung compliance, impaired gas exchange, and respiratory failure, while in less than a decade, two disease-modifying treatments have achieved global approval.
• "Inflammation and immunity in IPF pathogenesis and treatment." (2019) — PMID 30704705 — Repeated alveolar epithelial injury initiates fibrosis, while the causal role of inflammation remains debated, a view partly stemming from the failure of multi-center anti-inflammatory drug trials, though that perspective is slowly changing with new insights into macrophages and autoimmune responses.
• "Idiopathic Pulmonary Fibrosis: Review of Current Knowledge." (2024) — PMID 39264073 — Damage to the alveolar-capillary barrier reduces diffusing capacity, subsequently leading to decreased compliance, increased work of breathing, and gas exchange impairment alongside restrictive ventilatory defects, causing tissue hypoxia, muscle weakness, and progressive respiratory failure; the triggers and sustaining mechanisms of fibrosis remain unknown; drugs can only slow it down rather than reverse it, making early treatment initiation important; and transplantation is a difficult treatment dependent on candidate selection and donor lung availability.
• "[Idiopathic pulmonary fibrosis]." (2014) — PMID 25362769 — It is more common in individuals aged 60 or older, males, and current or former smokers; diagnosis relies on high-resolution computed tomography and multidisciplinary discussion; the median survival is 3 years; and one drug slows progression in mild-to-moderate disease, reduces the 1-year mortality risk, and causes skin and gastrointestinal side effects.
• "Association between smoking and prognosis in idiopathic pulmonary fibrosis: A systematic review and meta-analysis." (2026) — PMID 42339520 — Smoking is an established risk factor for the development of idiopathic pulmonary fibrosis; across 49 studies involving 32,974 individuals, all-cause mortality showed no difference based on smoking status (showing regional variation with higher rates outside East Asia and lower rates within East Asia); current smokers face a lung cancer risk hazard ratio of 1.88; and smoking is not associated with acute exacerbations or baseline forced vital capacity.
• "Smoking-related interstitial lung disease: A narrative review." (2024) — PMID 39423337 — Smoking is considered a risk factor for idiopathic pulmonary fibrosis, though its role and impact in development remain undetermined; for smoking-related interstitial lung disease, smoking cessation is the primary intervention, and following cessation, most cases follow a benign course, though some progress to chronic respiratory failure.
• "European Respiratory Society statement on familial pulmonary fibrosis." (2023) — PMID 36549714 — Several gene variants causing pulmonary fibrosis have been identified, and while some countries have introduced genetic testing for familial pulmonary fibrosis into clinical practice, international consensus is lacking.
• "Occupational interstitial lung diseases." (2023) — PMID 37535448 — Many workers are exposed to substances causing occupational interstitial lung disease, and because occupational and non-occupational interstitial lung disease may be indistinguishable clinically, functionally, and radiologically, occupational history is vital; removing exposure is the core treatment and can lead to significant improvement.
• "Hypersensitivity pneumonitis." (2020) — PMID 32764620 — This involves an exaggerated immune response to inhaled antigens (primarily bird proteins and molds); chronic forms often stem from long-term low-concentration exposure in the home; some fibrotic patients continue to progress even after complete avoidance of exposure; and antigen avoidance remains central to treatment.
• "Presentation, diagnosis and clinical course of the spectrum of progressive-fibrosing interstitial lung diseases." (2018) — PMID 30578335 — Some interstitial lung disease patients (including connective tissue disease-associated interstitial lung diseases such as rheumatoid arthritis-associated interstitial lung disease) follow a progressive fibrosing phenotype characterized by worsening symptoms, decline in lung function, limited response to immunomodulatory treatment, and early mortality.
• "Idiopathic pulmonary fibrosis: Current and future treatment." (2022) — PMID 35001525 — Idiopathic pulmonary fibrosis features a dry cough, fatigue, and progressive shortness of breath on exertion, leading to respiratory failure and death within 3 to 5 years of diagnosis; two oral antifibrotic drugs are the primary treatment; and single or double lung transplantation is the only confirmed intervention that extends life expectancy.
• "Acute Exacerbation of Idiopathic Pulmonary Fibrosis." (2019) — PMID 30884853 — Diagnosis of idiopathic pulmonary fibrosis combines medical history, physical examination, laboratory findings, pulmonary function tests, high-resolution computed tomography, and pathology; the clinical course is heterogeneous, ranging from asymptomatic stability to progressive respiratory failure and acute exacerbation; and differential diagnosis of acute exacerbation must rule out heart failure and fluid overload.
• "Pictorial Review of Fibrotic Interstitial Lung Disease on High-Resolution CT Scan and Updated Classification." (2024) — PMID 38056824 — High-resolution computed tomography forms the backbone of interstitial lung disease diagnosis, and interstitial lung abnormalities with high-risk features warrant evaluation and monitoring.
• "Current State of Fibrotic Interstitial Lung Disease Imaging." (2025) — PMID 40590695 — Diagnosis of interstitial lung disease increasingly relies on thin-section computed tomography, and multidisciplinary discussion incorporating imaging review provides high-confidence diagnoses for most patients and represents the current standard, while a significant proportion of interstitial lung abnormalities progress.
• "Diagnosis of Idiopathic Pulmonary Fibrosis. An Official ATS/ERS/JRS/ALAT Clinical Practice Guideline." (2018) — PMID 30168753 — If high-resolution computed tomography shows a usual interstitial pneumonia pattern, surgical lung biopsy carries a strong recommendation against it; if the pattern is probable usual interstitial pneumonia, indeterminate, or points to another diagnosis, bronchoalveolar lavage or surgical biopsy receives a conditional recommendation, as does multidisciplinary discussion.
• "Idiopathic pulmonary fibrosis: pathogenesis and management." (2018) — PMID 29471816 — Multidisciplinary discussion improves diagnostic accuracy, though strict application still leaves some patients unclassified; two drugs consistently reduce the rate of decline, though significant uncertainties remain regarding drug selection, timing, and combination therapy, alongside considerations for comorbidities and care priorities.
• "Diagnostic criteria for idiopathic pulmonary fibrosis: a Fleischner Society White Paper." (2018) — PMID 29154106 — When computed tomography shows probable usual interstitial pneumonia, diagnosis without biopsy is permitted in some cases; a multidisciplinary approach is important; and working diagnoses made without tissue confirmation may change and should be periodically reassessed.
• "Idiopathic pulmonary fibrosis." (2022) — PMID 35659420 — This is a progressive fibrosing interstitial pneumonia of unknown cause; the median survival for untreated patients is 3 to 5 years from symptom onset, though the natural history is variable and unpredictable; two antifibrotic drugs reduce disease progression; and a multidisciplinary approach encompasses nutrition, emotional support, physical conditioning, management of comorbidities, lung transplantation, and palliative care.
• "Advances in Concept and Imaging of Interstitial Lung Disease." (2025) — PMID 40358445 — Over 90% of idiopathic pulmonary fibrosis cases progress, while non-idiopathic pulmonary fibrosis interstitial lung diseases are classified as progressive pulmonary fibrosis based on disease behavior rather than etiology.
• "Progressive pulmonary fibrosis: a state-of-the-art review." (2026) — PMID 42276742 — A significant proportion of non-idiopathic pulmonary fibrosis fibrotic interstitial lung diseases exhibit an idiopathic pulmonary fibrosis-like progressive phenotype despite conventional treatment; this reflects the conditional recommendations of the 2022 guidelines, with newer classes of drugs further reducing progression as of 2025, emphasizing early detection, consideration of antifibrotic therapy, transplant evaluation, and holistic care.
• "Idiopathic Pulmonary Fibrosis (an Update) and Progressive Pulmonary Fibrosis in Adults: An Official ATS/ERS/JRS/ALAT Clinical Practice Guideline." (2022) — PMID 35486072 — Progressive pulmonary fibrosis in non-idiopathic pulmonary fibrosis interstitial lung diseases is defined by meeting at least two of the following criteria within the past year despite no alternative explanation: worsening symptoms, physiological progression, or radiological progression; one drug receives a conditional recommendation, while another drug warrants a recommendation for further research.
• "Acute exacerbations of progressive-fibrosing interstitial lung diseases." (2018) — PMID 30578331 — Acute exacerbations are proposed to be defined as a clinically significant respiratory worsening of typically less than one month's duration accompanied by new bilateral ground-glass opacities or consolidation against a background of fibrosis; no randomized trials exist, and preventative candidates include antifibrotic drugs, and reducing exposure to infections, irritants, and pollutants.
• "Acute exacerbation in fibrotic interstitial lung disease: an international working group report." (2026) — PMID 42447235 — Acute exacerbations carry high morbidity and mortality, with no proven effective treatments; acute respiratory worsening without diffuse alveolar damage (such as pulmonary edema, bronchitis, or pneumonia) is categorized separately, though severe pneumonia can trigger acute exacerbations.
• "[Not Available]." (2023) — PMID 37615152 — Diagnosis of acute exacerbation involves identifying new bilateral ground-glass opacities and consolidations while ruling out fluid overload and heart failure; treatment relies on glucocorticoids and supportive or palliative care despite low-quality evidence; and prognosis is poor.
• "Impact of Antifibrotic Therapy on Mortality and Acute Exacerbation in Idiopathic Pulmonary Fibrosis: A Systematic Review and Meta-Analysis." (2021) — PMID 34217681 — Involving 26 studies (8 randomized, 18 cohort) and 12,956 participants, antifibrotic treatment was associated with all-cause mortality relative risk of 0.55 ($I^2$ 82%) and acute exacerbation relative risk of 0.63, though the effect on acute exacerbations was consistent for only one drug.
• "Efficacy of antifibrotic drugs, nintedanib and pirfenidone, in treatment of progressive pulmonary fibrosis in both idiopathic pulmonary fibrosis (IPF) and non-IPF: a systematic review and meta-analysis." (2021) — PMID 34895203 — Across 13 trials, forced vital capacity decline was slowed to a similar degree in both idiopathic pulmonary fibrosis and non-idiopathic pulmonary fibrosis, though data for non-idiopathic pulmonary fibrosis alone were insufficient to assert a reduction in mortality.
• "Diagnosis, course and management of hypersensitivity pneumonitis." (2022) — PMID 35140104 — Management involves antigen avoidance, and while immunosuppressants are commonly used, they do not appear to slow the worsening of fibrotic disease, alongside oxygen therapy, pulmonary rehabilitation, and supportive care.
• "Pulmonary rehabilitation for interstitial lung disease." (2021) — PMID 34559419 — Across 21 studies, the 6-minute walk distance increased by an average of 40.07 meters (moderate certainty), with improvements in shortness of breath and quality of life; subgroups with idiopathic pulmonary fibrosis showed similar benefits; no adverse events were reported during rehabilitation; and while long-term survival effects remain uncertain, the certainty of evidence ranges from low to moderate.
• "Pulmonary Rehabilitation for Adults with Chronic Respiratory Disease: An Official American Thoracic Society Clinical Practice Guideline." (2023) — PMID 37581410 — Pulmonary rehabilitation receives a strong recommendation for adults with interstitial lung disease (moderate-quality evidence).
• "Pulmonary rehabilitation in idiopathic pulmonary fibrosis." (2024) — PMID 38958566 — In idiopathic pulmonary fibrosis, pulmonary rehabilitation improves exercise tolerance, symptoms, and quality of life; improvements in symptoms and quality of life are more sustained than functional capacity; and while supplemental oxygen during exercise is commonly used, further evidence is needed.
• "Progressive pulmonary fibrosis: an expert group consensus statement." (2023) — PMID 36517177 — This statement standardizes the definition of progressive pulmonary fibrosis, highlights the growing evidence for early antifibrotic therapy, and addresses care options when transplantation is not viable due to accessibility or contraindications.
• "The Future of Pharmacological Treatment in Idiopathic Pulmonary Fibrosis." (2019) — PMID 31253376 — Two drugs demonstrate high-level evidence for slowing progression and reducing annual mortality risk, supporting a care model that integrates nutrition, physical activity, education, emotional support, and symptom relief, with the goal of living well for as long as possible after diagnosis.
• "Interstitial lung abnormalities detected incidentally on CT: a Position Paper from the Fleischner Society." (2020) — PMID 32649920 — These changes are visible on computed tomography in individuals without clinical suspicion, appearing in 4% to 9% of older smokers and 2% to 7% of non-smokers, with detection increasing due to lung cancer screening; they are associated with radiological progression, increased mortality, and higher risks of chemotherapy and surgical complications, with subpleural fibrotic subtypes showing greater progression and mortality associations.
• "Prevalence, Risk Factors, and Outcomes of Adult Interstitial Lung Abnormalities: A Systematic Review and Meta-Analysis." (2023) — PMID 37534937 — Across 22 studies and 88,325 participants, prevalence was 7% in lung cancer screening cohorts, 7% in the general population, and 26% in familial cohorts; the mortality odds ratio was 3.56; interstitial lung abnormalities were associated with older age, male sex, and lower percent predicted forced vital capacity; and standardization is needed to identify individuals at the highest risk of progression.
• "Interstitial Lung Abnormalities." (2024) — PMID 38692806 — Some interstitial lung abnormalities progress to clinically significant interstitial lung disease, leaving knowledge gaps regarding optimal management.
• "Approach to the Evaluation and Management of Interstitial Lung Abnormalities: An Official American Thoracic Society Clinical Statement." (2025) — PMID 40387336 — This statement distinguishes interstitial lung abnormalities from interstitial lung disease based on the absence of symptoms, normal or declining lung function, and fibrotic or progressive imaging features; outlines evaluation strategies for lung cancer screening imaging; recommends screening first-degree relatives of adults with connective tissue disease or familial pulmonary fibrosis; assesses baseline symptoms and lung function; and suggests computed tomography follow-up every 2 to 3 years.
• "Combined Pulmonary Fibrosis and Emphysema." (2020) — PMID 32279289 — Characterized by upper-lobe emphysema and lower-lobe fibrosis, this condition preserves lung volumes relatively well while severely impairing carbon monoxide diffusing capacity; it carries a poor prognosis due to frequent overlaps with pulmonary hypertension, acute exacerbations, and lung cancer, with no established management guidelines.
• "Chronic Obstructive Pulmonary Disease Combined with Interstitial Lung Disease." (2022) — PMID 35385639 — Sharing common risk factors of smoking, male sex, and advanced age, chronic obstructive pulmonary disease combined with interstitial lung disease carries higher mortality than chronic obstructive pulmonary disease alone, though comparisons with idiopathic pulmonary fibrosis remain inconclusive; smoking cessation and regular pulmonary function testing are recommended, and pulmonary rehabilitation can be beneficial.
• "Pulmonary function testing in idiopathic interstitial pneumonias." (2006) — PMID 16738195 — Diffuse parenchymal lung diseases affect the space between the epithelial and endothelial basement membranes, typically presenting with common patterns of restrictive ventilatory defects and impaired diffusing capacity; pulmonary function tests serve as an aid in diagnosis, assessment of severity, prognosis, and monitoring of disease course.
• "Epidemiology of Digital Clubbing and Hypertrophic Osteoarthropathy: A Systematic Review and Meta-Analysis." (2022) — PMID 35067513 — Across 142 studies, the prevalence of clubbing in adults varies by underlying disease, being 33.4% in intestinal diseases, 31.3% in interstitial lung diseases, 27% in infective endocarditis, and 22.8% in liver diseases.
• "The incidence of acute exacerbation of idiopathic pulmonary fibrosis: a systematic review and meta-analysis." (2024) — PMID 39256473 — Across 10 cohort studies involving 11,855 participants, the incidence rate of acute exacerbation was 9% within 1 year, 13% within 2 years, and 19% within 3 years.
• "Why making smoking cessation a priority for rare interstitial lung disease smokers?" (2024) — PMID 39015486 — Smoking is a modifiable risk factor intertwined with the development and progression of interstitial lung disease, but due to nicotine addiction alongside psychological and social factors, smoking cessation maintenance is difficult and success rates among interstitial lung disease patients are low, underscoring the need for tailored assistance and sustained support.