Interstitial lung disease (ILD) is a group of more than 200 disorders that cause inflammation and scarring in the lung tissue between the air sacs. This scarring stiffens the lungs, making it progressively harder to breathe and get oxygen into the bloodstream. Globally, over 4.3 million people live with ILD, and that number has more than doubled since 1990.
What Happens Inside the Lungs
Your lungs contain millions of tiny air sacs surrounded by a thin layer of tissue called the interstitium. In a healthy lung, oxygen passes easily through this tissue into your blood. In ILD, repeated injury to the cells lining the air sacs triggers a repair process that goes wrong. Instead of healing normally, the body lays down thick, stiff scar tissue in the interstitium.
Specialized cells called fibroblasts drive this process. Once activated, they produce excess structural material that builds up between the air sacs, thickening the walls and making the lungs rigid. Over time, the scarring spreads, reducing the lungs’ ability to expand and transfer oxygen. In some forms of ILD, this scarring is irreversible and progressive. In others, catching the inflammation early can slow or stop the damage.
Types and Causes
ILD isn’t a single disease. It’s an umbrella term covering conditions that fall into several broad categories.
Autoimmune-Related ILD
Several autoimmune conditions can attack the lungs alongside other organs. Rheumatoid arthritis, lupus, scleroderma, Sjögren’s syndrome, and dermatomyositis all carry a risk of developing lung scarring. In these cases, the same immune dysfunction damaging joints, skin, or other tissues also injures lung tissue.
Environmental and Occupational Exposures
Breathing in certain dusts, molds, or chemicals over time can trigger a type of ILD called hypersensitivity pneumonitis. There are over 300 known triggers, and many are tied to specific occupations or hobbies. Farmers exposed to moldy hay can develop “farmer’s lung.” People who keep birds may react to proteins in feathers and droppings. Even contaminated hot tubs, humidifiers, and air conditioning systems can harbor bacteria or fungi that cause lung inflammation with repeated exposure. Asbestos, silica dust, and coal dust cause their own distinct forms of ILD.
Medication-Induced ILD
Certain drugs, including some chemotherapy agents, heart medications, and antibiotics, can damage lung tissue as a side effect. This form is sometimes reversible if the medication is stopped early enough.
Idiopathic Pulmonary Fibrosis
The most well-known form of ILD is idiopathic pulmonary fibrosis (IPF), where “idiopathic” simply means the cause is unknown. IPF tends to affect adults over 50, is more common in men, and carries the worst prognosis of any ILD type, with a median survival of 2 to 5 years from diagnosis. Smoking history is a significant risk factor.
Early Symptoms and How They Progress
The earliest sign is usually shortness of breath during physical activity, things like climbing stairs or walking uphill. A persistent dry cough that doesn’t produce mucus is also common. Fatigue and vague chest discomfort round out the typical early picture.
These symptoms are mild at first, which is part of the problem. Many people attribute them to aging, being out of shape, or allergies. The symptoms worsen over months or years, and by the time breathing becomes difficult during routine activities like getting dressed or having a conversation, significant scarring has often already occurred. Some people also develop clubbing, a widening and rounding of the fingertips, which signals chronic low oxygen levels.
How ILD Is Diagnosed
Diagnosis typically starts with a breathing test called pulmonary function testing. ILD produces what’s known as a restrictive pattern: the lungs can’t expand fully, so the total amount of air you can exhale in one breath is reduced. Doctors also measure how efficiently oxygen crosses from your lungs into your blood. In ILD, this gas transfer capacity drops, sometimes severely. A moderate decrease means it’s fallen to 40 to 60 percent of what’s expected for your age and size. Below 40 percent is considered severe.
High-resolution CT scanning is the cornerstone of ILD imaging. It can reveal several characteristic patterns that help pin down the specific type of ILD. Ground-glass opacities appear as hazy patches where the lung tissue looks cloudy but the underlying structure is still visible. These can indicate active inflammation. Reticular patterns show up as dense, web-like lines throughout the lung tissue, suggesting fibrosis. In advanced disease, the scan may show honeycombing, clusters of small cystic spaces with thick walls, typically 3 to 10 millimeters across, concentrated near the lung bases and outer edges. Honeycombing is a hallmark of advanced, irreversible scarring.
In some cases, a lung biopsy is needed to confirm the diagnosis, though the combination of clinical history, breathing tests, and CT findings is often enough.
Treatment Options
Treatment depends heavily on which type of ILD you have and whether the underlying cause is treatable.
For autoimmune-related ILD, medications that suppress the overactive immune system can reduce inflammation and slow lung damage. For hypersensitivity pneumonitis, the most important step is identifying and avoiding the trigger. If a contaminated humidifier or workplace exposure is the cause, removing it can halt progression and sometimes allow partial recovery.
For progressive fibrotic diseases like IPF, two antifibrotic medications are available. One works by slowing the activity of fibroblasts, the cells that produce scar tissue. The other blocks growth factor signals that drive the scarring process. Neither reverses existing damage, but they can meaningfully slow the decline. In studies of combination therapy, the rate of lung function loss dropped from about 17.7 milliliters per month to 10.6 milliliters per month. That difference translates to preserving breathing capacity over the course of years.
Supplemental oxygen becomes necessary when blood oxygen levels fall below a certain threshold at rest. Guidelines recommend long-term oxygen therapy for at least 15 hours a day when resting oxygen levels drop to severe ranges, or at moderately low levels if there’s evidence that organs like the heart are being affected by oxygen deprivation. Many people with ILD use portable oxygen concentrators to maintain their activity levels.
Pulmonary rehabilitation, a structured program of exercise training, breathing techniques, and education, helps people with ILD maintain their fitness and manage breathlessness more effectively. For younger patients with severe, progressive disease, lung transplantation may be considered.
Living With ILD
The trajectory of ILD varies enormously depending on the type. Some forms stabilize with treatment or after removing an environmental trigger. Others, particularly IPF, follow a downward course even with medication, though antifibrotic therapy and better management of related conditions like acid reflux, sleep apnea, and pulmonary hypertension have improved survival compared to a decade ago.
Oxygen levels tend to drop more dramatically during exercise than at rest, so monitoring with a pulse oximeter during activity can help you and your care team decide when supplemental oxygen is needed. Staying physically active within your limits, avoiding respiratory infections through vaccination, and addressing any ongoing exposures are the practical pillars of managing the disease long-term. The United States has the highest absolute number of ILD cases globally, with over 700,000 people affected, followed by India and China, so specialized ILD clinics and support networks are increasingly available in major medical centers.