The Difference between Interstitial Lung Disease and Pulmonary Fibrosis

Written by Todd Neff on behalf of National Jewish Health

Interstitial lung disease (ILD) is an umbrella term for more than 200 conditions. They cause inflammation or scarring in the airways (the “piping” of the lungs) and the tiny air sacs (alveoli (Opens in a new window)) that let our lungs collect oxygen and discard carbon dioxide. People with ILD have a harder and harder time breathing.

Physician reviewing an image of lungs

Pulmonary fibrosis (Opens in a new window) describes the scarring of the lungs (pulmonary = lung-related; fibrosis = scarring) that happens with many interstitial lung diseases. So, those with pulmonary fibrosis are a subset of those with ILD. It’s a big subset, says pulmonologist Joshua Solomon, MD, who leads the Center for Interstitial Lung Disease at National Jewish Health.

“When you’re dealing with scarring, that’s when we use pulmonary fibrosis,’” he says.

Because pulmonary fibrosis is not a disease, but rather something caused by lung diseases, the medical community tends to avoid the term.

That said, there’s much more to this story.

Interstitial lung disease can cause inflammation, fibrosis or both. ILD’s origins can be put into two main groups. The first is idiopathic, which means “the cause is not known.” Idiopathic ILD, like ILDs in general, can bring inflammation or fibrosis.

Idiopathic ILD with fibrosis is called idiopathic pulmonary fibrosis, or IPF. Even though “pulmonary fibrosis” refers to the scarring of the lungs caused by all sorts of interstitial lung diseases, “idiopathic pulmonary fibrosis” is, in fact, one of the ILDs that causes the scarring. Dr. Solomon estimates that IPF makes up about one-quarter to one-third of interstitial lung disease cases at National Jewish Health.

The second grouping of ILD’s origins are environmental. Hypersensitivity pneumonitis (HP) happens when there’s inflammation or scarring of the lungs caused by inhaling foreign substances such as animal proteins, fungus or molds. Bird Fancier’s Lung (Opens in a new window) is one example. There also are occupation-related ILDs such as asbestosis and silicosis. In addition to being caused by triggers in the environment or from occupations, ILDs can be associated with autoimmune diseases, smoking or medications such as bleomycin (Opens in a new window) and amiodarone (Opens in a new window).

To further complicate things, there’s also a new term on the block: progressive pulmonary fibrosis (Opens in a new window).

In 2019, a major clinical trial (Opens in a new window) of a drug called nintedanib concluded. The trial tested whether the drug helped patients with lung scarring that was getting worse – progressive fibrosis – regardless of what form of interstitial lung disease was causing it. The drug, better known now as Ofev®, slowed the progression of fibrosis. Given that the drug didn’t care what was causing fibrosis, the medical community came up with a term “progressive pulmonary fibrosis” to capture that idea.

“I think it’s good for patients to know that we are now focused on, ‘Hey, if you’re progressing – if you’re getting worse – you’re going to need to be treated regardless of what the cause is,” Dr. Solomon said.

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