Pulmonary fibrosis is a term that covers many different conditions that cause scar tissue to build up in your lungs.

This build-up of scar tissue, which makes your lungs stiff, is called fibrosis.

Pulmonary fibrosis is a type of interstitial lung disease (ILD). ‘Interstitial’ means the disease affects the interstitium, the lace-like network of tissue that supports the air sacs in your lungs. There are more than 200 different ILDs.

Some types of pulmonary fibrosis have an identifiable cause. But for many types, a definite cause cannot be found.

In ILDs, there can be scarring in your lungs or inflammation in your lungs. Some ILDs mostly cause scarring, some mostly cause inflammation. But often there is a combination of these processes going on. Which of these processes is dominant can determine what kind of treatment you may have.

The treatment and outlook for different types of pulmonary fibrosis vary considerably, so if you’re not sure about your diagnosis, check with your doctor or nurse for the exact name of your lung condition.

All types of pulmonary fibrosis are rare.

Although we do not always know what causes pulmonary fibrosis, we do know it is not a form of cancer or cystic fibrosis, and it is not contagious.

Those seen most often are:

• idiopathic pulmonary fibrosis or IPF
• hypersensitivity pneumonitis formerly called extrinsic allergic alveolitis
• pneumoconiosis, also known as an occupational interstitial lung disease
• pulmonary fibrosis associated with connective tissue and autoimmune diseases
• drug-induced pulmonary fibrosis

Sarcoidosis is a relatively common type of disease that usually affects the lungs. It can sometimes cause pulmonary fibrosis but we don’t know why. Cystic fibrosis is not a type of ILD or pulmonary fibrosis.

How does pulmonary fibrosis affect your breathing?

Pulmonary fibrosis scars your lungs and so reduces the efficiency of your breathing.

Scarring causes your lungs to become stiffer and less elastic so they are less able to move and take oxygen from the air you breathe.

Each time you breathe in, you draw air into your nose or mouth, down through your throat and into your windpipe, also called your trachea. Your windpipe splits into two smaller air tubes called bronchi, which go to your lungs. The air passes down the bronchi, which divide again and again, into thousands of smaller airways called bronchioles.

The bronchioles have many small air sacs, called alveoli. Inside the air sacs, oxygen moves across paper-thin walls to the capillaries – tiny blood vessels – and into your blood. The air sacs also pick up the waste gas, carbon dioxide from your blood, ready for you to breathe it out.

If you have pulmonary fibrosis, scarring affects the air sacs in your lungs. The air sacs are supported by the interstitium, a network of tissue, a bit like lace. Scarring fills in the gaps between and around the air sacs and limits the amount of oxygen that gets into the blood.

As scarring increases, your lungs are less able to expand to allow you to take deep breaths and the level of oxygen in your blood can start to drop. Breathing may feel like harder work and you can feel breathless from everyday activities like walking.

Causes of pulmonary fibrosis

In a small number of pulmonary fibrosis cases, it’s possible to identify a specific cause.

Some causes are:

• being exposed to certain types of dust – including wood or metal dusts or asbestos
• being exposed to allergens – such as bird feathers or mould
• a side effect of a drug

Some types of pulmonary fibrosis occur when you have another condition such as rheumatoid arthritis or scleroderma.

In a very few cases, two or more members of a family may develop pulmonary fibrosis. But current research suggests the genetics of familial interstitial lung disease is complex and there is no clear inherited predisposition to develop pulmonary fibrosis.

In most types of pulmonary fibrosis a specific cause cannot be found. One of the most frequently occurring forms of fibrosis is called idiopathic pulmonary fibrosis (IPF). The word ‘idiopathic’ means there is no known cause.

Researchers recently set out some common types of interstitial lung disease, grouped by what causes them, in the diagram below. It is not agreed by all doctors and there’s lots of research underway to improve our understanding of the causes.

Classification of common interstitial lung diseases

Adam Wallis and Katherine Spinks: The diagnosis and management of ILDs
British Medical Journal 2015