Pulmonary fibrosis

Hypersensitivity pneumonitis

Hypersensitivity pneumonitis, formerly called extrinsic allergic alveolitis, happens when your lungs develop an immune response – hypersensitivity - to something you breathe in which results in inflammation of the lung tissue - pneumonitis.

One example is farmer’s lung. This is caused by breathing in mould that grows on hay, straw and grain. Another is bird fancier’s lung, which is caused by breathing in particles from feathers or bird droppings. There are many other substances which can cause similar disease patterns. It can be very difficult to find the exact cause.


The symptoms, including cough, shortness of breath and sometimes fever and joint pains, can come on suddenly after you’ve been exposed. This is the acute form of the condition. It goes away without leading to fibrosis of the lung if you can permanently avoid the substance that caused the attack.

In other cases, symptoms of breathlessness and cough may only appear more gradually, perhaps over many years, as a result of permanent scarring of the lungs. This form of the condition is called chronic, or long-term, hypersensitivity pneumonitis and often a specific cause cannot be identified.


If a specific cause is identified, it is really important to completely avoid exposure to it.

You may need to take anti-inflammatory medication called steroids for a few weeks or months.

If you need steroids to control the condition for longer, your doctor may recommend additional drugs to reduce the risk of steroid associated side effects.

 “I may never know what’s causing my condition”

Jane, 61, was first diagnosed with hypersensitivity pneumonitis 10 years ago

When I moved to London, I developed a cough. It got so bad I ended up in hospital and was diagnosed with hypersensitivity pneumonitis. I took high dose steroids – and my symptoms disappeared! For seven years I had no symptoms at all.

But when the symptoms came back they didn’t go away. So now my hypersensitivity pneumonitis is long-term. Tests showed I have a hypersensitivity to pigeon and budgie droppings, but my doctor says there are thousands of other things that I could be reacting to. I may never know what’s causing my condition.

I get unpleasant bouts of coughing. And a small things makes me tired – like carrying shopping home.

I found my work as a lawyer more and more difficult. My employers suggested I claim on their permanent health insurance policy. My claim was accepted and I’m now on long-term sick leave.

I take steroids and immune-suppressing drugs every day. I’ll probably take drugs for the rest of my life.

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Last medically reviewed: August 2016. Due for review: August 2019

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.