Hypersensitivity pneumonitis

Hypersensitivity pneumonitis is a type of interstitial lung disease (ILD). It happens when your immune system overreacts to something you breathe in repeatedly from your environment, like your home or workplace. This reaction can cause inflammation and sometimes scarring in your lungs.

There are more than 200 substances that can trigger hypersensitivity pneumonitis. They include certain dusts, moulds, animal and plant proteins, chemicals and metals. 

Hypersensitivity pneumonitis used to be called extrinsic allergic alveolitis.

Who's at risk of hypersensitivity pneumonitis?

You may have a higher risk of developing hypersensitivity pneumonitis if:

• someone in your family has this condition or another ILD
• you're male or were assigned male at birth
• you have pet birds in your home
• you work with a substance like mouldy hay, feathers, wood dusts and metal fluids
• you're aged between 50 and 70 years old, but it can happen at any age.

What are the types of hypersensitivity pneumonitis?

Some examples of hypersensitivity pneumonitis are:

• farmer's lung caused by mould which grows on hay or straw for feeding animals - this is the most common type of hypersensitivity pneumonitis
• bird keeper's lung or pigeon fancier's lung caused by bird droppings and feathers
• chemical worker's lung
• metal worker's lung.

Fungi or bacteria growing in unclean humidifiers, air conditioners, heating systems and hot tubs can also cause hypersensitivity pneumonitis. 

Sometimes the cause of the reaction is not known. This is known as idiopathic or cryptogenic hypersensitivity pneumonitis.

You can reduce your risk of hypersensitivity pneumonitis by:

• wearing a protective face mask if you work with substances that can trigger the condition - find out about disposable dust masks to prevent farmer’s lung.
• avoiding compost heaps, rotting leaves or decaying plant material - Thrive has more information about gardening with a lung condition.
• making sure heating, ventilation and air conditioning systems are looked after and checked regularly
• having good hygiene
• keeping your home dry, warm and well-ventilated to prevent mould.

Your GP will ask when you get symptoms and if anything might trigger them. They’ll also ask about your medical history and family history of any lung conditions.

They may refer you for tests including:

• a chest X-ray
• a CT scan
• blood tests
• lung function tests, such as spirometry and gas transfer tests.

If they think you may have an ILD, they will refer you to a respiratory specialist. Your diagnosis will be confirmed by a team of experts, called a multi-disciplinary team.

It can take time to diagnose hypersensitivity pneumonitis because the symptoms are similar to other lung conditions. They may say you have an ILD, but often the cause is not clear.

Find out more about how ILDs are diagnosed.

Your specialist team can help you find the cause of your condition. Once you know what’s triggering your reaction, you can try to take steps to avoid it. 

You may be prescribed an immunosuppressant to help reduce inflammation in your lungs. Corticosteroids, such as prednisolone, are usually the first option. Other immunosuppressants include mycophenolate mofetil and azathioprine. 

If you have scarring in your lungs, you may be prescribed an anti-fibrotic such as nintedanib or pirfenidone. They can help slow down the build-up of scar tissue in the lungs.

Find out more about the treatments for ILDs.

Most people will have a review at least once a year. If you’re having treatment at a specialist centre, you may be seen every 3 to 4 months until your condition is stable.

Your specialist will review your symptoms and do some lung function tests to see how well your treatments are working. 

If you’re breathless, your specialist may refer you for pulmonary rehabilitation (PR). You may also be prescribed home oxygen therapy if you have low oxygen levels in your blood.

Your specialist can support you to make changes to your lifestyle to improve your symptoms, such as eating healthily and stopping smoking.

Read more about how to manage ILDs like hypersensitivity pneumonitis.

What's the outlook for hypersensitivity pneumonitis?

Hypersensitivity pneumonitis can stay the same for many years. But for some people it can progress more quickly.

How hypersensitivity pneumonitis develops can depend on:

• how often you breathe in the substance you're having a reaction to
• if you develop any scarring
• your general health and lifestyle habits. For example, if you smoke.

Acute hypersensitivity pneumonitis

Acute hypersensitivity pneumonitis is a short-term condition.

Symptoms can appear a few hours after breathing in the harmful substance.

If you can avoid the substance that triggers your symptoms, it's reversible. Most people can make a full recovery after a few weeks.

Chronic hypersensitivity pneumonitis

Chronic hypersensitivity pneumonitis is a long-term condition.

Treating hypersensitivity early can help prevent lung scarring, called pulmonary fibrosis. Pulmonary fibrosis can make breathing more difficult.

There are ways to manage your symptoms and improve inflammation. But once your lungs are scarred, it cannot be undone.

Meeting other people who are living with lung conditions, who relate with your experiences, can help you build a supportive network.

• Join one of our communities on the Health Unlocked forum to connect with other people with similar experiences to you.
• Find support in your area, including online and face to face support groups, mobility and wellbeing sessions, and singing for lung health sessions.
• Get in touch with our friendly helpline to talk about any aspects of your lung condition. We'll give you as much time as you need to talk about what's important to you.

The charity Action for Pulmonary Fibrosis also has information about support groups for anyone living with and affected by pulmonary fibrosis, including friends, family and carers.