Introduction

Hypersensitivity pneumonitis (HP) is a lung condition where the lung tissue (alveoli) and surrounding airways become inflamed after inhaling certain allergens or irritants.¹ This condition, also known as extrinsic allergic alveolitis, occurs when the immune system overreacts to regular and repeated exposure to specific triggers, such as mould, dust, or animal proteins.¹ HP can manifest in different ways, ranging from sudden acute reactions to chronic lung damage.  

Causes

HP is a complex lung disease where the immune system has an exaggerated reaction to certain environmental substances, which are also known as antigens.

Antigens typically enter our bodies via inhalation, and include:^1,2

• Organic dusts
• Moulds
• Fungi
• Animal proteins
• Chemicals

For example, individuals working in agriculture may develop HP from exposure to mouldy hay or bird droppings, while those in industrial environments may encounter chemicals that provoke an immune response. Common household contaminants like dust mites or pet dander can also contribute to HP development.²

When you inhale antigens, the immune system recognises them as harmful to your body and starts an inflammatory response in the lungs to destroy them. This leads to the respiratory symptoms of coughing, shortness of breath, and fatigue. If you continually inhale antigens over a longer period of time, the inflammatory response can damage your lungs - resulting in breathing difficulties.

Symptoms

Symptoms of HP can vary widely and are common to many other types of respiratory infections - making diagnosis more challenging.

Common symptoms of HP include:^2,3

• Coughing
• Shortness of breath
• Chest tightness
• Fatigue
• Unintentional weight loss 

In acute cases, symptoms may appear shortly after exposure to the triggering antigen and generally improve once you have left the source of the exposure. However, chronic HP may present with persistent or progressive respiratory symptoms, which can worsen over time. 

Types of hypersensitivity pneumonitis

HP manifests in three different forms, which are categorised based on the source of exposure and the immune response it triggers.⁶  

Acute hypersensitivity pneumonitis

Acute HP typically occurs shortly after intense or prolonged exposure to an offending antigen (the substance responsible for the immune response) and often occurs in occupational or environmental settings.^4,5 Common examples include farmer's lung (exposure to mouldy hay or grain), bird fancier's lung (bird droppings), and humidifier lung (exposure to contaminated water in humidifiers). 

Chronic hypersensitivity pneumonitis

Chronic HP develops over time with repeated exposure to lower levels of an antigen, resulting in persistent or progressive lung inflammation and damage.^4,5 However, only 5% of acute cases develop into chronic HP.³

Subacute hypersensitivity pneumonitis:

Some people may experience subacute HP, which is a combination of acute and chronic HP.^4,5 Subacute HP typically happens when someone is occasionally exposed to the antigen over an extended period of time. This leads to symptoms developing more slowly compared to acute HP, but the inflammation lasts longer, similar to chronic HP.⁶

Diagnosis

Diagnosing HP involves a comprehensive evaluation by a healthcare professional, including a detailed medical history, physical examination, and diagnostic tests. 

These tests may include:^4,5,6

• Pulmonary function tests to assess lung function   
• Imaging studies (chest X-rays or CT scans) to identify abnormalities in lung tissue
• Blood tests to measure levels of inflammatory markers or specific antibodies 
• Lung biopsy to confirm the presence of inflammation and tissue damage characteristic of HP

Treatment and management

The treatment and management of HP utilise two main approaches which can help ease your symptoms and prevent further lung damage. 

Firstly, you and your doctor should try to identify the trigger(s) of your HP so that you can avoid it.⁷ This may mean that you need to change your lifestyle to avoid places or jobs that expose you to the trigger. You should also ensure that the air quality in your home is good and the air is clear of allergens or pollutants through the use of proper ventilation and filters. Secondly, your healthcare provider may prescribe medication like corticosteroids to ease your symptoms and prevent your immune system from overreacting if you are still experiencing HP symptoms.⁷ 

In more severe cases of HP, or if your symptoms remain even after these steps, there may be additional treatments such as immunosuppressants and oxygen therapy, or even lung transplantation.⁷ It is important to consider your healthcare provider regularly to ensure that your treatment is working well.

Prognosis and complications

The prognosis (outcome) for people with HP depends on several factors, like the severity of their lung damage, the length of their exposure to the trigger, and how quickly they begin treatment.⁸ For most people, early diagnosis and intervention is enough to quickly see improvements in their symptoms and lung condition. However, if HP remains untreated for a long time (or there is continuous exposure to the trigger), it can lead to more lung damage and other complications over time.

Complications of untreated HP can include chronic respiratory issues like coughing, wheezing, and shortness of breath.⁸ In severe cases, it can even lead to serious issues such as the scarring of lung tissue or respiratory failure. Additionally, long-term exposure to allergens or irritants can increase the chance of developing other lung conditions, such as asthma or chronic obstructive pulmonary disease (COPD).⁸

Lifestyle and prevention

Alongside medical treatment, making certain lifestyle changes and taking preventive measures can help manage HP and reduce the risk of flare-ups. One key aspect is avoiding exposure to substances that trigger the allergic reaction.

Ways to avoid exposure to trigger substances:⁹

• Using air purifiers in the home or workplace
• Wearing protective masks or respirators in the workplace
• Regular cleaning of living space
• Maintaining proper ventilation in living spaces
• Reducing exposure to tobacco or strong chemicals
• Being mindful of your surroundings during outdoor activities
• Regular exercise
• Eating a balanced diet with adequate hydration

Summary

Hypersensitivity pneumonitis (HP) is a lung condition that causes inflammation and respiratory symptoms after inhaling certain allergens or triggers. HP can be a challenging condition to manage, especially if you have continuous exposure to your trigger. However, there are several treatment options available to ease your symptoms and improve your quality of life. Be sure to seek guidance from your healthcare providers to help you determine what your triggers are, what lifestyle adjustments you can make, and if you would benefit from taking any medication. These changes can help keep your HP controlled and lower the chance of any further problems developing.

FAQs

Can hypersensitivity pneumonitis be cured, or is it a lifelong condition?

HP varies from person to person. Some people find that their symptoms get better or even go away entirely after avoiding their triggers and getting appropriate treatment. However, others find that they have symptoms of HP for extended periods of their life.

Are there specific occupations or environments that pose a higher risk of developing hypersensitivity pneumonitis?

Yes. Certain occupations and environments are associated with a higher risk of developing HP.¹⁰ Places with lots of dust, mould, or other airborne particles are more likely to trigger HP. As such, working in agriculture, farming, or industries involving wood or textiles can increase your risk. Being in areas with poor ventilation or high humidity levels can also contribute to the development of HP. You can reduce your chances of developing HP by wearing protective gear and ensuring proper ventilation. 

Does age or genetics influence the development of hypersensitivity pneumonitis?

Yes - both age and genetics can affect your risk of developing hypersensitivity pneumonitis.¹⁰ The majority of HP cases are seen in adults between the age of 50-60, but it can develop in any age group. Some people might have a higher chance of getting it due to their genetics, as people in the same family often develop HP, but the specific genes responsible for this are still unknown.¹⁰  

Can pets or household allergens contribute to hypersensitivity pneumonitis?

Yes - pets and household allergens such as dust can trigger HP. Not everyone is triggered by animal dander or dust, but it is important to be aware of them as different potential triggers. If you do have pets, they could potentially make your HP worse. 

Are there any alternative therapies that may help manage hypersensitivity pneumonitis symptoms?

The primary treatment for managing HP symptoms generally involves medication that is prescribed by your healthcare provider, however, some people find relief from alternative therapies such as acupuncture, breathing exercises, or herbal remedies. These methods are not a substitute for medical treatment, but they may offer additional support in managing symptoms. You should consult with your healthcare provider before trying any alternative therapies to ensure they are safe and appropriate for you.

Is hypersensitivity pneumonitis contagious?

No, HP is not contagious. You cannot catch it from someone else the way you can catch a cold or flu. Instead, it's a condition triggered by exposure to certain substances in the environment, like dust or mould, which can cause inflammation in the lungs. However, if someone else is exposed to the same triggers, they might develop HP too.

What steps can I take to monitor and track symptoms of hypersensitivity pneumonitis?

There are several different steps you can take in order to help monitor and track your HP symptoms:

1. Keep a symptom journal: use a journal or diary to keep track of your symptoms regularly. Particularly, monitor any changes in your breathing, coughing, and your overall health. Try to be aware of any factors that may have triggered or increased your symptoms, such as certain environments or activities. 
2. Use a peak flow metre: if your symptoms include breathing difficulties, your healthcare provider may recommend you to use a peak flow metre to measure your lung function. A peak flow metre is a simple device that you breathe into, and it can monitor changes in your breathing and detect any decline in lung function. 
3. Track environmental triggers: try to identify potential triggers for your symptoms, particularly when you are in different environments. Be aware of your exposure to dust, allergens, or pollutants to see if any of them trigger your condition. If you find they do, try to avoid these areas as best as possible. 
4. Monitor physical activity: keep track of your physical activity and see if your breathing is affected when you exercise. If there are any activities that make you feel abnormally breathless or fatigued, consider modifying your exercise routine to avoid these.