Hypersensitivity pneumonitis (HP)

Key facts

Hypersensitivity pneumonitis occurs in people after they breathe in certain substances in the environment that trigger the immune system and cause an allergic reaction
In chronic hypersensitivity pneumonitis, the lungs might become scarred, making it harder for oxygen to enter the blood, resulting in shortness of breath
It is important to completely avoid exposure to the allergen, as continued exposure might result in the progression of the disease

Hypersensitivity pneumonitis is a rare disease that can cause a build-up of scarring in the lungs, known as pulmonary fibrosis. There are many different conditions associated with pulmonary fibrosis. Some are referred to as interstitial lung diseases (ILDs). ILDs with pulmonary fibrosis are a challenging group of diseases that affect the interstitium of the lung (the space between the air sacs in the lung) and lead to the formation of scar tissue.

In someone with hypersensitivity pneumonitis, the lungs can become scarred, thickened and stiff. This makes it harder for oxygen to enter the blood and results in shortness of breath.

Hypersensitivity pneumonitis occurs in people after they breathe in certain substances in their environment, like dust, mold, fungi, or chemicals. These substances trigger the body’s immune system and cause it to overreact, leading to an allergic reaction and inflammation in the lungs.

Quick Tip

Visit the conditions page to learn more about what an ILD is and how it is associated with pulmonary fibrosis.

Pulmonary fibrosis in hypersensitivity pneumonitis

Pulmonary fibrosis does not always occur in hypersensitivity pneumonitis. After exposure to an allergen, symptoms can be improved if there is complete avoidance of the allergen. Hypersensitivity pneumonitis can be characterized by three different types of responses:

A very sudden and severe (acute) attack that begins with heavy exposure to the trigger
A rapid response (sub-acute), which occurs most frequently in people exposed to relatively low levels of dust
A response over a long period (chronic), which develops after persistent acute attacks and recurrent sub-acute responses

The chronic response is marked by increasing cough, chronic bronchitis, shortness of breath, anorexia or weight loss, and lung fibrosis. The person may suffer permanent lung damage.

People who work in certain occupations and those who are repeatedly exposed to allergens can develop chronic hypersensitivity pneumonitis. However, sometimes the causative allergen isn’t identified. Other factors, such as some viral infections and genetics may increase the risk of chronic hypersensitivity pneumonitis, and cigarette smoking is considered to worsen the condition. It is not yet established why some people get chronic hypersensitivity pneumonitis and others don’t.

Symptoms of hypersensitivity pneumonitis

Most people who are continuously exposed to a small amount of allergen may not notice the changes to their lungs at first. These symptoms can develop over months or years.

Symptoms of chronic hypersensitivity pneumonitis may include:

Shortness of breath

Increasing cough

Fatigue and weakness

Anorexia or weight loss

Lung fibrosis

Important

If a specific allergen is identified as a cause, it is important to completely avoid exposure to it. It might help to speak with your employer to find out what steps they can take to help you with this in the workplace.

For patients who have chronic hypersensitivity pneumonitis, symptoms may not fully resolve, and research suggests that people live for about 7 years if this is the case. Some people might face progressive of the disease, which means that the scarring might get worse over time. However, currently there is no way of predicting if, or how fast, this will happen. Some patients might not have changes in their symptoms, while other patients might experience a faster worsening of their symptoms. It is not known why some people’s disease progresses faster than others.

If your symptoms are due to hypersensitivity pneumonitis, there are changes you can make to your home and environment to help take care of your lungs.

In particular, take steps to limit your exposure to certain dusts both at home and in your workplace:

Allergy-causing bacteria and fungi can thrive in stagnant, or still, water. It can also cause mold to grow. Be sure to remove any standing water inside and outside of your home
Try to keep the humidity in your home and workplace below 50 percent
Make sure your heating, ventilation and air conditioning systems are properly maintained, and that the water in these systems is not recirculated
Ask a loved one or your workplace to repair any water damage in both environments. This includes carpets, furniture and drywall
If you work with farm products, make sure to properly dry and store them

Speak to your healthcare team for guidance on how to speak with your employer about changes you may need to make to your workplace. They’ll be best able to advise you and give helpful suggestions.