Hypersensitivity pneumonitis

Medical quality assurance by Dr. Albrecht Nonnenmacher, MD at November 19, 2016
StartDiseasesHypersensitivity pneumonitis

Hypersensitivity pneumonitis is a serious medical condition that mainly affects people who work with or live around contaminated substances. If HP is not treated properly, it may lead to long-term damage.


Definition & Facts

Hypersensitivity pneumonitis is an inflammation of the lungs caused by sensitivity to inhaled organic dust. It is not precisely known how many people suffer from HP; the prevalence is generally low, but working age adults in certain occupations, such as farming, metal-working, and bird-handling, are most at risk of developing HP.

According to the Cleveland Clinic, the mortality rate of patients with HP is only slightly higher than those without HP; the total number of estimated deaths from HP between 1980 and 2002 was 814.

Symptoms & Complaints

Hypersensitivity pneumonitis is a disease of varying length and intensity. There are generally three different types: acute, subacute, and chronic. Acute HP may develop in as little as four hours after exposure. It is characterized by flu-like symptoms, such as fever, chills, chest tightness, a dry cough, and a headache.

Subacute HP is characterized by a persistent cough, shortness of breath, weight loss, and a crackling sound made by the lungs, typically while breathing in. Chronic HP, in addition to coughing, labored breathing, and weight loss, is also characterized by fatigue and deformities in the finger and toe nails.

If left untreated, chronic HP can cause serious long-term physical damage, such as lung fibrosis, which is the scarring of the lungs. It can also damage blood vessels in the lungs and in rare cases lead to heart failure.

Symptoms of HP can vary from person to person, depending on the type of HP involved and the level of exposure. Some individuals will only experience chronic symptoms after long-term exposure, while others may develop symptoms much sooner.


Hypersensitivity pneumonitis is caused by inhaling an environmental toxicant or harmful foreign substance, such as bacteria or mold, into the lungs. The presence of the foreign substance triggers an overly aggressive immune reaction in certain people, causing the small air sacs that link together the lungs and the blood vessels to become inflamed.

These dangerous foreign substances are present in certain environments, either at work or at home and from an occupation or hobby. For example, farmers can acquire HP from exposure to moldy or contaminated agricultural products. Contaminants can also show up in metal-working fluid or hardwood dust. Malt-workers, wine-makers, and cheese-makers are sometimes at risk because of contaminated food. People can even develop inflammation from contaminated sauna or hot tub water.

According to the National Heart, Lung, and Blood Institute, an estimated 85 to 95 percent of people exposed to potentially harmful substances never develop the disease, or they experience only a mild immune reaction with no obvious signs or symptoms. Certain genetic factors are believed to increase the chances of developing HP, but this phenomenon is not well-understood at this time.

Diagnosis & Tests

In order to properly diagnose hypersensitivity pneumonitis, doctors must first collect an exhaustive medical history from their patients and rule out other possible causes. That can take up to a few weeks or months at a time. Doctors will perform a thorough physical examination and attempt to determine if their patient has been exposed to any potential causative substances by checking for unusual lung sounds or abnormal oxygen levels in the blood. Assuming the causative substance can be identified quickly, acute HP is usually easier to diagnose than chronic HP, which develops progressively after long-term exposure to smaller amounts of foreign substances.

If a preliminary physical examination is not sufficient, doctors may order one or more of the following tests:

Treatment & Therapy

Once the foreign substance causing HP has been identified, patients should immediately either remove the offending substance, if at all possible, or completely avoid the source. Long-term damage to the lung can be averted if the cause of the inflammation is identified in time.

For patients who need immediate relief from their symptoms, doctors may prescribe either immunosuppressive drug, which suppresses the body's immune system response, or a bronchodilator, which is a substance that relaxes the muscles in the airways and increases airflow to the lungs.

If the patient has been diagnosed with low levels of oxygen in the blood, doctors may recommend providing additional oxygen through a mask, breathing tube, or portable tank or machine in the home. The long-term prognosis is generally good, but it depends upon the length and intensity of exposure to the foreign substance.

Hypersensitivity pneumonitis should be diagnosed and treated in the early stages, before chronic inflammation can cause irreparable damage that impairs the lung's capacity to properly function. Some patients with irreversibly scarred or damaged lungs may require a lung transplant.

Prevention & Prophylaxis

Hypersensitivity pneumonitis is not inevitable and can be prevented altogether by avoiding exposure to dangerous contaminants in the environment. According to the American Lung Association, the easiest way to limit exposure to contaminants is by taking these important steps:

  • Remove any stagnant water inside or outside the house
  • Keep humidity at home or work below 60 percent
  • Remove water-damaged carpeting, furnishings, and drywall
  • Properly maintain heating, ventilation, and air conditioning systems
  • Properly dry and store any potential hazardous products.

Wearing an air-purifying respirator or installing a dust filter, especially in a potentially hazardous job, may also prevent foreign substances from entering the lungs. People who experience recurring symptoms should see a doctor immediately.