Farmer’s lung is a type of allergic reaction that occurs as a response to mold from crops and grains. For this reason, it commonly affects those who work on farms.

There are different categories of farmer’s lung (FL), depending on the level of exposure to the mold.

This article discusses how FL affects the body and the different types that can occur.

It will also explore the treatment options available and the main methods of prevention.

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According to the American Lung Association (ALA), FL is a type of hypersensitivity pneumonitis. Hypersensitivity pneumonitis occurs when a person breathes in a substance, or the allergen, and has an allergic reaction to it.

FL occurs as a result of breathing in mold from hay, grain, and straw.

While it commonly affects farmers and cattle workers, FL can affect any person who experiences exposure to the mold.

Healthcare professionals also refer to FL as:

  • extrinsic allergic alveolitis
  • hypersensitivity alveolitis
  • hypersensitivity pneumonitis

FL occurs as a result of breathing in moldy dust, which contains microorganisms. Specifically, these are spores of bacteria that are able to tolerate heat.

The bacteria that most commonly cause FL are Micropolyspora faeni and Thermoactinomyces vulgaris.

A 2020 article notes that Aspergillus, which is a type of fungi, can also cause FL.

According to the Canadian Center for Occupational Health and Safety (CCOHS), in areas where people harvest crops in wet and rainy weather, the crops are likely to get damp and undergo “self-heating.” This allows for the heat-tolerant bacteria to grow and spoil the crops.

When they are moldy, the crops start to crumble, become dusty, and release spores. People then breathe in these spores.

The spores can trigger an allergic reaction, but not everyone will develop an allergic reaction to the spores.

The spores contain substances known as antigens, which the body’s immune system recognizes. The immune system identifies the mold spores as being foreign and produces substances known as antibodies, which fight the bacteria and fungus.

This occurs in the lungs. However, certain immune cells, such as white blood cells, will release poisons and chemicals that will damage lung tissue.

This results in the breathing and respiratory issues associated with FL. But the severity of these effects will depend on a person’s sensitivity to the mold.

The symptoms will vary depending on the person. Some people may have an abrupt, acute reaction to the substance.

Ultimately, this depends on the person’s sensitivity to the spores and how much enters the lungs.

There are three distinct types of FL: acute, sub-acute, and chronic.


According to the CCOHS, acute FL occurs in 1 in 3 cases.

It typically occurs after a person has experienced exposure to high concentrations of dust from the mold.

Symptoms of acute FL can be nonspecific and include:

  • shortness of breath
  • a low-grade fever
  • a fast heart rate
  • quick breathing
  • suddenly feeling sick
  • a dry cough

The ALA note that acute FL occurs 4–6 hours after the person breathes in the spores.

The CCOHS state that signs and symptoms usually decline after 12 hours if the person avoids repeated exposure to the moldy dust. But symptoms can last for up to 2 weeks.


Sub-acute FL occurs after a person has continuously inhaled moldy dust, but not a high concentration of it.

It can be more difficult to identify, as symptoms develop more subtly. Symptoms include:

  • coughing
  • shortness of breath
  • generally feeling unwell
  • muscle and joint pain and aches
  • loss of appetite
  • weight loss over several weeks
  • low-grade fever
  • occasional chills


Chronic FL occurs after long periods of exposure to the moldy dust. The inhaled concentration of moldy dust does not have to be high. The illness can occur over many months.

Symptoms include:

  • increasing shortness of breath
  • an occasional mild fever
  • significant weight loss
  • lack of energy

The 2020 article notes that the most important step to take is avoiding exposure to the spores.

If a person receives a diagnosis early, avoidance of the exposure may be enough to entirely reverse the damage. After a short time, a person’s lungs will return to normal.

For more severe cases, treatment can include corticosteroids and immunosuppressants. A person may need to take this medication for 3 months.

Research has shown that steroids are effective in acute FL. But there is no evidence to suggest that they are effective at preventing disease progression in the long term.

Additionally, people may require bronchodilators to make breathing easier, and oxygen therapy to raise the amount of oxygen in a person’s blood.

The CCOHS recommend not using medications long term, as doing so can hide symptoms of FL.

In cases where there is progressive disease with respiratory failure, a person may require lung transplantation.

The outlook of a person with FL will depend on:

  • the category of FL
  • the level of antigen exposure
  • the length of time a person has experienced exposure to the allergen

The 2020 article notes that FL has a mortality rate of 1% and the average lifespan is 8 years after a person has received a diagnosis.

If a person receives an early diagnosis, avoiding the allergen can reverse the damage, and the lungs will return to normal.

But there is no cure for those who become hypersensitive to the mold. Once a person has become hypersensitive, they will remain so for years, and perhaps for life.

A person can take steps to prevent FL, including:

  • taking precautions to avoid breathing in spores from moldy crops
  • drying wet hay, grain, or other crops at harvest
  • storing crops with a high risk of spoilage in a silage
  • ventilating buildings that have large amounts of dust
  • mechanizing the handling of hay or crops to reduce spore exposure
  • wetting down dust from moldy crops before removal to avoid spores becoming airborne

A person can also take the following steps at home:

  • Remove any standing water inside or outside the home.
  • Use a hygrometer at home to keep the humidity in the home below 50%.
  • Repair water damage as soon as possible.
  • Maintain the heating, ventilation, and air conditioning systems.

People working in similar conditions should wear properly fitted respirators that can protect against molds, bacteria, and dust.

A person who suspects that they have FL should contact a doctor if they experience any of the following symptoms:

  • suddenly feeling ill after handling moldy crop
  • a sudden continuous cough
  • a low-grade fever

People should let a healthcare professional know if they have experienced exposure to moldy crops or hay.

There is not a singular test that is able to detect FL. However, a healthcare professional will normally request a number of tests to obtain the correct diagnosis.

Some tests may include:

  • chest X-ray
  • high-resolution computerized tomography scan, which is helpful for investigating possible acute and chronic FL cases
  • serum antibody blood test
  • lung function test
  • lung allergy challenge test, which can help isolate the specific source of the allergic reaction

A healthcare professional may require further tests, as the symptoms of FL are similar to other conditions, such as viral illness.

FL is a type of hypersensitivity pneumonitis. It occurs after the inhalation of bacterial spores from moldy crops.

FL can cause a number of different signs and symptoms depending on whether it is acute, sub-acute, or chronic. This categorization also affects the type of treatment options available.

A person who suspects that they may have FL should contact a healthcare professional, as an early diagnosis in the acute phase may mean that the condition is reversible.