What Is Farmer's Lung?

Medically Reviewed by Melinda Ratini, MS, DO on April 26, 2023
4 min read

Farmer's lung is a disease caused by an allergy to the mold in certain crops. Farmers are most likely to get it because it's usually caused by breathing in dust from hay, corn, grass for animal feed, grain, tobacco, or some pesticides.

Not everyone gets farmer's lung after breathing in these things. It only happens if you have an allergic reaction.

Your doctor may use another name for your condition. It's also called extrinsic allergic alveolitis, hypersensitivity alveolitis, or hypersensitivity pneumonitis. The "-itis" at the end of these names means it causes inflammation. With farmer's lung, the inflammation, or swelling, is in your lungs.

Farmer's lung can cause three kinds of allergic reactions.

An acute attack is an intense reaction that happens 4 to 8 hours after you breathe in mold. Symptoms include:

  • Dry irritating cough
  • Fever and chills
  • Rapid breathing
  • Rapid heart rate
  • Shortness of breath
  • Sudden feeling that you're sick

A sub-acute attack is less intense and comes on more slowly than an acute attack. Symptoms include:

  • Achy muscles and joints
  • Coughing
  • Mild fever with some chills
  • No appetite
  • Shortness of breath

You may mistake acute or sub-acute farmer's lung for the flu, because many of the symptoms are the same.

Chronic farmer's lung happens after you've had many acute attacks and are around large amounts of moldy dust often. When you reach this point, your lungs may have permanent damage. Symptoms include:

  • Cough that won't go away
  • Depression
  • General aches and pains
  • Night sweats
  • No appetite and gradual weight loss
  • Occasional fever
  • Shortness of breath that gets worse over time
  • Weakness and loss of energy
  • Weight loss

Most people with acute or subacute farmer's lung get better; only a small percentage of people develop chronic farmer's lung. Chronic farmer's lung symptoms can be controlled/improved, but it can't be cured.

Farmers may notice that their symptoms get worse in the winter. Storing animal feed like hay, grass, or grain inside makes mold more likely to grow. Plus, there's no breeze or wind to clear it out of the air.

It's common for farmers to get this disease from moldy hay and other crops. But you also can get it from dust in things like:

  • Animal dander
  • Bacteria
  • Bark
  • Bird droppings
  • Dried rat urine
  • Feathers
  • Fungi
  • Husks
  • Insects
  • Wood

These allergens have to be very small -- around 5 millionths of a meter (5 microns) -- to affect you. Because the particles are so tiny, the normal defenses in your nose and throat miss them, and they go straight to your lungs. Your lungs then try to get rid of the dust, and your symptoms start when your immune system reacts to that.

The most important questions your doctor will ask you will be about your environment. If you're not a farmer, it may be harder to figure out that farmer's lung is causing your symptoms.

It can also be hard to know what's going on if you're not having an acute attack. Your doctor can give you a blood test to look for certain things that trigger your immune system (called antigens) or order a chest X-ray to look for signs that you've had acute attacks.

Other things your doctor can do to find out if you have farmer's lung include:

  • Pulmonary function test: This measures how much air you inhale and exhale.
  • Bronchoscopy: Your doctor uses a device called a bronchoscope to look at your airways and lungs and collect a sample of fluid for testing. You'll be given medicine to make you sleep through this.
  • Lung biopsy: If your doctor thinks you might have farmer's lung but can't be sure, they may want to take a sample of your lung tissue and send it to a lab for tests. You'll probably be given medicine to sleep through this as well.


There's no cure for farmer's lung, but you can control it by staying away from the allergen that causes your symptoms. You might:

  • Work outside as much as possible
  • Avoid dusty work
  • Wear a mask or other protective equipment
  • Use fans, filters, or exhaust blowers wherever you can

Steroid drugs like prednisone can help with your symptoms because they can slow down your immune system and help with inflammation. Your doctor may only prescribe these if you have a chronic case, though.

If steroids don't work, your doctor may want to prescribe and immune suppressant such as a drug called azathioprine (Azasan, Imuran).

In addition to recommending that you avoid your triggers, your doctor may also recommend bed rest or oxygen therapy, which involves getting extra oxygen through tubes in your nose or a mask, to help you feel better.