What Is Pneumonitis?

Medically Reviewed by Carmelita Swiner, MD on April 09, 2022
2 min read

Pneumonitis is when your lungs are irritated, or inflamed. Almost anything can cause it, including germs, medication, and allergies. Breathing in harsh chemicals, like bleach, can also bring on the condition. Typically, when your doctor says pneumonitis, they mean something has irritated your lungs rather than infected them.

It happens when tiny air sacs in your lungs, called alveoli, get inflamed and swollen. The cause isn’t always clear, but some reasons are:


Molds, chemicals, bacteria. This is when your lungs have an allergic reaction to a chemical, mold, or other substance. It’s sometimes called hypersensitivity pneumonitis. Common causes include:

  • Birds. You can get pigeon breeder’s disease (also called bird fancier’s disease) when you breathe in tiny particles in the air from bird feathers and poop. Poultry workers and bird breeders are more likely to get this.
  • Grain. Called farmer’s lung, your immune system reacts to a mold that grows on grain or hay. It’s more common on dairy farms, among cattle workers, and in places with high humidity.
  • Air conditioning. Mold can grow in cooling, heating, and humidifier systems. It may inflame your lungs. This is sometimes called humidifier lung.
  • Hot tubs. A certain mold that grows in hot tubs can inflame your lungs. It’s sometimes called hot tub lung.

Other causes. It could be that a virus or bacteria has infected your lungs. This is different from an allergic reaction. Your doctor will usually call that pneumonia.

You might find it harder to catch your breath when you walk up a flight of stairs, exercise, or do another activity. Other symptoms include:

If you don’t treat pneumonitis, it can start to scar your lungs. This is called pulmonary fibrosis, and it can be very serious.

Your doctor will ask about your symptoms, work, and hobbies. They’ll listen carefully to your lungs while you breathe. They might also do:

  • Imaging tests, like X-rays and CT scans
  • A spirometry test to show how much and how fast you breathe in and out
  • A blood oxygen test to show the percentage of oxygen in your blood
  • A bronchoscopy to look in your lung or take out liquid to look at
  • A biopsy to cut off a small piece of lung tissue to look at under a microscope

Your doctor will try to figure out what’s causing your pneumonitis and then help you to avoid it. That usually helps you get better. If the doctor can’t figure out the cause or your symptoms don’t improve, they may prescribe corticosteroids to lessen an allergic reaction or oxygen therapy to get more oxygen into your blood.