Hypersensitivity pneumonitis usually occurs in people who work in places where there are high levels of organic dusts, fungus, or molds. Bird fancier’s lung is the most common type of hypersensitivity pneumonitis. It is caused by repeated or intense exposure to proteins found in the feathers or droppings of many species of birds.

Farmer’s lung is caused by exposure to dust from moldy hay, straw, and grain. These exposures can lead to lung inflammation and acute lung disease. Over time, this acute Hypersensitivity pneumonitis may turn into long-lasting (chronic) lung disease.

Hypersensitivity pneumonitis may also be caused by fungi or bacteria in humidifiers, heating systems, and air Hypersensitivity pneumonitisers found in homes and offices. Exposure to certain chemicals, such as isocyanates or acid anhydrides, can also lead to hypersensitivity pneumonitis.