Interstitial lung disease (ILD) describes a large group of disorders characterized by progressive scarring of the lung tissue between and supporting the air sacs. The scarring associated with interstitial lung disease may cause progressive lung stiffness, eventually affecting your ability to breathe and get enough oxygen into your bloodstream.

Interstitial lung disease may be broadly categorized into known and unknown causes. Common known causes include autoimmune or rheumatologic diseases, occupational and organic exposures, medications, and radiation. Interstitial lung disease of unknown cause is predominated by idiopathic pulmonary fibrosis, a specific and progressive fibrotic lung disease, followed by the idiopathic interstitial pneumonias, such as nonspecific interstitial pneumonia (NSIP), and sarcoidosis.

Once lung scarring occurs, it's generally irreversible. Medications may slow the damage of interstitial lung disease, but many people never regain full use of their lungs. Lung transplant is an option for some people who have worsening interstitial lung disease despite treatment.

The primary signs and symptoms of interstitial lung disease include:

• Dry cough
• Shortness of breath at rest or with exertion

When to see a doctor

By the time symptoms appear, irreversible lung damage may have already occurred. Nevertheless, it's important to see your doctor at the first sign of breathing problems. Many conditions other than interstitial lung disease can affect your lungs, and getting an early and accurate diagnosis is important for proper treatment.

Taken from MAYO Clinic

Interstitial lung disease may occur when an injury to your lungs triggers an abnormal healing response. Ordinarily, your body generates just the right amount of tissue to repair damage. But in interstitial lung disease, the repair process goes awry and the tissue around the air sacs (alveoli) becomes scarred and thickened. This makes it more difficult for oxygen to pass into your bloodstream.

Interstitial lung disease can be triggered by many things — including autoimmune diseases, exposure to organic and inorganic agents in the home or workplace, medications, and some types of radiation. In some cases, the cause is unknown. 

Occupational and environmental factors

Long-term exposure to a number of organic and inorganic materials and agents can damage your lungs. These include:

• Asbestos fibers
• Bird protein (live pets and feather-containing products)
• Coal dust
• Grain dust
• Mold from indoor hot tubs, showers and prior water damage
• Silica dust

Medications and radiation

Many drugs can damage your lungs, especially:

• Chemotherapy/immunomodulating drugs, such as methotrexate and cyclophosphamide
• Heart medications, such as amiodarone (Cordarone, Nexterone, Pacerone) and propranolol (Inderal, Inderide, Innopran)
• Some antibiotics, such as nitrofurantoin (Macrobid, Macrodantin, others) and sulfasalazine (Azulfidine) 

Some people who have radiation therapy for lung or breast cancer show signs of lung damage months or sometimes years after the initial treatment. The severity of the damage depends on:

• How much of the lung was exposed to radiation
• The total amount of radiation administered
• Whether chemotherapy was also used
• The presence of underlying lung disease

Medical conditions

Lung damage can be associated with the following autoimmune diseases:

• Dermatomyositis/polymyositis
• Mixed-connective tissue disease
• Pulmonary vasculitis (microscopic polyangiitis)
• Rheumatoid arthritis
• Sarcoidosis
• Scleroderma
• Sjogren's syndrome
• Systemic lupus erythematosus
• Undifferentiated connective tissue disease

After extensive evaluation and testing, an explicit cause may not be found. Disorders without a known cause are grouped together under the label of idiopathic interstitial pneumonias, which are tissue-based classifications.

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