Interstitial lung diseases are a group of diseases, caused by inflammation and scarring of the alveoli (air sacs) and their supporting structures (the interstitium). This leads to the loss of the functional alveolar units and a reduction of the transfer of oxygen from air to blood.

There are dozens of different causes of ILD, and the classification systems are often confusing. In fact, many experts disagree on the best classification system for the interstitial lung diseases.

The interstitial lung diseases can be broken down into two large groups: those that have no known cause (idiopathic ILD) and those with an identifiable cause. One recent classification of idiopathic ILD includes the subclasses called: idiopathic pulmonary fibrosis/ usual interstitial pneumonitis (IPF/UIP), non-specific interstitial pneumonitis (NSIP), and acute interstitial pneumonitis (AIP). Sarcoidosis is a relatively common disorder that often causes interstitial lung disease.

Many occupational toxins, such as silica (sand) dust, asbestos fibers, coal dust, and cotton dust can cause ILD. Occasionally, hobbies such as keeping birds or bathing in hot tubs cause some forms of ILD.

Many connective tissue diseases (such as rheumatoid arthritis, scleroderma, and lupus) can cause interstitial lung disease. Some medications (examples include bleomycin, amiodarone, and methotrexate) may cause ILD.

The risk factors, genetic predispositions and incidence of ILD depend on the specific disease being considered. Sarcoidosis, for example, is relatively common in some ethnic groups (African Americans), whereas idiopathic pulmonary fibrosis is quite rare.

People who work in industries with heavy exposures to known causes of occupational lung disease (for example, coal miners, sand-blasters, ship workers) are usually screened routinely for lung disease.

Cigarette smoking increases the risk of developing ILD and can cause the disease to be more severe.