Interstitial fibrosis is scarring of the lungs that develops during an extended period of interstitial inflammation, explains MedicineNet. The interstitium is tissue that surrounds tiny air sacs in the lungs called alveoli. The tissue becomes inflamed in patients with interstitial lung disease, explains MedlinePlus.
When interstitial tissue develops inflammation and stiffness, it prevents the alveoli from expanding normally, and patients take in less oxygen, notes MedlinePlus. Interstitial lung disease encompasses many diseases that cause a thickened interstitium, which typically looks like fine lace or a sponge on imaging studies and X-rays, according to MedicineNet. The lung diseases have multiple causes, including environmental toxins, such as mold and asbestos, some chemotherapy medications and autoimmune diseases, such as lupus and rheumatoid arthritis.