Interstitial lung disease (ILD) is not a single condition but a diverse group of over 200 disorders that affect the tissue and space around the air sacs in the lungs. The defining feature of ILD is progressive inflammation and scarring (pulmonary fibrosis), which stiffens the lung tissue. This scarring reduces lung capacity and function, making it harder for the lungs to transfer oxygen into the bloodstream. While some forms are rare, the collective burden of these diseases is substantial, making it important to quantify how common ILD is globally and nationally.
Overall Prevalence and Incidence Rates
ILD is a relatively common group of respiratory disorders, with millions affected worldwide. Global Burden of Disease data from 2019 estimated that over 4.7 million people globally were living with ILD. Prevalence rates (total existing cases) range widely, from 6.3 to 71 cases per 100,000 people, reflecting differences in populations and diagnostic criteria.
Incidence (new cases diagnosed each year) also shows significant variation, ranging from 1 to 31.5 new cases per 100,000 person-years. In 2021, the worldwide number of new ILD cases was estimated to be over 390,000 people. The age-standardized incidence rate has increased slightly between 1990 and 2021, suggesting a rising trend in new diagnoses.
These reported numbers are often considered conservative estimates due to diagnostic challenges. ILD encompasses many distinct conditions, and early symptoms often overlap with common lung diseases, leading to frequent misdiagnosis and underreporting. Studies from high-income regions, which have better diagnostic capabilities, tend to report higher prevalence rates, such as 101.21 per 100,000 people in high Social Demographic Index (SDI) regions in 2019.
The Major Subtypes of Interstitial Lung Disease
The overall prevalence of ILD is a composite of dozens of distinct diseases, with the most frequently reported conditions determining the total burden. Idiopathic Pulmonary Fibrosis (IPF) is a widely studied subtype characterized by progressive scarring of the lungs with no identifiable cause.
Although IPF is considered rare, its severity and poor prognosis make it a major focus. Global prevalence is estimated between 10 and 60 cases per 100,000 individuals. IPF accounts for a significant proportion of total ILD cases, with incidence in North America and Europe estimated at three to nine new cases per 100,000 people annually.
Sarcoidosis is another common form of ILD, characterized by the growth of inflammatory cells (granulomas) in the lungs and other organs. Its incidence rate in some populations, such as the United Kingdom, is similar to IPF, around 5.0 per 100,000 person-years. In some cohorts, sarcoidosis is the single most common ILD diagnosis, sometimes accounting for over 40% of cases.
Interstitial lung disease related to connective tissue diseases (CTD-ILD) also contributes significantly. These conditions occur in people with autoimmune diseases, such as rheumatoid arthritis (RA) or systemic sclerosis (SSc). The frequency of CTD-ILD among all ILD cases ranges from 7.5% to over 34% depending on the region. Other subtypes, such as those caused by environmental or occupational exposures (e.g., asbestosis or hypersensitivity pneumonitis), account for the remaining population.
Demographic and Geographic Variations in Occurrence
The occurrence of ILD is not uniform, showing distinct patterns based on demographic and geographic factors. Age is a strong determinant, with both incidence and prevalence increasing sharply in older individuals. IPF, for example, is typically diagnosed in people between 50 and 70 years old, and its prevalence is higher in those over 75 compared to younger adults.
Sex differences are also apparent, with the overall prevalence of ILD being 20% higher in males than in females. This is partly driven by male predominance in certain subtypes, such as IPF, where up to 70% of those affected are male. Conversely, some autoimmune-related ILDs, like those linked to systemic sclerosis, are more frequently diagnosed in women.
Geographic location and environmental factors play a substantial role in determining the dominant ILD subtype in a region. The prevalence of occupational lung diseases is higher in areas with heavy industries, such as mining, contributing to higher male prevalence there. Conversely, the relative frequency of hypersensitivity pneumonitis is higher in some Asian countries, potentially due to different environmental or agricultural exposures.
Mortality and Survival Statistics
The commonality of ILD must be viewed alongside its significant mortality burden. For the most aggressive form, Idiopathic Pulmonary Fibrosis (IPF), the prognosis remains poor. Median survival is estimated at only three to five years following diagnosis, and the five-year cumulative survival rate is approximately 45.6%.
The introduction of antifibrotic medications has helped slow the rate of lung function decline and improved survival rates since 2010. However, IPF still has the highest mortality rate among all ILD subtypes. In contrast, other forms of ILD, such as sarcoidosis, have a significantly better prognosis.
For sarcoidosis, the five-year survival rate is substantially higher, with many patients experiencing mild or self-limiting disease. The overall mortality rate for sarcoidosis patients is approximately double that of the general population. This wide variability in survival underscores the importance of accurately diagnosing the specific ILD subtype to predict outcomes and guide treatment.