The cause of IPF is unknown but certain environmental factors and exposures have been shown to increase the risk of getting IPF. Cigarette smoking is the best recognized and most accepted risk factor for IPF, and increases the risk of IPF by about twofold. Other environmental and occupation exposures such as exposure to metal dust, wood dust, coal dust, silica, stone dust, biologic dusts coming from hay dust or mold spores or other agricultural products, and occupations related to farming/livestock have also been shown to increase the risk for IPF. There is some evidence that viral infections may be associated with idiopathic pulmonary fibrosis and other fibrotic lung diseases.

Pulmonary fibrosis may be a secondary effect of other diseases. Most of these are classified as interstitial lung diseases. Examples include autoimmune disorders, viral infections and bacterial infection like tuberculosis which may cause fibrotic changes in both lungs upper or lower lobes and other microscopic injuries to the lung. However, pulmonary fibrosis can also appear without any known cause. In this case, it is termed "idiopathic". Most idiopathic cases are diagnosed as "idiopathic pulmonary fibrosis". This is a diagnosis of exclusion of a characteristic set of histologic/pathologic features known as usual interstitial pneumonia (UIP). In either case, there is a growing body of evidence which points to a genetic predisposition in a subset of patients. For example, a mutation in surfactant protein C (SP-C) has been found to exist in some families with a history of pulmonary fibrosis.

Diseases and conditions that may cause pulmonary fibrosis as a secondary effect include:

- Inhalation of environmental and occupational pollutants, such as metals in asbestosis, silicosis and exposure to certain gases. Coal miners, ship workers and sand blasters among others are at higher risk.

- Hypersensitivity pneumonitis, most often resulting from inhaling dust contaminated with bacterial, fungal, or animal products.

- Cigarette smoking can increase the risk or make the illness worse.

- Some typical connective tissue diseases such as rheumatoid arthritis, SLE and scleroderma

- Other diseases that involve connective tissue, such as sarcoidosis and granulomatosis with polyangiitis.

- Infections

- Certain medications, e.g. amiodarone, bleomycin (pingyangmycin), busulfan, methotrexate, apomorphine, and nitrofurantoin

- Radiation therapy to the chest

Five million people worldwide are affected by pulmonary fibrosis. A wide range of incidence and prevalence rates have been reported for pulmonary fibrosis. The rates below are per 100,000 persons, and the ranges reflect narrow and broad inclusion criteria, respectively.

Based on these rates, pulmonary fibrosis prevalence in the United States could range from more than 29,000 to almost 132,000, based on the population in 2000 that was 18 years or older. The actual numbers may be significantly higher due to misdiagnosis. Typically, patients are in their forties and fifties when diagnosed while the incidence of idiopathic pulmonary fibrosis increases dramatically after the age of fifty. However, loss of pulmonary function is commonly ascribed to old age, heart disease or to more common lung diseases.

IPF has been recognized in several breeds of both dogs and cats, and has been best characterized in West Highland White Terriers. Veterinary patients with the condition share many of the same clinical signs as their human counterparts, including progressive exercise intolerance, increased respiratory rate, and eventual respiratory distress.

Prognosis is generally poor.

PAM is one of the rare lung diseases currently being studied by the Rare Lung Diseases Consortium (RLDC). Pulmonary Alveolar Microlithiasis patients, families, and caregivers are encouraged to join the NIH Rare Lung Diseases Consortium Contact Registry. This is a privacy protected site that provides up-to-date information for individuals interested in the latest scientific news, trials, and treatments related to rare lung diseases.

According to a recent study, the main risk factors for RA-ILD are advancing age, male sex, greater RA disease activity, rheumatoid factor (RF) positivity, and elevated titers of anticitrullinated protein antibodies such as anticyclic citrullinated peptide. Cigarette smoking also appears to increase risk of RA-ILD, especially in patients with human leukocyte antigen DRB1.

A recently published retrospective study by a team from Beijing Chao-Yang Hospital in Beijing, China, supported three of the risk factors listed for RA-ILD and identified an additional risk factor. In that study of 550 RA patients, logistic regression analysis of data collected on the 237 (43%) with ILD revealed that age, smoking, RF positivity, and elevated lactate dehydrogenase closely correlated with ILD.

Recent studies have identified risk factors for disease progression and mortality. A retrospective study of 167 patients with RA-ILD determined that the usual interstitial pneumonia (UIP) pattern on high-resolution computed tomography (HRCT) was a risk factor for progression, as were severe disease upon diagnosis and rate of change in pulmonary function test results in the first 6 months after diagnosis.

A study of 59 RA-ILD patients found no median survival difference between those with the UIP pattern and those without it. But the UIP group had more deaths, hospital admissions, need for supplemental oxygen, and decline in lung function.

The prognosis of pulmonary arterial hypertension (WHO Group I) has an "untreated" median survival of 2–3 years from time of diagnosis, with the cause of death usually being right ventricular failure (cor pulmonale). A recent outcome study of those patients who had started treatment with bosentan (Tracleer) showed that 89% patients were alive at 2 years. With new therapies, survival rates are increasing. For 2,635 patients enrolled in The Registry to Evaluate Early and Long-term Pulmonary Arterial Hypertension Disease Management (REVEAL Registry) from March 2006 to December 2009, 1-, 3-, 5-, and 7-year survival rates were 85%, 68%, 57%, and 49%, respectively. For patients with idiopathic/familial PAH, survival rates were 91%, 74%, 65%, and 59%. Levels of mortality are very high in pregnant women with severe pulmonary arterial hypertension (WHO Group I). Pregnancy is sometimes described as contraindicated in these women.

The epidemiology of IPAH is about 125–150 deaths per year in the U.S., and worldwide the incidence is similar to the U.S. at 4 cases per million. However, in parts of Europe (France) indications are 6 cases per million of IPAH. Females have a higher incidence rate than males (2–9:1).

Other forms of PH are far more common. In systemic scleroderma, the incidence has been estimated to be 8 to 12% of all patients; in rheumatoid arthritis it is rare. However, in systemic lupus erythematosus it is 4 to 14%, and in sickle cell disease, it ranges from 20 to 40%. Up to 4% of people who suffer a pulmonary embolism go on to develop chronic thromboembolic disease including pulmonary hypertension. A small percentage of patients with COPD develop pulmonary hypertension with no other disease to explain the high pressure. On the other hand, obesity-hypoventilation syndrome is very commonly associated with right heart failure due to pulmonary hypertension.

Regardless of cause, UIP is relentlessly progressive, usually leading to respiratory failure and death without a lung transplant. Some patients do well for a prolonged period of time, but then deteriorate rapidly because of a superimposed acute illness (so-called "accelerated UIP"). The outlook for long-term survival is poor. In most studies, the median survival is 3 to 4 years. Patients with UIP in the setting of rheumatoid arthritis have a slightly better prognosis than UIP without a known cause (IPF).

ILD may be classified according to the cause. One method of classification is as follows:

1. Inhaled substances

- Inorganic

- Silicosis

- Asbestosis

- Berylliosis

- printing workers (eg. carbon bblack, ink mist)

- Organic

- Hypersensitivity pneumonitis

2. Drug-induced

- Antibiotics

- Chemotherapeutic drugs

- Antiarrhythmic agents

3. Connective tissue and Autoimmune diseases

- Rheumatoid arthritis

- Systemic lupus erythematosus

- Systemic sclerosis

- Polymyositis

- Dermatomyositis

4. Infection

- Atypical pneumonia

- Pneumocystis pneumonia (PCP)

- Tuberculosis

- "Chlamydia" trachomatis

- Respiratory Syncytial Virus

5. Idiopathic

- Sarcoidosis

- Idiopathic pulmonary fibrosis

- Hamman-Rich syndrome

- Antisynthetase syndrome

6. Malignancy

- Lymphangitic carcinomatosis

7. Predominantly in children

- Diffuse developmental disorders

- Growth abnormalities deficient alveolarisation

- Infant conditions of undefined cause

- ILD related to alveolar surfactant region

Interstitial lung disease (ILD), or diffuse parenchymal lung disease (DPLD), is a group of lung diseases affecting the interstitium (the tissue and space around the air sacs of the lungs). It concerns alveolar epithelium, pulmonary capillary endothelium, basement membrane, perivascular and perilymphatic tissues. It may occur when an injury to the lungs triggers an abnormal healing response. Ordinarily, the 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 the bloodstream. The term ILD is used to distinguish these diseases from obstructive airways diseases.

In children, several unique forms of ILD exist which are specific for the young age groups. The acronym chILD is used for this group of diseases and is derived from the English name, Children’s Interstitial Lung Diseases – chILD.

Prolonged ILD may result in pulmonary fibrosis, but this is not always the case. Idiopathic pulmonary fibrosis is interstitial lung disease for which no obvious cause can be identified (idiopathic), and is associated with typical findings both radiographic (basal and pleural based fibrosis with honeycombing) and pathologic (temporally and spatially heterogeneous fibrosis, histopathologic honeycombing and fibroblastic foci).

In 2013 interstitial lung disease affected 595,000 people globally. This resulted in 471,000 deaths.

Since the disease was first described in 1918, over 500 case reports have appeared in the literature. PAM is associated with consanguinity. The incidence is higher in Turkey, Japan, India and Italy. The mean age at diagnosis is 35 years based on the cases reported in the literature.

Many cases of restrictive lung disease are idiopathic (have no known cause). Still, there is generally pulmonary fibrosis. Examples are:

- Idiopathic pulmonary fibrosis

- Idiopathic interstitial pneumonia, of which there are several types

- Sarcoidosis

- Eosinophilic pneumonia

- Lymphangioleiomyomatosis

- Pulmonary Langerhans' cell histiocytosis

- Pulmonary alveolar proteinosis

Conditions specifically affecting the interstitium are called interstitial lung diseases.

The exact cause of rheumatoid lung disease is unknown. However, associated factors could be due largely to smoking. Sometimes, the medicines used to treat rheumatoid arthritis, especially methotrexate, may result in lung disease.

Prevention's:

- Stop smoking: Chemicals found in cigarettes can irritate already delicate lung tissue, leading to further complications.

- Having regular checkups: The doctor could listen to lungs and monitor breathing, because lung problems that are detected early can be easier to treat.

Restrictive lung diseases may be due to specific causes which can be intrinsic to the parenchyma of the lung, or extrinsic to it.

Usual interstitial pneumonia (UIP) is a form of lung disease characterized by progressive scarring of both lungs. The scarring (fibrosis) involves the supporting framework (interstitium) of the lung. UIP is thus classified as a form of interstitial lung disease. The term "usual" refers to the fact that UIP is the most common form of interstitial fibrosis. "Pneumonia" indicates "lung abnormality", which includes fibrosis and inflammation. A term previously used for UIP in the British literature is cryptogenic fibrosing alveolitis, a term that has fallen out of favor since the basic underlying pathology is now thought to be fibrosis, not inflammation.

The National Institute of Occupational Safety and Health, Japan (JNIOSH) set limits for acceptable exposure at 0.0003 mg/m after the discovery of indium lung. Methods for reducing indium exposure are thought to be the best mode of protection. Medical surveillance of indium workers is also a method of prevention.

Diagnosis of obstructive disease requires several factors depending on the exact disease being diagnosed. However one commonalty between them is an FEV1/FVC ratio less than 0.7, i.e. the inability to exhale 70% of their breath within one second.

Following is an overview of the main obstructive lung diseases. "Chronic obstructive pulmonary disease" is mainly a combination of chronic bronchitis and emphysema, but may be more or less overlapping with all conditions.

The prognosis for cystic fibrosis has improved due to earlier diagnosis through screening and better treatment and access to health care. In 1959, the median age of survival of children with CF in the United States was six months. In 2010, survival is estimated to be 37 years for women and 40 for men. In Canada, median survival increased from 24 years in 1982 to 47.7 in 2007.

In the US, of those with CF who are more than 18 years old as of 2009, 92% had graduated from high school, 67% had at least some college education, 15% were disabled, 9% were unemployed, 56% were single, and 39% were married or living with a partner.

Asthma is an obstructive lung disease where the bronchial tubes (airways) are extra sensitive (hyperresponsive). The airways become inflamed and produce excess mucus and the muscles around the airways tighten making the airways narrower. Asthma is usually triggered by breathing in things in the air such as dust or pollen that produce an allergic reaction. It may be triggered by other things such as an upper respiratory tract infection, cold air, exercise or smoke. Asthma is a common condition and affects over 300 million people around the world.

Asthma causes recurring episodes of wheezing, breathlessness, chest tightness, and coughing, particularly at night or in the early morning.

- Exercise-Induced Asthma — is common in asthmatics, especially after participation in outdoor activities in cold weather.

- Occupational Asthma — An estimated 2% to 5% of all asthma episodes may be caused by exposure to a specific sensitizing agent in the workplace.

- Nocturnal Asthma — is a characteristic problem in poorly controlled asthma and is reported by more than two thirds of sub-optimally treated patients.

A peak flow meter can record variations in the severity of asthma over time. Spirometry, a measurement of lung function, can provide an assessment of the severity, reversibility, and variability of airflow limitation, and help confirm the diagnosis of asthma.

Cystic fibrosis is the most common life-limiting autosomal recessive disease among people of European heritage. In the United States, about 30,000 individuals have CF; most are diagnosed by six months of age. In Canada, about 4,000 people have CF. Around 1 in 25 people of European descent, and one in 30 of Caucasian Americans, is a carrier of a CF mutation. Although CF is less common in these groups, roughly one in 46 Hispanics, one in 65 Africans, and one in 90 Asians carry at least one abnormal "CFTR" gene. Ireland has the world's highest prevalence of CF, at one in 1353.

Although technically a rare disease, CF is ranked as one of the most widespread life-shortening genetic diseases. It is most common among nations in the Western world. An exception is Finland, where only one in 80 people carries a CF mutation. The World Health Organization states, "In the European Union, one in 2000–3000 newborns is found to be affected by CF". In the United States, one in 3,500 children is born with CF. In 1997, about one in 3,300 Caucasian children in the United States was born with CF. In contrast, only one in 15,000 African American children suffered from it, and in Asian Americans, the rate was even lower at one in 32,000.

Cystic fibrosis is diagnosed in males and females equally. For reasons that remain unclear, data have shown that males tend to have a longer life expectancy than females, but recent studies suggest this gender gap may no longer exist perhaps due to improvements in health care facilities, while a recent study from Ireland identified a link between the female hormone estrogen and worse outcomes in CF.

The distribution of CF alleles varies among populations. The frequency of ΔF508 carriers has been estimated at one in 200 in northern Sweden, one in 143 in Lithuanians, and one in 38 in Denmark. No ΔF508 carriers were found among 171 Finns and 151 Saami people. ΔF508 does occur in Finland, but it is a minority allele there. CF is known to occur in only 20 families (pedigrees) in Finland.

The most common cause is post-surgical atelectasis, characterized by splinting, i.e. restricted breathing after abdominal surgery.

Another common cause is pulmonary tuberculosis. Smokers and the elderly are also at an increased risk. Outside of this context, atelectasis implies some blockage of a bronchiole or bronchus, which can be within the airway (foreign body, mucus plug), from the wall (tumor, usually squamous cell carcinoma) or compressing from the outside (tumor, lymph node, tubercle). Another cause is poor surfactant spreading during inspiration, causing the surface tension to be at its highest which tends to collapse smaller alveoli. Atelectasis may also occur during suction, as along with sputum, air is withdrawn from the lungs. There are several types of atelectasis according to their underlying mechanisms or the distribution of alveolar collapse; resorption, compression, microatelectasis and contraction atelectasis.

The rate of BPD varies among institutions, which may reflect neonatal risk factors, care practices (e.g., target levels for acceptable oxygen saturation), and differences in the clinical definitions of BPD.

Brown induration is fibrosis and hemosiderin pigmentation of the lungs due to long standing pulmonary congestion (chronic passive congestion).

The pathogenesis of PMF is complicated, but involves two main routes - an immunological route, and a mechanical route.

Immunologically, disease is caused primarily through the activity of lung macrophages, which phagocytose dust particles after their deposition. These macrophages seek to eliminate the dust particle through either the mucociliary mechanism, or through lymphatic vessels which drain the lungs. Macrophages also produce an inflammatory mediator known as interleukin-1 (IL-1), which is part of the immune systems first line defenses against infecting particles. IL-1 is responsible for 'activation' of local vasculature, causing endothelial cells to express certain cell adhesion molecules, which help the cells of the bodies immune system to migrate into tissues. Macrophages exposed to dust have been shown to have markedly decreased chemotaxis. Production of inflammatory mediators - and the tissue damage that ensues as an effect of this, as well as reduced motility of cells, is fundamental to the pathogenesis of pneumoconiosis and the accompanying inflammation, fibrosis, and emphysema.

There are also some mechanical factors involved in the pathogenesis of Complex Pneumoconiosis that should be considered. The most notable indications are the fact that the disease tends to develop in the upper lobe of the lung - especially on the right, and its common occurrence in taller individuals.