It is most commonly caused by:

- Oesophageal rupture, for example in Boerhaave syndrome

- Asthma or other conditions leading to alveolar rupture

- Bowel rupture, where air in the abdominal cavity tracts up into the chest.

It has also been associated with:

- "Mycoplasma pneumoniae" pneumonia

- obesity

It can be induced to assist thoracoscopic surgery. It can be caused by a pulmonary barotrauma resulting when a person moves to or from a higher pressure environment, such as when a SCUBA diver, a free-diver or an airplane passenger ascends or descends.

In rare cases, pneumomediastinum may also arise as a result of blunt chest trauma (e.g. car accidents, fights, over pressure of breathing apparatus), while still evolving in the same fashion as the spontaneous form.

Pneumomediastinum is most commonly seen in otherwise healthy young male patients and may not be prefaced by a relevant medical history of similar ailments.

The annual age-adjusted incidence rate (AAIR) of PSP is thought to be three to six times as high in males as in females. Fishman cites AAIR's of 7.4 and 1.2 cases per 100,000 person-years in males and females, respectively. Significantly above-average height is also associated with increased risk of PSP – in people who are at least 76 inches (1.93 meters) tall, the AAIR is about 200 cases per 100,000 person-years. Slim build also seems to increase the risk of PSP.

The risk of contracting a first spontaneous pneumothorax is elevated among male and female smokers by factors of approximately 22 and 9, respectively, compared to matched non-smokers of the same sex. Individuals who smoke at higher intensity are at higher risk, with a "greater-than-linear" effect; men who smoke 10 cigarettes per day have an approximate 20-fold increased risk over comparable non-smokers, while smokers consuming 20 cigarettes per day show an estimated 100-fold increase in risk.

In secondary spontaneous pneumothorax, the estimated annual AAIR is 6.3 and 2.0 cases per 100,000 person-years for males and females, respectively, with the risk of recurrence depending on the presence and severity of any underlying lung disease. Once a second episode has occurred, there is a high likelihood of subsequent further episodes. The incidence in children has not been well studied, but is estimated to be between 5 and 10 cases per 100,000 person-years.

Death from pneumothorax is very uncommon (except in tension pneumothoraces). British statistics show an annual mortality rate of 1.26 and 0.62 deaths per million person-years in men and women, respectively. A significantly increased risk of death is seen in older victims and in those with secondary pneumothoraces.

Barrel chest generally refers to a , deep chest found on a man. A man described as barrel chested will usually have a naturally large ribcage, very round torso, large lung capacity, and can potentially have great upper body strength. It can sometimes be a sign of acromegaly (a syndrome resulting from excess levels of human growth hormone (HGH) in the body). It is most commonly related to osteoarthritis as individuals age. Arthritis can stiffen the chest causing the ribs to become fixed in their most expanded position, giving the appearance of a barrel chest.

Barrel chest also refers to an increase in the anterior posterior diameter of the chest wall resembling the shape of a barrel, most often associated with emphysema. There are two main causes of the barrel chest phenomenon in emphysema:

1. Increased compliance of the lungs leads to the accumulation of air pockets inside the thoracic cavity.

2. Increased compliance of the lungs increases the intrathoracic pressure. This increase in pressure allows the chest wall to naturally expand outward.

Barrel chest occurs naturally in native people who live at altitudes of over 5500 m, e.g. the Himalayas or the Andes. These natives also have polycythemia and other accommodations for high altitude life.

Secondary spontaneous pneumothorax occurs in the setting of a variety of lung diseases. The most common is chronic obstructive pulmonary disease (COPD), which accounts for approximately 70% of cases. Known lung diseases that may significantly increase the risk for pneumothorax are

In children, additional causes include measles, echinococcosis, inhalation of a foreign body, and certain congenital malformations (congenital cystic adenomatoid malformation and congenital lobar emphysema).

11.5% of people with a spontaneous pneumothorax have a family member who has previously experienced a pneumothorax. The hereditary conditions – Marfan syndrome, homocystinuria, Ehlers–Danlos syndrome, alpha 1-antitrypsin deficiency (which leads to emphysema), and Birt–Hogg–Dubé syndrome—have all been linked to familial pneumothorax. Generally, these conditions cause other signs and symptoms as well, and pneumothorax is not usually the primary finding. Birt–Hogg–Dubé syndrome is caused by mutations in the "FLCN" gene (located at chromosome 17p11.2), which encodes a protein named folliculin. "FLCN" mutations and lung lesions have also been identified in familial cases of pneumothorax where other features of Birt–Hogg–Dubé syndrome are absent. In addition to the genetic associations, the HLA haplotype AB is also a genetic predisposition to PSP.

The death rate of people with flail chest depends on the severity of their condition, ranging from 10 to 25%.

The primary risk factor for COPD globally is tobacco smoking. Of those who smoke, about 20% will get COPD, and of those who are lifelong smokers, about half will get COPD. In the United States and United Kingdom, of those with COPD, 80–95% are either current smokers or previously smoked. The likelihood of developing COPD increases with the total smoke exposure. Additionally, women are more susceptible to the harmful effects of smoke than men. In nonsmokers, secondhand smoke is the cause of about 20% of cases. Other types of smoke, such as, marijuana, cigar, and water-pipe smoke, also confer a risk. Water-pipe smoke appears to be as harmful as smoking cigarettes. Problems from marijuana smoke may only be with heavy use. Women who smoke during pregnancy may increase the risk of COPD in their child. For the same amount of cigarette smoking, women have a higher risk of COPD than men.

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.

Approximately 1 out of 13 people admitted to the hospital with fractured ribs are found to have flail chest.

Shortness of breath is the primary reason 3.5% of people present to the emergency department in the United States. Of these individuals, approximately 51% are admitted to the hospital and 13% are dead within a year. Some studies have suggested that up to 27% of people suffer from dyspnea, while in dying patients 75% will experience it. Acute shortness of breath is the most common reason people requiring palliative care visit an emergency department.

When a pleural effusion has been determined to be exudative, additional evaluation is needed to determine its cause, and amylase, glucose, pH and cell counts should be measured.

- Red blood cell counts are elevated in cases of bloody effusions (for example after heart surgery or hemothorax from incomplete evacuation of blood).

- Amylase levels are elevated in cases of esophageal rupture, pancreatic pleural effusion, or cancer.

- Glucose is decreased with cancer, bacterial infections, or rheumatoid pleuritis.

- pH is low in empyema (<7.2) and may be low in cancer.

- If cancer is suspected, the pleural fluid is sent for cytology. If cytology is negative, and cancer is still suspected, either a thoracoscopy, or needle biopsy of the pleura may be performed.

- Gram staining and culture should also be done.

- If tuberculosis is possible, examination for "Mycobacterium tuberculosis" (either a Ziehl–Neelsen or Kinyoun stain, and mycobacterial cultures) should be done. A polymerase chain reaction for tuberculous DNA may be done, or adenosine deaminase or interferon gamma levels may also be checked.

The most common causes of exudative pleural effusions are bacterial pneumonia, cancer (with lung cancer, breast cancer, and lymphoma causing approximately 75% of all malignant pleural effusions), viral infection, and pulmonary embolism.

Another common cause is after heart surgery, when incompletely drained blood can lead to an inflammatory response that causes exudative pleural fluid.

Conditions associated with exudative pleural effusions:

- Parapneumonic effusion due to pneumonia

- Malignancy (either lung cancer or metastases to the pleura from elsewhere)

- Infection (empyema due to bacterial pneumonia)

- Trauma

- Pulmonary infarction

- Pulmonary embolism

- Autoimmune disorders

- Pancreatitis

- Ruptured esophagus (Boerhaave's syndrome)

- Rheumatoid pleurisy

- Drug-induced lupus

Pulmonary contusion is found in 30–75% of severe cases of chest injury, making it the most common serious injury to occur in association with thoracic trauma. Of people who have multiple injuries with an injury severity score of over 15, pulmonary contusion occurs in about 17%. It is difficult to determine the death rate (mortality) because pulmonary contusion rarely occurs by itself. Usually, deaths of people with pulmonary contusion result from other injuries, commonly traumatic brain injury. It is controversial whether pulmonary contusion with flail chest is a major factor in mortality on its own or whether it merely contributes to mortality in people with multiple injuries. The estimated mortality rate of pulmonary contusion ranges from 14–40%, depending on the severity of the contusion itself and on associated injuries. When the contusions are small, they do not normally increase the chance of death or poor outcome for people with blunt chest trauma; however, these chances increase with the size of the contusion. One study found that 35% of people with multiple significant injuries including pulmonary contusion die. In another study, 11% of people with pulmonary contusion alone died, while the number rose to 22% in those with additional injuries. Pulmonary contusion is thought to be the direct cause of death in a quarter to a half of people with multiple injuries (polytrauma) who die. An accompanying flail chest increases the morbidity and mortality to more than twice that of pulmonary contusion alone.

Pulmonary contusion is the most common cause of death among vehicle occupants involved in accidents, and it is thought to contribute significantly in about a quarter of deaths resulting from vehicle collisions. As vehicle use has increased, so has the number of auto accidents, and with it the number of chest injuries. However an increase in the number of airbags installed in modern cars may be decreasing the incidence of pulmonary contusion. Use of child restraint systems has brought the approximate incidence of pulmonary contusion in children in vehicle accidents from 22% to 10%.

Differences in the bodies of children and adults lead to different manifestations of pulmonary contusion and associated injuries; for example, children have less body mass, so the same force is more likely to lead to trauma in multiple body systems. Since their chest walls are more flexible, children are more vulnerable to pulmonary contusion than adults are, and thus suffer from the injury more commonly. Pulmonary contusion has been found in 53% of children with chest injuries requiring hospitalization. Children in forceful impacts suffer twice as many pulmonary contusions as adults with similar injury mechanisms, yet have proportionately fewer rib fractures. The rates of certain types of injury mechanisms differ between children and adults; for example, children are more often hit by cars as pedestrians. Some differences in children's physiology might be advantageous (for example they are less likely to have other medical conditions), and thus they have been predicted to have a better outcome. However, despite these differences, children with pulmonary contusion have similar mortality rates to adults.

Pneumomediastinum (from Greek "pneuma" – "air", also known as mediastinal emphysema) is (abnormal presence of air or other gas) in the mediastinum. First described in 1819 by René Laennec, the condition can result from physical trauma or other situations that lead to air escaping from the lungs, airways, or bowel into the chest cavity.

Air in subcutaneous tissue does not usually pose a lethal threat; small amounts of air are reabsorbed by the body. Once the pneumothorax or pneumomediastinum that causes the subcutaneous emphysema is resolved, with or without medical intervention, the subcutaneous emphysema will usually clear. However, spontaneous subcutaneous emphysema can, in rare cases, progress to a life-threatening condition, and subcutaneous emphysema due to mechanical ventilation may induce ventilatory failure.

Pulmonary contusion can result in respiratory failure—about half of such cases occur within a few hours of the initial trauma. Other severe complications, including infections and acute respiratory distress syndrome (ARDS) occur in up to half of cases. Elderly people and those who have heart, lung, or kidney disease prior to the injury are more likely to stay longer in hospital and have complications from the injury. Complications occur in 55% of people with heart or lung disease and 13% of those without. Of people with pulmonary contusion alone, 17% develop ARDS, while 78% of people with at least two additional injuries develop the condition. A larger contusion is associated with an increased risk. In one study, 82% of people with 20% or more of the lung volume affected developed ARDS, while only 22% of people with less than 20% did so.

Pneumonia, another potential complication, develops in as many as 20% of people with pulmonary contusion. Contused lungs are less able to remove bacteria than uninjured lungs, predisposing them to infection. Intubation and mechanical ventilation further increase the risk of developing pneumonia; the tube is passed through the nose or mouth into the airways, potentially tracking bacteria from the mouth or sinuses into them. Also, intubation prevents coughing, which would clear bacteria-laden secretions from the airways, and secretions pool near the tube's cuff and allow bacteria to grow. The sooner the endotracheal tube is removed, the lower the risk of pneumonia, but if it is removed too early and has to be put back in, the risk of pneumonia rises. People who are at risk for pulmonary aspiration (e.g. those with lowered level of consciousness due to head injuries) are especially likely to get pneumonia. As with ARDS, the chances of developing pneumonia increase with the size of the contusion. Children and adults have been found to have similar rates of complication with pneumonia and ARDS.

Subcutaneous emphysema is a common result of certain types of surgery; for example it is not unusual in chest surgery. It may also occur from surgery around the esophagus, and is particularly likely in prolonged surgery. Other potential causes are positive pressure ventilation for any reason and by any technique, in which its occurrence is frequently unexpected. It may also occur as a result of oral surgery, laparoscopy, and cricothyrotomy. In a pneumonectomy, in which an entire lung is removed, the remaining bronchial stump may leak air, a rare but very serious condition that leads to progressive subcutaneous emphysema. Air can leak out of the pleural space through an incision made for a thoracotomy to cause subcutaneous emphysema. On infrequent occasions, the condition can result from dental surgery, usually due to use of high-speed tools that are air driven. These cases result in usually painless swelling of the face and neck, with an immediate onset, the crepitus (crunching sound) typical of subcutaneous emphysema, and often with subcutaneous air visible on X-ray.

One of the main causes of subcutaneous emphysema, along with pneumothorax, is an improperly functioning chest tube. Thus subcutaneous emphysema is often a sign that something is wrong with a chest tube; it may be clogged, clamped, or out of place. The tube may need to be replaced, or, when large amounts of air are leaking, a new tube may be added.

Since mechanical ventilation can worsen a pneumothorax, it can force air into the tissues; when subcutaneous emphysema occurs in a ventilated patient, it is an indication that the ventilation may have caused a pneumothorax. It is not unusual for subcutaneous emphysema to result from positive pressure ventilation. Another possible cause is a ruptured trachea. The trachea may be injured by tracheostomy or tracheal intubation; in cases of tracheal injury, large amounts of air can enter the subcutaneous space. An endotracheal tube can puncture the trachea or bronchi and cause subcutaneous emphysema.

The most common causes of transudative pleural effusions in the United States are heart failure and cirrhosis. Nephrotic syndrome, leading to the loss of large amounts of albumin in urine and resultant low albumin levels in the blood and reduced colloid osmotic pressure, is another less common cause of pleural effusion. Pulmonary emboli were once thought to cause transudative effusions, but have been recently shown to be exudative.

The mechanism for the exudative pleural effusion in pulmonary thromboembolism is probably related to increased permeability of the capillaries in the lung, which results from the release of cytokines or inflammatory mediators (e.g. vascular endothelial growth factor) from the platelet-rich blood clots. The excessive interstitial lung fluid traverses the visceral pleura and accumulates in the pleural space.

Conditions associated with transudative pleural effusions include:

- Congestive heart failure

- Liver cirrhosis

- Severe hypoalbuminemia

- Nephrotic syndrome

- Acute atelectasis

- Myxedema

- Peritoneal dialysis

- Meigs' syndrome

- Obstructive uropathy

- End-stage kidney disease

Poorly ventilated cooking fires, often fueled by coal or biomass fuels such as wood and dung, lead to indoor air pollution and are one of the most common causes of COPD in developing countries. These fires are a method of cooking and heating for nearly 3 billion people, with their health effects being greater among women due to more exposure. They are used as the main source of energy in 80% of homes in India, China and sub-Saharan Africa.

People who live in large cities have a higher rate of COPD compared to people who live in rural areas. While urban air pollution is a contributing factor in exacerbations, its overall role as a cause of COPD is unclear. Areas with poor outdoor air quality, including that from exhaust gas, generally have higher rates of COPD. The overall effect in relation to smoking, however, is believed to be small.

Its cause is usually traumatic, from a blunt or penetrating injury to the thorax, resulting in a rupture of the serous membrane either lining the thorax or covering the lungs. This rupture allows blood to spill into the pleural space, equalizing the pressures between it and the lungs. Blood loss may be massive in people with these conditions, as each side of the thorax can hold 30 to 40% of a person's blood volume or 1.5 to 2 L per side in the average adult. Even minor injury to the chest wall can lead to significant hemothorax.

Less frequently, hemothorax occurs spontaneously. A major vascular cause of hemothorax is aortic dissection or rupture of thoracic aortic aneurysms. It may also follow surgical intervention in the thoracic area. Infrequently, patients with pneumothorax may develop spontaneous hemothorax. Spontaneous hemothorax or hemopneumothorax may be occur with endometriosis, if endometrial tissue implants on the pleural surface, then bleeds in response to cyclical hormonal changes in menstruating women.

If left untreated, the condition can progress to a point where the blood accumulation begins to put pressure on the mediastinum and the trachea, effectively limiting the amount that the heart's ventricles are able to fill. The condition can cause the trachea to deviate, or move, toward the unaffected side.

Atelectasis is the collapse or closure of a lung resulting in reduced or absent gas exchange. It may affect part or all of a lung. It is usually unilateral. It is a condition where the alveoli are deflated down to little or no volume, as distinct from pulmonary consolidation, in which they are filled with liquid. It is often called a collapsed lung, although that term may also refer to pneumothorax.

It is a very common finding in chest x-rays and other radiological studies, and may be caused by normal exhalation or by various medical conditions. Although frequently described as a collapse of lung tissue, atelectasis is not synonymous with a pneumothorax, which is a more specific condition that features atelectasis. Acute atelectasis may occur as a post-operative complication or as a result of surfactant deficiency. In premature neonates, this leads to infant respiratory distress syndrome.

The term uses combining forms of "atel-" + "", from , "incomplete" + ἔκτασις, "extension".

Asbestos can cause lung cancer that is identical to lung cancer from other causes. Exposure to asbestos is associated with all major histological types of lung carcinoma (adenocarcinoma, squamous cell carcinoma, large-cell carcinoma and small-cell carcinoma). The latency period between exposure and development of lung cancer is 20 to 30 years. It is estimated that 3%-8% of all lung cancers are related to asbestos. The risk of developing lung cancer depends on the level, duration, and frequency of asbestos exposure (cumulative exposure). Smoking and individual susceptibility are other contributing factors towards lung cancer. Smokers who have been exposed to asbestos are at far greater risk of lung cancer. Smoking and asbestos exposure have a multiplicative (synergistic) effect on the risk of lung cancer. Symptoms include chronic cough, chest pain, breathlessness, haemoptysis (coughing up blood), wheezing or hoarseness of the voice, weight loss and fatigue. Treatment involves surgical removal of the cancer, chemotherapy, radiotherapy, or a combination of these (multimodality treatment). Prognosis is generally poor unless the cancer is detected in its early stages. Out of all patients diagnosed with lung cancer, only 15% survive for five years after diagnosis.

Treatment for this condition is the same as for hemothorax and pneumothorax independently: by tube thoracostomy, the insertion of a chest drain through an incision made between the ribs, into the intercostal space. A chest tube must be inserted to drain blood and air from the pleural space so it can return to a state of negative pressure and function normally.

Commonly, surgery is needed to close off whatever injuries caused the blood and air to enter the cavity (e.g. stabbing, broken ribs).

Pneumothorax presents typically with pleuritic chest pain of acute onset and shortness of breath not improved with oxygen. Physical findings may include absent breath sounds on one side of the chest, jugular venous distension, and tracheal deviation.

Hemopneumothorax, or haemopneumothorax, is a medical term describing the combination of two conditions: pneumothorax, or air in the chest cavity, and hemothorax (also called hæmothorax), or blood in the chest cavity.

A hemothorax, pneumothorax or both can occur if the chest wall is punctured. To understand the ramifications of this it is important to have an understanding of the role of the pleural space. The pleural space is located anatomically between the visceral membrane, which is firmly attached to the lungs, and the parietal membrane which is firmly attached to the chest wall (a.k.a. ribcage and intercostal muscles, muscles between the ribs). The pleural space contains pleural fluid. This fluid holds the two membranes together by surface tension, as much as a drop of water between two sheets of glass prevents them from separating. Because of this, when the intercostal muscles move the ribcage outward, the lungs are pulled out as well, dropping the pressure in the lungs and pulling air into the bronchi, when we 'breathe in'. The pleural space is maintained in a constant state of negative pressure (in comparison to atmospheric pressure).

Rupture of the trachea or bronchus is the most common type of blunt injury to the airway. It is difficult to determine the incidence of TBI: in as many as 30–80% of cases, death occurs before the person reaches a hospital, and these people may not be included in studies. On the other hand, some TBI are so small that they do not cause significant symptoms and are therefore never noticed. In addition, the injury sometimes is not associated with symptoms until complications develop later, further hindering estimation of the true incidence. However, autopsy studies have revealed TBI in 2.5–3.2% of people who died after trauma. Of all neck and chest traumas, including people that died immediately, TBI is estimated to occur in 0.5–2%. An estimated 0.5% of polytrauma patients treated in trauma centers have TBI. The incidence is estimated at 2% in blunt chest and neck trauma and 1–2% in penetrating chest trauma. Laryngotracheal injuries occur in 8% of patients with penetrating injury to the neck, and TBI occurs in 2.8% of blunt chest trauma deaths. In people with blunt trauma who do reach a hospital alive, reports have found incidences of 2.1% and 5.3%. Another study of blunt chest trauma revealed an incidence of only 0.3%, but a mortality rate of 67% (possibly due in part to associated injuries). The incidence of iatrogenic TBI (that caused by medical procedures) is rising, and the risk may be higher for women and the elderly. TBI results about once every 20,000 times someone is intubated through the mouth, but when intubation is performed emergently, the incidence may be as high as 15%.

The mortality rate for people who reach a hospital alive was estimated at 30% in 1966; more recent estimates place this number at 9%. The number of people reaching a hospital alive has increased, perhaps due to improved prehospital care or specialized treatment centers. Of those who reach the hospital alive but then die, most do so within the first two hours of arrival. The sooner a TBI is diagnosed, the higher the mortality rate; this is likely due to other accompanying injuries that prove fatal.

Accompanying injuries often play a key role in the outcome. Injuries that may accompany TBI include pulmonary contusion and laceration; and fractures of the sternum, ribs and clavicles. Spinal cord injury, facial trauma, traumatic aortic rupture, injuries to the abdomen, lung, and head are present in 40–100%. The most common accompanying injury is esophageal perforation or rupture (known as Boerhaave syndrome), which occurs in as many as 43% of the penetrating injuries to the neck that cause tracheal injury.