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.

Spontaneous pneumothoraces are divided into two types: "primary", which occurs in the absence of known lung disease, and "secondary", which occurs in someone with underlying lung disease. The cause of primary spontaneous pneumothorax is unknown, but established risk factors include male sex, smoking, and a family history of pneumothorax. Smoking either cannabis or tobacco increases the risk. The various suspected underlying mechanisms are discussed below.

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.

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.

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.

Conditions that cause subcutaneous emphysema may result from both blunt and penetrating trauma; SCE is often the result of a stabbing or gunshot wound.

Subcutaneous emphysema is often found in car accident victims because of the force of the crash.

Chest trauma, a major cause of subcutaneous emphysema, can cause air to enter the skin of the chest wall from the neck or lung. When the pleural membranes are punctured, as occurs in penetrating trauma of the chest, air may travel from the lung to the muscles and subcutaneous tissue of the chest wall. When the alveoli of the lung are ruptured, as occurs in pulmonary laceration, air may travel beneath the visceral pleura (the membrane lining the lung), to the hilum of the lung, up to the trachea, to the neck and then to the chest wall. The condition may also occur when a fractured rib punctures a lung; in fact, 27% of patients who have rib fractures also have subcutaneous emphysema. Rib fractures may tear the parietal pleura, the membrane lining the inside of chest wall, allowing air to escape into the subcutaneous tissues.

Subcutaneous emphysema is frequently found in pneumothorax (air outside of the lung in the chest cavity) and may also result from air in the mediastinum, pneumopericardium (air in the pericardial cavity around the heart). A tension pneumothorax, in which air builds up in the pleural cavity and exerts pressure on the organs within the chest, makes it more likely that air will enter the subcutaneous tissues through pleura torn by a broken rib. When subcutaneous emphysema results from pneumothorax, air may enter tissues including those of the face, neck, chest, armpits, or abdomen.

Pneumomediastinum can result from a number of events. For example, foreign body aspiration, in which someone inhales an object, can cause pneumomediastinum (and lead to subcutaneous emphysema) by puncturing the airways or by increasing the pressure in the affected lung(s) enough to cause them to burst.

Subcutaneous emphysema of the chest wall is commonly among the first signs to appear that barotrauma, damage caused by excessive pressure, has occurred, and it is an indication that the lung was subjected to significant barotrauma. Thus the phenomenon may occur in diving injuries.

Trauma to parts of the respiratory system other than the lungs, such as rupture of a bronchial tube, may also cause subcutaneous emphysema. Air may travel upward to the neck from a pneumomediastinum that results from a bronchial rupture, or downward from a torn trachea or larynx into the soft tissues of the chest. It may also occur with fractures of the facial bones, neoplasms, during asthma attacks, when the Heimlich maneuver is used, and during childbirth.

Injury with pneumatic tools, those that are driven by air, is also known to cause subcutaneous emphysema, even in extremities (the arms and legs). It can also occur as a result of rupture of the esophagus; when it does, it is usually as a late sign.

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.

As with other chest injuries such as pulmonary contusion, hemothorax, and pneumothorax, pulmonary laceration can often be treated with just supplemental oxygen, ventilation, and drainage of fluids from the chest cavity. A thoracostomy tube can be used to remove blood and air from the chest cavity. About 5% of cases require surgery, called thoracotomy. Thoracotomy is especially likely to be needed if a lung fails to re-expand; if pneumothorax, bleeding, or coughing up blood persist; or in order to remove clotted blood from a hemothorax. Surgical treatment includes suturing, stapling, oversewing, and wedging out of the laceration. Occasionally, surgeons must perform a lobectomy, in which a lobe of the lung is removed, or a pneumonectomy, in which an entire lung is removed.

Complications are not common but include infection, pulmonary abscess, and bronchopleural fistula (a fistula between the pleural space and the bronchial tree). A bronchopleural fistula results when there is a communication between the laceration, a bronchiole, and the pleura; it can cause air to leak into the pleural space despite the placement of a chest tube. The laceration can also enlarge, as may occur when the injury creates a valve that allows air to enter the laceration, progressively expanding it. One complication, air embolism, in which air enters the bloodstream, is potentially fatal, especially when it occurs on the left side of the heart. Air can enter the circulatory system through a damaged vein in the injured chest and can travel to any organ; it is especially deadly in the heart or brain. Positive pressure ventilation can cause pulmonary embolism by forcing air out of injured lungs and into blood vessels.

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).

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).

Isolated mechanical forces may not adequately explain ventilator induced lung injury (VILI). The damage is affected by the interaction of these forces and the pre-existing state of the lung tissues, and dynamic changes in alveolar structure may be involved. Factors such as plateau pressure and positive end-expiratory pressure (PEEP) alone do not adequately predict injury. Cyclic deformation of lung tissue may play a large part in the cause of VILI, and contributory factors probably include tidal volume, positive end-expiratory pressure and respiratory rate. There is no protocol guaranteed to avoid all risk in all applications.

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.

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.

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

Professional divers are screened for risk factors during initial and periodical medical examination for fitness to dive. In most cases recreational divers are not medically screened, but are required to provide a medical statement before acceptance for training in which the most common and easy to identify risk factors must be declared. If these factors are declared, the diver may be required to be examined by a medical practitioner, and may be disqualified from diving if the conditions indicate.

Asthma, Marfan syndrome, and COPD pose a very high risk of pneumothorax. In some countries these may be considered absolute contraindications, while in others the severity may be taken into consideration. Asthmatics with a mild and well controlled condition may be permitted to dive under restricted circumstances.

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.

The atmosphere is composed of 78% nitrogen and 21% oxygen. Since oxygen is exchanged at the alveoli-capillary membrane, nitrogen is a major component for the alveoli's state of inflation. If a large volume of nitrogen in the lungs is replaced with oxygen, the oxygen may subsequently be absorbed into the blood, reducing the volume of the alveoli, resulting in a form of alveolar collapse known as absorption atelectasis.

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

In terms of the epidemiology of air embolisms one finds that the "intra-operative" period to have the highest incidence. For example, VAE in neurological cases ranges up to 80%, and OBGYN surgeries incidence can climb to 97% for VAE (vascular air embolism). In divers the incidence rate is 7/100,000 per dive.

Most people with TBI who die do so within minutes of the injury, due to complications such as pneumothorax and insufficient airway and to other injuries that occurred at the same time. Most late deaths that occur in TBI are attributed to sepsis or multiple organ dysfunction syndrome (MODS). If the condition is not recognized and treated early, serious complications are more likely to occur; for example, pneumonia and bronchiectasis may occur as late complications. Years can pass before the condition is recognized. Some TBI are so small that they do not have significant clinical manifestations; they may never be noticed or diagnosed and may heal without intervention.

If granulation tissue grows over the injured site, it can cause stenosis of the airway, after a week to a month. The granulation tissue must be surgically excised. Delayed diagnosis of a bronchial rupture increases risk of infection and lengthens hospital stay. People with a narrowed airway may suffer dyspnea, coughing, wheezing, respiratory tract infection, and difficulty with clearing secretions. If the bronchiole is completely obstructed, atelectasis occurs: the alveoli of the lung collapse. Lung tissue distal to a completely obstructed bronchiole often does not become infected. Because it is filled with mucus, this tissue remains functional. When the secretions are removed, the affected portion of the lung is commonly able to function almost normally. However, infection is common in lungs distal to a partially obstructed bronchiole. Infected lung tissue distal to a stricture can be damaged, and wheezing and coughing may develop due to the narrowing. In addition to pneumonia, the stenosis may cause bronchiectasis, in which bronchi are dilated, to develop. Even after an airway with a stricture is restored to normal, the resulting loss of lung function may be permanent.

Complications may also occur with treatment; for example a granuloma can form at the suture site. Also, the sutured wound can tear again, as occurs when there is excessive pressure in the airways from ventilation. However, for people who do receive surgery soon after the injury to repair the lesion, outcome is usually good; the long-term outcome is good for over 90% of people who have TBI surgically repaired early in treatment. Even when surgery is performed years after the injury, the outlook is good, with low rates of death and disability and good chances of preserving lung function.

Respiratory disease is a common and significant cause of illness and death around the world. In the US, approximately 1 billion "common colds" occur each year. A study found that in 2010, there were approximately 6.8 million emergency department visits for respiratory disorders in the U.S. for patients under the age of 18. In 2012, respiratory conditions were the most frequent reasons for hospital stays among children.

In the UK, approximately 1 in 7 individuals are affected by some form of chronic lung disease, most commonly chronic obstructive pulmonary disease, which includes asthma, chronic bronchitis and emphysema.

Respiratory diseases (including lung cancer) are responsible for over 10% of hospitalizations and over 16% of deaths in Canada.

In 2011, respiratory disease with ventilator support accounted for 93.3% of ICU utilization in the United States.

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

For individuals who survive the initial crush injury, survival rates are high for traumatic asphyxia.

Pulmonary diseases may also impact newborns, such as pulmonary hyperplasia, pulmonary interstitial emphysema (usually preterm births), and infant respiratory distress syndrome,