Acute respiratory distress syndrome (ARDS) is triggered when a severe illness or injury causes widespread inflammation in the lungs, flooding the air sacs with fluid and making it difficult to get oxygen into the bloodstream. Sepsis and pneumonia are the two most common causes, but ARDS can develop from a wide range of triggers, from aspiration to major burns to blood transfusion reactions. In the United States alone, an estimated 64 to 79 cases occur per 100,000 people each year, and mortality ranges from 27% in mild cases to 45% in severe ones.
The Most Common Causes
Sepsis, the body’s life-threatening response to infection, is the single most common trigger. It accounts for roughly 31 to 32% of all ARDS cases and is especially dangerous because the inflammation originates throughout the body, not just in the lungs. When the immune system spirals out of control fighting an infection in the blood, abdomen, or urinary tract, the lungs often become collateral damage.
Pneumonia is the leading lung-specific cause. Bacterial, viral, and fungal pneumonias can all directly injure lung tissue enough to trigger ARDS. COVID-19 brought widespread attention to this pathway, but influenza, bacterial pneumonia, and other respiratory infections have long been major contributors. One large study found that pulmonary infections accounted for 46% of cases when sepsis originating in the lungs was included.
Aspiration, where stomach contents, food, or liquid enters the airways, is the third most frequent trigger, responsible for around 11% of cases. This type of injury is especially common in people who are unconscious, heavily sedated, or have difficulty swallowing.
Direct vs. Indirect Lung Injury
Doctors classify ARDS causes into two broad categories based on where the initial damage occurs. Direct lung injuries harm the lung tissue itself. Indirect injuries start somewhere else in the body and reach the lungs through the bloodstream.
Direct causes include:
- Pneumonia (bacterial, viral, or fungal)
- Aspiration of stomach contents or liquids
- Inhaling toxic fumes, smoke, or chemicals
- Lung contusion from chest trauma
- Near-drowning
- Burn injury to the airways
- Ventilator-induced injury from excessive pressure or volume during mechanical breathing support
Indirect causes include:
- Sepsis from infections outside the lungs
- Major trauma to other parts of the body
- Blood transfusion reactions (known as TRALI)
- Pancreatitis
- Drug overdose
- Fat emboli, which can occur after bone fractures
- Cardiopulmonary bypass during heart surgery
This distinction matters because direct and indirect injuries can behave differently in the lungs, potentially affecting how well someone responds to treatment. But the end result is the same: the lungs fill with fluid and can no longer move oxygen efficiently into the blood.
What Happens Inside the Lungs
To understand why ARDS is so dangerous, it helps to know what’s happening at the microscopic level. Your lungs contain millions of tiny air sacs called alveoli, each surrounded by a thin membrane and a web of blood vessels. Oxygen passes through this membrane into the blood, and carbon dioxide passes back out. Normally, the barrier is extremely tight, keeping fluid on one side and air on the other.
When an injury or infection activates the immune system, specialized immune cells in the lungs (alveolar macrophages) begin releasing chemical signals that recruit large numbers of white blood cells called neutrophils. These neutrophils swarm into the lung tissue and release a wave of inflammatory molecules. While this response is meant to fight infection, it also damages the delicate membrane between the air sacs and blood vessels, breaking down the tight seals between cells.
Once that barrier is breached, protein-rich fluid floods into the air sacs. This inflammatory fluid is thick, not the watery buildup you might see with heart failure. It fills the spaces where gas exchange should happen, causing air sacs to collapse. The lungs become stiff, heavy, and increasingly unable to deliver oxygen. This “exudative phase” is what makes ARDS patients so critically ill and is the primary driver of how severe the condition becomes.
Burns and Trauma
Major burns are a particularly complex trigger because they can cause ARDS through multiple pathways at once. If someone inhales smoke or hot fumes, that directly damages airway and lung tissue, ramping up local inflammation and often requiring mechanical ventilation. But even without smoke inhalation, extensive skin burns cause a systemic inflammatory response that increases blood vessel permeability throughout the body, not just at the burn site. Fluid leaks into the lungs from the inside, and any secondary infections (common in burn patients) add yet another layer of risk.
Non-thoracic trauma, such as major fractures, abdominal injuries, or significant blood loss, triggers ARDS through the indirect pathway. The body’s massive inflammatory response to severe tissue damage sends inflammatory signals through the bloodstream that ultimately compromise the lung’s barrier. Polytrauma accounts for roughly 7% of ARDS cases. Blood transfusions given during trauma resuscitation carry their own risk: transfusion-related acute lung injury (TRALI) is a recognized and sometimes fatal complication where antibodies or other biological signals in donated blood trigger sudden lung inflammation.
Chemical and Inhalation Triggers
Breathing in toxic substances can cause acute lung injury that progresses to ARDS. Occupational exposures to metals, solvents, acids, chlorine, ozone, and phosgene are well-documented triggers. Community-level events like factory explosions, chemical spills, or train derailments carrying hazardous materials have caused clusters of cases. Even high exposure to common household cleaning agents can injure lung tissue.
Vaping-related lung injury drew significant attention in recent years. E-cigarette fluids contain at least seven groups of potentially toxic compounds, including carbonyls, volatile organic compounds like benzene and toluene, trace metals, and bacterial endotoxins. Some flavoring chemicals have been shown to disrupt normal function in the cells lining the airways. Depending on the type and amount of material inhaled, the resulting damage can range from mild airway irritation to full-blown respiratory failure with alveolar inflammation and fluid buildup, following the same pathway as other forms of ARDS.
Risk Factors That Increase Vulnerability
Not everyone exposed to an ARDS trigger develops the syndrome. Certain pre-existing factors make the lungs more vulnerable to tipping into full-blown respiratory failure once an initial insult occurs.
Chronic alcohol use is one of the strongest risk factors identified. In one study of 351 patients already at risk for ARDS, 43% of those who chronically abused alcohol went on to develop the condition, compared to just 22% of those who did not. Another analysis found the gap was even wider: 70% versus 31%. The effect is most pronounced in patients with sepsis. Alcohol also increases the risk of transfusion-related lung injury. Chronic alcohol users who received blood products developed TRALI at roughly double the rate of non-drinkers (36.5% vs. 17.6% in one matched comparison).
Active smoking is an independent risk factor as well. One large study found that smoking more than 20 cigarettes per day carried an even greater risk than lighter smoking, suggesting a dose-dependent relationship. Passive smoke exposure also increased the odds, even after adjusting for alcohol use. In lung transplant patients, donor smoking history predicted higher rates of a form of acute lung injury that develops within 72 hours of transplant.
How Severity Is Classified
ARDS is formally diagnosed using a set of criteria established in 2012 known as the Berlin definition. A diagnosis requires that respiratory failure develop within one week of a known trigger, that chest imaging show fluid in both lungs not explained by other causes, and that the breathing difficulty not be fully explained by heart failure. Doctors may use echocardiography to rule out a cardiac cause.
Severity is graded based on how well oxygen is getting into the blood relative to how much supplemental oxygen is being delivered. Mild ARDS carries a mortality rate of about 27%, moderate ARDS about 32%, and severe ARDS about 45%. These numbers reflect just how dangerous the condition is across the spectrum, and why identifying and treating the underlying cause quickly is critical to improving the odds of survival.