Acute respiratory distress syndrome (ARDS) develops rapidly and includes severe dyspnea and hypoxemia; it typically causes respiratory failure. Key diagnostic criteria for ARDS include (1) diffuse bilateral pulmonary infiltrates on chest x-ray (CXR); (2) Pa_O2 (arterial partial pressure of oxygen in mmHg) FI_O2 (inspired O₂ fraction) ≤200 mmHg; and (3) absence of elevated left atrial pressure (pulmonary capillary wedge pressure ≤18 mmHg). Acute lung injury is a related but milder syndrome, with less profound hypoxemia (Pa_O2/FI_O2 ≤300 mmHg), that can develop into ARDS. Although many medical and surgical conditions can cause ARDS, most cases (>80%) result from sepsis, bacterial pneumonia, trauma, multiple transfusions, gastric acid aspiration, and drug overdose. Individuals with more than one predisposing factor have a greater risk of developing ARDS. Other risk factors include older age, chronic alcohol abuse, metabolic acidosis, and overall severity of critical illness.