Fluid in the air sacs (alveoli) of the lung. The fluid moves from the small blood vessels in the lung (capillaries) into the air sacs when there is damage to the lung, or if there is a backup of pressure from the heart. This fluid (edema) in the air sacs prevents normal oxygen exchange and if left untreated can be lethal. Common causes include: heart failure, heart attack, heart valve dysfunction, pneumonia, toxins, smoke inhalation (from a fire), kidney failure, high altitude exposure, or adverse drug reactions.