Dyspnea (shortness of breath) is a very common symptom. Although nearly everyone has had the experience of being short of breath when exercising hard, the occurrence of shortness of breath at an inappropriately low exercise level or at rest is abnormal, and is termed dyspnea. It is estimated that 25% of the population over 40 years old experiences of dyspnea. Half of cancer patients report dyspnea, and in many cases it is severe. It is a major symptom of heart attack and heart failure. One large study showed that half the patients in a major hospital experience dyspnea, equal to the number that experience pain. Up to 80% of terminally ill patients experience dyspnea, often severe dyspnea, and it is one of the hardest problems for their caregivers to deal with. EMTs (ambulance staff) tell us that dyspnea is one of the most common symptoms that causes people to summon their aid. In addition to being an uncomfortable experience, dyspnea also has benefit (like pain) in warning us of danger. For instance, asthmatics must attend to their dyspnea in order to get medical treatment when appropriate, and dyspnea is one of the most common symptoms of heart attack (in fact, women are more likely to feel short of breath than chest pain when having a heart attack).