Acute and chronic dyspnea differ in how quickly they start and how long they last. They have different causes. Acute dyspnea Acute dyspnea can come on quickly and doesn’t last very long (hours to days). Allergies, anxiety, exercise and illness (like the common cold or the flu) can cause acute dyspnea. More serious conditions, like a heart attack, sudden airway narrowing (anaphylaxis) or blood clot (pulmonary embolism) can also cause acute dyspnea. Chronic dyspnea Chronic dyspnea is shortness of breath that lasts a long time (several weeks or longer) or keeps coming back. Ongoing health conditions like asthma, heart failure and COPD can cause chronic dyspnea. Not getting enough exercise can also make you feel breathless all the time because your muscles are trying to get more oxygen.