There are multiple reasons a person's leg swells below the knee or in any area, notes Healthgrades. Mild swelling is very commonly caused by standing for long periods, while leg swelling that is chronic is often a sign of a serious health condition, such as congestive heart failure.
A person with edema, or leg swelling, must see a doctor to determine the cause, as there are many possibilities, according to Healthgrades. Edema may be caused by serious infections, arthritis or hypothyroidism. Liver failure, obesity and an infection in the heart may also cause edema. Symptoms such as dizziness, difficulty breathing or chest pain that accompany edema require immediate medical attention, and 911 must be called. Confusion and shortness of breath also signal an emergency situation.