Exertional Hyponatremia occurs during or up to 24 hours after prolonged physical activity. It is defined by a serum, plasma or blood sodium concentration below 135 millequivalents per liter. This infographic provides an update on Exertional Hyponatremia among U.S. Armed Forces, information on service members at high risk. Exertional hyponatremia can result from loss of sodium and/or potassium as well as relative excess of body water.