Update: Exertional hyponatremia, active component, U.S. Armed Forces, 2001-2016.

From 2001 through 2016, there were 1,519 incident diagnoses of exertional hyponatremia among active component service members (incidence rate: 6.9 cases per 100,000 person-years [p-yrs]). The incidence rate in 2016 (6.6 cases per 100,000 p-yrs) represented a decrease of 23.3% from 2015. Compared to their respective counterparts, overall incidence rates of exertional hyponatremia were higher among females, those aged 19 years or younger, and recruit trainees. The overall incidence rate during the surveillance period was highest in the Marine Corps, intermediate in the Army and Air Force, and lowest in the Navy. Overall incidence rates were lowest among black, non-Hispanic service members and highest among white, non-Hispanic and Asian/Pacific Islander service members. Service members (particularly recruit trainees) and their supervisors must be vigilant for early signs of heat-related illnesses and must be knowledgeable of the dangers of excessive water consumption and the prescribed limits for water intake during prolonged physical activity (e.g., field training exercises, personal fitness training, recreational activities) in hot, humid weather.