<PERSON>, Davis-Monthan Air Force Base Health and Wellness Center program coordinator, instructs U.S. Air Force Senior Airman <PERSON>, 355th Aircraft Maintenance Squadron crew chief, how to wear a VO2 max facemask at Davis-Monthan AFB, Ariz., Sept. 2, 2015. The mask covers both the mouth and nose while it measures the body's oxygen intake though a tube connected to both the facemask and computer. (U.S. Air Force photo by <PERSON> <PERSON>/Released)