An Orlando clinic is using a $3 million grant from the U.S. Department of Defense to develop virtual reality (VR) technology for treating post-traumatic stress disorder (PTSD) in veterans and emergency services workers.

The University of Central Florida's RESTORES clinic has been testing VR therapy, which uses the tech to expose veterans to the scenes and sounds of warzones, since 2010.

After the 2016 Pulse nightclub shooting in Orlando, RESTORES expanded its services to treat first response personnel and mass shooting survivors suffering with PTSD. Since the clinic opened, it has treated 450 veterans and first responders.

"This is cutting-edge technology for people in the fire service, police officers and so on. They suffer with PTSD as much as veterans, but there's no VR technology available for first responders right now," Deborah Beidel, founder and director of the clinic, told CNBC over the phone this month.

The PTSD treatment is a three-week program that combines VR with group therapy sessions. VR sessions expose patients to videos and scenes that trigger their trauma, such as warzones, which are coupled with the sounds and smells of gunfire and smoke. Exposure to these sensations is intended to reduce the stress associated with the patients' memories of them.

In October, the Department of Defense donated $3 million to the clinic to help it continue developing the treatment. RESTORES plans to invest the funds into new tech, with a view to roll the system out commercially by 2021. Developing software in-house will allow clinicians to create VR scenes for more non-military patients.