Before these units were founded, paramedics had to wait for other services - such as the fire brigade - to bring patients to them from the heart of an incident. But, by working at the centre of an event, the HART teams can save more lives during the early stages of an emergency.
The team are trained to cope with a number of hazardous environments and situations. They can enter burning buildings, deal with the victims of terrorist attacks, brave toxic gas clouds and access patients using ropes.
The team's advanced medical capabilities allow them to perform surgical procedures in a range of locations from the street to the cramped confines of a collapsed building or down a Pennine pothole, keeping patients alive long enough to reach specialised medical care.
Our 10-part series will also capture the banter in the mess hall and the blood, sweat and tears of a training regime that demands high levels of physical fitness and skills ranging from nuclear decontamination to using high-speed flood relief boats.
Using drone cameras at real emergency scenes thanks to a ground-breaking agreement with ambulance and fire service officers, 999 Rescue Squad captures dramatic footage of major fires, river rescues and road accidents.