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Two burning teenagers fled a house fire after the gas bottles from which they were allegedly "huffing" exploded in Mosgiel yesterday afternoon.
Kate Nicol said she was working at Enliven Taieri when she heard an explosion and rushed outside to see the brick Gordon Rd property on fire and saw "a man on fire screaming", followed by a second explosion, just after 2pm.
"[He was] on the ground still burning and smouldering, and there was another one from the house yelling 'help me, help me.
"I need some warm water'."
Mrs Nicol said she helped the shocked man - a "tall skinny teenager" - to the footpath, as the Outram Volunteer Fire Brigade and an off-duty Christchurch firefighter arrived at the scene.
A 17-year-old and a 18-year-old remained in a critical condition in the intensive care unit at Dunedin Hospital last night.
Fire Service East Otago assistant area commander Trevor Tilyard said the pair had been "huffing" - getting high by inhaling a propellant - from two 9kg gas bottles before the explosion.
The force of the explosions blew off much of the tiled roof, displaced walls and shattered windows, and left glass strewn on the other side of the street, while a car parked outside the property had its rear window smashed.
Last night a fire investigator and Dunedin police remained at the cordoned-off scene to determine what triggered the explosion.
Mr Tilyard said given it was only 5degC about the time of the explosion, a heater or hot-water cylinder thermostat could have triggered the blast.
"If they were huffing out of 9kg cylinders, then they would not be able to stop it [the fire] spreading."
It was the first time he could recall an explosion as a result of huffing, and he had a simple message for those wanting to try the practice: "Don't".
"It is just so damn dangerous."
One of the teenagers suffered burns to his internal airways and lungs as a result of inhaling at the time of the explosion, with lpg burning at an estimated 900degC.
"If he ever fully recovers, he is going to spend months and months, if not years, to recover, and the other young man is in for a long rehabilitation as well."
Mr Tilyard praised the Outram volunteer firefighters, who were covering for Mosgiel at the time of the explosion, and said it was lucky they were so close.
Dr Leo Schep, from the NZ National Poisons Centre, said he had given two recent speeches about the dangers of huffing, and "was distressed" about the incident.
At the level people got a narcotic effect from huffing, "you are getting close to the lower flammability limit, that point where you get sufficient concentration to get an explosion".
Two South Island teenagers have died from huffing-related incidents in the past six years.