They didn't suffer a lot - it was an instantaneous death

Alberto Incoronato, University of Naples Federico II

And they warn that the study has modern-day lessons for the two million people living in the shadow of Vesuvius.

The researchers draw their conclusions from a study of 80 skeletons entombed in ash in boat chambers on the beach at the foot of Mount Vesuvius.

All were in natural, relaxed postures, and showed no signs of distress, said Alberto Incoronato, of the University of Naples Federico II, and colleagues.

"There was no time for displaying any defensive reaction," he told BBC News Online. "They didn't suffer a lot - it was an instantaneous death."

'Volcanic risk'

The study reveals that the deadly eruption sent a fiery cloud of ash and gases racing through the town and into the shelters.

But rather than suffocating the victims, the shock of the heatwave would have stopped their vital organs faster than they could react.

"Contrary to what is believed so far, they died of heat exposure and not suffocation," Professor Incoronato told BBC News Online.

"This has implications as far as the volcanic risk of Vesuvius is concerned," he added. "Even people sheltered from direct impact can die, as the study at Herculaneum shows."

The team is working to determine the risks for the two million people living in the area now, he said.

Ruins of the city of Pompeii

The ancient city of Herculaneum lay at the foot of Vesuvius on a cliff overlooking the sea.

When the mountain erupted, both it and Pompeii were buried by a succession of avalanches of volcanic ash.

Many of the inhabitants of both towns were killed by suffocation. But those in the boat chambers died "in less than a fraction of a second" from the blasting heatwave, the Italian researchers report in the journal Nature.

The patterns of tooth enamel cracks and bone colouration indicated that the victims were exposed to temperatures of about 500C.

"Their life-like stance reflects their posture at the time when the first surge emplaced," the scientists write.

"These individuals, who did not suffer mechanical impact, do not display any evidence of voluntary self-protective reaction or agony contortions, indicating that the activity of their vital organs must have stopped within a shorter time than the conscious reaction time, a state known as fulminant shock."