A spokesman from Great Western Ambulance Service said one man had suffered first degree burns and another third degree burns.
A police spokesman said officers were called to a report that a lorry carrying gas cylinders had exploded.
Both men, believed to be in their 50s, were taken to Frenchay Hospital.
An Avon Fire and Rescue Service spokesman said they had suffered burns to their upper bodies and faces.
He added that their injuries were serious but were not thought to be life-threatening.
Gas testingThe incident happened at about 1650 BST. The A370 was closed and has since reopened.
Neville Levy, operations manager for the ambulance service, said the men had been carrying out testing on LPG (liquefied petroleum gas) tanks when the explosion happened.
The fire service spokesman said firefighters had found a small fire in the lorry trailer on arrival.
Stancombe Quarry, run by Tarmac, produces limestone which is used for concrete, building blocks and asphalt.
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