An employee was cutting a piece of duct work to size using a grinder with a cut-off wheel. The pipe was staged on the back of a truck parked approximately 5 feet from two manhole covers. As the employee was cutting, vapors from an underground tank beneath the manhole covers ignited, causing first degree flash burns to the employee's torso, chest, hands, arms, head, face, neck, and back and second degree burns to the chest and hands.