An employee was taking gas readings on a natural gas line during pigging operations. Natural gas escaped around the pigging device as it traveled down the pipeline. The pig was being propelled by the flammable gas to the end point where the work-site was located. When the pig reached the end point, back pressure propelled the pig out of the line causing a spark that resulted in a flash fire produced by the natural gas that had escaped from the line. The employee was hospitalized for second and third degree burns to the hands, face and neck.