A method of removing a residual gas from inside a conventional shipping container is disclosed. The method first involves the step of accessing the container by opening a door of the container and fitting a panel to the open door space. Next the method involves extracting at least some of the residual gas present in the container via an outlet valve located at the panel using an external suction pump. Finally the method involves providing a flow of a flushing gas into the container via an inlet valve located at the panel to flush residual gas from the container. The total pressure of gases in the container can be monitored and controlled by adjusting the flow of gases in the inlet and outlet holes or valves.