A novice scuba (潜水初学者) diver practicing in a swimming pool takes enough air from his tank to fully expand his lungs before abandoning the tank at depth L and swimming to the surface. He ignores instructions and fails to exhale (呼气) during his ascent (上升) . When he reaches the surface, the difference between the external pressure on him and the air pressure in his lungs is 9.3kPa. From what depth L does he start? Is the diver safe when he reaches the surface?