Ultraviolet radiation (UVR) emitted during arc welding frequently causes acute health effects such as keratoconjunctivitis and erythema. The extent of the hazard of the UVR varies depending on the welding method and on the welding conditions. For this reason, it is important to identify the levels of UVR that are present under various conditions. In this study, we experimentally evaluated the hazard of UVR emitted in tungsten inert gas welding (TIG) and metal inert gas welding (MIG) of magnesium alloys according to the American Conference of Governmental Industrial Hygienists (ACGIH) guidelines. The distance between the arc and the UVR detector was 500 mm. The effective irradiances measured in this study are in the range 0.85～8.9 mW/cm2. The maximum allowable exposure times corresponding to these levels are 0.3～3.5 s/day, extremely small values for daily accumulated exposure times. This demonstrates that exposure to UVR emitted by TIG and MIG of magnesium alloys is quite hazardous in practice. In addition, we found the following properties of the hazard of UVR. (1) Hazards of ultraviolet radiation generated during welding of magnesium alloy are stronger MIG than TIG. (2) It is more hazardous at higher welding currents. (3) Hazard increases in TIG when the shielding gas includes helium.