Recently, open-type push-pull ventilation systems have been widely employed as effective substitutes for the conventional local exhaust ventilation system, and have prevailed at many welding workshops in Japan. In this study, the effect of the uniform velocity on carbon dioxide (CO₂) shielding arc welding was examined by laboratory experiments. The ventilation system examined in the experiments successfully fulfilled the requirement for open-type push-pull ventilators prescribed in Japanese regulations (ordinances). It was proved that the velocity at any points in the capture zone fell in the range of 50 to 150% of the average capture zone velocity. Welding defects could be avoided by controlling the flow rate of shielding gas. Unless the capture velocity exceeded a 0.8 m/s, the formation of blow-holes in the welded metal could be prevented at the shielding gas flow rate of 20 L/min. If the flow rate was provided at 30 L/min and 40 L/min, blow-holes didn't form at the capture velocity of 1.2 m/s and 1.6 m/s, respectively. At a capture velocity of faster than 0.3 m/s, the fume concentration at welder's breathing zone was reduced to a level below the limit values: ACGIH (TLV) and Japan Welding Engineering Society (CLV^#). These data are important for designing open-type push-pull ventilation in the welding workshop. The other engineering countermeasures currently employed in the welding work in Japan, such as fume collecting torch and general ventilation, are also concerned in this report. ^#: Control Limit Value