In this work, rectangular visualization channels with different widths were used to study the shock wave and ignition from pressure release of hydrogen into air. The minimum pressure required for ignition was determined for each channel. Shock wave speed was also measured from the high-speed shadowgraph. The results found that with diminishing channel width, the minimum release pressure required for ignition is also decreased which suggests that most accidental release from a thin crack will favors spontaneous ignition. This result is in consistent with our observation that most accidental release from a leak of a pressure source did result in prompt ignition.