The upper flammability limits (UFL) of hydrogen air, methane air, ethane air, n-butane air, and ethylene air were determined experimentally at room temperature (20 degrees C) and initial pressure of 1.0, 0.7, 0.5, 0.3, 0.1, and 0.05 atm. Experiments were conducted in a closed cylindrical stainless steel vessel (i.d. 10.22 cm, length 100 cm) with upward flame propagation. The UFL of hydrogen was observed to be inversely proportional to the initial pressure in the range from 1.0 to 0.3 atm and proportional to the initial pressure from 0.3 to 0.05 atm. In contrast, the UFLs of the lower alkanes and ethylene decreased with the initial pressure. The average flame propagation velocities at UFL concentrations of hydrogen, methane, ethane, n-butane, and ethylene in air at reduced pressures were also examined. It was found that the flame propagation velocity of hydrogen was larger than those of the hydrocarbons, increased when the initial pressure decreased from 1.0 to 0.3 atm, and then decreased with further decrease of pressure. Flame propagation velocities at UFL concentrations of the hydrocarbons decreased with the initial pressure. Finally, based on the behavior of the UFLs and flame propagation velocities, the relative risk and hazards of ignition and flame escalation of hydrogen and the light hydrocarbons at subatmospheric pressures were discussed.