There are a number of situations when fires may occur at low pressures and oxygen concentrations that are different than standard atmospheric conditions, such as in buildings at high elevation, airplanes, and spacecraft. The flammability of materials may be affected by these environmental conditions. Since ignition delay is a measure of material flammability and directly influences whether a fire will occur, experiments were conducted to assess the variation of the ignition delay of PMMA in sub-atmospheric pressures and elevated oxygen concentrations. Three sets of experiments were performed at different pressures and in air, in an atmosphere having 30% oxygen/70% nitrogen by volume, and in a 'normoxic' atmosphere (constant oxygen partial pressure). It was observed that as the pressure is reduced, the ignition time decreased, reached a minimum, and then increased until ignition did not occur. Several mechanisms were considered to explain the 'U-shaped' dependence of ignition time on pressure, and three regimes were identified each having a different controlling mechanism: the transport regime where the ignition delay is controlled by changes in convection heat losses and critical mass flux for ignition; the chemical kinetic regime where the ignition delay is controlled by gas-phase chemical kinetics; and an overlap region where both the transport and chemistry effects are seen. The results provide further insight about the effect of the environmental conditions on the flammability of materials, and guidance about fire safety in low pressure environments.