It is Kinetics science which deals with how fast things happen: instantly (i.e. more quickly than monitored, as in explosions), evolving at a sizeable pace (i.e. in the monitoring time-span, as in combustion), or at a negligible rate (i.e. more slowly than monitored, as in slow oxidation). For instance, a piece of paper enclosed in a transparent container with more air than the theoretical one, may not burn completely if ignited (e.g. by a concentrated light), because, as oxygen concentration gets reduced, convection and diffusion might not supply enough oxygen to maintain the minimum heat release needed for propagation. Hydrocarbon fuels cannot burn in N2/O2 mixtures if xO2<12% (<5% for H2 fuel), and hydrocarbon fuels cannot burn in CO2/air mixtures if xO2<15% (<6% for H2 fuel). At room temperature, without ignition, the piece of paper in air will oxidize very slowly (unnoticeable to the eye).