We have investigated the mechanism of scanning tunnelling microscope (STM)-induced oxidation of a hydrogen passivated silicon surface in air. The influence of the relative air humidity, the Si doping concentration and the type and applied tip-sample voltage on the oxidation process was studied. The relative air humidity is crucial, if the relative air humidity drops below 10% no oxidation is observed. The width of the oxide lines increases with increasing tip-sample voltage, The voltage required to produce oxide lines on p-type Si is -1.1 V and does not depend on the doping concentration. For n-type Si, the threshold voltage for oxidation varies from -2.4 V for heavily doped to -3.5 V for lightly doped. A simple model is developed in which the electrical field between the tip (estimated as a sphere) and the substrate was calculated. We assume that a critical field is required to induce the oxidation. With this model, a good fit can be obtained with the experimentally found line-width dependence of the voltage. In addition, the effect of the doping type and concentration can be understood if depletion, in the case of n-type silicon, is included.