Nitrogen dioxide gas sensor devices, based on porous silicon, have been investigated in the sub-ppm range at room temperature (RT) monitoring humidity, illumination, gas concentration and flow rate. The devices, realized changing silicon doping, resistivity and etching parameters, have been characterized measuring the electrical conductance versus NC; in air at constant relative humidity (RH). It has been found that hysteresis and the sensor performances improve if a prolonged exposure to high concentrations of NO, is adopted as pre-treatment method. The investigation on stability and selectivity has been cēmpleted recording the device performances over several days in ambient air,