The electrical injection in porous silicon fabricated with heavily doped p-type silicon is very sensitive to NO2. The known effect is an injection increase associated to NO2. We show experimentally a strong correlation between two structural properties and the sensitivity of electrical injection to NO2. The first property is the microstructure, i.e. the pore morphology at nm scale. A structure with straight, elongated pores shows large sensitivity, as opposed to a branching structure. The second property is the layer thickness, which determines the sign of the effect of NO2. If the thickness is sufficiently low – of the order of few μm – the injection in presence of NO2 decreases, instead of increasing.