The local approximation is usually used for the electron distribution function (EDF) calculation, which is the most important plasma characteristic. This application greatly simplifies solving the Boltzmann kinetic equation and it is base of the widely used fluid model for plasma modeling. It is shown that at the plasma periphery local approximation is not valid even at high gas pressure because the ambipolar electric field practically always is larger than heating electric field. As a result, the applicability of local approximation for EDF depends on ratio between heating and ambipolar electric fields.