The aim of this work is to improve the treatment of density effects in non local thermodynamic equilibrium plasmas. The density effect on atomic structure (wavefunctions and energy levels) is modeled by an ion-sphere potential. The modification of the atomic potential, continuum lowering and appearance of resonances are shown. In particular, we show that the continuum resonances are linked to the electrons in the subshells passed into the continuum. Their presence determine the existence of partially bound configurations, which must be taken into account in the collisional-radiative model. We introduce in the set of rate equations a supplementary ionization process due to the plasma environment. This process (and its inverse) enters into the balance of all of the other process. It is equivalent to tunneling ionization where an outer electron located above the ionization threshold (and trapped by the potential barrier) crosses the barrier. As an application, we studied the influence of temperature and density on the average ionization and the ionic populations of a carbon plasma. We compared these calculations with the traditional method based on the chemical picture with continuum lowering.