A self-consistent relation between metal-impurity and light-impurity concentrations in an Ohmically heated tokamak plasma is obtained in a zero-dimensional model with approximations in which the sputtering yields of the constituent metal are proportional to the boundary plasma temperature and the radiation loss power of an impurity atom is constant. Comparison of this relation with data from actual tokamak discharges supports these assumptions. A parametric study of the relation demonstrates its contribution to the understanding of fundamental aspects of plasma-wall interaction.