Abstract In the radiative regime non-intrinsic impurities have been used to produce divertor power loads which would be considered acceptable when extrapolated to ITER. However, it has been a matter of concern that the maximum Z eff currently deemed acceptable for ITER has been exceeded by a large margin in radiative plasma experiments in large divertor machines such as JET, JT60-U, AUG and DIII-D. This paper points out that without a suitable scaling law, comparisons of Z eff between current machines and ITER are meaningless. Results from a multi-machine database are presented which show that there appears to be a remarkably simple and robust scaling which relates Z eff to line averaged density, total radiated power and main plasma surface area. A similar scaling has been found in code simulations with EDGE2D and DIVIMP. The consequences for ITER are discussed.