Experiments show that benign termination is only possible within a finite range of neutral pressures. When the neutral pressure is too low, the temperature cannot be reduced sufficiently for the companion plasma to recombine, while at too high neutral pressures runaway electron impact ionization leads to a re-ionization of the companion plasma. In this contribution, we analyze experimental data from TCV using a particle balance model, and show that the upper neutral pressure limit is caused by the enhanced ionization from runaway electrons. We put constraints on the deuterium and neon densities in the vessel and formulate a number of questions which must be answered in order to quantify the upper neutral pressure limit in reactor-scale tokamaks.