  An ionization front expanding into a neutral medium can be slowed-down
significantly by recombinations. In cosmological numerical simulations the
recombination rate is often computed using a 'clumping factor', that takes into
account that not all scales in the simulated density field are resolved. Here
we demonstrate that using a single value of the clumping factor significantly
overestimates the recombination rate, and how a local estimate of the clumping
factor is both easy to compute, and gives significantly better numerical
convergence. We argue that this lower value of the recombination rate is more
relevant during the reionization process and hence that the importance of
recombinations during reionization has been overestimated.
