  In this Letter, we argue that the standard analytical derivations of
properties of HII regions, such as the speed, shape and asymptotic position of
ionisation fronts require a more precise treatment. These derivations use the
on the spot approximation, which in effect ignores the diffuse component of the
radiation field. We show that, in fact, HII regions are diffusion dominated.
This has as a result that the morphology of inhomogeneous HII regions will be
drastically different, because shadowing effects have a less profound impact on
the apparent shape. Moreover, it will have influence on the propagation speed
of ionisation fronts. We quantify our claims by analytically deriving the
internal radiation structure of HII regions, taking diffusion fully into
account for several different cosmologically relevant density distributions.
