  Inhomogeneous big bang nucleosynthesis (BBN) produces a spatially
inhomogeneous distribution of element abundances at $T \sim 10^9$ K, but
subsequent element diffusion will tend to erase these inhomogeneities. We
calculate the cosmological comoving diffusion length for the BBN elements. This
diffusion length is limited by atomic scattering and is therefore dominated by
diffusion when the atoms are neutral, between the redshifts of recombination
and reionization. We find that the comoving diffusion length today is $d_{com}
\approx 70$ pc for all of the elements of interest except $^7$Li, for which
$d_{com}$ is an order of magnitude smaller because $^7$Li remains ionized
throughout the relevant epoch. This comoving diffusion length corresponds to a
substellar baryonic mass scale and is roughly equal to the horizon scale at
BBN. These results lend support to the possibility that inhomogeneities on
scales larger than the horizon at BBN could lead to a spatially inhomogeneous
distribution of elements today, while purely subhorizon fluctuations at BBN can
result only in a homogeneous element distribution at present.
