  We present results from Monte Carlo simulations describing the radiation
transfer of $H\alpha$ line emission, produced both by HII regions in the disk
and in the diffuse ionized gas (DIG), through the dust layer of the galaxy
NGC891. This allows us to calculate the amount of light originating in the HII
regions of the disk and scattered by dust at high $z$, and compare it with the
emission produced by recombinations in the DIG. The cuts of photometric and
polarimetric maps along the $z$-axis show that scattered light from HII regions
is still 10\% of that of the DIG at $z\sim 600$~pc, whereas the the degree of
linear polarization is small ($<1$\%). The importance of these results for the
determination of intrinsic emission line ratios is emphasized, and the
significance and possible implications of dust at high $z$ are discussed.
