  Star formation rates in galaxies are frequently estimated using the Balmer
line fluxes. However, these can be systematically underestimated because dust
competes for the absorption of Lyman continuum photons in the ionized gas. Here
we present theoretical correction factors in a simple analytic form. T These
factors scale as the product of the ionization parameter, ${\cal U}$, and the
nebular O/H abundance ratio, both of which can now be derived from the
observation of bright nebular line ratios. The correction factors are only
somewhat dependent upon the photoelectron production by grains, but are very
sensitive to the presence of complex PAH-like carbonaceous molecules in the
ionized gas, providing that these can survive in such an environment.
