Abstract
Star formation rates in galaxies are frequently estimated using the Balmer line fluxes of their H II regions. However, these can be systematically underestimated because dust competes for the absorption of Lyman continuum photons in the ionized gas. This factor cannot be neglected, and in this paper we present theoretical correction factors in a simple analytic form in order to allow observers to take this effect into account when estimating star formation rates from Balmer lines. These factors scale as the product of the ionization parameter, 
, 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 polycyclic aromatic hydrocarbon-like carbonaceous molecules in the ionized gas, providing that these can survive in such an environment.
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