  We investigate how the typical dust extinction of H-alpha luminosity from a
star-forming galaxy depends upon star formation rate (SFR), metallicity and
stellar mass independently, using a sample of ~90,000 galaxies from Data
Release 7 of the Sloan Digital Sky Survey (SDSS). We measure extinctions
directly from the Balmer decrement of each source, and while higher values of
extinction are associated with an increase in any of the three parameters, we
demonstrate that the fundamental property that governs extinction is stellar
mass. After this mass-dependent relationship is removed, there is very little
systematic dependence of the residual extinctions with either SFR or
metallicity, and no significant improvement is obtained from a more general
parameterisation. In contrast to this, if either a SFR-dependent or
metallicity-dependent extinction relationship is applied, the residual
extinctions show significant trends that correlate with the other parameters.
Using the SDSS data, we present a relationship to predict the median dust
extinction of a sample of galaxies from its stellar mass, which has a scatter
of ~0.3 mag. The relationship was calibrated for H-alpha emission, but can be
more generally applied to radiation emitted at other wavelengths. These results
have important applications for studies of high-redshift galaxies, where
individual extinction measurements are hard to obtain but stellar mass
estimates can be relatively easily estimated from long-wavelength data.
