  We present a model for nebular emission in star forming galaxies, which takes
into account the effects of dust reprocessing. The nebular emissions have been
computed with CLOUDY and then included into GRASIL, our spectrophotometric code
specifically developed for dusty galaxies. The interface between nebular
emission and population synthesis is based on a set of pre-computed HII region
emission models covering a wide range of physical quantities. Concerning the
extinction properties of normal star forming galaxies, we are able to interpret
the observed lack of correlation between the attenuation measured at Halpha and
in the UV band as a consequence of age selective extinction. We also find that,
for these galaxies with modest SFR, the ratio FIR/UV provides the best
constraints on the UV attenuation. Our model also allows to deal with different
SFR estimators in a consistent way, from the UV to radio wavelengths, and to
discuss the uncertainties arising from the different physical conditions
encountered in star forming galaxies. We provide our best estimates of
SFR/luminosity calibrations, together with their expected range of variation.
It results that SFR derived through Halpha, even when corrected for extinction
using the Balmer decrement, is affected by important uncertainties due to age
selective extinction. Another remarkable result is that SFR from UV luminosity
corrected by means of the ratio FIR/UV has a small uncertainty. Finally, our
model provides a calibration of SFR from radio luminosity; we are also able to
reproduce the observed FIR/radio ratio.
