  Dust and associated depletion of heavy elements from the gas phase can modify
the thermal properties of HII regions from the dust-free case, with significant
consequences for the emergent optical spectrum. We present the results of
theoretical calculations illustrating the effects of grains on the spectra of
giant, extragalactic HII regions, with emphasis on high metallicity systems
(i.e. solar and higher Z). Dust provides a simple explanation for the
observational absence of pure Balmer-line spectra that are expected on
theoretical grounds for dust-free, chemically enriched nebulae. Grains may also
play a role in enhancements of forbidden-line emission observed in HII regions
in the enriched nuclei of normal galaxies. In most cases, depletion introduces
the strongest perturbations to the optical spectrum. Selective absorption of
the ionizing continuum as well as heating by grain photoelectrons are important
in some instances, however, and grain heating can be particularly important for
enhancing emission in high-ionization lines. Allowing for depletion, the
presence of dust is unlikely to introduce large errors in global metallicity
indicators, although uncertainties in depletion factors coupled with the
sensitivity of infrared cooling to electron density will make accurate
calibrations difficult at high Z. The present calculations establish further
that previous relative abundance analyses that fail to take into account dust
effects in a self-consistent way (grain heating as well as depletion) may
overestimate temperature gradients in high-Z nebulae, resulting in errors in
relative abundances for different elements.
