  We derive the chlorine abundances in a sample of nearby planetary nebulae
(PNe) and H II regions that have some of the best available spectra. We use a
nearly homogeneous procedure to derive the abundance in each object and find
that the Cl/H abundance ratio shows similar values in H II regions and PNe.
This supports our previous interpretation that the underabundance we found for
oxygen in the H II regions is due to the depletion of their oxygen atoms into
organic refractory dust components. For other elements, the bias introduced by
ionization correction factors in their derived abundances can be very
important, as we illustrate here for sulfur using photoionization models. Even
for low-ionization PNe, the derived sulfur abundances can be lower than the
real ones by up to 0.3 dex, and the differences found with the abundances
derived for H II regions that have similar S/H can reach 0.4 dex.
