  The sensitive infrared telescopes, Spitzer and Herschel, have been used to
target low-metallicity star-forming galaxies, allowing us to investigate the
properties of their interstellar medium (ISM) in unprecedented detail.
Interpretation of the observations in physical terms relies on careful modeling
of those properties. We have employed a multiphase approach to model the ISM
phases (HII region and photodissociation region) with the spectral synthesis
code Cloudy. Our goal is to characterize the physical conditions (gas
densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel
Dwarf Galaxy Survey. We are particularly interested in correlations between
those physical conditions and metallicity or star-formation rate. Other key
issues we have addressed are the contribution of different ISM phases to the
total line emission, especially of the [CII]157um line, and the
characterization of the porosity of the ISM. We find that the lower-metallicity
galaxies of our sample tend to have higher ionization parameters and galaxies
with higher specific star-formation rates have higher gas densities. The [CII]
emission arises mainly from PDRs and the contribution from the ionized gas
phases is small, typically less than 30% of the observed emission. We also find
correlation - though with scatter - between metallicity and both the PDR
covering factor and the fraction of [CII] from the ionized gas. Overall, the
low metal abundances appear to be driving most of the changes in the ISM
structure and conditions of these galaxies, and not the high specific
star-formation rates. These results demonstrate in a quantitative way the
increase of ISM porosity at low metallicity. Such porosity may be typical of
galaxies in the young Universe.
