Metallicity Effects in PDRs
  Almost all properties of a photodissociation region (PDR) depend on its metallicity. The heating and cooling efficiencies that determine the temperature of the gas and dust, the dust composition, as well as the elemental abundances that influence the chemical structure of the PDR are just three examples that demonstrate the importance of metallicity effects in PDRs. PDRs are often associated with sites of star formation. If we want to understand the star formation history of our own Galaxy and of distant low-metallicity objects we need to understanding how metallicity acts on PDR physics and chemistry. 
Markus Roellig