The high brightness of the mid-J 13CO, mid-J and high-J 12CO
 lines observed in many UV excited sources require the presence
 of very high density clumps in PDRs. The presence of these
 extremely high density clumps within massive star forming
 regions can be tested unambigiuously through observations of
 certain PDR species, whose chemical abundance and excitation
 require high (n(H) < 10^7 cm^-3) or very high densities
 (n(H) < 10^8 cm-3). Such tracers are the H2O, OH and CH
 rotational lines. Theoretical models predict that these
 molecules are chemically produced very efficiently in the
 outer hot layers of high density PDRs.