  Molecules containing one or a few hydrogen atoms and a heavier atom
(hydrides) have been predicted to trace FUV radiation. In some chemical models,
FUV emission by the central object or protostar of a star forming region
greatly enhances some of the hydride abundances. Two massive regions, W3 IRS5
and AFGL 2591, have been observed in hydride lines by HIFI onboard the {\it
Herschel Space Observatory}. We use published results as well as new
observations of CH$^+$ towards W3 IRS5. Molecular column densities are derived
from ground state absorption lines, radiative transfer modeling or rotational
diagrams. Models assuming no internal FUV are compared with two-dimensional
models including FUV irradiation of outflow walls. We confirm that the effect
of FUV is clearly noticeable and greatly improves the fit. The most sensitive
molecules to FUV irradiation are CH$^+$ and OH$^+$, enhanced in abundance by
many orders of magnitude. Modeling in addition also full line radiative
transfer, Bruderer et al (2010b) achieve good agreement of a two-dimensional
FUV model with observations of CH$^+$ in AFGL 2591. It is concluded that CH$^+$
and OH$^+$ are good FUV tracers in star-forming regions.
