text: Very high densities are required to thermalize the rotational transitions of HO, so it is difficult to detect far-infrared emission lines from a quiescent molecular cloud. Even at H densities of 10 cm, dust must be optically thick at infrared wavelengths.  But the passage of a shock wave through a molecular cloud is precisely the process which can bring the molecular gas out of equilibrium with the dust, making observations of far-infrared emission lines possible.  A moderately fast shock may produce a transient raise in the HO abundance relative to hydrogen.  So, it is possible that far-infrared emission lines of HO can be a good diagnostic of shock conditions.