OH abundance X_OH = N_OH∕N_H vs. N_H. The total column density of hydrogen nuclei, N_H, is inferred from ¹³CO(1–0) emission and H I absorption. As the H I column density represents lower limits, the OH abundances represent upper limits. Colors and symbols are as in Fig. 8. The right axis shows the OH abundance in units of the typical OH abundance in diffuse clouds of 4 × 10⁻⁸ (Crutcher 1979, indicated also by the dashed gray line).