OH abundance vs. . N_OH is inferred from 1665 MHz (squares) or 1667 MHz absorption (circles) and from ¹³CO(1–0) emission. Absorption features associated with H II regions are shown in black; those not associated are shown in red. Triangles denote upper limits on (inferred from the non-detection of ¹³CO). The right axis shows the OH abundance in units of literature (molecular) OH abundance of 1 × 10⁻⁷ (e.g., Liszt & Lucas 1999, 2002, indicated also by the dashed gray line). The black error bars in the lower left corner show the systematic errors (only the upper halfs of the error bars are shown here).