Percentage of the different formation routes of OH as a function of the visual extinction for the model P_th/k= 10⁸K cm^–3 and . The blue line represents the percentage of the ion–neutral formation route, and the red line represents the percentage of the neutral–neutral formation route, which requires high temperatures (T_k ≳ 300 K). The green line represents the percentage of the formation of OH via H₂O photodissociation. The orange line represents the percentage of the formation of OH via radiative association.