Percentage of the different formation routes of H₂O as a function of the visual extinction for the model P_th/k= 10⁸ K 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 increase in the ion–neutral formation route around A_v = 0.2 is due to the increase in OH⁺ following neutral-neutral formation of OH. It is therefore associated with warm chemistry.