The net timescales (after subtraction of any reverse reaction rate) of the principle reactions that form or destruct H₂ are plotted versus altitude along a streamline launched at 1 AU for our Class I model. The flow crossing timescale t_f is also plotted at the bottom for comparison. Thin curves denote reactions whose net effect leads to destruction, and thick curves those that lead to net creation. The reactions are H + H = H₂: catalytic formation of H₂ on grains; H₂ + hν = H + H: photodissociation (with the minimum self-shielded rate shown on a separate curve); : X-ray ionization; O + 2H₂ = H₂O + 2H: combined oxydation reactions with O and OH; H₂ + H and H₂ + H₂: collisional dissociation; The latter becomes very efficient near z ≃ 2 AU, where the wind temperature reaches 4000 K.