a) Model of the evolution of EW_Hα in a star-forming galaxy under two different conditions for dust attenuation. The attenuation in the V band during the major star formation epoch is A_V = 1 mag. and the SFR is constant. The full drawn line shows how the equivalent width changes with age assuming constant dust attenuation. In this case EW_Hα is identical to the dust free case. In the second case, shown in the dashed line, we assume an age dependency in the attenuation. b) The ratio between EW_Hα in the two cases. The figure shows that EW_Hα in the age dependent dust attenuation case drops off up to ~50%.