Mean cross-section of the dust extinction per hydrogen atom as assumed in the radiative transfer model. Its functional dependence on wavelength is compared with the observed mean extinction for the diffuse ISM (Fitzpatrick 1999) (although extended to the FIR) and the mean extinction curve provided by Weingartner & Draine (2001) (corrected by a factor 0.93). Also shown are the contributions of the several dust components in the model.