Metal-to-dust ratio versus sight-line extinction for GRB afterglows. The horizontal dashed lines marks the selection criterion for GRBs to enter this sample. Vertical dashed lines illustrate metal-to-dust ratios of 1, 10 or 100 times the Galactic value. Solid lines show the toy model of two physically independent absorbers, where one is fully devoid of dust with a H-equivalent metal column of N_H,X = 10^21.7 cm^-2 and represented by the dotted line, while the other is neutral and has metal-to-dust ratios of 1, 2 or 3 times the Local Group value. Two individual cases (GRBs 080605 and 080607) illustrate the scatter between the analysis of different data sets.