The effect of the combination of various values of intrinsic extinction A_V (lines with labels) and redshift, producing an effective dimming in the R band as given on the y-axis. The solid lines have been computed assuming a dust extinction curve as described in Reichart (1999) but with no 2175 Å bump and an opacity due to intergalactic hydrogen approximated by τ(HI) = 2.6    ×    (1 + z)^3.3 for 700 nm/(1 + z) ≤ 121.6 nm (Valageas, Schaeffer & Silk 1999). Dots represent the GRBs of our sample for which we have an A_V measurement from the GROND SED. GRBs with effective R-band reduction of  > 0.5 mag are labeled; GRBs 080218B and 080915 lie above the correspondingly labeled dashed lines which correspond to the pairs of (z,A_V) values described in the text. Moderate A_V at moderate redshifts can easily produce a dimming of 1–3 mag in the R band.