Metals-to-dust ratio versus metallicity for the GRB sub-sample. The metallicities based on Zn are marked with red stars. The black circles represent Si-derived metallicity (see Table 6). GRB 080607 is marked by a red square. The small circles and stars illustrate metallicity or metals-to-dust ratio upper limits. MW, LMC and SMC environments are denoted with diamonds from right to left. The blue triangles correspond to four data points taken from Schady et al. (2010). The blue dashed line represents the best fit to the three Local Group environments and the four GRBs from Schady et al. (2010). The blue triangle with dotted error bars is GRB 050730 which is also included in both Schady et al. (2010) and our sample.