We have experimentally demonstrated the robustness and immunity of the surface states to surface impurities and disorder in topological insulator (TI) Bi0.9Sb0.1 in comparison with those of semi-metallic Sb(111) surface by measuring the scattering rates of the quasiparticles (QP) via angle-resolved photoemission spectroscopy. The surface impurities on TIs increase the scattering rate of the QPs but their effects are extrinsic and limited regardless of their magnetic properties, while in the semimetallic Sb case they open new scattering channels making the surface states fragile.