Lower limits to the grammage inferred for the Cygnus region (left) and Westerlund 1 (right) if the observed gamma rays are of hadronic origin in three models of gamma ray production as described in the text. These limits are shown together with an up-to-date computation of the Galactic CR grammage inferred from secondary/primary ratios (Schroer et al. 2021). The yellow shadowed region shows the maximum source grammage still compatible with secondary to primary ratios (≃0.4 g cm^–2).