Comparison of a solar metallicity (metal-rich) molecular cloud and a low-metallicity cloud impacted by the UV photons of nearby star clusters. The decrease in dust shielding in the case of the low-metallicity cloud leads to further photodissociation of the molecular gas, leaving a layer of self-shielded H₂ outside of the small CO-emitting cores. This CO-dark H₂ can, in principle, be traced by [C II] or [C I].