Clump-envelope structure of a molecular cloud in an environment of (a) high metallicity and (b) low metallicity, suggested by Koda et al. (2022) based on previous studies (e.g., Maloney & Black 1988; Wolfire et al. 2010). In the low-metallicity environment (b), CO molecules are selectively photo-dissociated in the envelope (orange) by the external UV radiation field, producing a thick CO-deficient H₂ layer (gray), which substantially reduces the CO(2–1) and CO(1–0) emission. The dense star-forming clumps (brown and their immediate surroundings) reside at the hearts of the clouds; the CO molecules are protected there, and the clumps can remain bright in CO(3–2).