It is broadly accepted that CO is a reliable tracer of H2 in massive IR (LIR ≳ 109 L⊙) galaxies, and that there are clear correlations between LIR and L'CO that are qualitatively independent of environment and even redshift. We present two tales on the search for 12CO emission from dusty star-forming galaxies in both field (Lockman Hole, z < 0.1) and cluster (Zw Cl0024.1+1652, z ∼ 0.4) environments, according to the capabilities of the EMIR receiver at the IRAM-30m telescope. The observed galaxies are part of two follow-up programs in the millimetre regime of the spectroscopic Lockman-SpReSO and GLACE surveys in the optical (OSIRIS / 10.4m GTC). From these data we derived LIR and L'CO estimations and put them in the framework of the historic records according to the literature for each environmental case. We provide insights about some practical limits of the current facilities (IRAM observatories, ALMA, LMT) to get reliable estimations for IR at low and intermediate redshifts. Our results suggest that the amount of cold gas and the star formation efficiency increase with the cluster-centric distance, hence pointing to an environmental dependency.