CO line fluxes for the observed transitions. The ¹²CO and ¹³CO lines are normalized relative to the J = 4–3 and J = 6–5 lines, respectively. Observations of the Milky Way (Wright et al. 1991), the dense Orion Bar PDR (Habart et al. 2010), the ultraluminous galaxy Mrk231 (van der Werf et al. 2010), and the high redshift quasar APM08279+5255 (Weiss et al. 2007a) are compared. In IRAS 4A and 4B, the available maps are convolved to 20″ in order to compare similar spatial regions.