Observed ¹²CO(4–3)/¹²CO(2–1) and ¹²CO(4–3)/¹²CO(3–2) ratios compared with predictions by the PDR Toolbox. The shaded areas, constructed by logarithmic interpolation between the points in the PDR Toolbox density grid, show the evolution of the ¹²CO(4–3)/¹²CO(2–1) and ¹²CO(4–3)/¹²CO(3–2) brightness temperature ratio as a function of the FUV field in the allowed density range of the Musca cloud. At a typical density = 4 × 10² cm⁻³ of the ambient cloud (dashed line) for the allowed FUV field strength (<1 G₀), the predicted line ratios are only a fraction of the observed ones in the blueshifted component.