¹²CO and ¹³CO excitation ladders of NGC 253 with the flux of each transition plotted as black asterisks with red error bars. In dotted blue, green, and yellow lines we plot the LVG I, II, and III ISM phases with their filling factors. The composite model is plotted with a solid red line. The model density, temperature, and column density are shown in the legend along with the relative contribution of each phase in terms of emission and column density.