Ratio of line fluxes of CO 14–13 to CO 16–15 as a function of logarithm of H₂ density. The lines show non-LTE radiative-transfer models for gas kinetic temperatures of 200 K (light blue), 300 K (blue), 500 K (orange), and 1000 K (red), assuming collisions with H₂. The observations are shown as light blue boxes with the horizontal gray line indicating the median values of ratios for all boxes along the DR21 outflow.