Ratio of line fluxes of [O III] lines at 52 and 88 μm in units of erg cm⁻² s⁻¹ as a function of logarithm of electron density. The lines show non-LTE radiative-transfer models for a gas electron temperature of 8000 ± 400 K derived from the GLOSTAR survey (Khan et al. 2024). The observed ratios toward boxes 3–6 along the DR21 outflow, where both [O III] lines are firmly detected, are shown as light blue lines, and for the area of the H II region in GLOSTAR (see Fig. 4) as dashed black line.