[CII] excitation (or spin) temperature, T^ex, as a function of total gas density n and kinetic temperature on interstellar gas, T^kin (computed for T^bg = 2.726 K). The upper (lower) panel is dedicated to optically thin (thick) medium. The black solid and dot-dash lines correspond to T^ex = 16 K and T^ex = 21 K, that are the CMB temperature at z = 5.0 and z = 6.7, respectively. At these redshifts, [CII] emission is suppressed for kinetic temperature below these lines, due to the CMB. For optically thin emission, this suppression affects mostly the cold neutral medium (T^kin ~ 50–120 K, n ~ 20–200 cm^-3).