Radiative losses from optically thin plasma as a function of electron temperature for selected values of ionization age, derived from SPEX version 3.08.01 (Kaastra et al. 1996, 2018). The upper left panel illustrates the cooling function assuming solar metal abundances from Lodders et al. (2009), highlighting the dependence on ionization time. The remaining panels show radiative losses for plasmas composed of the individual elements indicated in the labels.