Comparison between Spitzer MIPS24 and WISE W4 photometry as a function of W4 S/N (see colour bar at right). The open circles in the background denote upper limits. W4 measurements systematically overestimate the actual fluxes at low S/N. The black solid (dotted) horizontal line marks the median (lowest 5 th percentile) flux for the objects in our sample without performing the quality cuts described in the text. The same holds for the red lines where the S/N≥2 cut is also applied. A significant amount of our sources would have systematically overestimated W4 fluxes.