Trapping efficiencies of CO and CO₂ in water ($F_{{\mathrm{H}}_2\mathrm{O}}$(CO,CO₂)), and CO in CO₂ ($F_{{\mathrm{C}\mathrm{O}}_2}$(CO)) plotted against the host: volatile ice ratio (left) and the ice layer thickness (right). The thickness is given in monolayer (ML), where 1 ML = 10¹⁵ molecules cm⁻². The grey shaded boxes show the H₂O:CO and H₂O:CO₂ ice ratios observed in a variety of interstellar sources, while the dashed lines show their largest and smallest median values for data presented in Boogert et al. (2015). The laboratory data used in this plot are presented in Table A.1.