Thermal o/p ratios for ﹩{{\rm{H}}}_{2}\mathrm{CO}﹩, ﹩{{\rm{H}}}_{2}{\rm{O}}﹩ and ﹩{\mathrm{NH}}_{3}﹩, as given by the Boltzmann distribution of the ortho and para levels. The different markers show measurements toward disks (4: orange and red), comets (1, 2, 3: gray), low-mass protostars (5, 6: light blue), high-mass protostars (7–9: pink), PDRs (10–12: green and blue diamonds), and prestellar cores (5: blue). Sources with o/p ratios measured for ﹩{{\rm{H}}}_{2}\mathrm{CO}﹩ and another molecule are highlighted with red edges. References: (1) Shinnaka et al. (2016), (2) Bonev et al. (2013), (3) Gicquel et al. (2014), (4) Salinas et al. (2016), (5) Jørgensen et al. (2005), (6) Umemoto et al. (1999), (7) Emprechtinger et al. (2010), (8) Choi et al. (2015), (9) Melnick et al. (2010), (10) Choi et al. (2014), (11) Cuadrado et al. (2017), (12) Guzmán et al. (2011).