Literature comparison of fits best describing HCN/CO as a function of Σ_mol. We include estimates from Milky Way clouds (Tafalla et al. 2023, dashed green line), fits of individual environments and full FoV in M51 (this work, solid lines), fits based on HCN/CO at 260 pc resolution in NGC 4321 (Neumann et al. 2024), and fits based on 31 local spiral galaxies from the ALMOND (Neumann et al. 2023a) and EMPIRE (Jiménez-Donaire et al. 2019) surveys at ∼kiloparsec resolution (Neumann et al. 2025). We add our HCN/CO observations for visual comparison for each environment within M51 (open colored circles).