Top: Milky Way space-averaged mean atomic and molecular densities from Pineda et al. (2013). Middle: mean densities 2⟨ n(H₂)⟩ derived from CH absorption with X(CH) = 3.5 × 10^-8 shown as a solid line, color coded as in previous figures); ⟨ n(H)⟩ derived from C II absorption using n(C⁺)/n(H) = 1.4 × 10^-4 (filled light gray rectangles); and n(H I) derived from from a nearby NGPS survey H I emission profile assuming a constant spin temperature of 135 K, shown as a dashed gray line. Bottom: the CO-H₂ conversion factor derived by dividing the mean molecular density ⟨ n(H₂)⟩ by the CO abundance A_CO assuming that the CO-emitting gas is on the near side. The histogram of A_CO is overlaid and heavily shaded with an arbitrary vertical scale in the bottom panel.