Isotopic intensity ratios for the traditional dense-gas tracers as a function of H₂ column density for California, Perseus, and Orion A. The N(H₂) range has been reduced compared to other plots due to the low signal-to-noise ratio of the data with low N(H₂). The dashed horizontal lines represent)he standard interstellar medium isotopic ratios (77 for ¹²C/¹³C and 22 for ³²S/³⁴S), andthe low value of the observed ratios indicates that the emission of the main isotopotog is optically thick. Note how the outflow points from Orion A (near 2 × 10²³ cm⁻²) have lower optical depths in some tracers but are still optically thick. No temperature correction has been applied to the data.