Comparison of integrated intensities HCO⁺/HCN vs. HNC/HCO⁺ in M 33 (red filled circles and square; arrows indicate upper/lower limits) with values found in the LMC (Chin et al. 1997, 1998) (CH97, CH98; blue diamonds) and in luminous infrared galaxies compiled by Baan et al. (2008) (BA08; open symbols) and by Costagliola et al. (2011) (CO11; crosses). The dotted, dashed, and dot-dashed lines shows HNC/HCN = 1, 0.4, and 0.17 (stacked value of M 33), respectively. The gray shaded area shows the range of the observed HCO⁺/HCN ratios in M 31 by BR05.