The CH⁺ line strength per unit reddening is shown as a function of the ISRF strength, I_UV. Highest values for I_UV are found for low CH⁺ abundances per unit reddening. Because N(CH)  ∝  N(H₂) and E_(B−V)  ∝  H i Eq. (2) gives I_UV/n_H  ∝  N(CH⁺)/E_(B−V). There is some evidence for enhanced CH⁺ production (UV pumping?) in clouds with moderate I_UV  ~  5.