Cosmic ray horizons of ¹⁴N (left) and ⁵⁶Fe (right) in the background photon fields. The widely used energy loss length (dashed red) overestimates the effect of interactions. The L_FD horizons correspond to the distance where the full disintegration probability is 99%: in dot-dashed green the values for the homogeneous case, in solid purple the values assuming coherent inhomogeneities, and in dotted orange the values obtained numerically treating the redshift dependence of the rates and adiabatic losses.