Abstract
Considering the prototypical case of NGC 6153, it is shown that photoionization models comprising two components with different chemical compositions can account quantitatively for most of the spectral features of those planetary nebulae in which the optical recombination lines seem to indicate abundances much larger than the collisionally excited lines. Contrary to a common belief, the recombination lines then no longer reflect the average nebular abundances. In particular, fluctuations of composition within PNe may lead to overestimation of the helium abundance.
