  Context: Cosmic rays are present in almost all phases of the ISM. PAHs and
cosmic rays represent an abundant and ubiquitous component of the interstellar
medium. However, the interaction between them has never before been fully
investigated. Aims: To study the effects of cosmic ray ion (H, He, CNO and
Fe-Co-Ni) and electron bombardment of PAHs in galactic and extragalactic
environments. Methods: We calculate the nuclear and electronic interactions for
collisions between PAHs and cosmic ray ions and electrons with energies between
5 MeV/nucleon and 10 GeV, above the threshold for carbon atom loss, in normal
galaxies, starburst galaxies and cooling flow galaxy clusters. Results: The
timescale for PAH destruction by cosmic ray ions depends on the electronic
excitation energy Eo and on the amount of energy available for dissociation.
Small PAHs are destroyed faster, with He and the CNO group being the more
effective projectiles. For electron collisions, the lifetime is independent of
the PAH size and varies with the threshold energy To. Conclusions: Cosmic rays
process the PAHs in diffuse clouds, where the destruction due to interstellar
shocks is less efficient. In the hot gas filling galactic halos, outflows of
starburst galaxies and intra-cluster medium, PAH destruction is dominated by
collisions with thermal ions and electrons, but this mechanism is ineffective
if the molecules are in denser cloudlets and isolated from the hot gas. Cosmic
rays can access the denser clouds and together with X-rays will set the
lifetime of those protected PAHs. This limits the use of PAHs as a`dye' for
tracing the presence of cold entrained material.
