  We have investigated the role of the cyclotron emission associated to cosmic
magnetic fields on the evolution of CMB spectral distortions by considering the
contributions by spontaneous and stimulated emission and by absorption in the
computation of the photon and energy injection rates. These cyclotron emission
rates have been numerically compared with those of the relevant radiative
processes operating in the cosmic plasma, bremsstrahlung and double Compton
scattering, for realistic CMB distorted spectra at early and late epochs. For
reasonable magnetic field strengths we find that the cyclotron emission
contribution is much smaller than the bremsstrahlung and double Compton
contributions, because of their different frequency locations and the high
bremsstrahlung and double Compton efficiency to keep the long wavelength region
of the CMB spectrum close to a blackbody (at electron temperature) during the
formation of the spectral distortion. Differently from previous analyses, we
find that for a very large set of dissipation mechanisms the role of cyclotron
emission in the evolution of CMB spectral distortions is negligible and, in
particular, it cannot re-establish a blackbody spectrum after the generation of
a realistic early distortion. The constraints on the energy exchanges at
various cosmic times can be then derived, under quite general assumptions, by
considering only Compton scattering, bremsstrahlung, and double Compton, other
than, obviously, the considered dissipation process. Finally, upper limits to
the CMB polarization degree induced by cyclotron emission have been estimated.
