  We discuss the effective action for QCD gauge fields at finite temperatures
and densities, obtained after integrating out the hardest momentum scales from
the system. We show that a non-vanishing baryon density induces a charge
conjugation (C) odd operator to the gauge field action, proportional to the
chemical potential. Even though it is parametrically smaller than the leading C
even operator, it could have an important effect on C odd observables. The same
operator appears to be produced by classical kinetic theory, allowing in
principle for a non-perturbative study of such processes.
