  We study the effect of finite temperatures and Fermion density on the
effective pseudo-scalar-photon vertex induced by the triangle diagram. The
manifestly covariant calculations show that when the pseudo-scalar mass is much
less than the temperature, then there is a large enhancement of the decay rate.
Alternatively, when the temperature is much higher than the Fermion mass, or
the Fermion chemical potential is large, the lifetime is enhanced. Other
related processes, and applications of these results to cosmology and
astrophysics, are discussed.
