  Long standing problem in out-of-equilibrium thermal field theories are
pinching singularities. We find that the expressions suspect of pinching
require loop particles to be on the mass shell. This fact, with the help of
threshold effect and similar effect due to spin, leads to the elimination of
pinching in single self-energy insertion approximation to propagator in all
propagators appearing in QED and QCD under very mild restrictions on particle
densities.
  This, together with the cancellation of collinear singularities, allows the
extraction of useful physical information contained in the imaginary parts of
the two loop diagrams.
  In some cases of interest ($\pi-\rho $ interaction, electro-weak interaction,
decay of Higgs particle, ...) none of the mentioned mechanisms works and one
has to resort to the resummed Schwinger-Dyson series. These cases are more
sensitive to the limitations related to the finite time range.
