At the last scattering surface (LSS), the energy density of neutrinos is argued to be ~0.68 of the energy density of blackbody photons, based on a thermodynamic equilibrium argument. This is also required to obtain the correct total energy density for stable expansion. At that time, the average BB photon energy (2.7kT) was ~0.7 ev with the neutrino energy a little lower. If neutrinos have mass, there is evidence that the combined mass of the 3 varieties is < 0.3 eV. I believe that particles with a significant interaction cross section are unstable (or exist in reduced numbers) at temperatures much higher than the energy eqivalent of their rest mass. Because of the low cross section of neutrinos, will this heat-instability effect not reduce the ~0.68 factor, or are neutrinos intrinsically stable with heat simply because they're leptons?