I will summarize a recent estimation (arXiv:1605.07720) of the thermal masses and damping rates of active (m < eV) and sterile (M ∼ GeV)
neutrinos with thermal momenta k ∼ 3T at temperatures below the electroweak crossover
(5 GeV < T < 160 GeV). These quantities in turn fix the washout rates of Standard
Model lepton number densities and the thermal production rate of sterile neutrinos. These interact via direct scatterings mediated
by Yukawa couplings, and via their overlap with active neutrinos. I will review the calculation, which includes all leading order reactions. I will show that the resulting washout rate generally exceeds the Hubble rate for 5 GeV
< T < 30 GeV. Therefore it is challenging to generate a large lepton asymmetry facilitating dark matter computations operating at T < 5 GeV, whereas the generation of a baryon asymmetry at T > 130 GeV remains an option.