  We point out that the Pauli blocking of neutrinos by cosmological relic
neutrinos can be a significant effect. For zero-energy neutrinos, the standard
parameters for the neutrino background temperature and density give a
suppression of approximately 1/2. We show the effect this has on three-body
beta decays. The size of the effect is of the same order as the recently
suggested neutrino capture on beta-decaying nuclei.
