The phenomena of neutrino oscillations can proceed only in the case of the coherent superposition of neutrino mass states. An external environment can modify a neutrino evolution in a way that conditions for the coherent superposition of neutrino mass states are violated. Such a violation results in quantum decoherence of neutrino states and leads to suppression of flavor neutrino oscillations. We сonsider the influence of the neutrino radioactive decay in dense media and an external electromagnetic field on neutrino oscillations, and the corresponding damping of neutrino oscillations in the presence of an electron media and electromagnetic field is calculated. The formalism of quantum electrodynamics of open systems is used in the performed evaluations. The studied phenomena can be significant for description of neutrino oscillations in extreme conditions of astrophysical environments peculiar to supernovae, neutron stars or quasars.