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We discuss the evolution of the ratio in number of recombinations due to 2stwo photon escape and due to the escape of Lyman-$\alpha$ photons from theresonance during the epoch of cosmological recombination, within the width ofthe last scattering surface and near its boundaries. We discuss how this ratioevolves in time, and how it defines the profile of the Lyman-$\alpha$ line inthe spectrum of CMB. One of the key reasons for explaining its time dependenceis the strong overpopulation of the 2p level relative to the 2s level atredshifts $z \la 750$.

Relative contribution of the hydrogen 2S two-photon decay and Lyman-alpha escape channels during the epoch of cosmological recombination