  The cosmic microwave background (CMB) polarization, whose experimental
detection must become the next natural step in the study of the CMB radiation,
depends on the dynamics of hydrogen recombination in the Universe, on the
relation between the power of scalar perturbations and gravitational waves, and
on the presence or absence of secondary ionization. The spectrum of the CMB
anisotropy is currently known with an accuracy that is high enough to draw some
conclusions about the cosmological parameters and about the presence of
secondary ionization of matter. The reduction of observational data strongly
suggests the presence of secondary ionization. We consider the effect of this
ionization at redshifts $z<100$ on the CMB polarization generated by the scalar
mode on various angular scales.
