Two different effects of the blackbody-radiation-induced atomic line broadening are compared. The first one
(stimulated Raman scattering) has been discussed by many authors; the second one (quadratic level mixing) was
discussed in our previous work. It is shown that the mixing effect gives the most significant contribution to the
line broadening and it is indicated how to distinguish these two effects in laboratory experiments. The influence
of the level mixing on the recombination history of primordial plasma is also discussed.