Two different effects of the blackbody radiation (BBR)-induced atomic line broadening are compared. The first one (stimulated Raman scattering) was discussed by many authors, the second one (quadratic level mixing) was predicted earlier in our publication. 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.The influence of external fields on atomic characteristics is still one of the interesting subjects for investigations in modern atomic physics. In particular a question about the blackbody radiation (BBR) influence on atoms is widely discussed. First the BBR induced effects were observed experimentally and then the theoretical description was given in [1,2] within the frameworks of quantum mechanical (QM) approach. In particular, it was shown that the blackbody radiation induces the ac-Stark shift of energy levels and an additional line broadening in atoms. Theoretical calculations of the dynamic Stark shifts and depopulation rates of Rydberg energy levels caused by the BBR and the corresponding experimental measurements were widely discussed in literature [3]- [8]. The most important consequence of these investigations corresponds to the improvement of atomic clocks and the development of optical standards of frequency measurements [9].Finally, in [10] the effect of level mixing induced by the blackbody radiation was firstly described theoretically within the rigorous quantum electrodynamic (QED) theory. The mixing effect for the states of opposite parity arising in the presence of an external electric field leads to a significant changes of the decay rates, see, for example, [11,12]. We should note that all effects in the presence of the BBR are similar to the phenomena which take place in an external electric field. Similar to the Stark (static or dynamic) effect in the presence of 'ordinary' external electric field the energy shift of atomic levels induced by the BBR can be estimated with the use of rootmean square value of the field strength of thermal radiation (in a.u.):where E 2 (ω) is rms electric field strength, ω is the radiation frequency. The Planck's distribution function is presented by n β (ω) with β = 1/k B T , k B is the Boltzman's constant, T is the temperature in Kelvin and α is the fine structure constant. Then the integral rms value of electric field isIn conjunction with the expression (2) the level mixing effect induced by the thermal radiation can be introduced. The level mixing effect in an external electric field was considered in connection with the Lamb shift measurements in hydrogen and hydrogen-like ions [13,14] and the corresponding theoretical analysis of the electric field influence on atomic levels can be found in [15,16]. An accurate description of level mixing effect in hydrogen atom was given in [17]. In particular, the authors of [17] have sh...