Photon-photon interactions can be a source of CMB circular polarization
  Photon-photon interactions, as described by the standard Heisenberg-Euler interaction, can transform plane polarization of the CMB into circular polarization, in the period right after last scattering. We estimate the standard deviation of the resulting circular polarization parameter, as constrained by confining observations to angular regions of large plane polarization, and find results of the order of $10^{-9}$ for the Stokes parameter $V$. 
R. F. Sawyer
astro-ph.CO