  Gigaparsec scale alignments of the quasar optical polarization vectors have
been proven to be robust against a scenario of contamination by the Galactic
interstellar medium (ISM). This claim has been established by means of optical
polarization measurements of the starlight surrounding the lines of sight of
the 355 quasars for which reliable optical polarization measurements are
available. In this paper, we take advantage of the full-sky and high quality
polarization data released by the \textit{Planck} satellite to provide an
independent, complementary, and up-to-date estimation of the contamination
level of the quasar optical polarization data by the Galactic dust. Our
analysis reveals signatures of Galactic dust contamination at the two sigma
level for about 30 per cent of the quasar optical polarization data sample. The
remaining 70 per cent of the lines of sight do not show Galactic dust
contamination above the two sigma level, suggesting low to negligible
contamination of the quasar optical polarization signal. We further found
arguments suggesting that Galactic thermal dust cannot fully account for the
reported quasar optical polarization alignments. Based on the measurements of
the ratio of the polarized intensity of the dust in the submillimeter to the
degree of linear polarization of the quasar in the optical, we provide a new
and independent quality criteria to apply to the quasar optical polarization
sample. We argue that, unless correction is applied, such a criterion should be
imposed on the data for future investigations in the framework of the
cosmological-scale correlations of quasar optical polarization vector
orientations that still could compete with the isotropic principle of the
cosmological paradigm.
