Scatter plot in the plane (R_P/p,  R_S/V) of the 355 measurements of the polarization ratios measured from the Planck maps smoothed with a Gaussian kernel of FWHM of 15′ and the quasar data. Horizontal and vertical dashed lines mark the cuts suggested in Sect. 4.1 to retrieve a Δ_S/V distribution that agrees with the hypothesis of uniformity at the 2σ level. Our analysis does not allow us to argue for dust contamination for the quasars corresponding to polarization ratio values below these thresholds.