We study the contamination of the B-mode of the cosmic microwave background polarization (CMBP) by Galactic synchrotron in the lowest emission regions of the sky. The 22.8-GHz polarization map of the 3-yr Wilkinson Microwave Anisotropy Probe (WMAP) data release is used to identify and analyse such regions. Two areas are selected with signal-to-noise ratio S/N < 2 and S/N < 3, covering ∼16 and ∼26 per cent fraction of the sky, respectively. The polarization power spectra of these two areas are dominated by the sky signal on large angular scales (multipoles ℓ < 15), while the noise prevails on degree scales. Angular extrapolations show that the synchrotron emission competes with the CMBP B-mode signal for tensor-to-scalar perturbation power ratio T/S= 10−3–10−2 at 70 GHz in the 16 per cent lowest emission sky (S/N < 2 area). These values worsen by a factor ∼5 in the S/N < 3 region. The novelty is that our estimates regard the whole lowest emission regions and outline a contamination better than that of the whole high Galactic latitude sky found by the WMAP team (T/S > 0.3). Such regions allow T/S∼ 10−3 to be measured directly which approximately corresponds to the limit imposed by using a sky coverage of 15 per cent. This opens interesting perspectives on the investigation of the inflationary model space in lowest emission regions.