  We evaluate the degradation of the accuracy of the component separation
between the cosmic microwave background (CMB) and foreground components caused
by neglect of absorption of the monopole component of the CMB by the galactic
interstellar matter. The amplitude of the temperature anisotropy caused by the
CMB shadow, due to dust components, is about 1 uK. This value is comparable to
the required noise level necessary to probe non-Gaussianity studies with
upcoming CMB experiments. In addition, the amplitude of the polarization caused
by the CMB shadow due to dust is comparable to or larger than the RMS value of
the CMB B-mode polarization, imprinted by primordial gravitational waves. We
show that applying a single-power law model as the dust spectrum to observed
multifrequency data introduces systematic errors, which are comparable to or
larger than the required noise level for forthcoming CMB B-mode polarization
experiments. Deducing the intrinsic spectrum of dust emission from the
submillimeter waveband data reduces systematic error below the required noise
level. However, this method requires dust temperature measurements with an
accuracy of better than a few percent. We conclude that the CMB shadow due to
dust must be considered in future CMB missions for achieving their targeted
sensitivity. Our results will be important to detect the primordial CMB B-mode
polarization, with the amplitude of the tensor-to-scalar ratio of r=10^{-3}.
